Showing papers in "Biogeochemistry in 2004"

Nitrogen cycles: past, present, and future

Abstract: This paper contrasts the natural and anthropogenic controls on the conversion of unreactive N2 to more reactive forms of nitrogen (Nr). A variety of data sets are used to construct global N budgets for 1860 and the early 1990s and to make projections for the global N budget in 2050. Regional N budgets for Asia, North America, and other major regions for the early 1990s, as well as the marine N budget, are presented to highlight the dominant fluxes of nitrogen in each region. Important findings are that human activities increasingly dominate the N budget at the global and at most regional scales, the terrestrial and open ocean N budgets are essentially dis- connected, and the fixed forms of N are accumulating in most environmental reservoirs. The largest uncertainties in our understanding of the N budget at most scales are the rates of natural biological nitrogen fixation, the amount of Nr storage in most environmental reservoirs, and the production rates of N2 by denitrification.

Nitrogen cycling in a northern hardwood forest: Do species matter?

Abstract: To investigate the influence of individual tree species on nitrogen (N) cycling in forests, we measured key characteristics of the N cycle in small single-species plots of five dominant tree species in the Catskill Mountains of New York State. The species studied were sugar maple (Acer saccharum), American beech (Fagus grandifolia), yellow birch (Betula alleghaniensis), eastern hemlock (Tsuga ca- nadensis), and red oak (Quercus rubra). The five species varied markedly in N cycling characteristics. For example, hemlock plots consistently showed characteristics associated with slow N cycling, in- cluding low foliar and litter N, high soil C:N, low extractable N pools, low rates of potential net N mineralization and nitrification and low NO3 amounts trapped in ion-exchange resin bags buried in the mineral soil. Sugar maple plots had the lowest soil C:N, and the highest levels of soil characteristics associated with NO3 production and loss (nitrification, extractable NO3 , and resin bag NO3 ). In con- trast, red oak plots had near-average net mineralization rates and soil C:N ratios, but very low values of the variables associated with NO3 production and loss. Correlations between soil N transformations and litter concentrations of N, lignin, lignin:N ratio, or phenolic constituents were generally weak. The in- verse correlation between net nitrification rate and soil C:N that has been reported in the literature was present in this data set only if red oak plots were excluded from the analysis. This study indicates that tree species can exert a strong control on N cycling in forest ecosystems that appears to be mediated through the quality of soil organic matter, but that standard measures of litter quality cannot explain the mechanism of control.

Water table elevation controls on soil nitrogen cycling in riparian wetlands along a European climatic gradient

Abstract: Riparian zones have long been considered as nitrate sinks in landscapes. Yet, riparian zones are also known to be very productive ecosystems with a high rate of nitrogen cycling. A key factor regulating processes in the N cycle in these zones is groundwater table fluctuation, which controls aerobic/anaerobic conditions in the soil. Nitrification and denitrification, key processes regulating plant productivity and nitrogen buffering capacities are strictly aerobic and anaerobic processes, respectively. In this study we compared the effects of these factors on the nitrogen cycling in riparian zones under different climatic conditions and N loading at the European scale. No significant differences in nitrification and denitrification rates were found either between climatic regions or between vegetation types. On the other hand, water table elevation turned out to be the prime determinant of the N dynamics and its end product. Three consistent water table thresholds were identified. In sites where the water table level is within −10 cm of the soil surface, ammonification is the main process and ammonium accumulates in the topsoils. Average water tables between −10 and −30 cm favour denitrification and therefore reduce the nitrogen availability in soils. In drier sites, that is, water table level below −30 cm, nitrate accumulates as a result of high net nitrification. At these latter sites, denitrification only occurs in fine textured soils probably triggered by rainfall events. Such a threshold could be used to provide a proxy to translate the consequences of stream flow regime change to nitrogen cycling in riparian zones and consequently, to potential changes in nitrogen mitigation.

Pre-industrial and contemporary fluxes of nitrogen through rivers: a global assessment based on typology

Abstract: This paper provides a global synthesis of reactive nitrogen (Nr) loading to the continental landmass and subsequent riverine nitrogen fluxes under a gradient of anthropogenic disturbance, from pre-industrial to contemporary. A mass balance model of nitrogen loading to the landmass is employed to account for transfers of Nr between atmospheric input sources (as food and feed products) and sub- sequent consumer output loads. This calculation produces a gridded surface of nitrogen loading ulti- mately mobilizable to aquatic systems (Nmob). Compared to the pre-industrial condition, nitrogen loading to the landmass has doubled from 111 to 223 Tg/year due to anthropogenic activities. This is particularly evident in the industrialized areas of the globe where contemporary levels of nitrogen loading have increased up to 6-fold in many areas. The quantity of nitrogen loaded to the landscape has shifted from a chiefly fixation-based system (89% of total loads) in the pre-industrial state to a het- erogeneous mix in contemporary times where fertilizer (15%), livestock (24%) and atmospheric de- position (15%) dominate in many parts of the industrialized and developing world. A nitrogen transport model is developed from a global database of drainage basin characteristics and a comprehensive compendium of river chemistry observations. The model utilizes constituent delivery coefficients based on basin temperature and hydraulic residence times in soils, rivers, lakes and reservoirs to transport nitrogen loads to river mouths. Fluxes are estimated for total nitrogen, dissolved inorganic nitrogen, and total organic nitrogen. Model results show that total nitrogen fluxes from river basins have doubled from 21 Tg/year in the pre-industrial to 40 Tg/year in the contemporary period, with many industrialized areas of the globe showing an increase up to 5-fold. DIN fluxes from river basins have increased 6-fold from 2.4 Tg/year in the pre-industrial to 14.5 Tg/year in the contemporary period. The amount of nitrogen loading delivered to river mouth as flux is greatly influenced by both basin temperatures and hydraulic residence times suggesting a regional sensitivity to loading. The global, aggregate nitrogen retention on the continental land mass is 82%, with a range of 0-100% for individual basins. We also present the first seasonal estimates of riverine nitrogen fluxes at the global scale based on monthly discharge as the primary driver.

Carbon turnover in peatland mesocosms exposed to different water table levels

Abstract: Changes of water table position influence carbon cycling in peatlands, but effects on the sources and sinks of carbon are difficult to isolate and quantify in field investigations due to seasonal dynamics and covariance of variables. We thus investigated carbon fluxes and dissolved carbon produc- tion in peatland mesocosms from two acidic and oligotrophic peatlands under steady state conditions at two different water table positions. Exchange rates and CO2 ,C H4 and DOC production rates were si- multaneously determined in the peat from diffusive-advective mass-balances of dissolved CO2 ,C H4 and DOC in the pore water. Incubation experiments were used to quantify potential CO2, CH4, and DOC production rates. The carbon turnover in the saturated peat was dominated by the production of DOC (10-15 mmol m 2 d 1 ) with lower rates of DIC (6.1-8.5 mmol m 2 d 1 ) and CH4 (2.2-4.2 mmol m 2 d 1 ) production. All production rates strongly decreased with depth indicating the impor- tance of fresh plant tissue for dissolved C release. A lower water table decreased area based rates of photosynthesis (24-42%), CH4 production (factor 2.5-3.5) and emission, increased rates of soil respi- ration and microbial biomass C, and did not change DOC release. Due to the changes in process rates the C net balance of the mesocosms shifted by 36 mmol m 2 d 1 . According to our estimates the change in C mineralization contributed most to this change. Anaerobic rates of CO2 production rates deeper in the peat increased significantly by a factor of 2-3.5 (DOC), 2.9-3.9 (CO2), and 3-14 (CH4) when the water table was lowered by 30 cm. This phenomenon might have been caused by easing an inhibiting effect by the accumulation of CO2 and CH4 when the water table was at the moss surface.

Trends in Dissolved Organic Carbon in UK Rivers and Lakes

Abstract: Several studies have highlighted an increase in DOC concentration in streams and lakes of UK upland catchments though the causal mechanisms controlling the increase have yet to be fully explained. This study, compiles a comprehensive data set of DOC concentration records for UK catchments to evaluate trends and test whether observed increases are ubiquitous over time and space. The study analysed monthly DOC time series from 198 sites, including 29 lakes, 8 water supply reservoirs and 161 rivers. The records vary in length from 8 to 42 years going back as far as 1961. Of the 198 sites, 153 (77%) show an upward trend in DOC concentration significant at the 95% level, the remaining 45 (23%) show no significant trend and no sites show a significant decrease in DOC concentration. The average annual increase in DOC concentration was 0.17 mg C/l/year. The dataset shows: (i) a spatial consistent upward trend in the DOC concentration independent of regional effects of rainfall, acid and nitrogen deposition, and local effects of land-use change; (ii) a temporally consistent increase in DOC concentration for period back as far as the 1960s; (iii) the increase in DOC concentration means an estimated DOC flux from the UK as 0.86 Mt C for the year 2002 and is increasing at 0.02 Mt C/year. Possible reasons for the increasing DOC concentration are discussed.

Chronic nitrate additions dramatically increase the export of carbon and nitrogen from northern hardwood ecosystems

Abstract: A long-term field experiment was initiated to simulate chronic atmospheric N deposition, a widespread phenomenon in industrial regions of the world. Eight years of experimental nitrate ( $${\text{NO}}_{\text{3}}^-- $$ ) additions (3 g $${\text{NO}}_{\text{3}}^-- $$ -N m−2 per year) to four different northern hardwood forests located along a 500 km geographic gradient dramatically increased leaching losses of $${\text{NO}}_{\text{3}}^-- $$ -N, dissolved organic carbon (DOC), and dissolved organic nitrogen (DON). During the last two water years, the average increase in solution $${\text{NO}}_{\text{3}}^-- $$ -N and DON leaching from the $${\text{NO}}_{\text{3}}^-- $$ -amended plots was 2.2 g N m−2, equivalent to 72% of the annual experimental N addition. Results indicate that atmospheric N deposition may rapidly saturate some northern hardwood ecosystems across an entire biome in the upper Great Lakes Region of the USA. Changes in soil C and N cycling induced by chronic N deposition have the potential in this landscape to significantly alter the flux of DOC and DON from upland to aquatic ecosystems. Michigan Gradient study site characteristics are similar to those of European forests most susceptible to N saturation.

Effect of microrelief and vegetation on methane emission from wet polygonal tundra, Lena Delta, Northern Siberia

Abstract: The effect of microrelief and vegetation on methane (CH4) emission was investigated in a wet polygonal tundra of the Lena Delta, Northern Siberia (72.37N, 126.47E). Total and plant-mediated CH4 fluxes were measured by closed-chamber techniques at two typical sites within a low-centred polygon. During the study period, total CH4 flux averaged 28.0 ± 5.4 mg m−2 d−1 in the depressed polygon centre and only 4.3 ± 0.8 mg m−2 d−1 at the elevated polygon rim. This substantial small-scale spatial variability of CH4 emission was caused by strong differences of hydrologic conditions within the microrelief of the polygon, which affected aeration status and organic matter content of the soils as well as the vegetation cover. Beside water table position, the vegetation cover was a major factor controlling CH4 emission from polygonal tundra. It was shown that the dominant vascular plant of the study area, Carex aquatilis, possesses large aerenchyma, which serve as pathways for substantial plant-mediated CH4 transport. The importance of plant-mediated CH4 flux was strongly influenced by the position of the water table relative to the main root horizon. Plant-mediated CH4 transport accounted for about two-thirds of the total flux in the wet polygon centre and for less than one-third of the total flux at the moist polygon rim. A clipping experiment and microscopic-anatomical studies suggested that plant-mediated CH4 transport via C. aquatilis plants is driven only by diffusion and is limited by the high diffusion resistance of the dense root exodermes.

Altered utilization patterns of young and old soil C by microorganisms caused by temperature shifts and N additions

Abstract: To determine if changes in microbial community composition and metabolic capacity alter decomposition patterns of young and old soil carbon pools, we incubated soils under conditions of varying temperature, N-availability, and water content. We used a soil from a pineapple plantation (CAM; δ13C litter = −14.1‰) that had previously been under tropical forest (C3; δ13C soil carbon = −26.5‰). Forest derived carbon represented 'old' carbon and plantation inputs represented 'new' carbon. In order to differentiate utilization of young ( 14 years) soil carbon, we measured the δ13C of respired CO2 and microbial phospholipid fatty acids (PLFAs) during a 103 day laboratory incubation. We determined community composition (PLFA and bacterial intergenic transcribed spacer (ITS) analysis) in addition to carbon degrading and nutrient releasing enzyme activities. We observed that greater quantities of older carbon were respired at higher temperatures (20 and 35 °C) compared to the lower temperature (5 °C). This effect could be explained by changes in microbial community composition and accompanying changes in enzyme activities that affect C degradation. Nitrogen addition stimulated the utilization of older soil carbon, possibly due to greater peroxidase activity, but microbial community composition was unaffected by this treatment. Increasing soil moisture had no effect on the utilization of older SOM, but enzyme activity typically declined. Increased oxidative enzyme activities in response to elevated temperature and nitrogen additions point to a plausible mechanism for alterations in C resource utilization patterns.

Sources and sinks of aquatic carbon in a peatland stream continuum

Abstract: Streams draining peatland systems contain a number of different C-species, all of which are linked either directly or indirectly to the cycling of C in the terrestrial environment. Concentrations and fluxes of dissolved, particulate and gaseous forms of carbon were measured along a network of streams draining an acidic peatland catchment (46.3 km2) in NE Scotland. The main aim was to identify sources and sinks of all the major forms of C in the drainage network and use this to develop a conceptual understanding of the evolution of streamwater chemistry along a peatland stream continuum. The investigation included a small-scale intra-catchment study of three contiguous sites in a 1.3 km2 headwater catchment (Brocky Burn) and a larger scale integrated study of seven sites. Mean annual fluxes of the main carbon species varied from 115–215 (DOC), 8.15–97.0 (POC), 0.32–6.90 (HCO3--C) and 2.62–10.4 (free CO2-C) kg C ha−1 year−1; all contributed to the overall carbon flux to varying degrees. Methane-C was only measurable at sites within areas of deep peat (<0.01–0.09 kg C ha−1 year−1). Downstream spatial changes in the intra-catchment study (Brocky Burn) were characterised by a decrease in DOC, CO2-C and CH4-C and an increase in POC fluxes over a distance of 1.1 km from the Upper to the Lower sites. In the context of the integrated catchment study estimated losses and gains of carbon from the water column showed no net change in DOC, a large decrease in POC (−55%) and a slight increase in (HCO3--C) (+7.7%) and CO2-C (+4.5%). A significant decrease in the CO2-C flux: HCO3-C flux ratio with distance downstream from the stream source, illustrates the importance of outgassing of CO2 from streams draining peatland C reservoirs. These data are interpreted in the context of losses and gains of the various components of the aquatic C flux along the peatland stream continuum.

Denitrification and patterns of electron donors and acceptors in eight riparian zones with contrasting hydrogeology

Abstract: A better understanding of nitrate removal mechanisms is important for managing the water quality function of stream riparian zones. We examined the linkages between hydrologic flow paths, patterns of electron donors and acceptors and the importance of denitrification as a nitrate removal mechanism in eight riparian zones on glacial till and outwash landscapes in southern Ontario, Canada. Nitrate-N concentrations in shallow groundwater from adjacent cropland declined from levels that were often 10–30 mg L−1 near the field-riparian edge to < 1 mg L−1 in the riparian zones throughout the year. Chloride data suggest that dilution cannot account for most of this nitrate decline. Despite contrasting hydrogeologic settings, these riparian zones displayed a well-organized pattern of electron donors and acceptors that resulted from the transport of oxic nitrate-rich groundwater to portions of the riparian zones where low DO concentrations and an increase in DOC concentrations were encountered. The natural abundances of d15N and in situ acetylene injection to piezometers indicate that denitrification is the primary mechanism of nitrate removal in all of the riparian zones. Our data indicate that effective nitrate removal by denitrification occurs in riparian zones with hydric soils as well as in non-hydric riparian zones and that a shallow water table is not always necessary for efficient nitrate removal by denitrification. The location of ‘hot spots’ of denitrification within riparian areas can be explained by the influence of key landscape variables such as slope, sediment texture and depth of confining layers on hydrologic pathways that link supplies of electron donors and acceptors.

The importance of in-stream uptake for regulating stream concentrations and outputs of N and P from a forested watershed: evidence from long-term chemistry records for Walker Branch Watershed*

Abstract: Long-term, weekly measurements of streamwater nitrogen and phosphorus concentra- tions in the West Fork of Walker Branch, a 1st order forested stream in eastern Tennessee, were used to assess the importance of in-stream processes for controlling stream concentrations and watershed exports. Over the period from 1991 to 2002, there was a slight declining trend in watershed export of dissolved inorganic N via streamflow, despite relatively high and constant wet N deposition rates (5 kg/ha/y). The watershed retains >90% of N deposition inputs. Concentra- tions of NO3 � and soluble reactive phosphorus (SRP) showed distinct seasonal patterns with autumn and early spring minima and summer maxima. An end-member mixing analysis indicated that these seasonal concentration patterns were largely a result of seasonal variations in in-stream uptake processes, with net uptake of NO3 � and SRP having the greatest impact on streamwater concentrations in November (reductions of 29 lg N/l and 2.5 lg P/l, respectively). This was likely a result of high rates of uptake by microbes colonizing new inputs of leaf detritus. For NO3 � there was a secondary peak in net uptake in March and April (about 9 lg N/l) resulting from increased uptake by stream algae and bryophytes. Summer was a period of net release of NO3 � to stream- water (peaking at 9 lg N/l in July) and minimal net effects on SRP concentrations. On average, in-stream processes resulted in removal of about 20% of the NO3 � and 30% of the SRP entering the stream from the catchment annually. This study, as well as other recent work, suggests that in-stream processes are important buffers on stream nutrient concentrations and exports reducing the effects of changes in inputs and retention in terrestrial portions of watersheds.

Changes in dissolved lignin-derived phenols, neutral sugars, uronic acids, and amino sugars with depth in forested Haplic Arenosols and Rendzic Leptosols

Abstract: A major part of the dissolved organic matter produced in the organic layers of forest ecosystems and leached into the mineral soil is retained by the upper subsoil horizons. The retention is selective and thus dissolved organic matter in the subsoils has different composition than dissolved organic matter leached from the forest floor. Here we report on changes in the composition of dissolved organic matter with soil depth based on C-to-N ratios, XAD-8 fractionation, wet-chemical analyses (lignin-derived CuO oxidation products, hydrolysable sugars and amino sugars) and liquid-state 13C nuclear magnetic resonance (NMR) spectroscopy. Dissolved organic matter was sampled directly beneath the forest floor using tension-free lysimeters and at 90 cm depth by suction cups in Haplic Arenosols under Scots pine (Pinus sylvestris L.) and Rendzic Leptosols under European beech (Fagus sylvatica L.) forest. At both sites, the concentrations of dissolved organic carbon (DOC) decreased but not as strongly as reported for deeply weathered soils. The decrease in DOC was accompanied by strong changes in the composition of dissolved organic matter. The proportion of the XAD-8-adsorbable (hydrophobic) fraction, carboxyl and aromatic C, and the concentrations of lignin-derived phenols decreased whereas the concentrations of sugars, amino sugars, and nitrogen remained either constant or increased. A general feature of the compositional changes within the tested compound classes was that the ratios of neutral to acidic compounds increased with depth. These results indicate that during the transport of dissolved organic matter through the soils, oxidatively degraded lignin-derived compounds were preferentially retained while potentially labile material high in nitrogen and carbohydrates tended to remain dissolved. Despite the studied soils' small capacity to sorb organic matter, the preferential retention of potentially refractory and acidic compounds suggests sorption by the mineral soil matrix rather than biodegradation to govern the retention of dissolved organic matter even in soils with a low sorption capacity.

Phosphorus Retention at the Redox Interface of Peatlands Adjacent to Surface Waters in Northeast Germany

Abstract: It is demanded currently in public discussions to rewet peatlands and re-establish their function as nutrient sinks. But due to high phosphorus (P) concentrations in the pore water of rewetted peatlands (40–420 μM) it is hypothesized that they can act as a surplus P source for adjacent surface waters and consequently support the eutrophication of such waters. Our detailed investigations of processes at the redox interface in four fens with different geochemical character show the dependence of P retention from the chemistry of the pore water. The precipitation of Fe(III) oxyhydroxide led to high retention of phosphorus and other substances such as DOC and sulphate in the eutrophic fens. When molar Fe/P ratios were larger than about 3 the initially high P concentrations in the anaerobic pore water (20–210 μM) decreased to concentrations below 1 μM under aerobic conditions. Thus, after rewetting high pore water concentrations of P do not automatically result in an increased P load to adjacent surface waters compared to pre-rewetting conditions. An enhanced P export to adjacent surface waters from eutrophic fens can be expected when the Fe/P ratio is smaller than 3 in the anaerobic pore water. In our investigations of natural, oligotrophic to mesotrophic fens the precipitation of Fe(III) oxyhydroxide was inhibited by the formation of stable dissolved Fe ∼ humic complexes. P retention in these fens was only related to the DOC concentrations at the redox interface, so that lower DOC concentrations concurred with higher P retention. The P equilibrium concentrations in an aerobic environment can be higher than that of eutrophic fens with Fe/P ratios larger than about 3 in the anaerobic pore water.

Variations of bioavailable Sr concentration and 87Sr/86Sr ratio in boreal forest ecosystems

Abstract: The mean depth of Sr and water uptake in mixed Norway spruce (Picea abies) and Scots pine (Pinus sylvestris) stands was investigated, using natural variations of 87Sr/86Sr and 18O/16O in soils in relation to depth. Three spruce-pine pairs were studied on a podzol and a peat site in Northern Sweden. Tree leaf and wood, as well as soils, soil solutions and roots below each tree were analysed for Sr and Ca concentrations and 87Sr/86Sr ratio. The 18O/16O ratio was also determined in xylem sap and soil solutions in relation to depth. Soil solution 18O/16O decreased in relation to depth. Comparing with xylem sap 18O/16O data indicated a deeper uptake of soil water by pine than spruce on the podzol site and a superficial uptake by both species on the peat. The 87Sr/86Sr ratio of bioavailable Sr generally increased in soils in relation to depth. Contrastingly, the 87Sr/86Sr ratio in spruce wood was generally higher than in pine wood suggesting a deeper uptake of Sr by spruce. But the 87Sr/86Sr ratio and concentrations of bioavailable Sr were systematically higher below spruce than below pine. In order to explain these unexpected results, we built a simple flux model to investigate the possible effects of interspecific variations in Sr cycling, soil mineral weathering and depth of Sr uptake on soil and tree 87Sr/86Sr ratio. At the study sites, spruce cycled in litterfall up to 12 times more strontium than pine. The use of the model showed that this difference in Sr cycling could alone explain higher isotopic signatures of trees and topsoils below spruce. Besides, high isotopic signatures of roots in the A/E horizons below spruce led us to hypothesise a species-specific weathering process. Finally, the comparison between the 87Sr/86Sr ratios in wood and root or soil solutions below each species suggested that the average depth of Sr and water uptake were close, but irregular variations of the Sr isotopic ratio with depth reduce the accuracy of the results. Tree species strongly influence Sr isotopic ratios in boreal forest soils through differences in Sr cycling, and possibly through specific mineral weathering.

Natural controls and human impacts on stream nutrient concentrations in a deforested region of the Brazilian Amazon basin

Abstract: This study documents regional patterns in stream nitrogen and phosphorus concentrations in the Brazilian state of Rondonia in the southwestern Amazon basin, and interprets the patterns as functions of watershed soil properties, deforestation extent, and urban population density. The survey includes 77 different locations sampled in the dry and wet seasons, with a watershed size range from 1.8 to 33,000 km2 over a total area of approximately 140,000 km2. A sequential regression technique is used to separate the effects of natural watersheds properties and anthropogenic disturbance on nutrients and chloride. Natural variation in soil texture explains most of the variance in stream nitrate concentrations, while deforestation extent and urban population density explain most of the variance in stream chloride (Cl) and total dissolved nitrogen (TDN) concentrations. Stream TDN, total dissolved phosphorus (TDP), particulate phosphorus (PP) and Cl concentrations all increase non-linearly with deforestation extent in the dry season after controlling for natural variability due to soil type. Stream nutrient and Cl disturbances are observed only in watersheds more than 66–75% deforested (watershed area range 2–300 km2), suggesting stream nutrient concentrations are resistant to perturbation from vegetation conversion below a 66–75% threshold. In heavily deforested watersheds, stream Cl shows the largest changes in concentration (12 ± 6 times forested background), followed by TDP (2.3 ± 1.5), PP (1.9 ± 0.8) and TDN (1.7 ± 0.5). Wet season signals in Cl and TDP are diluted relative to the dry season, and no land use signal is observed in wet season TDN, PN, or PP. Stream TDN and TDP concentrations in non-urban watersheds both correlate with stream Cl, suggesting that sources other than vegetation and soil organic matter contribute to enhanced nutrient concentrations. Small, urbanized watersheds (5–20 km2) have up to 40 times the chloride and 10 times the TDN concentrations of forested catchments in the dry season. Several large watersheds (∼1000–3000 km2) with urban populations show higher Cl, TDN and TDP levels than any small pasture watershed, suggesting that human impacts on nutrient concentrations in large river systems may be dominated by urban areas. Anthropogenic disturbance of dry-season stream Cl and TDN is detectable in large streams draining deforested and urbanized watersheds up to 33,000 km2. We conclude that regional deforestation and urbanization result in changes in stream Cl, N and P concentrations at wide range of scales, from small pasture streams to large river systems.

Nitrogen processing in the hyporheic zone of a pastoral stream

Abstract: The distribution of nitrogen-transforming processes, and factors controlling their rates, were determined within the hyporheic zone of a lowland stream draining agricultural land. In the field, physicochemical parameters were measured along a 10 m-long hyporheic flow line between downwelling and upwelling zones. Sediment cores were retrieved from the stream bed surface, and from 20, 40 and 60 cm deep in each zone, and in the laboratory, water from the corresponding depth was percolated through each core at the natural flow rate. Concentrations of nitrogen species and oxygen were measured before and after flow through each core. Denitrification was measured using a 15N-nitrate tracer. Shallow and downwelling zone samples were clearly distinct from deeper and upwelling zone samples in terms of physicochemical conditions, microbial processes and factors controlling nitrogen processing. Denitrification was highest in surface and downwelling zone cores, despite high oxygen levels, probably due to high pore-water nitrate concentrations in these cores and isolation of the denitrifying bacteria from oxygen in the bulk water by the hyporheic biofilms. Denitrification was limited by oxygen inhibition in the downwelling group, and by nitrate availability in the upwelling group. Strong evidence indicated that dissimilatory nitrate reduction to ammonium, occurred in almost all cores, and outcompeted denitrification for nitrate. In contrast, nitrification was undetectable in all but two cores, probably because of intense competition for oxygen. Field patterns and lab experiments indicated that the hyporheic zone at this moderately N-rich site is a strong sink for nitrate, fitting current theories that predict where hyporheic zones are nitrate sinks or nitrate sources.

The effect of climate and cultivation on soil organic C and N

Abstract: Here, we investigate the response of soil organic carbon (SOC) and soil organic nitrogen (SON) to cultivation within two different climatic regimes by comparing large soil data sets from India and the Great Plains. Multiple regression models for both regions show that SOC and SON, as well as C/N ratios, increase with decreasing temperatures and increasing precipitation, trends also noted in soil data organized by Holdridge life zones. The calculated difference between natural and cultivated soils in India revealed that the greatest absolute SOC and SON losses occurred in regions of low temperatures and high precipitation, while the C/N ratio decreased during cultivation only with decreasing temperature. In India, the fractional loss of SOC relative to undisturbed soils increases with decreasing temperature whereas, in the Great Plains, it increases with increasing precipitation. Also, the fractional loss of SOC increased in India with increasing amounts of original C, whereas no relationship between fractional loss and original C was noted for the Great Plains. The differential response of each region to cultivation is hypothesized to be due to differences in both climate and management practices (crop cycles, fertilization). These findings suggest that estimates of soil C loss due to cultivation should be based on an array of factors, and that it is unlikely that a constant relative C loss occurs in any region.

The role of ammonia volatilization in controlling the natural 15N abundance of a grazed grassland

Abstract: Although the variation in natural 15N abundance in plants and soils is well characterized, mechanisms controlling N isotopic composition of organic matter are still poorly understood. The primary goal of this study was to examine the role of NH3 volatilization from ungulate urine patches in determining 15N abundance in grassland plants and soil in Yellowstone National Park. We additionally used isotopic measurements to explore the pathways that plants in urine patches take up N. Plant, soil, and volatilized NH3δ15N were measured on grassland plots for 10 days following the addition of simulated urine. Simulated urine increased 15N of roots and soil and reduced 15N of shoots. Soil enrichment was due to the volatilization of isotopically light NH3. Acid-trapped NH3δ15N increased from −28‰ (day 1) to −0.3‰ (day 10), and was lighter than the original urea-N added (1.2‰). A mass balance analysis of urea-derived N assimilated by plants indicated that most of the N taken up by plants was in the form of ammonium through roots. However, isotope data also showed that shoots directly absorbed 15N – depleted NH3-N that was volatilized from simulated urine patches. These results indicate that NH3 volatilization from urine patches enriches grassland soil with 15N and shoots are a sink for volatilized NH3, which likely leads to accelerated cycling of excreted N back to herbivores.

Effects of elevated CO2 on nutrient cycling in a sweetgum plantation

Abstract: The effects of elevated CO2 on nutrient cycling and selected belowground processes in the closed-canopy sweetgum plantation were assessed as part of a free-air CO2 enrichment (FACE) ex- periment at Oak Ridge, Tennessee. We hypothesized that nitrogen (N) constraints to growth response to elevated CO2 would be mitigated primarily by reduced tissue concentrations (resulting in increased biomass production per unit uptake) rather than increased uptake. Conversely, we hypothesized that the constraints of other nutrients to growth response to elevated CO2 would be mitigated primarily by increased uptake because of adequate soil supplies. The first hypothesis was not supported: although elevated CO2 caused reduced foliar N concentrations, it also resulted in increased uptake and require- ment of N, primarily because of greater root turnover. The additional N uptake with elevated CO2 constituted between 10 and 40% of the estimated soil mineralizeable N pool. The second hypothesis was largely supported: elevated CO2 had no significant effects on tissue concentrations of P, K, Ca, or Mg and caused significantly increased uptake and requirement of K, Ca, and Mg. Soil exchangeable pools of these nutrients are large and should pose no constraint to continued growth responses. Elevated CO2 also caused increased microbial biomass, reduced N leaching and increased P leaching from O horizons (measured by resin lysimeters), reduced soil solution NH þ ,S O 2� 4 , and Ca 2þ concentrations, and in- creased soil solution pH. There were no statistically significant treatment effects on soil nutrient availability as measured by resin capsules, resin stakes, or in situ incubations. Despite significantly lower litterfall N concentrations in the elevated CO2 treatment, there were no significant treatment effects on translocation or forest floor biomass or nutrient contents. There were also no significant treatment effects on the rate of decomposition of fine roots. In general, the effects of elevated CO2 on nutrient cycling in this study were not large; future constraints on growth responses imposed by N limitations will depend on changes in N demand, atmospheric N deposition, and soil mineralization rates.

Net N input and riverine N export from Illinois agricultural watersheds with and without extensive tile drainage

Abstract: Some of the largest riverine N fluxes in the continental USA have been observed in agricultural regions with extensive artificial subsurface drainage, commonly called tile drainage. The degree to which high riverine N fluxes in these settings are due to high net N inputs (NNI), greater transport efficiency caused by the drainage systems, or other factors is not known. The objective of this study was to evaluate the role of tile drainage by comparing NNI and riverine N fluxes in regions of Illinois with similar climate and crop production practices but with different intensities of tile drainage. Annual values of NNI between 1940 and 1999 were estimated from county level agricultural production statistics and census estimates of human population. During 1945–1961, riverine nitrate flux in the extensively tile drained region averaged 6.6 kg N ha−1 year−1 compared to 1.3 to 3.1 kg N ha−1 for the non-tile drained region, even though NNI was greater in the non-tile drained region. During 1977–1997, NNI to the tile-drained region had increased to 27 kg N ha−1 year−1 and riverine N flux was approximately 100% of this value. In the non-tile-drained region, NNI was approximately 23 kg N ha−1 year−1 and riverine N flux was between 25% and 37% of this value (5 to 9 kg N ha−1 year−1). Denitrification is not included in NNI and, therefore, any denitrification losses from tile-drained watersheds must be balanced by other N sources, such as depletion of soil organic N or underestimation of biological N fixation. If denitrification and depletion of soil organic N are significant in these basins, marginal reductions in NNI may have little influence on riverine N flux. If tile drained cropland in Illinois is representative of the estimated 11 million ha of tile drained cropland throughout the Mississippi River Basin, this 16% of the drainage area contributed approximately 30% of the increased nitrate N flux in the Lower Mississippi River that occurred between 1955 and the 1990s.

Is the Iron Gate I reservoir on the Danube River a sink for dissolved silica

Abstract: Damming rivers changes sediment and nutrient cycles downstream of a dam in many direct and indirect ways. The Iron Gates I reservoir on the Yugoslavian-Romanian border is the largest impoundment by volume on the Danube River holding 3.2 billion m3 of water. Silica retention within the reservoir in the form of diatom frustules was postulated to be as high as 600 kt year−1 in previous studies using indirect methods. This amount of dissolved silicate was not delivered to the coastal Black Sea, and presumably caused a shift in the phytoplankton community there, and subsequent drastic decline in fishery. We directly quantified the amount of dissolved silicate (DSi) entering and leaving the reservoir for 11 continuous months. The budget based on these data reveals two important facts: (1) only about 4% of incoming DSi was retained in the reservoir; (2) the DSi concentrations were relatively low in the rivers upstream of the reservoir compared to regional and global averages. Thus damming the Danube at the Iron Gates could not have caused the decline in DSi concentrations documented downstream of the impoundment. Rather, this change in DSi must have occurred in the headwaters of the Danube River. Potential reasons include the construction of many dams upstream of the Iron Gates, hydrologic changes resulting in lower groundwater levels, and clogging of the riverbed limiting groundwater–river exchange.

Microbial population dynamics in an extreme environment: controlling factors in talus soils at 3750 m in the Colorado Rocky Mountains

Abstract: High elevation talus soils are extreme environments for life. They are pertinent to the hydrology and biochemical cycling of high elevation ecosystems that supply drinking water to major metropolitan areas, and are undergoing change as a result of N deposition. As biological systems, they are virtually unstudied. In order to gain a basic understanding of these important systems, we studied the seasonal timing of C inputs and microclimate, and how they corresponded to microbial biomass dynamics, in vegetated and unvegetated soils from a high altitude talus slope at 3750 m in the Colorado Front Range. The soil microclimate was described by soil moisture and temperature measurements; C inputs were estimated with measures of eolian dust inputs and photosynthetically active radiation (PAR). The biomass of different microbial functional groups (glutamate and salicylate mineralizers adapted to different temperatures) was estimated seasonally over 3 years. We found that the soil microclimate can be divided into three distinct seasons: Winter, with free water and temperature between −2.9 and 0 °C; spring, characterized by wet isothermal soils (0 °C); and summer, characterized by hot (mean 10.3 °C; range 0–29.3 °C) dry soils occasionally wetted by precipitation. The highest OM inputs to unvegetated soils occurred in spring; PAR only reached soils in the summer. In unvegetated soils, the biomass of glutamate mineralizers (GM) was highest in spring and summer. In contrast, the GM of vegetated soils had highest biomass in the winter when plants were senesced. In both unvegetated and vegetated soils, salicylate mineralizers (SM) had greater biomass levels in the summer when temperatures were highest.

Early stage litter decomposition rates for Swiss forests

Abstract: The decomposition of belowground and aboveground tree litter was studied on five forest sites across Switzerland, ranging from 480 to 1500 m in altitude, and including calcareous and acidic soils. In addition to decomposition of local litter types (Picea abies, Fagus sylvatica, Castanea sativa), the decomposition of a standard beech litter was studied on all sites. After 2 years of decomposition, mass loss ranged from 18 to 71% across the different sites and litter types. The lowest decomposition rates were observed for beech roots, while mass loss was greatest for both spruce needles and spruce roots at the low-altitude site. Mass loss during the first winter correlated best with the content of water-soluble substances. After 1 year of incubation, mass loss of the standard litter varied less than did mass loss of local litter, but variance increased during the second year for aboveground litter. These observations indicate a smaller climatic influence on litter breakdown at the beginning of the decomposition process. Litter mass loss could be described using an exponential model with a decay constant depending on either lignin/N ratio or Mn content of the litter and annual soil temperature and throughfall precipitation as climatic variables. Modelling the observed mass loss indicated a strong influence of litter quality in the first 2 years of decomposition, confirming the field data from the standard litter experiment. The experiment will continue for some years and is expected to yield additional data on long-term decomposition.

Solute fluxes in throughfall and stemflow in four forest ecosystems in northwest Amazonia

Abstract: The contribution of throughfall and stemflow as pathways for solute inputs into the forest floor in four mature forests in northwest Amazonia was investigated. Total solute inputs, resulting from the changes of atmospheric deposition after rainfall passes through the forest canopy, are presented in the form of throughfall and stemflow nutrient inputs and their possible sources are discussed. Throughfall is by far the most important solute input into the forest floor of the forests studied. On average, it represents about 98% of the total solute inputs. Although trends in solute enrichment varied among the forests, there is a general tendency in all ecosystems towards a distinct enrichment of SO4, K, Cl, NO3 and NH4 in throughfall and stemflow and a small increase of protons, Mn, orthoP and Fe. When comparing the net enrichment between the forests, the relative increase of solutes in throughfall and stemflow was higher in the flood plain and low terrace than in the high terrace and sedimentary plain forests. While highest values for total cation inputs were observed in the flood plain, the low terrace showed the highest value for total inorganic anions. The length of the antecedent dry period was the main factor affecting throughfall and stemflow composition, concentrations increasing with increasing length. A second, less important factor was the amount of throughfall and stemflow, which showed a poor and negative correlation with solute concentrations. The increased activity of frugivores in the canopy during fruiting periods seemed to lead to temporary increased solute concentrations in throughfall and stemflow as a result of the wash off of deposited faecal materials and detritus in the canopy. Leaching from leaves and wash off of exudes, of solutes deposited on the foliage after evaporation of intercepted rainfall and of dry deposited materials were all found to contribute to the concentration of solutes in the throughfall and stemflow. Gross rainfall enrichment after passing the forest canopy, mainly by nutrient leaching, is considerably lower than the amounts of nutrients released in litterfall implying a tight nutrient cycling and nutrient conserving mechanisms by forests studied.

High groundwater nitrate concentrations inhibit eutrophication of sulphate-rich freshwater wetlands

Abstract: During the last 60 years, pollution of the groundwater with $${\text{NO}}_{\text{3}}^-- $$ has greatly increased in many parts of Europe, as a consequence of excessive use of manure and synthetic fertilisers. Monitoring of groundwater-fed wetlands indicated that sediments with high $${\text{NO}}_{\text{3}}^-- $$ concentrations had the lowest Fe and $${\text{PO}}_{\text{4}}^{3--} $$ concentrations in the pore water. A comparison of two restored open water fens, differing in $${\text{NO}}_{\text{3}}^-- $$ supply via the groundwater, indicated that the redox potential and the sulphate ( $${\text{SO}}_{\text{4}}^{2--} $$ ) reduction rate were lower when the groundwater contained not only $${\text{SO}}_{\text{4}}^{2--} $$ but also high $${\text{NO}}_{\text{3}}^-- $$ concentrations. The lower $${\text{SO}}_{\text{4}}^{2--} $$ reduction rates in the $${\text{NO}}_{\text{3}}^-- $$ -rich open water fen were associated with lower $${\text{PO}}_{\text{4}}^{3--} $$ concentrations and the presence of plant species characteristic of clear water. In contrast, the higher $${\text{SO}}_{\text{4}}^{2--} $$ reduction rates in the $${\text{NO}}_{\text{3}}^-- $$ -poor open water fen were associated with very high $${\text{PO}}_{\text{4}}^{3--} $$ concentrations and massive development of plant species characteristic of eutrophic environments. Investigations at $${\text{NO}}_{\text{3}}^-- $$ -rich seepage sites in black alder carrs, showed that high $${\text{NO}}_{\text{3}}^-- $$ concentrations in the pore water caused chlorosis in the alder carr vegetation, due to lower availability of Fe in the pore water and less Fe uptake by the plants. Experimental desiccation of sediments proved that the $${\text{NO}}_{\text{3}}^-- $$ -rich seepage sites contained no oxidisable FeS x , contrary to $${\text{NO}}_{\text{3}}^-- $$ -poor locations, which became acidified and mobilised extremely high amounts of $${\text{SO}}_{\text{4}}^{2--} $$ due to FeS x oxidation. A laboratory experiment showed that $${\text{NO}}_{\text{3}}^-- $$ addition to sediments led to reduced releases of Fe, $${\text{SO}}_{\text{4}}^{2--} $$ and S2−, very likely due to the oxidation of reduced Fe and S compounds. Overall, the results confirmed that $${\text{NO}}_{\text{3}}^-- $$ is an energetically more favourable electron acceptor in anaerobic sediments than Fe and $${\text{SO}}_{\text{4}}^{2--} $$ , and that high $${\text{NO}}_{\text{3}}^-- $$ loads function as a redox buffer, preventing reduction of Fe and $${\text{SO}}_{\text{4}}^{2--} $$ . Limited $${\text{SO}}_{\text{4}}^{2--} $$ reduction prevents S2− -mediated mobilisation of $${\text{PO}}_{\text{4}}^{3--} $$ from Fe- $${\text{PO}}_{\text{4}}^{3--} $$ complexes. At a higher redox potential, reduced Fe, including FeS x , was oxidised, increasing the content of Fe(III) capable of binding $${\text{PO}}_{\text{4}}^{3--} $$ . This prevented increased $${\text{PO}}_{\text{4}}^{3--} $$ availability and the concomitant massive development of plant species characteristic of eutrophic environments.

Influences of land use and stream size on particulate and dissolved materials in a small Amazonian stream network

Abstract: We investigated the influences of forest or pasture land use and stream size on particulate and dissolved material concentrations in a network of second to third order streams in Rondonia, in the Brazilian Amazon During the dry season, a second order stream originating in pasture had lower concentrations of dissolved oxygen and nitrate, higher concentrations of chlorophyll, total suspended solids, particulate organic carbon, particular organic nitrogen, ammonium, and phosphate than a second order stream originating in forest Where the second order forest stream exited forest and entered pasture, concentrations of dissolved oxygen dropped from 6 mg/L to almost 0 mg/L and nitrate concentrations dropped from 12 μM to 2 μM over a reach of 2 km These changes indicated a strong influence of land use During the rainy season, differences among reaches of all particulate and dissolved materials were diminished Concentrations of oxygen, chlorophyll, total suspended solids, particulate organic carbon and nitrogen, nitrate, ammonium, and phosphate in the third order pasture stream more closely resembled the second order forest stream than the second order pasture stream, suggesting that conditions in the channels of larger pasture streams more strongly control concentrations of these materials If this pattern is widespread in stream networks of regions that consist of a mosaic of forest and pasture lands, it may have important consequences for understanding the effects of deforestation on larger rivers of the Amazon Basin This would indicate that the effects of forest clearing on the concentrations of many suspended and dissolved materials will be most easily detected in very small streams but potentially difficult to detect in larger streams and rivers

Dissolution of wollastonite during the experimental manipulation of Hubbard Brook Watershed 1

Abstract: Powdered and pelletized wollastonite (CaSiO3) was applied to an 11.8 ha forested watershed at the Hubbard Brook Experimental Forest (HBEF) in northern New Hampshire, U.S.A. during October of 1999. The dissolution of wollastonite was studied using watershed solute mass balances, and a 87Sr/86Sr isotopic tracer. The wollastonite (87Sr/86Sr = 0.70554) that was deposited directly into the stream channel began to dissolve immediately, resulting in marked increases in stream water Ca concentrations and decreases in the 87Sr/86Sr ratios from pre-application values of 0.872 mg/L and 0.72032 to values of ∼2.6 mg/L and 0.71818 respectively. After one calendar year, 401 kg of the initial 631 kg of wollastonite applied to the stream channel was exported as stream dissolved load, and 230 kg remained within the stream channel as residual CaSiO3 and/or adsorbed on streambed exchange sites. Using previously established values for streambed Ca exchange capacity at the HBEF, the dissolution rate for wollastonite was found to be consistent with dissolution rates measured in laboratory experiments. Initially, Ca was released from the mineral lattice faster than Si, resulting in the development of a Ca-depleted leached layer on mineral grains. The degree of preferential Ca release decreased with time and reached stoichiometric proportions after ∼6 months. Using Sr as a proxy for Ca, the Ca from wollastonite dissolution can be accurately tracked as it is transported through the aquatic and terrestrial ecosystems of this watershed.

Urban nitrogen biogeochemistry: status and processes in green retention basins

Abstract: Nitrogen (N) cycling has been poorly characterized in urban ecosystems. Processes involving N are of specific concern due to increasing anthropogenic inputs from fertilizer uses and fossil fuel combustion in cities. Here we report on a study of N biogeochemistry in city green retention basins and city parks in the Phoenix metropolitan area, Arizona, USA. City retention basins receive N inputs from street runoff, and along with city parks, fertilizer input from management, making these urban patches potential hot spots for biogeochemical cycling. We sampled soils from six retention basins and two non- retention city parks and measured soil organic matter (SOM) content, net N mineralization, net ni- trification, denitrification potential, and intact core denitrification flux and nitrate retention. Our results showed significantly higher SOM, extractable nitrate, nitrification rates and potential denitrification rates in surface soils (0-7.5 cm; soil that is directly affected by fertilizer N input, irrigation, and storm runoff) than in deeper soils. We also observed a distinct horizontal trend of decreasing SOM and denitrification potentials from inlet to outlet (dry well) in the retention basins. Denitrification rates, measured both as potential rates with substrate amendment (390-1151 ng N2O-N g � 1 soil h � 1 ), and as intact core fluxes (3.3-57.6 mg N m � 2 d � 1 ), were comparable to the highest rates reported in literature for other ecosys- tems. Management practices that affect biogeochemical processes in urban retention basins thus could affect the whole-city N cycling.

Major element fluxes from a coniferous catchment in central Ontario, 1983–1999

Abstract: Acid deposition over time scales of decades may deplete essential base cation (BC) reserves in soils to the extent that forest health may be affected. In order to assess the nutrient status of soils in central Ontario, input–output budgets for calcium (Ca), magnesium (Mg), potassium (K) and nitrogen (N) were calculated over a 17-year period (1983–1999) for a coniferous catchment in the Muskoka-Haliburton region. Inputs through deposition and weathering (BCs only), were compared with outputs through stream export and net accumulation in forest biomass. Despite a lack of forest growth at PC1, annual NO3–N concentrations in the stream were low (<0.1 mg/l) over the 17-year period, and over 80% of the atmospheric N input was retained in the catchment, indicating this catchment has not reached N-saturation. Stream export of Mg, and in particular Ca, exceeded input of these elements through deposition and weathering, indicating a net loss from the catchment over the 17-year period. Mass balance calculations indicated there was no net loss of K from the catchment. Soil re-sampling measurements confirmed large losses of Ca, but not Mg, and there were significant decreases in both NaCl-exchangeable Ca concentrations and soil pH between 1983 and 1999. The measured decline in soil Ca concentration amounted to a loss of approximately 85 kg/ha Ca from the exchangeable pool over the 17-year period. Similarly, input–output budget calculations indicated a net loss of Ca from the catchment in the range of 76 to 88 kg/ha between 1983 and 1999. Although the magnitude of Ca export decreased over the 17-year period, current stream export continues to exceed Ca input through deposition and weathering.