Showing papers in "Global Biogeochemical Cycles in 1999"

Estimating historical changes in global land cover: Croplands from 1700 to 1992

Abstract: Human activities over the last three centuries have significantly transformed the Earth's environment, primarily through the conversion of natural ecosystems to agriculture. This study presents a simple approach to derive geographically explicit changes in global croplands from 1700 to 1992. By calibrating a remotely sensed land cover classification data set against cropland inventory data, we derived a global representation of permanent croplands in 1992, at 5 min spatial resolution [Ramankutty and Foley, 1998]. To reconstruct historical croplands, we first compile an extensive database of historical cropland inventory data, at the national and subnational level, from a variety of sources. Then we use our 1992 cropland data within a simple land cover change model, along with the historical inventory data, to reconstruct global 5 min resolution data on permanent cropland areas from 1992 back to 1700. The reconstructed changes in historical croplands are consistent with the history of human settlement and patterns of economic development. By overlaying our historical cropland data set over a newly derived potential vegetation data set, we analyze our results in terms of the extent to which different natural vegetation types have been converted for agriculture. We further examine the extent to which croplands have been abandoned in different parts of the world. Our data sets could be used within global climate models and global ecosystem models to understand the impacts of land cover change on climate and on the cycling of carbon and water. Such an analysis is a crucial aid to sharpen our thinking about a sustainable future.

Nitrogen in crop production: An account of global flows

Abstract: Human activities have roughly doubled the amount of reactive N that enters the element's biospheric cycle. Crop production is by far the single largest cause of this anthropogenic alteration. Inorganic fertilizers now provide 80 Tg N yr−1 (Tg = 1012 g), managed (symbiotic) biofixation adds about 20 Tg N yr−1, and between 28 and 36 Tg N yr−1 are recycled in organic wastes. Anthropogenic inputs (including N in seeds and irrigation water) now supply about 85% of 170 (151–186) Tg N reaching the world's cropland every year. About half of this input, 85 Tg N yr−1, is taken up by harvested crops and their residues. Quantification of N losses from crop fields is beset by major uncertainties. Losses to the atmosphere (denitrification and volatilization) amount to 26–60 Tg N yr−1, while waters receive (from leaching and erosion) 32–45 Tg N yr−1. These N losses are the major reason behind the growing concerns about the enrichment of the biosphere with reactive N. The best evidence suggests that in spite of some significant local and regional losses, the world's agricultural land accumulates N. The addition of 3–4 billion people before the year 2050 will require further substantial increases of N input in cropping, but a large share of this demand can come from improved efficiency of N fertilizer use.

Global patterns of terrestrial biological nitrogen (N2) fixation in natural ecosystems

Abstract: Human activities have clearly caused dramatic alterations of the terrestrial nitrogen cycle, and analyses of the extent and effects of such changes are now common in the scientific literature. However, any attempt to evaluate N cycling processes within ecosystems, as well as anthropogenic influences on the N cycle, requires an understanding of the magnitude of inputs via biological nitrogen fixation (BNF). Although there have been many studies addressing the microbiology, physiology, and magnitude of N fixation at local scales, there are very few estimates of BNF over large scales. We utilized >100 preexisting published estimates of BNF to generate biome- and global-level estimates of biological N fixation. We also used net primary productivity (NPP) and evapotranspiration (ET) estimates from the Century terrestrial ecosystem model to examine global relationships between these variables and BNF as well as to compare observed and Century-modeled BNF. Our data-based estimates showed a strong positive relationship between ecosystem ET and BNF, and our analyses suggest that while the model's simple relationships for BNF predict broad scale patterns, they do not capture much of the variability or magnitude of published rates. Patterns of BNF were also similar to patterns of ecosystem NPP. Our “best estimate” of potential nitrogen fixation by natural ecosystems is ∼195 Tg N yr−1, with a range of 100–290 Tg N yr−1. Although these estimates do not account for the decrease in natural N fixation due to cultivation, this would not dramatically alter our estimate, as the greatest reductions in area have occurred in systems characterized by relatively low rates of N fixation (e.g., grasslands). Although our estimate of BNF in natural ecosystems is similar to previously published estimates of terrestrial BNF, we believe that this study provides a more documented, constrained estimate of this important flux.

A global database of sea surface dimethylsulfide (DMS) measurements and a procedure to predict sea surface DMS as a function of latitude, longitude, and month

Abstract: A database of 15,617 point measurements of dimethylsulfide (DMS) in surface waters along with lesser amounts of data for aqueous and particulate dimethylsulfoniopropionate concentration, chlorophyll concentration, sea surface salinity and temperature, and wind speed has been assembled. The database was processed to create a series of climatological annual and monthly 1°×1° latitude-longitude squares of data. The results were compared to published fields of geophysical and biological parameters. No significant correlation was found between DMS and these parameters, and no simple algorithm could be found to create monthly fields of sea surface DMS concentration based on these parameters. Instead, an annual map of sea surface DMS was produced using an algorithm similar to that employed by Conkright et al. [1994]. In this approach, a first-guess field of DMS sea surface concentration measurements is created and then a correction to this field is generated based on actual measurements. Monthly sea surface grids of DMS were obtained using a similar scheme, but the sparsity of DMS measurements made the method difficult to implement. A scheme was used which projected actual data into months of the year where no data were otherwise present.

Closing the global N2O budget: A retrospective analysis 1500–1994

Abstract: We present new estimates of global nitrous oxide (N2O) emissions for the period 1500–1994 based on revised Intergovernmental Panel on Climate Change guidelines [Intergovernmental Panel on Climate Change (IPCC), 1997; Mosier et al., 1998]. Use of these estimates as input to a simple atmospheric box model resulted in a closed N2O budget over time, showing that increases in atmospheric N2O can be primarily attributed to changes in food production systems. We hypothesize that before the ninetheenth century conversion of natural land to agriculture had no net effect on N2O. During the twentieth century a fast expansion of agricultural land coupled with intensification of land use may have caused a net increase in N2O. In our base scenario the total N2O emissions increased from 11 Tg N yr−1 in 1850 to 15 Tg N yr−1 in 1970 and to 18 Tg N yr−1 in 1994.

Combining satellite data and biogeochemical models to estimate global effects of human-induced land cover change on carbon emissions and primary productivity

Abstract: This study uses a global terrestrial carbon cycle model (the Carnegie-Ames-Stanford Approach (CASA) model), a satellite-derived map of existing vegetation, and global maps of natural vegetation to estimate the effects of human-induced land cover change on carbon emissions to the atmosphere and net primary production. We derived two maps approximating global land cover that would exist for current climate in the absence of human disturbance of the landscape, using a procedure that minimizes disagreements between maps of existing and natural vegetation that represent artifacts in the data. Similarly, we simulated monthly fields of the Normalized Difference Vegetation Index, required as input to CASA, for the undisturbed land cover case. Model results estimate total carbon losses from human-induced land cover changes of 182 and 199 Pg for the two simulations, compared with an estimate of 124 Pg for total flux between 1850 and 1990 [Houghton, 1999], suggesting that land cover change prior to 1850 accounted for approximately one-third of total carbon emissions from land use change. Estimates of global carbon loss from the two independent methods, the modeling approach used in this paper and the accounting approach of Houghton [1999], are comparable taking into account carbon losses from agricultural expansion prior to 1850 estimated at 48–57 Pg. However, estimates of regional carbon losses vary considerably, notably in temperate midlatitudes where our estimates indicate higher cumulative carbon loss. Overall, land cover changes reduced global annual net primary productivity (NPP) by approximately 5%, with large regional variations. High-input agriculture in North America and Europe display higher annual NPP than the natural vegetation that would exist in the absence of cropland. However, NPP has been depleted in localized areas in South Asia and Africa by up to 90%. These results provide initial crude estimates, limited by the spatial resolution of the data sets used as input to the model and by the lack of information about transient changes in land cover. The results suggest that a modeling approach can be used to estimate spatially-explicit effects of land cover change on biosphere-atmosphere interactions.

Controls on CH4 emissions from a northern peatland

Abstract: We examined the controls on summer CH4 emission from five sites in a peatland complex near Thompson, Manitoba, Canada, representing a minerotrophic gradient from bog to rich fen at wet sites, where the water table positions ranged from −10 to −1 cm. Average CH4 flux, determined by static chambers on collars, ranged from 22 to 239 mg CH4−C m−2 d−1 and was related to peat temperature. There was an inverse relationship between water table position and CH4 flux: higher water tables led to smaller fluxes. The determination of anaerobic CH4 production and aerobic CH4 consumption potentials in laboratory incubations of peat samples was unable to explain much of the variation in CH4 flux. Average net ecosystem exchange of CO2 ranged from 1.4 to 2.5 g CO2−C m−2 d−1 and was strongly correlated with CH4 flux; CH4 emission averaged 4% of CO2 uptake. End-of-season sedge biomass was also strongly related to CH4 flux, indicating the important role that vascular plants play in regulating CH4 flux. Determination of isotopic signatures in peat pore water CH4 revealed average δ13C values of between −50 and −73‰ and δD of between −368 and −388‰. Sites with large CH4 emission rates also had high CO2 exchange rates and enriched δ13C CH4 signatures, suggesting the importance of the acetate fermentation pathway of methanogenesis. Comparison of δD and δ13C signatures in pore water CH4 revealed a slope shallow enough to suggest that oxidation is not an important overall control on CH4 emissions at these sites, though it appeared to be important at one site. Analysis of 14C in pore water CH4 showed that most of the CH4 was of recent origin with percent of modern carbon values of between 112 and 128%. The study has shown the importance of vascular plant activities in controlling CH4 emissions from these wetland sites through influences on the availability of fresh plant material for methanogenesis, rhizospheric oxidation, and plant transport of CH4.

An assessment of vegetation fire in Africa (1981–1991): Burned areas, burned biomass, and atmospheric emissions

Abstract: This paper presents the first published time series of burned area maps of Africa, covering an 8 year period, 1981–1983 and 1985–1991. These maps were derived from the analysis of the advanced very high resolution radiometer (AVHRR) global area coverage (GAC) images at 5 km resolution. The burned area maps for the period 1985–1991 were used with biomass density and burning efficiency figures, to estimate the quantity of burned biomass during this 6 year period. Emission factors were further used to estimate the trace gas and aerosol emissions produced by vegetation fires. Biomass density was estimated based on values found in the literature and on the accumulated normalized difference vegetation index (NDVI) as derived from the remote sensing images. Burning efficiency was assessed with a dryness index that was based on the relative greenness index (RGI), also derived from the NDVI. Average emission factors were retrieved from the literature. The uncertainties in the burned area, biomass density, combustion efficiency, and emission factors were considered, with a total error of 51% for the burned biomass and 58% for the emission estimates. The results obtained for the burned biomass in Africa were compared with other values found in the literature and showed values lower by a factor of 1.1–3.3. The annual burned biomass from vegetation fires in Africa on average was estimated between 704 and 2168 Tg . In the same way, the atmospheric emissions on average ranges are as follows: CO2 (990–3726 Tg), CO (40–151 Tg), CH4 (1.2–4.4 Tg), NOx (2.8–10.6 Tg), and PM (< 2.5 μm) (3.3–12.4Tg).

The isotopic composition of atmospheric methane

Abstract: Measurements of the 13C/12C, D/H and 14C composition of atmospheric methane (CH4) between 1988 and 1995 are presented. The 13C/12C measurements represent the first global data set with time series records presented for Point Barrow, Alaska (71°N), Olympic Peninsula, Washington (48°N), Mauna Loa, Hawaii (20°N), American Samoa (14°S), Cape Grim, Australia (41°S), and Baring Head, New Zealand (41°S). North-south trends of the 13C/12C and D/H of atmospheric CH4 from air samples collected during oceanographic research cruises in the Pacific Ocean are also presented. The mean annual δ13C increased southward from about −47.7 ‰ at 71°N to −41.2 ‰ at 41°S. The amplitude of the seasonal cycle in δ13C ranged from about 0.4 ‰ at 71°N to 0.1 ‰ at 14°S. The seasonal δ13C cycle at sites in tropical latitudes could be explained by CH4 loss via reaction with OH radical, whereas at temperate and polar latitudes in the northern hemisphere seasonal changes in the δ13C of the CH4 source were needed to explain the seasonal cycle. The higher δ13C value in the southern (−47.2 ‰) versus northern (−47.4 ‰) hemisphere was a result of interhemispheric transport of CH4. A slight interannual δ13C increase of 0.02±0.005 ‰ yr−1 was measured at all sites between 1990 and 1995. The mean δD of atmospheric CH4 was −86±3 ‰ between 1989 and 1995 with a 10 ‰ depletion in the northern versus southern hemisphere. The 14C content of CH4 measured at 48°N increased from 122 to 128 percent modern between 1987 and 1995. The proportion of CH4 released from fossil sources was 18±9% in the early 1990s as derived from the 14C content of CH4.

Land use change and biogeochemical controls of nitrogen oxide emissions from soils in eastern Amazonia

Abstract: The objectives of this study were (1) to determine the effects of land use change on N oxide fluxes from soil in seasonally dry, eastern Amazonia and (2) evaluate the “hole-in-the-pipe” model in a field setting where N availability varies among land uses and soil moisture varies among seasons. We measured N oxide flux from an old-growth forest, a 20-year-old secondary forest, an active pasture, and a degraded pasture. We also measured soil water content, soil inorganic N stocks, net N mineralization and nitrification potential. To determine the effects of pasture age on N oxide flux, we measured gas fluxes at a chronosequence of pastures (0-13 years). In the land use study, N2O fluxes followed the order: primary forest (2.4 kg N ha−1 yr−1) > secondary forest (0.9 kg N ha−1 yr−1) > active pasture (0.3 kg N ha−1 yr−1) ≥ degraded pasture (0.1 kg N ha−1 yr−1), and NO fluxes followed the order: primary forest (1.5 kg N ha y−11) > degraded pasture (0.7 kg N ha−1 yr−1) > active pasture (0.5 kg N ha−1 yr−1) ≥ secondary forest (0.3 kg N ha−1 yr−1). In the chronosequence study, no trend in N oxide emissions with pasture age was apparent, but emissions from pastures were lower than from the forest.Total N oxide flux correlated with a laboratory measure of nitrification potential (r2 = 0.85).The ratio N2O:NO correlated with soil water content (r2 = 0.56). Parameterization of the model accounted for variability in N oxide emissions across land uses and seasons and the model application revealed the importance of studying both N oxide gases simultaneously. Model predictions for six independent sites agreed well with observed fluxes, suggesting that the model may be applicable at a broader scale. The consistently low annual emissions of N2O estimated for all of the Amazonian pastures that we studied suggest that conversions of tropical forests to cattle pastures may not in the long term cause a significant increase in the contribution of soil emissions to atmospheric N2O or NO.

The δ15N of nitrate in the southern ocean: Consumption of nitrate in surface waters

Abstract: We report nitrogen isotope data for nitrate from transects of hydrocast and surface samples collected in the eastern Indian and Pacific sectors of the Southern Ocean, focusing here on the data from the upper water column to study the effect of nitrate consumption by phytoplankton. The δ15N of nitrate increases by 1–2‰ from deep water into the Antarctic summertime surface layer, due to kinetic isotopic fractionation during nitrate uptake. Estimation of the nitrate uptake isotope effect from Antarctic depth profiles yields values in the range of 5–6‰ in east Indian sector and 4–5‰ in the east Pacific sector. Surface transect data from the Pacific sector also yield values of 4–5‰. The major uncertainty in the profile-based estimation of the isotope effect involves the δ15N of nitrate from the temperature minimum layer below the summertime Antarctic surface layer, which deviates significantly from the predictions of simple models of isotope fractionation. For the Subantarctic surface, it is possible to distinguish between nitrate supplied laterally from the surface Antarctic and nitrate supplied vertically from the Subantarctic thermocline because of the distinctive relationships between the δ15N and concentration of nitrate in these two potential sources. Our Subantarctic samples, collected during the summer and fall, indicate that nitrate is supplied to the Subantarctic surface largely by northward transport of Antarctic surface water. Isotopic data from the Pacific sector of the Subantarctic suggest an isotope effect of 4.5‰, indistinguishable from the Antarctic estimates in this sector.

Spatiotemporal patterns of carbon‐13 in the global surface oceans and the oceanic suess effect

Abstract: A global synthesis of the 13C/12C ratio of dissolved inorganic carbon (DIC) in the surface ocean is attempted by summarizing high-precision data obtained from 1978 to 1997 in all major ocean basins. The data, mainly along transects but including three subtropical time series, are accompanied by simultaneous, precise measurements of DIC concentration and titration alkalinity. The reduced isotopic ratio, δ13C, in the surface ocean water is governed by a balance between biological and thermodynamic processes. These processes have strongly opposing tendencies, which result in a complex spatial pattern in δ13C with relatively little variability. The most distinctive feature in the spatial distribution of δ13C seen in our data is a maximum of δ13C near the subantarctic front with sharply falling values to the south. We attribute this feature to a combination of biological uptake of CO2 depleted in 13C (low δ13C) and air-sea exchange near the front and upwelling further south of waters with low δ13C resulting from the remineralization of organic matter. Additional features are maxima in δ13C downstream of upwelling regions, reflecting biological uptake, and minima in the subtropical gyres caused by strongly temperature dependent thermodynamic isotopic fractionation. At the time series stations, two in the North Atlantic Ocean and one in the North Pacific, distinct seasonal cycles in δ13C are observed, the Pacific data exhibiting only about half the amplitude of the Atlantic. Secular decreases in δ13C caused by the invasion of isotopically light anthropogenic CO2 into the ocean (the 13C Suess effect) have been identified at these time series stations and also in data from repeated transects in the Indian Ocean and the tropical Pacific. A tentative global extrapolation of these secular decreases yields a surface oceanic 13C Suess effect of approximately −0.018‰ yr−1 from 1980 to 1995. This effect is nearly the same as the 13C Suess effect observed globally in the atmosphere over the same period. We attribute this response to a deceleration in the growth rate of anthropogenic CO2 emissions after 1979, which subsequently has reduced the atmospheric 13C Suess effect more than the surface ocean effect.

Stability of elemental carbon in a savanna soil

Abstract: We have investigated the stability of oxidation-resistant elemental carbon (OREC) in a sandy savanna soil at the Matopos fire trial site, Zimbabwe. The protection of some soil plots from fire for the last 50 years at this site has enabled a comparison of OREC abundances between those plots which have been protected from fire and plots which have continued to be burnt. The total 0–5 cm OREC inventory of the soil protected from fire is estimated to be 2.0±0.5 mg cm−2; approximately half the “natural” OREC inventory at the study site of 3.8±0.5 mg cm−2 (the mean for plots burnt every 1–5 years). The associated half-life for natural OREC loss from the 0–5 cm interval of the protected plots is calculated to be 2000 μm) in the soil being considerably <50 years. These results suggest that at least in well-aerated tropical soil environments, charcoal and OREC can be can be significantly degraded on decadal to centennial timescales. OREC abundance and carbon-isotope data suggest that OREC in coarse particles is progressively degraded into finer particle sizes, with a concomitant increase in resistance to oxidative degradation of OREC in the finer particle sizes due to the progressive loss of more readily degraded OREC. It remains unclear whether the OREC that is degraded is oxidized completely to CO2 and subsequently emitted from the soil, reduced to a sufficiently small particle size to be illuviated to deeper parts of the soil profile, solubilized and lost from the profile as dissolved organic carbon or transmuted into a chemical form which is susceptible to attack by the acid-dichromate reagent. The conclusion that a significant proportion of OREC can undergo natural degradation in well-aerated environments on decadal/centennial timescales suggests that only a fraction of the total production of OREC from biomass burning and fossil fuel combustion is likely to be sequestered in the slow-cycling “geological” carbon reservoir.

Dynamic replacement and loss of soil carbon on eroding cropland

Abstract: Links between erosion/sedimentation history and soil carbon cycling were examined in a highly erosive setting in Mississippi loess soils. We sampled soils on (relatively) undisturbed and cropped hillslopes and measured C, N, 14C, and CO2 flux to characterize carbon storage and dynamics and to parameterize Century and spreadsheet 14C models for different erosion and tillage histories. For this site, where 100 years of intensive cotton cropping were followed by fertilization and contour plowing, there was an initial and dramatic decline in soil carbon content from 1870 to 1950, followed by a dramatic increase in soil carbon. Soil erosion amplifies C loss and recovery: About 100% of the original, prehistoric soil carbon was likely lost over 127 years of intensive land use, but about 30% of that carbon was replaced after 1950. The eroded cropland was therefore a local sink for CO2 since the 1950s. However, a net CO2 sink requires a full accounting of eroded carbon, which in turn requires that decomposition rates in lower slopes or wetlands be reduced to about 20% of the upland value. As a result, erosion may induce unaccounted sinks or sources of CO2, depending on the fate of eroded carbon and its protection from decomposition. For erosion rates typical of the United States, the sink terms may be large enough (1 Gt yr−1, back-of-the-envelope) to warrant a careful accounting of site management, cropping, and fertilization histories, as well as burial rates, for a more meaningful global assessment.

Acetone, methanol, and other partially oxidized volatile organic emissions from dead plant matter by abiological processes: Significance for atmospheric HOx chemistry

Abstract: In this paper, attention is called to the significance of abiological production of partially oxidized volatile organic carbons (POVOCs) from the decay of dead plant material. Measured relative emission of acetone and methanol can be at least 10−4 and 3 - 5 × 10−4 g g−1of decaying dry plant matter, respectively. If these results may be extrapolated, global annual emissions of 6–8 Tg of acetone and 18 – 40 Tg of methanol would result, adding strongly to the estimated total emissions of these compounds to the atmosphere. Because acetone and methanol, through OH and HO2 formation, play significant roles in the chemistry of the atmosphere, further research is strongly needed to quantify the emissions of acetone, methanol, and other POVOCs

Anthropogenic CO2 inventory of the Indian Ocean

Abstract: This study presents basin-wide anthropogenic CO2 inventory estimates for the Indian Ocean based on measurements from the World Ocean Circulation Experiment/Joint Global Ocean Flux Study global survey. These estimates employed slightly modified ΔC* and time series techniques originally proposed by Gruber et al. [1996] and Wallace [1995], respectively. Together, the two methods yield the total oceanic anthropogenic CO2 and the carbon increase over the past 2 decades. The highest concentrations and the deepest penetrations of anthropogenic carbon are associated with the Subtropical Convergence at around 30° to 40°S. With both techniques, the lowest anthropogenic CO2 column inventories are observed south of 50°S. The total anthropogenic CO2 inventory north of 35°S was 13.6±2 Pg C in 1995. The inventory increase since GEOSECS (Geochemical Ocean Sections Program) was 4.1±1 Pg C for the same area. Approximately 6.7±1 Pg C are stored in the Indian sector of the Southern Ocean, giving a total Indian Ocean inventory of 20.3 ±3 Pg C for 1995. These estimates are compared to anthropogenic CO2 inventories estimated by the Princeton ocean biogeochemistry model. The model predicts an Indian Ocean sink north of 35°S that is only 0.61–0.68 times the results presented here; while the Southern Ocean sink is nearly 2.6 times higher than the measurement-based estimate. These results clearly identify areas in the models that need further examination and provide a good baseline for future studies of the anthropogenic inventory.

Hierarchical control on nitrous oxide emission in forest ecosystems

Abstract: While much is known about process level control on N2O production by nitrification and denitrification, knowledge of the environmental controls responsible for site variation in annual N2O fluxes on ecosystem level is low. Our goal was to improve existing concepts of controls on N2O fluxes. We measured N2O emission weekly or biweekly during 1 year in 11 temperate forest ecosystems using closed chambers. We identified three types of forest with different temporal emission patterns: forest with seasonal, event-based and background emission patterns. Comparison of annual data sets from literature showed that most temperate forests had low N2O emissions throughout the year (background emission pattern) with mean annual fluxes of 0.39 ±0.27 kg N ha−1 yr−1 (n = 21). Event-based emission patterns were observed during frost/thaw periods and after rewetting. Highest fluxes up to 72 kg N ha−1 were emitted from a drained alder forest with organic soil in 46 weeks, followed by well drained tropical and temperate forests with seasonal emission patterns and fluxes between 2 − 6 (n = 3) and 1 − 5 kg N ha−1 yr−1 (n = 4), respectively. Seasonal emission patterns were explained by combined effect of high annual precipitations; broad leave trees; amount and structure of organic upper horizon; high mineral bulk densities; and plant community. These state variables reduce gas diffusivity so that oxygen demand by microorganism and roots exceeded oxygen supply during wet and warm periods (>10° C). The resultant upper mean level was about 100 μg N2O−N m−2 h−1 in both temperate and tropical forests. Annual N2O losses of the seasonal emission type were controlled by both duration and upper mean level of the periods with high emissions. We conclude that “short-term controls” of climate determine the duration of high emissions, whereas “long-term controls” by state variables determine the difference between background and seasonal emission types.

An eddy‐permitting coupled physical‐biological model of the North Atlantic: 1. Sensitivity to advection numerics and mixed layer physics

Abstract: Physical influences on biological primary production in the North Atlantic are investigated by coupling a four-component pelagic ecosystem model with a high-resolution numerical circulation model. A series of sensitivity experiments demonstrates the important role of an accurate formulation of upper ocean turbulence and advection numerics. The unrealistically large diffusivity implicit in upstream advection approximately doubles primary production when compared with a less diffusive, higher-order, positive-definite advection scheme.This is of particular concern in the equatorial upwelling region where upstream advection leads to a considerable increase of upper ocean nitrate concentrations. Counteracting this effect of unrealistically large implicit diffusion by changes in the biological model could easily lead to misconceptions in the interpretation of ecosystem dynamics. Subgrid-scale diapycnal diffusion strongly controls biological production in the subtropical gyre where winter mixing does not reach the nutricline. The parameterization of vertical viscosity is important mainly in the equatorial region where friction becomes an important agent in the momentum balance.

Consumption of atmospheric methane by soils: A process‐based model

Abstract: A process-based model for the consumption of atmospheric methane (CH4) by soils was developed to identify the most important factors affecting uptake rates and to determine whether the current uncertainties in the estimated size of the global soil sink might be reduced. Descriptions of diffusion and microbial oxidation processes, which together determine the CH4 flux, were included. The results suggest that the global sink strength lies within the range 20–51 Tg yr−1 CH4, with a preferred value of 38 Tg yr−1 CH4. Dry tropical ecosystems account for almost a third of this total. Here microbial activity rather than diffusion is limiting uptake. It is also in these areas that the impact of any intensification in agriculture will be the most pronounced, with a possible future reduction in uptake in excess of 3 Tg yr−1 CH4. This is in contrast to the overall impact of global warming, which is expected to leave the size of the global soil sink relatively unchanged.

Human impacts on the large world rivers: Would the Changjiang (Yangtze River) be an illustration?

Abstract: The “Three Gorges Project” (TGP) in the upstream of Changjiang (Yangtze River) has resulted in great concern of scientific society and public conversations on the economic, environmental, and human health issues. Data of nutrients from main stream and 15 large tributaries indicate that Changjiang receives a large part of its nutrient burden from the drainage area upstream the “Three Gorges Dam” (TGD). A model calculation shows that the construction of TGD may cause further eutrophication in the upstream region with phosphate as a limiting factor relative to nitrogen for photosynthesis. The estimated carbon fixation within the reservoir may equal to 10–20% of the actual particulate organic carbon (POC) budget for the Changjiang. Taking into account the fact that dissolved inorganic nitrogen (DlN) in the Changjiang has increased by a factor of 2 in the last 10–20 years, the expected N:P ratio of the river would reach 300–400 after the year 2010. Such a change in nutrient and organic carbon budgets of the Changjiang will significantly influence the environment and health of ecosystems of the adjacent shelf region.

In situ sulphate stimulation of mercury methylation in a boreal peatland: Toward a link between acid rain and methylmercury contamination in remote environments

Abstract: Recent studies have found that “pristine” peatlands have high peat and pore water methylmercury (MeHg) concentrations and that peatlands may act as large sources of MeHg to the downstream aquatic system, depending upon the degree of hydrologie connectivity and catchment physiography. Sulphate-reducing bacteria have been implicated as principal methylators of inorganic mercury in many environments with previous research focused primarily on mercury methylation in aquatic sediments. Experiments in a poor fen in the Experimental Lakes Area, northwestern Ontario, Canada, demonstrated that the in situ addition of sulphate to peat and peat pore water resulted in a significant increase in pore water MeHg concentrations. As peatlands cover a large area of the Northern Hemisphere, this finding has potentially far ranging implications for the global mercury cycle, particularly in areas impacted by anthropogenically derived sulphate where the methylmercury fraction of total mercury species may be much larger than in nonimpacted environments.

Wintertime CO2 efflux from Arctic soils: Implications for annual carbon budgets

Abstract: Estimates of annual carbon loss from arctic tundra ecosystems are based nearly entirely on measurements taken during the growing season in part because of methodological limitations but also reflecting the assumption that respiration during winter is near zero. Measurements of CO2 flux during winter, however, indicate significant amounts of carbon loss from tundra ecosystems throughout the 240-day nongrowing season. In our study during the 1996 and 1997 nongrowing seasons, winter carbon losses ranged from 2.0 g CO2 m−2 season−1 in moist dwarf shrub communities to 97 g CO2 m−2 season−1 in natural snowdrift communities, with an average wintertime CO2 efflux of 45 g CO2 m−2 for all Low Arctic tundra communities (0.14 Pg CO2 yr−1 worldwide). These measurements indicate that current estimates of annual carbon loss from tundra ecosystems are low. Inclusion of wintertime losses of CO2 into annual carbon budgets increases the annual carbon efflux of arctic tundra ecosystems by 17% and changes some ecosystems from net annual sinks to net sources of CO2 to the atmosphere.

Major challenges confronting marine biogeochemical modeling

Abstract: Substantial improvements are required in the current suite of numerical models if we are to better understand the present ocean biogeochemical state and predict potential future responses to anthropogenic perturbations. At present, major impediments to marine biogeochemical modeling include the inadequate representation of multi-element cycling and community structure, large-scale physical circulation, mesoscale space and time variability, and mass exchange between the open ocean and the atmosphere, land, and coastal ocean. Marine biogeochemical modeling is inherently data driven, and significant progress on any of these topics will require close collaboration between the observational and modeling communities. Two main thrusts should be to develop improved, mechanistically based parameterizations of specific biogeochemical processes and to test the overall skill of integrated system models through detailed model-data comparison of both the mean state and seasonal to interdecadal variability.

A global oceanic sediment model for long-term climate studies

Abstract: A chemically reactive 10-layer sediment module was coupled to a geochemical ocean general circulation model (the Hamburg Oceanic Carbon Cycle Model). The sediment model includes four solid sediment components (CaCO3, opal, organic carbon, and clay), and five pore water substances (dissolved inorganic carbon, total alkalinity, PO43−, O2, Si(OH)4) plus corresponding species containing 13C and 14C instead of 12C. The processes, namely, particle deposition, pore water reactions, pore water diffusion and interaction with the open water column, vertical sediment advection, sediment accumulation, and bioturbation, are simulated through basic parametrizations. For the water column part the Si and C cycles are coupled by a formulation of the “rain ratio” Si:C(CaCO3):C(POC), where POC is particulate organic carbon, in biogenic particle export production, with CaCO3 frustrule production growing in parallel to a weakening of opal production during progressing deficiency of dissolved silicate in the surface layer. For two preindustrial velocity fields the model reproduces major features of observed water column and sediment tracer distributions parallel to a correct preindustrial CO2 level close to 280 ppm. The model reacts sensitively to the formulation of the POC flux parametrization, the rain ratio, as well as the solubility of opal but is fairly insensitive to changes in the bioturbation rate as well as the amount of clay deposition. A simulation of the sediment distribution by use of a velocity field, which represents the ocean at conditions during the last glacial maximum, yields realistic glacial-interglacial changes for the Atlantic Ocean, while discrepancies remain for the Indo-Pacific region. A significant decrease of the atmospheric pCO2 could be achieved through an additional change of water column inventories by a change in weathering input of Si and alkalinity.

Long‐term greenhouse gas emissions from hydroelectric reservoirs in tropical forest regions

Abstract: The objective of this work is to quantify long-term emissions of two major greenhouse gases, CO2 and CH4, produced by the decomposition of the flooded organic matter in tropical artificial reservoirs. In a previous paper [Galy-Lacaux et al., 1997], gas emissions from the tropical reservoir of Petit Saut (French Guiana) were quantified over the first two years after impounding. This work presents emission fluxes and distributions of dissolved methane and carbon dioxide measured in the reservoir of Petit Saut over three and a half years, since the beginning of impounding (1994) and during operation (1995–1997). To assess long term emissions, an experimental campaign was conducted on four hydroelectric reservoirs (Taabo, Buyo, and Ayame I and II) built between 1960 and 1980 in the Ivory Coast. Average dissolved CH4 concentration in the water column of the Petit Saut reservoir first increased, up to a maximum of 14 mg L−1, in May 1995. Then the time course of dissolved CH4 over the three and a half year period, showed periodical variations. These changes were related to changes in the inlet water flow and the residence time of water in the reservoir. In the older African reservoirs, average dissolved methane concentrations were lower and ranged between 0.20 and 0.32 mg L−1. The whole data set allows us to propose an analytical algorithm in order to predict the time course of dissolved CH4 concentration in the Petit Saut reservoir. Temporal variations of total CH4 and CO2 emissions from the reservoir over three and a half years were extrapolated with this algorithm to calculate long term carbon losses. Over a 20-year period the estimated carbon losses in the form of CO2 and CH4 were dominated by the outlet fluxes of dissolved gases (2160 ± 400 Gg (C)), and they correspond to a total net carbon loss of 3.2 Tg (C). The contribution of the Petit Saut reservoir to greenhouse gas emission, over 20 years, is estimated to be 66 ± 20 Tg of CO2 equivalent (56 Tg as CH4 and 9.7 Tg as CO2).

Modeling the geochemical cycle of iron in the oceans and its impact on atmospheric CO2 concentrations

Abstract: Iron occurs at very low concentrations in seawater and seems to be a limiting factor for primary production in the equatorial Pacific and the Southern Ocean. The global distribution of iron is still not well understood because of a lack of data and the complex chemistry of iron. We develop a 10-box model to study the oceanic distribution of iron and its effect on atmospheric CO2 concentration. Subject to our assumptions, we find that a lack of interocean fractionation of deep sea iron concentrations, as suggested by Johnson et al. [1997a], is not readily explained by a balance of eolian deposition, scavenging, and regeneration. Incorporation of organic complexation in the model, as suggested by Johnson et al., to reduce the scavenging rate of iron when concentrations fall below some ligand-stabilized concentration, is one solution to this difficulty. Alternatively, the deep-sea concentration may be more variable than the current, rather sparse data coverage suggests. In the model, deep-sea iron concentrations are responsive to the atmospheric source, even if we adopt stabilization of concentrations by a ligand as modeled by Johnson et al. [l997a]. In the Southern Ocean, where the model suggests iron supply has an important limiting effect on the biota, more than 99% of the iron supply to the surface in the present day comes from upwelling and not from the local atmospheric flux. In the context of glacial-interglacial changes to atmospheric CO2 the model suggests that increasing atmospheric iron to the entire global ocean by a factor of 2, leads to decreases in atmospheric CO2 of 10–30 ppm, depending on assumptions. However, in our model, CO2 concentrations are almost unaffected by changes in Southern Ocean atmospheric fluxes alone, unless these are unrealistically large (> 100 times present day). The effect on atmospheric carbon dioxide is slightly stronger if accompanied by increased stratification of the Southern Ocean. The model suggests that eolian “iron fertilization” of the ocean could have importantly influenced glacial atmospheric CO2 concentrations but that other processes must also be at work to account for the full magnitude of the glacial-interglacial change.

Net CO2 and H2O fluxes of terrestrial ecosystems

Abstract: Using 139 flux studies, we addressed the variability of net ecosystem surface assimilation (Asmax), net ecosystem surface respiration (Rsmax), as well as net surface evapotranspiration (Esmax) among and within vegetation types. While forests and C3 crops, particularly in the northern hemisphere, have been preferentially investigated, information on tropical forests, C4 grasslands or wetlands is rather limited. Almost no data are available for disturbed sites. Despite large variations within a vegetation type, enclosure studies tended to give highest Asmax rates compared to micrometeorological techniques. Excluding enclosure studies, we tested the effect of stand age and leaf area index (LAI) on net ecosystem gas exchange. For grasslands, Asmax increased by 7 μmol m−2 s−1 per unit LAI, for C4 crops by 11 μmol m−2 s−1, and for coniferous forests by 0.9 μmol m−2 s−1 per unit LAI. In contrast, Asmax of broad-leaved forests and C3 crops as well as Rsmax stayed constant over a wide range of LAI. Asmax and Rsmax of forests were lowest in young stands ( 160 years) was within the same range as those of 30- to 80-year-old forests, and always higher than those of regenerating stands. Rsmax seemed to decrease with age. Asmax increased linearly with ecosystem surface conductance for all vegetation types (r2 = 0.65). Asmax of forests and grasslands was closely related to Esmax (r2 = 0.87), with a slope of 0.082 μmol CO2 m−2 s−1/mmol H2O m−2 s−1. The results clearly illustrated where gaps in our knowledge exist and how ecosystem properties affect the capacity of net ecosystem gas exchange.

Controls on the carbon isotopic composition of Southern Ocean phytoplankton

Abstract: Carbon isotopic compositions of suspended organic matter and biomarker compounds were determined for 59 samples filtered from Southern Ocean surface waters in January 1994 along two north-south transects (WOCE SR3 from Tasmania to Antarctica, and across the Princess Elizabeth Trough (PET) east of Prydz Bay, Antarctica) Along the SR3 line, bulk organic matter show generally decreasing 13C contents southward, which are well correlated with increasing dissolved molecular carbon dioxide concentrations, CO2(aq) This relationship does not hold along the PET transect Using concentrations and isotopic compositions of molecular compounds, we evaluate the relative roles of several factors affecting the δ13C of Southern Ocean suspended particulate organic matter Along the WOCE SR3 transect, the concentration of CO2(aq) plays an important role It is well described by a supply versus demand model for the extent of cellular CO2 utilization and its associated linear dependence of isotopic fractionation (EP) on the reciprocal of CO2(aq) An equally important factor appears to be changes in algal assemblages along the SR3 transect, with their contribution to isotopic fractionation also well described by the supply and demand model, when formulated to include the cell surface/volume control of supply Changes in microalgal growth rates appear to have a minor effect on EP Along the PET transect, algal assemblage changes and possibly changes in microalgal growth rates appear to strongly affect the carbon isotopic variations of suspended organic matter These results can be used to improve the formulation of modern carbon cycle models that include phytoplankton carbon isotopic fractionation

Effect of seasonal changes in the pathways of methanogenesis on the δ13C values of pore water methane in a Michigan peatland

Abstract: The δ13C value of pore water methane produced in a Michigan peatland varied by 11‰ during the year. This isotopic shift resulted from large seasonal changes in the pathways of methane production. On the basis of mass balance calculations, the δ13C value of methane from CO2 reduction (average = −71.4 ± 1.8‰) was depleted in 13C compared to that produced from acetate (−44.4 ± 8.2‰). The dissolved methane at the site remained heavy (approximately −51‰) during most of the year. Tracer experiments using 14C-labeled CO2 indicated that during January 110 ± 25% of the methane was produced by CO2 reduction. Because of low-methane production rates during the winter, this 13C-depleted methane had only a slight effect on the isotopic composition of the methane pool. In early spring when peat temperatures and methane production rates increased, the δ13C value of the dissolved methane in shallow peat was influenced by the isotopically light methane and approached −61‰. Peat incubation experiments conducted at 15°C in May and June (when the peat reaches its maximum temperature) indicated that an average of 84 ± 9% of the methane production was from acetate and had an average δ13C value of −48.7 ± 5.6‰. Rising acetate concentrations during April-May (approaching 1 mmol L−1(mM)) followed by a rapid decrease in acetate concentrations during May-June reflected the shift toward methane production dominated by acetate fermentation. During this period, dissolved methane in shallow peat at the site returned to heavier values (approximately −51‰) similar to that produced in the incubation experiments.

Climate and litter quality controls on decomposition: An analysis of modeling approaches

Abstract: Four mathematical models simulated decay of two litter types of contrasting quality over a 2-year period at four sites in North America. The litter types were Drypetes glauca and Triticum aestivum, representing litter with high and low nitrogen:lignin ratios, respectively. The field sites were an Arctic tussock tundra (Alaska, United States), a warm desert (New Mexico, United States), a temperate deciduous forest (New York, United States) and a tropical rain forest (Puerto Rico). Models captured the overall patterns of site and litter quality controls on decomposition; both simulated and observed mass losses were higher in warm, moist environments (both forests) than in cold (tundra) or dry sites (desert), and simulated and observed decay was more rapid for Drypetes than Triticum. However, predictions tended to underestimate litter mass loss in the tropical forest and overestimate decay in the desert and tundra, suggesting that site controls in model formulations require refinement for use under such a broad range of conditions. Also, predicted nitrogen content of litter residues was lower than observed in Drypetes litter and higher than observed for Triticum. Thus mechanisms describing loss of nitrogen from high-quality litter and nitrogen immobilization by low-quality litter were not captured by model structure. Individual model behaviors revealed different sensitivities to controlling factors that were related to differences in model formulation. As these models represent working hypotheses regarding the process of litter decay, results emphasize the need for greater resolution of climate and litter quality controls. Results also demonstrate the need for finer resolution of the relationships between carbon and nitrogen dynamics during decomposition.