Showing papers in "Global Biogeochemical Cycles in 2003"

Global carbon sequestration in tidal, saline wetland soils

Abstract: [1] Wetlands represent the largest component of the terrestrial biological carbon pool and thus play an important role in global carbon cycles. Most global carbon budgets, however, have focused on dry land ecosystems that extend over large areas and have not accounted for the many small, scattered carbon-storing ecosystems such as tidal saline wetlands. We compiled data for 154 sites in mangroves and salt marshes from the western and eastern Atlantic and Pacific coasts, as well as the Indian Ocean, Mediterranean Ocean, and Gulf of Mexico. The set of sites spans a latitudinal range from 22.4°S in the Indian Ocean to 55.5°N in the northeastern Atlantic. The average soil carbon density of mangrove swamps (0.055 ± 0.004 g cm−3) is significantly higher than the salt marsh average (0.039 ± 0.003 g cm−3). Soil carbon density in mangrove swamps and Spartina patens marshes declines with increasing average annual temperature, probably due to increased decay rates at higher temperatures. In contrast, carbon sequestration rates were not significantly different between mangrove swamps and salt marshes. Variability in sediment accumulation rates within marshes is a major control of carbon sequestration rates masking any relationship with climatic parameters. Globally, these combined wetlands store at least 44.6 Tg C yr−1 and probably more, as detailed areal inventories are not available for salt marshes in China and South America. Much attention has been given to the role of freshwater wetlands, particularly northern peatlands, as carbon sinks. In contrast to peatlands, salt marshes and mangroves release negligible amounts of greenhouse gases and store more carbon per unit area.

Global patterns of the isotopic composition of soil and plant nitrogen

Abstract: [1] We compiled new and published data on the natural abundance N isotope composition (δ15N values) of soil and plant organic matter from around the world. Across a broad range of climate and ecosystem types, we found that soil and plant δ15N values systematically decreased with increasing mean annual precipitation (MAP) and decreasing mean annual temperature (MAT). Because most undisturbed soils are near N steady state, the observations suggest that an increasing fraction of ecosystem N losses are 15N-depleted forms (NO3, N2O, etc.) with decreasing MAP and increasing MAT. Wetter and colder ecosystems appear to be more efficient in conserving and recycling mineral N. Globally, plant δ15N values are more negative than soils, but the difference (δ15Nplant-δ15Nsoil) increases with decreasing MAT (and secondarily increasing MAP), suggesting a systematic change in the source of plant-available N (organic/NH4+ versus NO3−) with climate. Nitrogen isotopes reflect time integrated measures of the controls on N storage that are critical for predictions of how these ecosystems will respond to human-mediated disturbances of the global N cycle.

Global distribution of C3 and C4 vegetation: Carbon cycle implications

Abstract: [1] The global distribution of C3 and C4 plants is required for accurately simulating exchanges of CO2, water, and energy between the land surface and atmosphere. It is also important to know the C3/C4 distribution for simulations of the carbon isotope composition of atmospheric CO2 owing to the distinct fractionations displayed by each photosynthetic type. Large areas of the land surface are spatial and temporal mosaics of both photosynthetic types. We developed an approach for capturing this heterogeneity by combining remote sensing products, physiological modeling, a spatial distribution of global crop fractions, and national harvest area data for major crop types. Our C3/C4 distribution predicts the global coverage of C4 vegetation to be 18.8 million km2, while C3 vegetation covers 87.4 million km2. We incorporated our distribution into the SiB2 model and simulated carbon fluxes for each photosynthetic type. The gross primary production (GPP) of C4 plants is 35.3 Pg C yr−1, or ∼23% of total GPP, while that of C3 plants is 114.7 Pg C yr−1. The assimilation-weighted terrestrial discrimination against 13CO2 is −16.5‰. If the terrestrial component of the carbon sink is proportional to GPP, this implies a net uptake of 2.4 Pg C yr−1 on land and 1.4 Pg C yr−1 in the ocean using a 13C budgeting approach and average carbon cycle parameter values for the 1990s. We also simulated the biomass of each photosynthetic type using the CASA model. The simulated biomass values of C3 and C4 vegetation are 389.3 and 18.6 Pg C, respectively.

Biomass burning in Asia: Annual and seasonal estimates and atmospheric emissions

Abstract: Estimates of biomass burning in Asia are developed to facilitate the modeling of Asian and global air quality. A survey of national, regional, and international publications on biomass burning is conducted to yield consensus estimates of 'typical' (i.e., non-year-specific) estimates of open burning (excluding biofuels). We conclude that 730 Tg of biomass are burned in a typical year from both anthropogenic and natural causes. Forest burning comprises 45% of the total, the burning of crop residues in the field comprises 34%, and 20% comes from the burning of grassland and savanna. China contributes 25% of the total, India 18%, Indonesia 13%, and Myanmar 8%. Regionally, forest burning in Southeast Asia dominates. National, annual totals are converted to daily and monthly estimates at 1{sup o} x 1{sup o} spatial resolution using distributions based on AVHRR fire counts for 1999--2000. Several adjustment schemes are applied to correct for the deficiencies of AVHRR data, including the use of moving averages, normalization, TOMS Aerosol Index, and masks for dust, clouds, landcover, and other fire sources. Good agreement between the national estimates of biomass burning and adjusted fire counts is obtained (R{sup 2} = 0.71--0.78). Biomass burning amounts are converted to atmospheric emissions, yielding themore » following estimates: 0.37 Tg of SO{sub 2}, 2.8 Tg of NO{sub x}, 1100 Tg of CO{sub 2}, 67 Tg of CO, 3.1 Tg of CH{sub 4}, 12 Tg of NMVOC, 0.45 Tg of BC, 3.3 Tg of OC, and 0.92 Tg of NH{sub 3}. Uncertainties in the emission estimates, measured as 95% confidence intervals, range from a low of {+-}65% for CO{sub 2} emissions in Japan to a high of {+-}700% for BC emissions in India.« less

An assessment of biofuel use and burning of agricultural waste in the developing world

Abstract: [1] We present an assessment of biofuel use and agricultural field burning in the developing world. We used information from government statistics, energy assessments from the World Bank, and many technical reports, as well as from discussions with experts in agronomy, forestry, and agro-industries. We estimate that 2060 Tg biomass fuel was used in the developing world in 1985; of this, 66% was burned in Asia, and 21% and 13% in Africa and Latin America, respectively. Agricultural waste supplies about 33% of total biofuel use, providing 39%, 29%, and 13% of biofuel use in Asia, Latin America, and Africa, and 41% and 51% of the biofuel use in India and China. We find that 400 Tg of crop residues are burned in the fields, with the fraction of available residue burned in 1985 ranging from 1% in China, 16–30% in the Middle East and India, to about 70% in Indonesia; in Africa about 1% residue is burned in the fields of the northern drylands, but up to 50% in the humid tropics. We distributed this biomass burning on a spatial grid with resolution of 1 1, and applied emission factors to the amount of dry matter burned to give maps of trace gas emissions in the developing world. The emissions of CO from biofuel use in the developing world, 156 Tg, are about 50% of the estimated global CO emissions from fossil fuel use and industry. The emission of 0.9 Pg C (as CO2) from burning of biofuels and field residues together is small, but nonnegligible when compared with the emissions of CO2 from fossil fuel use and industry, 5.3 Pg C. The biomass burning source of 10 Tg/yr for CH4 and 2.2 Tg N/yr of NOx are relatively small when compared with total CH4 and NOx sources; this source of NOx may be important on a regional basis. INDEX TERMS: 1610 Global Change: Atmosphere (0315, 0325); 1803 Hydrology: Anthropogenic effects; 1615 Global Change: Biogeochemical processes (4805); KEYWORDS: biofuel use, global biofuel burning, agricultural waste burning, biofuel emissions

The application and interpretation of Keeling plots in terrestrial carbon cycle research

Abstract: [1] Photosynthesis and respiration impart distinct isotopic signatures to the atmosphere that are used to constrain global carbon source/sink estimates and partition ecosystem fluxes. Increasingly, the ‘‘Keeling plot’’ method is being used to determine the carbon isotope composition of ecosystem respiration (d 13 CR) in order to better understand the processes controlling ecosystem isotope discrimination. In this paper we synthesize emergent patterns in d 13 CR by analyzing 146 Keeling plots constructed at 33 sites across North and South America. In order to interpret results from disparate studies, we discuss the assumptions underlying the Keeling plot method and recommend standardized methods for determining d 13 CR. These include the use of regression calculations that account for error in the x variable, and constraining estimates of d 13 CR to nighttime periods. We then recalculate d 13 CR uniformly for all sites. We found a high degree of temporal and spatial variability in C3 ecosystems, with individual observations ranging from � 19.0 to � 32.6%. Mean C3 ecosystem discrimination was 18.3%. Precipitation was a major driver of both temporal and spatial variability of d 13 CR, suggesting (1) a large influence of recently fixed carbon on ecosystem respiration and (2) a significant effect of previous climatic effects on d 13 CR. These results illustrate the importance of water availability as a key control on atmospheric 13 CO2 and highlight the potential of d 13 CR as a useful tool for integrating environmental effects on dynamic canopy and ecosystem processes. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 0322 Atmospheric Composition and Structure: Constituent sources and sinks; 1615 Global Change: Biogeochemical processes (4805); 1694 Global Change: Instruments and techniques; 3322 Meteorology and Atmospheric Dynamics: Land/atmosphere interactions; KEYWORDS: carbon cycle, carbon isotopes, ecosystem respiration, carbon dioxide, terrestrial ecosystems

Modeling temporal and large‐scale spatial variability of soil respiration from soil water availability, temperature and vegetation productivity indices

Abstract: [1] Field-chamber measurements of soil respiration from 17 different forest and shrubland sites in Europe and North America were summarized and analyzed with the goal to develop a model describing seasonal, interannual and spatial variability of soil respiration as affected by water availability, temperature, and site properties. The analysis was performed at a daily and at a monthly time step. With the daily time step, the relative soil water content in the upper soil layer expressed as a fraction of field capacity was a good predictor of soil respiration at all sites. Among the site variables tested, those related to site productivity (e.g., leaf area index) correlated significantly with soil respiration, while carbon pool variables like standing biomass or the litter and soil carbon stocks did not show a clear relationship with soil respiration. Furthermore, it was evidenced that the effect of precipitation on soil respiration stretched beyond its direct effect via soil moisture. A general statistical nonlinear regression model was developed to describe soil respiration as dependent on soil temperature, soil water content, and site-specific maximum leaf area index. The model explained nearly two thirds of the temporal and intersite variability of soil respiration with a mean absolute error of 0.82 μmol m−2 s−1. The parameterized model exhibits the following principal properties: (1) At a relative amount of upper-layer soil water of 16% of field capacity, half-maximal soil respiration rates are reached. (2) The apparent temperature sensitivity of soil respiration measured as Q10 varies between 1 and 5 depending on soil temperature and water content. (3) Soil respiration under reference moisture and temperature conditions is linearly related to maximum site leaf area index. At a monthly timescale, we employed the approach by Raich et al. [2002] that used monthly precipitation and air temperature to globally predict soil respiration (T&P model). While this model was able to explain some of the month-to-month variability of soil respiration, it failed to capture the intersite variability, regardless of whether the original or a new optimized model parameterization was used. In both cases, the residuals were strongly related to maximum site leaf area index. Thus, for a monthly timescale, we developed a simple T&P&LAI model that includes leaf area index as an additional predictor of soil respiration. This extended but still simple model performed nearly as well as the more detailed time step model and explained 50% of the overall and 65% of the site-to-site variability. Consequently, better estimates of globally distributed soil respiration should be obtained with the new model driven by satellite estimates of leaf area index. Before application at the continental or global scale, this approach should be further tested in boreal, cold-temperate, and tropical biomes as well as for non-woody vegetation.

A parameterization of sea-salt aerosol source function for sub- and super-micron particles

Abstract: [1] A parameterization of a sea-salt source function for both sub- and super-micron particles was developed based on the semi-empirical formulation of Monahan et al. [1986]. This new parameterization extends the range of Monahan's equation to below 0.2 μm in diameter where it has been found to overestimate submicron sea-salt aerosols, especially the sea-salt number concentrations. The new parameterization was used in a one-dimensional (1-D) column model to predict the number size distributions and compared with reasonable agreement to the observed distributions at various wind speeds reported by O'Dowd et al. [1997]. A global 3-D sea-salt simulation with this parameterization was also made and a much better dependence of sea-salt on surface wind speed was predicted than other schemes compared to observations. For an indirect impact assessment of sea-salt aerosols on climate where submicron particles may have a dominant contribution to aerosol-cloud interactions, this scheme provides the most realistic number flux of sea-salt particles between ocean and atmosphere.

Worldwide distribution of continental rock lithology: Implications for the atmospheric/soil CO2 uptake by continental weathering and alkalinity river transport to the oceans

Abstract: [1] The silicate rock weathering followed by the formation of carbonate rocks in the ocean, transfers CO2 from the atmosphere to the lithosphere. This CO2 uptake plays a major role in the regulation of atmospheric CO2 concentrations at the geologic timescale and is mainly controlled by the chemical properties of rocks. This leads us to develop the first world lithological map with a grid resolution of 1° × 1°. This paper analyzes the spatial distribution of the six main rock types by latitude, continents, and ocean drainage basins and for 49 large river basins. Coupling our digital map with the GEM-CO2 model, we have also calculated the amount of atmospheric/soil CO2 consumed by rock weathering and alkalinity river transport to the ocean. Among all silicate rocks, shales and basalts appear to have a significant influence on the amount of CO2 uptake by chemical weathering.

An ecosystem model of the global ocean including Fe, Si, P colimitations

Abstract: [1] Observations have shown that large areas of the world ocean are characterized by lower than expected chlorophyll concentrations given the ambient phosphate and nitrate levels. In these High Nutrient-Low Chlorophyll regions, limitations of phytoplankton growth by other nutrients like silicate or iron have been hypothesized and further evidenced by in situ experiments. To explore these limitations, a nine-component ecosystem model has been embedded in the Hamburg model of the oceanic carbon cycle (HAMOCC5). This model includes phosphate, silicate, dissolved iron, two phytoplankton size fractions (nanophytoplankton and diatoms), two zooplankton size fractions (microzooplankton and mesozooplankton), one detritus and semilabile dissolved organic matter. The model is able to reproduce the main characteristics of two of the three main HNLC areas, i.e., the Southern Ocean and the equatorial Pacific. In the subarctic Pacific, silicate and phosphate surface concentrations are largely underestimated because of deficiencies in ocean dynamics. The low chlorophyll concentrations in HNLC areas are explained by the traditional hypothesis of a simultaneous iron-grazing limitation: Diatoms are limited by iron whereas nanophytoplankton is controlled by very efficient grazing by microzooplankton. Phytoplankton assimilates 18 × 109 mol Fe yr−1 of which 73% is supplied by regeneration within the euphotic zone. The model predicts that the ocean carries with it about 75% of the phytoplankton demand for new iron, assuming a 1% solubility for atmospheric iron. Finally, it is shown that a higher supply of iron to surface water leads to a higher export production but paradoxically to a lower primary productivity.

The contribution of ocean‐leaving DMS to the global atmospheric burdens of DMS, MSA, SO2, and NSS SO4=

Abstract: [1] The contribution of ocean-derived DMS to the atmospheric burdens of a variety of sulphur compounds (DMS, MSA, SO2, and nss SO4=) is quantified from season to season. Such quantification, especially for nss SO4= (the climate-relevant product of DMS oxidation), is essential for the quantification of the radiative forcing of climate that may be attributable to marine phytoplankton under possible future climate conditions. Three-dimensional chemical transport modeling up to the stratosphere is used as a tool in realizing this aim. Global data sets on oceanic and terrestrial sulphur sources are used as input. We find that the contribution of ocean-leaving DMS to the global annually averaged column burdens of the modeled compounds is considerable: 11.9 mumol m(-2) (98% of total global burden) for DMS; 0.95 mumol m(-2) (94% of total global burden) for MSA; 2.8 mumol m(-2) (32% of total global burden) for SO2 and 2.5 mumol m(-2) (18% of total global burden) for nss SO4=. The mean annual contribution of DMS to the climate-relevant nss SO4= column burden is greatest in the relatively pristine Southern Hemisphere, where it is estimated at 43%. This contribution is only 9% in the Northern Hemisphere, where anthropogenic sulphur sources are overwhelming. The marine algal-derived input of the other modeled sulphur compounds ( DMS, MSA, and SO2) is also greatest in the Southern Hemisphere where a lower oxidative capacity of the atmosphere, a larger sea-to-air transfer of DMS and a larger emission surface area lead to an elevation of the atmospheric DMS burden.

Interannual variability in the peatland-atmosphere carbon dioxide exchange at an ombrotrophic bog

Abstract: This loss was equivalent to between 30 and 70% of the net CO2 uptake during the growing season. During the first 3 years of study, the bog was an annual sink for CO2 (260 g CO2 m 2 yr 1 ). In the fourth year, with the dry summer, however, annual NEE was only 34 g CO2 m 2 yr 1 , which is not significantly different from zero. We examined the performance of a peatland carbon simulator (PCARS) model against the tower measurements of NEE and derived ecosystem respiration (ER) and photosynthesis (PSN). PCARS ER and PSN were highly correlated with tower-derived fluxes, but the model consistently overestimated both ER and PSN, with slightly poorer comparisons in the dry year. As a result of both component fluxes being overestimated, PCARS simulated the tower NEE reasonably well. Simulated decomposition and autotrophic respiration contributed about equal proportions to ER. Shrubs accounted for the greatest proportion of PSN (85%); moss PSN declined to near zero during the summer period due to surface drying. INDEX TERMS: 0315 Atmospheric Composition and Structure: Biosphere/atmosphere interactions; 1615 Global Change: Biogeochemical processes (4805); 1890 Hydrology: Wetlands; KEYWORDS: peatland, bog, net ecosystem exchange, eddy covariance, carbon dioxide

Substrate regulation of soil respiration in a tallgrass prairie: Results of a clipping and shading experiment

Abstract: [1] Changes in soil respiration, one of the major fluxes of global carbon cycling, could significantly slow down or accelerate the increase in atmospheric CO2, with consequent feedbacks to climate change. It is critical to understand how substrate availability regulates soil respiration in projecting the response of carbon cycling to changed climate. We conducted a clipping and shading experiment for 1 year in a tallgrass prairie of the Great Plains, United States, to manipulate substrate supply to soil respiration. Our results showed that reduced substrate supply under clipping and/or shading significantly decreased soil respiration at all the timescales (diurnal, transient, and annual) irrespective of the minor concurrent changes in soil temperature and moisture. Annual mean soil respiration decreased significantly by 33, 23, and 43% for the clipping, shading, and clipping plus shading treatments, respectively. Temperature sensitivity of soil respiration decreased from 1.93 in the control plots to 1.88, 1.75, and 1.83 in the clipped, shaded, and clipped plus shaded plots, respectively. Rhizosphere respiration, respiration from decomposition of aboveground litter, and respiration from oxidation of soil organic matter and dead roots accounted for 30, 14, and 56% of annual mean soil respiration, respectively. Rhizosphere respiration was more sensitive to temperature than the other two components. Our results suggest a critical role of substrate supply in regulating soil respiration and its temperature sensitivity.

An approach to understanding hydrologic connectivity on the hillslope and the implications for nutrient transport

Abstract: [1] Hydrologic processes control much of the export of organic matter and nutrients from the land surface. It is the variability of these hydrologic processes that produces variable patterns of nutrient transport in both space and time. In this paper, we explore how hydrologic ‘‘connectivity’’ potentially affects nutrient transport. Hydrologic connectivity is defined as the condition by which disparate regions on the hillslope are linked via subsurface water flow. We present simulations that suggest that for much of the year, water draining through a catchment is spatially isolated. Only rarely, during storm and snowmelt events when antecedent soil moisture is high, do our simulations suggest that mid-slope saturation (or near saturation) occurs and that a catchment connects from ridge to valley. Observations during snowmelt at a small headwater catchment in Idaho are consistent with these model simulations. During early season discharge episodes, in which the mid-slope soil column is not saturated, the electrical conductivity in the stream remains low, reflecting a restricted, local (lower slope) source of stream water and the continued isolation of upper and mid-slope soil water and nutrients from the stream system. Increased streamflow and higher stream water electrical conductivity, presumably reflecting the release of water from the upper reaches of the catchment, are simultaneously observed when the mid-slope becomes sufficiently wet. This study provides preliminary evidence that the seasonal timing of hydrologic connectivity may affect a range of ecological processes, including downslope nutrient transport, C/N cycling, and biological productivity along the toposequence. A better elucidation of hydrologic connectivity will be necessary for understanding local processes as well as material export from land to water at regional and global scales. INDEX TERMS: 1615 Global Change: Biogeochemical processes (4805); 1860 Hydrology: Runoff and streamflow; 1866 Hydrology: Soil moisture; 1899 Hydrology: General or miscellaneous; KEYWORDS: carbon and nitrogen transport, hydrologic connectivity, TOPMODEL

Long‐term changes in dissolved oxygen concentrations in the ocean caused by protracted global warming

Abstract: [i] In the Earth's geological record massive marine ecological change has been attributed to the occurrence of widespread anoxia in the ocean [Jahren, 2002; White, 2002; Wignall and Twitchett, 1996]. Climate change projection till the end of this century predict a 4 to 7% decline in the dissolve oxygen in the ocean [Bopp et al., 2002; Matear et al., 2000; Plattner et al., 2001; Sarmiento et al., 1998] suggesting the potential for global warming to eventually drive the deep ocean anoxic. To examine the multicentury impact of protracted global warming on oceanic concentrations of dissolved oxygen, we use a climate system model and a low-order oceanic biogeochemical model. The models are integrated for an atmospheric equivalent CO 2 concentration, which is specified to triple according to a standard scenario from the late nineteenth to the late twenty-first century, and then is subsequently held constant at that elevated level for an additional 6 centuries. For the present day, the model successfully reproduced the large-scale features of the dissolved oxygen field in the ocean. In the global warming simulation, the physical model displays marked changes in high-latitude oceanic stratification and overturning, including near-cessation of deep water renewal for depths greater than about 1.5 km during the period of elevated stable CO 2 concentration. Our model predicts a decline in oxygen concentration through most of the subsurface ocean. Concentration changes in the thermocline waters result mainly from solubility changes in the upstream source waters, while changes in the deep waters result mainly from lack of ventilation and ongoing consumption of oxygen by remineralization of sinking particulate organic matter. Changes in the upper 2 km of the ocean generally show signs ofequi libration by the end ofthe integration, but at greater depths, there occurs a slow but steady decline through to the end of the integration. By the end of the integration, we simulate a doubling ofthe volume ofhypoxic water (less than 10 μmol/kg) in the thermocline of the eastern equatorial Pacific Ocean. During the integration deep ocean oxygen concentrations generally decline by between 20 and 40%, but, significantly, no extensive deep ocean anoxia develops during the period of integration, nor does it appear that it would likely do so for at least a further 4000 years of integration. Subsurface oxygen decline is moderated by an overall reduction in export production of particulate organic matter, which reduces oxygen consumption in the ocean interior due to the remineralization of this material.

Effect of elevated CO2 on the community metabolism of an experimental coral reef

Abstract: [1] The effect of elevated pCO2 on the metabolism of a coral reef community dominated by macroalgae has been investigated utilizing the large 2650 m3 coral reef mesocosm at the Biosphere-2 facility near Tucson, Arizona. The carbonate chemistry of the water was manipulated to simulate present-day and a doubled CO2 future condition. Each experiment consisted of a 1–2 month preconditioning period followed by a 7–9 day observational period. The pCO2 was 404 ± 63 μatm during the present-day pCO2 experiment and 658 ± 59 μatm during the elevated pCO2 experiment. Nutrient levels were low and typical of natural reefs waters (NO3− 0.5–0.9 μM, NH4+ 0.4 μM, PO43− 0.07–0.09 μM). The temperature and salinity of the water were held constant at 26.5 ± 0.2°C and 34.4 ± 0.2 ppt. Photosynthetically available irradiance was 10 ± 2 during the present-day experiment and 7.4 ± 0.5 mol photons m−2 d−1 during the elevated pCO2 experiment. The primary producer biomass in the mesocosm was dominated by four species of macroalgae; Haptilon cubense, Amphiroa fragillisima, Gelidiopsis intricata and Chondria dasyphylla. Algal biomass was 10.4 mol C m−2 during the present-day and 8.7 mol C m−2 and during the elevated pCO2 experiments. As previously observed, the increase in pCO2 resulted in a decrease in calcification from 0.041 ± 0.007 to 0.006 ± 0.003 mol CaCO3 m−2 d−1. Net community production (NCP) and dark respiration did not change in response to elevated pCO2. Light respiration measured by a new radiocarbon isotope dilution method exceeded dark respiration by a factor of 1.2 ± 0.3 to 2.1 ± 0.4 on a daily basis and by 2.2 ± 0.6 to 3.9 ± 0.8 on an hourly basis. The 1.8-fold increase with increasing pCO2 indicates that the enhanced respiration in the light was not due to photorespiration. Gross production (GPP) computed as the sum of NCP plus daily respiration (light + dark) increased significantly (0.24 ± 0.03 vs. 0.32 ± 0.04 mol C m−2 d−1). However, the conventional calculation of GPP based on the assumption that respiration in the light proceeds at the same rate as the dark underestimated the true rate of GPP by 41–100% and completely missed the increased rate of carbon cycling due to elevated pCO2. We conclude that under natural, undisturbed, nutrient-limited conditions elevated CO2 depresses calcification, stimulates the rate of turnover of organic carbon, particularly in the light, but has no effect on net organic production. The hypothesis that an increase pCO2 would produce an increase in net production that would counterbalance the effect of decreasing saturation state on calcification is not supported by these data.

Air‐sea flux of bromoform: Controls, rates, and implications

Abstract: Bromoform (CHBr3) is the largest single source of atmospheric organic bromine and therefore of importance as a source of reactive halogens to the troposphere and lower stratosphere The sea-to-air flux, originating with macroalgal and planktonic sources, is the main source for atmospheric bromoform We review bromoform's contribution to atmospheric chemistry, its atmospheric and oceanic distributions and its oceanic sources and sinks We have reassessed oceanic emissions, based on published aqueous and airborne concentration data, global climatological parameters, and information concerning coastal and biogenic sources The goals are to attempt an estimate of the global source strength and partly to identify key regions that require further investigation The sea-to-air flux is spatially and temporally variable with tropical, subtropical and shelf waters identified as potentially important source regions We obtain an annual global flux of bromoform of ∼10 Gmol Br yr−1 (3–22 Gmol Br yr−1) This estimate is associated with significant uncertainty, arising from data precision and coverage, choice of air-sea exchange parameterizations and model assumptions Anthropogenic sources of ∼03 (to 11) Gmol Br yr−1 (as CHBr3) can be locally significant, but are globally negligible Our estimate of the global oceanic source is three to four times higher than recent estimates based on the modeling of atmospheric sinks The reasons for this discrepancy could lie with the limited regional and temporal data available and the broad assumptions that underlie our flux calculations Alternatively, atmospheric sink calculations, often made on the basis of background CHBr3 levels, may neglect the influence of strong but highly localized sources (eg, from some coastal and shelf regions) The strongly variable and poorly characterized source of CHBr3, together with its short atmospheric lifetime, complicates model-based estimation of the distribution of reactive Br resulting from its atmospheric degradation An integrated program of marine and atmospheric observations, atmospheric modeling and mechanistic studies of oceanic bromoform production is required to better constrain present and future Br delivery to the atmosphere

Modeling carbon dynamics in vegetation and soil under the impact of soil erosion and deposition

Abstract: [1] Soil erosion and deposition may play important roles in balancing the global atmospheric carbon budget through their impacts on the net exchange of carbon between terrestrial ecosystems and the atmosphere. Few models and studies have been designed to assess these impacts. In this study, we developed a general ecosystem model, Erosion-Deposition-Carbon-Model (EDCM), to dynamically simulate the influences of rainfall-induced soil erosion and deposition on soil organic carbon (SOC) dynamics in soil profiles. EDCM was applied to several landscape positions in the Nelson Farm watershed in Mississippi, including ridge top (without erosion or deposition), eroding hillslopes, and depositional sites that had been converted from native forests to croplands in 1870. Erosion reduced the SOC storage at the eroding sites and deposition increased the SOC storage at the depositional areas compared with the site without erosion or deposition. Results indicated that soils were consistently carbon sources to the atmosphere at all landscape positions from 1870 to 1950, with lowest source strength at the eroding sites (13 to 24 gC m−2 yr−1), intermediate at the ridge top (34 gC m−2 yr−1), and highest at the depositional sites (42 to 49 gC m−2 yr−1). During this period, erosion reduced carbon emissions via dynamically replacing surface soil with subsurface soil that had lower SOC contents (quantity change) and higher passive SOC fractions (quality change). Soils at all landscape positions became carbon sinks from 1950 to 1997 due to changes in management practices (e.g., intensification of fertilization and crop genetic improvement). The sink strengths were highest at the eroding sites (42 to 44 gC m−2 yr−1), intermediate at the ridge top (35 gC m−2 yr−1), and lowest at the depositional sites (26 to 29 gC m−2 yr−1). During this period, erosion enhanced carbon uptake at the eroding sites by continuously taking away a fraction of SOC that can be replenished with enhanced plant residue input. Overall, soil erosion and deposition reduced CO2 emissions from the soil into the atmosphere by exposing low carbon-bearing soil at eroding sites and by burying SOC at depositional sites. The results suggest that failing to account for the impact of soil erosion and deposition may potentially contribute to an overestimation of both the total historical carbon released from soils owing to land use change and the contemporary carbon sequestration rates at the eroding sites.

Global modeling of the fate of nitrogen from point and nonpoint sources in soils, groundwater, and surface water

Abstract: [1] We present a global model that describes the fate of nitrogen (N) from point and nonpoint sources in the hydrological system up to the river mouths at the 0.5° by 0.5° spatial and annual temporal resolution. Estimates for point sources are based on population densities, per capita human N emissions, and data on sanitation coverage and wastewater treatment. For nonpoint sources, we use spatial information on land use, climate, hydrology, geology, and soils, combined with data on N inputs (fertilizers and animal manure, biological N fixation, and atmospheric deposition), and outputs (N removal in harvested agricultural products, ammonia emissions). Denitrification in the root zone and nitrate leaching to groundwater are calculated with a model that combines the effect of temperature, crop type, soil properties, and hydrological conditions. The nitrate concentration of the outflow for shallow and deep groundwater layers is based on historical inputs of fertilizer N and the effects of residence time and denitrification. In-stream N retention is based on a global estimate of 30% of the N discharged to surface water. Calculated and reported total N concentrations of discharge near the river outlet agree fairly well. However, our model systematically overestimates total N concentrations for river basins with mean annual temperature >0°C.

Distribution and storage of soil organic carbon in China

Abstract: organic carbon density in soils varies from 0.73 to 70.79 kg C/m 2 with the majority ranging between 4.00 and 11.00 kg C/m 2 . Carbon density decreases from east to west. A general southward increase is obvious for western China, while carbon density decreases from north to south in eastern China. Highest values are observed in forest soils in northeast China and in subalpine soils in the southeastern part of the Tibetan Plateau. The average density of � 8.01 kg C/m 2 in China is lower than the world’s mean organic carbon density in soil (� 10.60 kg C/m 2 ), mainly due to the extended arid and semi-arid regions. Total organic carbon storage in soils in China is estimated to be � 70.31 Pg C, representing � 4.7% of the world storage. Carbon storage in the surface organic horizons which is most sensitive to interactions with the atmosphere and environmental change is � 32.54 Pg C. INDEX TERMS: 0330 Atmospheric Composition and Structure: Geochemical cycles; 1615 Global Change: Biogeochemical processes (4805); 1815 Hydrology: Erosion and sedimentation; 1625 Global Change: Geomorphology and weathering (1824, 1886);

Eddy-driven sources and sinks of nutrients in the upper ocean: Results from a 0.1° resolution model of the North Atlantic

Abstract: [1] A nitrate-based model of new production is incorporated into eddy-resolving (0.1°) simulations of the North Atlantic. The biological model consists of light and nutrient limited production within the euphotic zone and relaxation of the nitrate field to climatology below. Sensitivity of the solutions to the parameters of the biological model is assessed in a series of simulations. Model skill is quantitatively evaluated with observations using an objective error metric; simulated new production falls within the range of observed values at several sites throughout the basin. Results from the “best fit” model are diagnosed in detail. Mean and eddying components of the nutrient fluxes are separated via Reynolds decomposition. In the subtropical gyre, eddy-driven vertical advection of nutrients is sufficient to overcome the mean wind-driven downwelling in the region and fuels a significant fraction of the annual new production in that area. In contrast, eddies constitute a net sink of nutrients in the subpolar gyre. Geostrophic adjustment to deep winter convection through mesoscale processes causes a net flux of nutrients out of the euphotic zone; the magnitude of this sink is sufficient to counterbalance the mean wind-driven upwelling of nutrients over much of the region. On the basis of these simulations it appears that the oceanic mesoscale has major impacts on nutrient supply to, and removal from, the euphotic zone.

Substantial advective iron loss diminishes phytoplankton production in the Antarctic Zone

Abstract: [1] After 1 decade of research it is a well-established fact that iron limits photosynthetic CO2 fixation and phytoplankton growth in the Southern Ocean; intense blooms are scarce However, the input of iron to the Southern Ocean is considerable An important factor for diminished phytoplankton production refers to the meridional circulation of the Southern Ocean Intense, spatially relatively homogeneous upwelling of Upper Circumpolar Deep Water (UCDW) causes a large iron flux into the surface layer However, the main entrainment of upwelled UCDW into the surface layer occurs in autumn and winter, which strongly restricts the usefullness of iron supply for phytoplankton due to unfavorable light conditions Moreover, the meridional transport within the Ekman layer is intense enough to export at least 25% of the iron input away from the Antarctic Zone before it can be used by phytoplankton This also depresses the potential phytoplankton primary production by at least 25% Most iron that crosses the Polar Front unused probably leaves the surface ocean north of the Polar Front because the surface water participates in Antarctic Intermediate Water/Mode Water formation

Global distribution of N2O and the ΔN2O‐AOU yield in the subsurface ocean

Abstract: [1] We present and analyze a data set of subsurface N2O from a range of oceanic regions. Observed N2O concentrations are highest in the eastern tropical Pacific (ETP), intermediate in the northern Pacific and Indian Oceans, and relatively low in the Southern and Atlantic Oceans. Tongues of high N2O, which propagate along sigma surfaces, provide evidence that N2O from the ETP is exported widely. Correlation slopes of ΔN2O (the level above atmospheric equilibrium) versus apparent oxygen utilization (AOU) are found to be an unreliable gauge of the biological N2O yield per mole O2 consumed because the slopes are strongly influenced by mixing gradients. Most features of the subsurface data set are consistent with an N2O source dominated by nitrification, including the widespread, robust ΔN2O-AOU correlation and the lack of a widespread anticorrelation between ΔN2O and N*. In addition, ΔN2O/NO3− ratios tend to increase with decreasing O2 in a manner consistent with laboratory studies of nitrifying bacteria. The sensitivity of the nitrifier N2O/NO3− yield to O2 can explain much of the variability in ΔN2O/AOU observed in the ocean. A parameterization is derived for the instantaneous production of N2O per mole O2 consumed as a nonlinear function of O2 and depth. The parameterization is based on laboratory and oceanic data and is designed for use in ocean biogeochemistry models. It is coupled to a global dissolved O2 climatology and ocean carbon model output to estimate a total oceanic N2O inventory of 610–840 Tg N and a global production rate of ∼5.8 ± 2 Tg N/y.

Dissolved inorganic phosphorus, dissolved iron, and Trichodesmium in the oligotrophic South China Sea

Abstract: [1] Dissolved inorganic phosphorus (DIP) concentrations in the oligotrophic surface waters of the South China Sea decrease from ∼20 nM in March 2000 to ∼5 nM in July 2000, in response to seasonal water column stratification. These minimum DIP concentrations are one order of magnitude higher than those in the P-limited, iron-replete stratified surface waters of the western North Atlantic, suggesting that the ecosystem in the South China Sea may be limited by bioavailable nitrogen or some trace nutrient rather than DIP. Nutrient enrichment experiments using either nitrate, phosphate or both indicate that nitrogen limits the net growth of phytoplankton in the South China Sea, at least during March and July 2000. The fixed nitrogen limitation may result from the excess phosphate (N:P<16) transported into the South China Sea from the North Pacific relative to microbial population needs, or from iron control of nitrogen fixation. The iron-limited nitrogen fixation hypothesis is supported by the observation of low population densities of Trichodesmium spp. (<48 × 103 trichomes/m3), the putative N2 fixing cyanobacterium, and with low concentrations of dissolved iron (∼0.2–0.3 nM) in the South China Sea surface water. Our results suggest that nitrogen fixation can be limited by available iron even in regions with a high rate of atmospheric dust deposition such as in the South China Sea.

Sustainability of terrestrial carbon sequestration: A case study in Duke Forest with inversion approach

Abstract: [1] A sound understanding of the sustainability of terrestrial carbon (C) sequestration is critical for the success of any policies geared toward stabilizing atmospheric greenhouse concentrations. This includes the Kyoto Protocol and/or other greenhouse strategies implemented by individual countries. However, the sustainability of C sinks and pools has not been carefully studied with either empirical or theoretical approaches. This study was intended to develop a conceptual framework to define the sustainability based on C influx and residence time (τ). The latter τ quantifies the capacity for C storage in various plant and soil pools. We estimated τ via inverse analysis of multiple data sets from a Free-Air CO2 Enrichment (FACE) experiment in Duke Forest, North Carolina, United States. This study suggested that estimated residence times at elevated CO2 decreased for plant C pools and increased for litter and soil pools in comparison to those at ambient CO2. The ensemble of the residence times from all the pools at elevated CO2, however, was well correlated with that at ambient CO2. We then used the estimated residence times, combined with C influx, to simulate C sequestration rates in response to a gradual increase in atmospheric CO2 concentration (Ca). The simulated C sequestration rate gradually increased from 69 g m−2 yr−1 in 2000 when Ca was 378 ppm to 201 g m−2 yr−1 in 2100 when Ca was at 710 ppm. Thus, the current evidence from both experimental observations and inverse analysis suggested that C sequestration in the forest ecosystem was likely to increase gradually as Ca gradually increases. The model projection of the C sequestration will improve as more data on long-term processes become available in coming years. In addition, such a modeled increase in terrestrial C sequestration is too small to balance the anthropogenic C emission.

Changes in the 13C/12C of dissolved inorganic carbon in the ocean as a tracer of anthropogenic CO2 uptake

Abstract: [1] Measurements of the δ13C of dissolved inorganic carbon primarily during World Ocean Circulation Experiment and the Ocean Atmosphere Carbon Exchange Study cruises in the 1990s are used to determine ocean-wide changes in the δ13C that have occurred due to uptake of anthropogenic CO2. This new ocean-wide δ13C data set (∼25,000 measurements) substantially improves the usefulness of δ13C as a tracer of the anthropogenic CO2 perturbation. The global mean δ13C change in the surface ocean is estimated at −0.16 ± 0.02‰ per decade between the 1970s and 1990s with the greatest changes observed in the subtropics and the smallest changes in the polar and southern oceans. The global mean air-sea δ13C disequilibrium in 1995 is estimated at 0.60 ± 0.10‰ with basin-wide disequilibrium values of 0.73, 0.63, and 0.23‰ for the Pacific, Atlantic, and Indian oceans, respectively. The global mean depth-integrated anthropogenic change in δ13C between the 1970s and 1990s was estimated at −65 ± 33‰ m per decade. These new estimates of air-sea δ13C disequilibrium and depth-integrated δ13C changes yield an oceanic CO2 uptake rate of 1.5 ± 0.6 Gt C yr−1 between 1970 and 1990 based on the atmospheric CO2 and 13CO2 budget approaches of Quay et al. [1992] and Tans et al. [1993] and the dynamic method of Heimann and Maier-Reimer [1996]. Box-diffusion model simulations of the oceanic uptake of anthropogenic CO2 and its δ13C perturbation indicate that a CO2 uptake rate of 1.9 ± 0.4 Gt C yr−1 (1970–1990) explains both the observed surface ocean and depth-integrated δ13C changes. Constraining a box diffusion ocean model to match both the observed δ13C and bomb 14C changes yields an oceanic CO2 uptake rate of 1.7 ± 0.2 Gt C yr−1 (1970–1990). The oceanic CO2 uptake rates derived from anthropogenic changes in ocean δ13C are similar to rates determined by atmospheric CO2 and O2 budgets [Battle et al., 2000], atmospheric δ13C and CO2 measurements [Ciais et al., 1995], and GCM simulations [Orr et al., 2001].

Four years' observations of terrestrial lipid class compounds in marine aerosols from the western North Pacific

Abstract: [1] In order to understand the long-range atmospheric transport of terrestrial organic matter over the open ocean, marine aerosol samples were collected on a biweekly basis from 1990 to 1993 in a remote island, Chichi-Jima, in the western North Pacific. The samples were analyzed for lipid class compounds using a capillary gas chromatography (GC) and GC/mass spectrometry (GC/MS). A homologous series of n-alkanes (C20-C40), alcohols (C13-C34), fatty acids (C9-C34) and α,ω-dicarboxylic acids (C7-C28) were detected in the aerosol samples. Distributions of n-alkanes (0.17–14 ngm−3, average 1.7 ngm−3) are characterized by a strong odd-carbon number predominance (CPI ratios, average 4.5) with a maximum at C29 or C31, indicating that n-alkanes are mainly derived from terrestrial higher plant waxes. Fatty alcohols (0.19–23 ngm−3, average 2.0 ngm−3) show an even-carbon number predominance with a maximum generally at C26 or C28, again indicating a contribution from terrestrial higher plants. On the other hand, fatty acids (2.5–38 ngm−3, average 14 ngm−3) show a bimodal distribution with two maxima at C16 and C24 or C28. Lower molecular weight fatty acids (generally

Gross primary production and light response parameters of four Southern Plains ecosystems estimated using long-term CO2-flux tower measurements

Abstract: [1] Gross primary production (GPP) is one of the most important characteristics of an ecosystem. At present, no empirically based method to estimate GPP is available, other than measurements of net CO2 exchange and calculations of respiration. Data sets from continuous CO2 flux measurements in a number of ecosystems (Ameriflux, AgriFlux, etc.) for the first time provide an opportunity to obtain empirically based estimates of GPP. In this paper, using the results of CO2 flux tower measurements during the 1997 season at four sites in Oklahoma (tallgrass prairie, mixed prairie, pasture, and winter wheat crop), we describe a method to evaluate the average daytime rate of ecosystem respiration, Rd, by estimation of the respiration term of the nonrectangular hyperbolic model of the ecosystem-scale light-response curve. Comparison of these predicted daytime respiration rates with directly measured corresponding nighttime values, Rn, after appropriate length of the night and temperature correction, demonstrated close linear relationship, with 0.82 ≤ R2 ≤ 0.98 for weekly averaged fluxes. Daily gross primary productivity, Pg, can be calculated as Pg = Pd + Rd, where Pd is the daytime integral of the net ecosystem CO2 exchange, obtained directly from measurements. Annual GPP for the sites, obtained as the sum of Pg over the whole period with Pg > 0 were: tallgrass prairie, 5223 g CO2 m−2; winter wheat, 2853 g CO2 m−2; mixed prairie, 3037 g CO2 m−2; and pasture, 2333 g CO2 m−2. These values are in agreement with published GPP estimates for nonforest terrestrial ecosystems.

Peatland responses to varying interannual moisture conditions as measured by automatic CO2 chambers

Abstract: [1] Net ecosystem CO2 exchange (NEE) was measured from June 2000 through October 2001 by 10 automatic chambers at a peatland in southeastern New Hampshire. The high temporal frequency of this sampling method permitted detailed examination of NEE as it varied daily and seasonally. Summer of 2001 was significantly drier than the 30-year average, while summer of 2000 was wetter than normal. Although NEE varied spatially across the peatland with differences in plant species composition and biomass, maximum CO2 uptake was 30–40% larger in the drier summer in evergreen and deciduous shrub communities but the same or lower in sedge sites. Ecosystem respiration rates were 13–84% larger in the drier summer depending on plant growth form with water table and temperature as strong predictors. Ecosystem respiration was also correlated with maximum ecosystem productivity and foliar biomass suggesting that plant processes, water table, and temperature are tightly linked in their control of respiratory losses. The ratio between maximum productivity and respiration declined for evergreen shrub and sedge sites between the wet and dry summer, but increased in deciduous shrub sites. A drier climate may reduce the CO2 sink function of peatlands for some growth forms and increase it for others, suggesting that ecosystem carbon and climate models should account for differences in growth form responses to climate change. It also implies that plant functional types respond on short timescales to changes in moisture, and that the transition from sedges to shrubs could occur rapidly in peatlands under a drier and warmer climate.

Carbon isotopic fractionation during decomposition of plant materials of different quality

Abstract: [1] Changes in isotopic 13C composition of solid residues and CO2 evolved during decomposition of C3 and C4 plant materials were monitored over 10 months to determine carbon isotopic fractionation at successive stages of biodegradation We selected plant materials of different chemical quality, eg, Zea mays (leaves, stems, coarse roots, and fine roots), Lolium perenne (leaves and roots), Pinus pinaster (needles), and Cocos nucifera (coconut shell) and also characterized these by solid-state 13C NMR Roots were more lignified than aerial parts of the same species Lignin was always depleted in 13C (up to 52‰) as compared with cellulose from the same sample Proteins were enriched in 13C in C3 plants but depleted in maize Cumulative CO2 evolved fitted a double-exponential model with two C pools of different lability During early stages of decomposition, the CO2-C released was usually 13C depleted as compared with the initial substrate but enriched at posterior stages Consequently, with ongoing decomposition, the solid residue became 13C depleted, which could only partly be explained by an accumulation of lignin-C The extension of the initial 13C depleted CO2-C phase was significantly correlated with the labile substrate C content, acid-detergent soluble fraction, and total N, pointing to a direct influence of plant quality on C isotopic dynamics during early stages of biodegradation This isotopic fractionation can also lead to an underestimation of the contribution of plant residues to CO2-C when incubated in soils We discuss possible implications of these mechanisms of 13C fractionation in ecosystems