Global Biogeochemical Cycles
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Soil carbon turnover in a recovering temperate forest Soil radiocarbon measurements show that mineral soil carbon under a recovering temperate forest in South Carolina turns over twice as fast as carbon in undisturbed soil. The observed 12‐year turnover time influences the design and interpretation of CO2 fertilization experiments. Experiments conducted on formerly disturbed sites will show a soil carbon fertilization response considerably faster than experiments conducted on native sites. Calculating the soil carbon CO2 fertilization factor from observed increases in soil carbon requires values for the turnover time and inventory of active soil carbon. We also use the observed turnover time to estimate the rate of atmospheric carbon dioxide sequestration in soil following agricultural abandonment. Although using the observed turnover rate increases estimates of soil carbon uptake on abandoned land, the amount of carbon sequestered globally is minimal because the net area of land being abandoned is small.
Global Biogeochemical Cycles - Tập 9 Số 4 - Trang 449-454 - 1995
Basin‐wide particulate carbon flux in the Atlantic Ocean: Regional export patterns and potential for atmospheric CO<sub>2</sub> sequestration Particle flux data from 27 sites in the Atlantic Ocean have been compiled in order to determine regional variations in the strength and efficiency of the biological pump and to quantify carbon fluxes over the ocean basin, thus estimating the potential oceanic sequestration of atmospheric CO2 . An algorithm is derived relating annual particulate organic carbon (POC) flux to primary production and depth that yields variations in the export ratio (ER = POC flux/primary production) at 125 m of between 0.08 and 0.38 over the range of production from 50 to 400 g C m−2 yr−1 . Significant regional differences in changes of the export ratio with depth are related to the temporal stability of flux. Sites with more pulsed export have higher export ratios at 125 m but show more rapid decreases of POC flux with depth, resulting in little geographic variation in fluxes below ∼3000 m. The opposing effects of organic carbon production and calcification on Δp CO2 of surface seawater are considered to calculate an “effective carbon flux” at the depth of the euphotic zone and at the base of the winter mixed layer. POC flux at the base of the euphotic zone integrated over the Atlantic Ocean between 65°N and 65°S amounts to 3.14 Gt C yr−1 . Of this, 5.7% is remineralized above the winter mixed layer and thus does not contribute to CO2 sequestration on climatically relevant timescales. The effective carbon flux, termed J eff , amounts to 2.47 Gt C yr−1 and is a measure of the potential sequestration of atmospheric CO2 for the area considered. A shift in the composition of sedimenting particles (seen in a decrease of the opal:carbonate ratio) is seen across the entire North Atlantic, indicating a basin‐wide phenomenon that may be related to large‐scale changes in climatic forcing.
Global Biogeochemical Cycles - Tập 15 Số 4 - Trang 845-862 - 2001
Rapid biologically mediated oxygen isotope exchange between water and phosphate In order to better constrain the rate of oxygen isotope exchange between water and phosphate via biochemical reactions a set of controlled experiments were conducted in 1988 at the Aquaculture Plant in Elat, Israel. Different species of algae and other organisms were grown in seawater tanks under controlled conditions, and the water temperature and oxygen isotopic composition (δ18 Ow ) were monitored. The oxygen isotopic composition of phosphate (δ18 Op ) in the organisms' food source, tissues, and the δ18 Op of dissolved inorganic phosphate (DIP) were measured at different stages of the experiments. Results indicate that intracellular oxygen isotope exchange between phosphorus compounds and water is very rapid and occurs at all levels of the food chain. Through these reactions the soft tissue δ18 Op values become 23–26‰ higher than δ18 Ow , and δ18 Op values of DIP become ∼20‰ higher than δ18 Ow . No correlation between δ18 Op values and either temperature or P concentrations in these experiments was observed. Our data imply that biogenic recycling and intracellular phosphorus turnover, which involves kinetic fractionation effects, are the major parameters controlling the δ18 Op values of P compounds dissolved in aquatic systems. This information is fundamental to any application of δ18 Op of dissolved organic or inorganic phosphate to quantify the dynamics of phosphorus cycling in aquatic systems.
Global Biogeochemical Cycles - Tập 16 Số 1 - 2002
Potential availability of sedimentary phosphorus to sediment resuspension in Florida Bay Several studies have suggested that phosphorus is a limiting nutrient for seagrass and phytoplankton growth in much of Florida Bay. In fact, soluble reactive phosphate concentrations in Florida Bay waters can be as low as a few nM. Sediments represent the largest phosphorus reservoir because Florida Bay sediments are dominated by carbonate, which has a strong capacity to retain phosphorus. The supply of phosphorus to the water column from sediment resuspension is potentially important in providing the nutrients required for phytoplankton production. Applying an improved sequential extraction technique to sediments collected from 40 geographically representative stations in Florida Bay, this study provides the first detailed spatial distribution of total sedimentary phosphorus (TSP) and its partitioning into five chemically distinguishable pools in the surface, fine‐grained sediments of the bay. A strong gradient of decreasing TSP concentration was observed from the west (14.6 μmol g−1 ) to east (1.2 μmol g−1 ) across the central bay. The spatial pattern of TSP is consistent with distribution of both seagrass and phytoplankton that are limited by available phosphorus in Florida Bay. Among the five pools, the authigenic carbonate fluorapatite, biogenic apatite and CaCO3 ‐bound phosphorus account for the largest fraction (45%) of TSP, of which inorganic phosphorus is the dominant form, and organic phosphorus accounts for about 30% in the western and north central regions and less than 10% in other areas of Florida Bay. The second largest pools are the refractory organic phosphorus (24% of TSP) and reductant‐soluble inorganic phosphorus (19% of TSP). Readily exchangeable phosphorus accounts for 8% of TSP, of which organic phosphorus is 60%. Detrital apatite phosphorus of igneous or metamorphic origin represents the smallest fraction, only 5% of TSP. Spatial distribution of phosphorus and iron in sediments indicates that external sources of these two essential plant nutrients to Florida Bay are spatially separated with phosphorus introduced by west coast waters across the western margin of Florida Bay and iron from freshwater flow into the eastern region.
Global Biogeochemical Cycles - Tập 18 Số 4 - 2004
Soil Carbon Dynamics Following Land Use Changes and Conversion to Oil Palm Plantations in Tropical Lowlands Inferred From Radiocarbon Abstract Tropical forests account for a large portion of the Earth's terrestrial carbon pool. However, rapid deforestation threatens the stability of this carbon. We examine radiocarbon (Δ14 C) and stable carbon (δ13 C) isotopes of soil organic matter to provide insight into rates of carbon turnover, inputs, and losses of pasture‐derived (C4) versus forest or oil palm‐derived (C3) carbon. Data are presented for natural lowland forests on mineral soil converted to pastures in Peru and to oil palm plantations in Peru, Indonesia, and Cameroon. We additionally examine plots of secondary forests following agricultural use. There were large losses in carbon stocks under both pasture and oil palms. In the plots converted to pasture, our data indicate a preferential loss of relatively young carbon, and a greater loss of forest‐derived carbon than replacement with pasture‐derived carbon. Natural forests converted directly to oil palm plantations sustained losses in carbon, but Δ14 C values suggest that the soil may retain a sufficient amount of newly acquired carbon to offset initial losses of young carbon. Furthermore, replacement of pastures with oil palm plantations facilitates the accumulation of young carbon, which may lead to a gradual increase in carbon stocks. The sites examined here are representative of the biophysical characteristics in roughly half of the humid tropics, suggesting that these findings may be applicable to a large area of similarly managed mineral soils in lowland tropical forests.
Global Biogeochemical Cycles - Tập 34 Số 9 - 2020
Modeling methane emissions from rice paddies The quantification of methane (CH4 ) emissions from global rice paddies is still highly uncertain. The extrapolation of CH4 release rates from point measurements to regional level or empirically correlating methane emission rates with a few factors (e.g., temperature and rice primary production) is unlikely to yield reliable results. Reducing the uncertainties in estimates of current CH4 emission and predicting its future change require a process model to simulate CH4 emissions from various paddyland environments. CH4 emission is an ecosystem process closely coupled to plant growth and soil organic matter decomposition. In this study a methane emission model was developed based on supplies of carbon substrate for methanogens by rice primary production and soil organic matter degradation, direct environmental controls on methanogenesis, and the balance between CH4 production and consumption by methanotrophic oxidation. A validation of the model indicated its reasonable ability to calculate CH4 emission rates and their seasonal variations. The model, when coupled with supporting data sets, would complement spatial and dynamic analysis of CH4 emissions from rice paddies in the framework of climate‐plant‐soil interactions.
Global Biogeochemical Cycles - Tập 9 Số 2 - Trang 183-195 - 1995
Factors affecting methane production under rice To understand why atmospheric methane is increasing worldwide, accurate estimates are needed of the global input from rice fields. We report greenhouse and laboratory studies over three growing seasons to isolate and control factors that might affect methane emission from rice paddies, including soil texture, added exogenous organic matter (OM), nitrogen and sulfate ion, and water management. Without added OM, methane production was relatively low, increasing during the growing season, and continuing after harvest, provided the soil remained water‐logged. If ground rice straw was added to the soil prior to planting, methane production began shortly after flooding, with an initial burst of the gas after 3 to 5 weeks, and then a gradual increase to a second peak later in the season (and after harvest), with rates considerably higher than in treatments without added OM.
Global Biogeochemical Cycles - Tập 7 Số 1 - Trang 143-155 - 1993
An assessment of biofuel use and burning of agricultural waste in the developing world 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.
Global Biogeochemical Cycles - Tập 17 Số 4 - 2003
Net ecosystem production and organic carbon balance of U.S. East Coast estuaries: A synthesis approach Abstract Net ecosystem production (NEP) and the overall organic carbon budget for the estuaries along the East Coast of the United States are estimated. We focus on the open estuarine waters, excluding the fringing wetlands. We developed empirical models relating NEP to loading ratios of dissolved inorganic nitrogen to total organic carbon, and carbon burial in the sediment to estuarine water residence time and total nitrogen input across the landward boundary. Output from a data‐constrained water quality model was used to estimate inputs of total nitrogen and organic carbon to the estuaries across the landward boundary, including fluvial and tidal‐wetland sources. Organic carbon export from the estuaries to the continental shelf was computed by difference, assuming steady state. Uncertainties in the budget were estimated by allowing uncertainties in the supporting model relations. Collectively, U.S. East Coast estuaries are net heterotrophic, with the area‐integrated NEP of −1.5 (−2.8, −1.0) Tg C yr−1 (best estimate and 95% confidence interval) and area‐normalized NEP of −3.2 (−6.1, −2.3) mol C m−2 yr−1 . East Coast estuaries serve as a source of organic carbon to the shelf, exporting 3.4 (2.0, 4.3) Tg C yr−1 or 7.6 (4.4, 9.5) mol C m−2 yr−1 . Organic carbon inputs from fluvial and tidal‐wetland sources for the region are estimated at 5.4 (4.6, 6.5) Tg C yr−1 or 12 (10, 14) mol C m−2 yr−1 and carbon burial in the open estuarine waters at 0.50 (0.33, 0.78) Tg C yr−1 or 1.1 (0.73, 1.7) mol C m−2 yr−1 . Our results highlight the importance of estuarine systems in the overall coastal budget of organic carbon, suggesting that in the aggregate, U.S. East Coast estuaries assimilate (via respiration and burial) ~40% of organic carbon inputs from fluvial and tidal‐wetland sources and allow ~60% to be exported to the shelf.
Global Biogeochemical Cycles - Tập 29 Số 1 - Trang 96-111 - 2015
Methane emissions from beaver ponds: Rates, patterns, and transport mechanisms Net methane fluxes as well as methane fluxes via gas bubbles and via molecular diffusion were measured separately at nine different sampling sites throughout a beaver pond located in the low boreal forest region of central Ontario, Canada. In 1990, the average daily methane emission rate was 37.2 ± 30.4 mg CH4 m−2 d−1 , yielding a total annual methane efflux of 5.8 g CH4 m−2 of which 1.1 g CH4 m−2 were produced in the wintertime when the pond was ice covered. On average, 65% of the net methane flux was released via gas bubble emission, the remaining 35% via molecular diffusion. Gas bubble flux rates were highly variable in space and time, suggesting that low sampling frequencies, common in many methane emission studies, could introduce large uncertainties to the estimated annual flux rates for different wetland systems. The effects of changes in environmental variables on methane emissions were found to be strongly dependent on which transport mechanism dominated the release of methane from the sediments. While the gas bubble flux volume was mainly affected by changes in atmospheric pressure and sediment temperatures, the diffusive flux component was found to be more sensitive to changes in the pond water level. Overall, magnitudes of methane fluxes from this pond were similar to other beaver ponds and moderate to high when compared to other wetland types in the low boreal forest region.
Global Biogeochemical Cycles - Tập 13 Số 4 - Trang 1079-1090 - 1999
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