Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO 2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C·ha −1 ·yr −1 ·°C −1 . Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr −1 among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C·ha −1 ·°C −1 . Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with a slope of −8 Mg C·ha −1 ·°C −1 , indicating that decomposition rates of soil organic matter increased with MAT faster than did rates of NPP. Turnover rates of surface litter also increased with temperature among forests. We found no detectable effect of temperature on total carbon storage among moist tropical evergreen forests, but rather a shift in ecosystem structure, from low-biomass forests with relatively large accumulations of detritus in cooler sites, to largebiomass forests with relatively smaller detrital stocks in warmer locations. These results imply that, in a warmer climate, conservation of forest biomass will be critical to the maintenance of carbon stocks in moist tropical forests. Keywordsbelowground carbon allocation, carbon cycle, carbon turnover, decomposition, forest biomass, forest productivity, global warming, mean annual temperature, meta-analysis, net primary productivity, soil organic matter, tropical evergreen forests Abstract. Evergreen broad-leaved tropical forests can have high rates of productivity and large accumulations of carbon in plant biomass and soils. They can therefore play an important role in the global carbon cycle, influencing atmospheric CO 2 concentrations if climate warms. We applied meta-analyses to published data to evaluate the apparent effects of temperature on carbon fluxes and storages in mature, moist tropical evergreen forest ecosystems. Among forests, litter production, tree growth, and belowground carbon allocation all increased significantly with site mean annual temperature (MAT); total net primary productivity (NPP) increased by an estimated 0.2-0.7 Mg C·ha Ϫ1 ·yr Ϫ1 ·ЊC Ϫ1 . Temperature had no discernible effect on the turnover rate of aboveground forest biomass, which averaged 0.014 yr Ϫ1 among sites. Consistent with these findings, forest biomass increased with site MAT at a rate of 5-13 Mg C·ha Ϫ1 ·ЊC Ϫ1 . Despite greater productivity in warmer forests, soil organic matter accumulations decreased with site MAT, with ...

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We measured aboveground plant biomass, aboveground net primary productivity (ANPP), detritus accumulation, and nitrogen and phosphorus uptake by aboveground vegetation in six Metrosideros polymorpha stands on the windward slopes of Mauna Loa, Hawai‘i, USA. Our objective was to quantify the effects of elevation (primarily temperature) on ecosystem properties during primary succession, as a key to understanding ecosystem–climate interactions. Four study sites were on 111‐ to 136‐yr‐old lava flows at elevations of 290, 700, 1130, and 1660 m. Two additional sites on 3400‐yr‐old lava were at 700 and 1660 m elevations. All sites were on solid pahoehoe (smooth or ropy‐textured) lava substrates with gentle relief, were free of significant human disturbance, received abundant precipitation, and had similar vegetation composition. Total aboveground biomass, soil organic matter mass, and aboveground net primary production (ANPP) were all greater in the old sites than in young sites. Differences between young and old sites in aboveground live biomass, detrital mass, and ANPP all supported the conclusion that ecosystem development proceeded relatively faster at 700 m elevation than at 1660 m. However, aboveground biomass in the old sites (81 Mg/ha at 1660 m elevation and 123 Mg/ha at 700 m) was low in comparison with other wet tropical forests. Accumulations of N and P in live biomass and detritus followed the same trends as were observed for organic matter. Rates of soil carbon accumulation over the first 3400 yr of succession averaged 2.1 g·m−2·yr−1, similar to other reported soil chronosequences. Observed rates of N accumulation ranged from 0.1 to 0.6 g·m−2·yr−1 over the first 136 yr of succession. There were no monotonic elevational trends among young sites with respect to live biomass, detritus mass, or total N or P accumulation. Foliar nitrogen concentrations in the young sites were among the lowest reported from any tropical forests and tended to decline with increasing elevation. The growth and biomass of individual plant species varied in distinctive ways along the elevational gradient. Nevertheless, among young sites there was a direct, linear relationship between total ANPP and mean annual temperature of the site, with a similar pattern in the two old sites. For each 1°C increase in mean annual temperature, total ANPP increased by 54 g·m−2·yr−1. Community‐level ANPP also was directly correlated with rates of N and P uptake by the vegetation, regardless of site age or elevation.

Growing interest in the potential for agricultural soils to provide a sink for atmospheric C has prompted studies of effects of management on soil organic carbon (SOC) sequestration. We analyzed the impact on SOC of four N fertilization rates (0-270 kg N ha −1 ) and four cropping systems: continuous corn (CC) (Zea mays L.); corn-soybean [Glycine max (L.) Merr.] (CS); corn-corn-oat-alfalfa (oat, Avena sativa L.; alfalfa, Medicago sativa L.) (CCOA), and corn-oat-alfalfa-alfalfa (COAA). Soils were sampled in 2002, Years 23 and 48 of the experiments located in northeast and north-central Iowa, respectively. The experiments were conducted using a replicated split-plot design under conventional tillage. A native prairie was sampled to provide a reference (for one site only). Cropping systems that contained alfalfa had the highest SOC stocks, whereas the CS system generally had the lowest SOC stocks. Concentrations of SOC increased significantly between 1990 and 2002 in only two of the nine systems for which historical data were available, the fertilized CC and COAA systems at one site. Soil quality indices such as particulate organic carbon (POC) were influenced by cropping system, with CS < CC < CCOA. In the native prairie, SOC, POC, and resistant C concentrations were 2.8, 2.6, and 3.9 times, respectively, the highest values in cropped soil, indicating that cultivated soils had not recovered to precultivation conditions. Although corn yields increased with N additions, N fertilization increased SOC stocks only in the CC system at one site. Considering the C cost for N fertilizer production, N fertilization generally had a net negative effect on C sequestration. RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted.

A single ecosystem dominates the Midwestern United States, occupying 26 million hectares in five states alone: the corn-soybean agroecosystem [Zea mays L.-Glycine max (L.) Merr.]. Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97-100% of farms, at an average rate of 135 kg N·ha −1 ·yr −1 . We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental sites in Mollisols in Iowa, USA. We compared the corn-soybean system with other experimental cropping systems fertilized with N in the corn phases only: continuous corn for grain; corn-corn-oats (Avena sativa L.)-alfalfa (Medicago sativa L.; corn-oats-alfalfa-alfalfa; and continuous soybean. In all systems, we estimated long-term OC inputs and decay rates over all phases of the rotations, based on long-term yield data, harvest indices (HI), and root : shoot data. For corn, we measured these two ratios in the four N treatments in a single year in each site; for other crops we used published ratios. Total OC inputs were calculated as aboveground plus belowground net primary production (NPP) minus harvested yield. For corn, measured total OC inputs increased with N fertilization (P < 0.05, both sites). Belowground NPP, comprising only 6-22% of total corn NPP, was not significantly influenced by N fertilization. When all phases of the crop rotations were evaluated over the long term, OC decay rates increased concomitantly with OC input rates in several systems. Increases in decay rates with N fertilization apparently offset gains in carbon inputs to the soil in such a way that soil C sequestration was virtually nil in 78% of the systems studied, despite up to 48 years of N additions. The quantity of belowground OC inputs was the best predictor of long-term soil C storage. This indicates that, in these systems, in comparison with increased N-fertilizer additions, selection of crops with high belowground NPP is a more effective management practice for increasing soil C sequestration. Keywords agroecosystems, carbon mineralization, corn, oats, alfalfa, and soybean crop rotations, Midwestern U.S. corn-soybean ecosystem, Nashua and Kanawha sites, Iowa, USA, net primary production, nitrogen fertilization, root production, soil carbon sequestration RightsWorks produced by employees of the U.S. Government as part of their official duties are not copyrighted within the U.S. The content of this document is not copyrighted. . Nitrogen (N) fertilization could influence the soil carbon (C) balance in this system because the corn phase is fertilized in 97-100% of farms, at an average rate of 135 kg NÁha À1 Áyr À1 . We evaluated the impacts on two major processes that determine the soil C balance, the rates of organic-carbon (OC) inputs and decay, at four levels of N fertilization, 0, 90, 180, and 270 kg/ha, in two long-term experimental...