The Accumulation of Organic Carbon in Mineral Soils by Afforestation of Abandoned Farmland

The afforestation of abandoned farmland significantly influences soil organic carbon (OC). However, the dynamics between OC inputs after afforestation and the original OC are not well understood. To learn more about soil OC dynamics after afforestation of farmland, we measured the soil OC content in paired forest and farmland plots in Shaanxi Province, China. The forest plots had been established on farmland 18, 24, 48, 100, and 200 yr previously. The natural 13C abundance of soil organic matter was also analyzed to distinguish between crop- and forest-derived C in the afforested soils. We observed a nonlinear accumulation of total OC in the 0–80 cm depth of the mineral soil across time. Total soil OC accumulated more rapidly under forest stands aged 18 to 48 yr than under forest stands aged 100 or 200 yrs. The rate of OC accumulation was also greater in the 0–10 cm depth than in the 10–80 cm depth. Forest-derived OC in afforested soils also accumulated nonlinearly across time, with the greatest increase in the 0–20 cm depth. Forest-derived OC in afforest soils accounted for 52–86% of the total OC in the 0–10 cm depth, 36–61% of the total OC in the 10–20 cm depth, and 11–50% of the total OC in the 20–80 cm depth. Crop-derived OC concentrations in the 0–20 cm depth decreased slightly after afforestation, but there was no change in crop-derived OC concentrations in the 20–80 cm depth. The results of our study support the claim that afforestation of farmland can sequester atmospheric CO2 by increasing soil OC stocks. Changes in the OC stocks of mineral soils after afforestation appear to be influenced mainly by the input of forest-derived C rather than by the loss of original OC

Isotopic study of mercury sources and transfer between a freshwater lake and adjacent forest food web

Studies of monomethylmercury (MMHg) sources and biogeochemical pathways have been extensive in aquatic ecosystems, but limited in forest ecosystems. Increasing evidence suggests that there is significant mercury (Hg) exchange between aquatic and forest ecosystems. We use Hg stable isotope ratios (delta Hg-202 and Delta Hg-199) to investigate the relative importance of MMHg sources and assess Hg transfer pathways between Douglas Lake and adjacent forests located at the University of Michigan Biological Station, USA. We characterize Hg isotopic compositions of basal resources and use linear regression of % MMHg versus delta Hg-202 and Delta Hg-199 to estimate Hg isotope values for inorganic mercury (IHg) and MMHg in the aquatic and adjacent forest food webs. In the aquatic ecosystem, we found that lake sediment represents a mixture of IHg pools deposited via watershed runoff and precipitation. The delta Hg-202 and Delta Hg-199 values estimated for IHg are consistent with other studies that measured forest floor in temperate forests. The Delta Hg-199 value estimated for MMHg in the aquatic food web indicates that MMHg is subjected to similar to 20% photochemical degradation prior to bioaccumulation. In the forest ecosystem, we found a significant negative relationship between total Hg and delta Hg-202 and Delta Hg-199 of soil collected at multiple distances from the lakeshore and lake sediment. This suggests that IHg input from watershed runoff provides an important Hg transfer pathway between the forest and aquatic ecosystems. We measured Delta Hg-199 values for high trophic level insects and compared these insects at multiple distances perpendicular to the lake shoreline. The Delta Hg-199 values correspond to the % canopy cover suggesting that forest MMHg is subjected to varying extents of photochemical degradation and the extent may be controlled by sunlight. Our study demonstrates that the use of Hg isotopes adds important new insight into the relative importance of MMHg sources and complex Hg transfer pathways across ecosystem boundaries. (C) 2015 Elsevier B.V. All rights reserved.1120Nsciescopu

Mercury stable isotopes for understanding sources & biogeochemical pathways in natural ecosystems

Mercury (Hg) is a widely distributed, toxic, and bioaccumulative compound that poses serious health threats to wildlife and humans consuming fish. Thus far, studies of Hg sources and biogeochemical pathways have been extensive in aquatic ecosystems. Increasing evidence suggests that there is significant amount of Hg in forest ecosystems, but the input of multiple sources and complex biogeochemical processing of Hg make it difficult to link sources to receptors. For instance, forest ecosystems may receive Hg directly from atmospheric deposition. Another potentially important Hg source to forest ecosystems is through complex ecological processes such as emergent-insect mediated processes, which provide important energy and nutrient subsidies to forest ecosystems. On the flip side, Hg associated and/or accumulated in terrestrial resources can travel to aquatic ecosystems through watershed runoff or movements of organisms. Here, I demonstrate how the measurements of Hg stable isotope ratios in natural samples can be used to distinguish the relative importance of Hg sources, biogeochemical, and transfer processes between streams, freshwater lakes, and forest ecosystems. Hg stable isotopes are increasingly being used to provide insight into the sources, biogeochemical, and exposure pathways of Hg in diverse ecosystem food webs. Many environmental samples and Hg sources have distinct isotope ratios for Hg. Hg stable isotopes undergo fractionation via environmentally relevant processes, which allow precise source tracing and biogeochemical processing of Hg. Recent studies have also confirmed the utility as ecological and biological tracers for exploring processes such as bioaccumulation and trophic transfer. In particular, with the combination of traditional ecological tracers of stable carbon and nitrogen isotopes, which serve as proxies for feeding behaviors and trophic positions, respectively, Hg stable isotopes can aid understanding of various Hg sources, biogeochemical processes, and ecosystem fate in natural environment.2

Isotopic study of mercury sources and transfer between a freshwater lake and adjacent forest food web

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Soil organic matter dynamics and nitrogen availability in response to site preparation and management during revegetation in tropical Central Queensland, Australia

There is considerable interest in finding a cost-effective method of site preparation that effectively controls weeds during planting and further reduces the need for recurring herbicide applications. In this study, two weed control methods, herbicide and scalping, were examined. Both methods may have implications for soil organic matter (SOM) dynamics and nitrogen (N) which could consequently affect plant survival and vegetation establishment. This study aimed to investigate the dynamics of SOM, carbon (C) and N pools under site manipulation practices and the associated early plant survival and growth in tropical Australia

The Human Footprint in the Carbon Cycle of Temperate and Boreal Forests

Temperate and boreal forests in the Northern Hemisphere cover an area of about 23107 square kilometres and act as a substantial carbon sink (0.6¿0.7 petagrams of carbon per year)1. Although forest expansion following agricultural abandonment is certainly responsible for an important fraction of this carbon sink activity, the additional effects on the carbon balance of established forests of increased atmospheric carbon dioxide, increasing temperatures, changes in management practices and nitrogen deposition are difficult to disentangle, despite an extensive network of measurement stations2,3. The relevance of this measurement effort has also been questioned4, because spot measurements fail to take into account the role of disturbances, either natural (fire, pests, windstorms) or anthropogenic (forest harvesting). Here we show that the temporal dynamics following stand-replacing disturbances do indeed account for a very large fraction of the overall variability in forest carbon sequestration. After the confounding effects of disturbance have been factored out, however, forest net carbon sequestration is found to be overwhelmingly driven by nitrogen deposition, largely the result of anthropogenic activities5. The effect is always positive over the range of nitrogen deposition covered by currently available data sets, casting doubts on the risk of widespread ecosystem nitrogen saturation6 under natural conditions. The results demonstrate that mankind is ultimately controlling the carbon balance of temperate and boreal forests, either directly (through forest management) or indirectly (through nitrogen deposition).JRC.H.2-Climate chang