Global soil resources under stress The future of humanity is intertwined with the future of Earth's soil resources. Soil provides for agriculture, improves water quality, and buffers greenhouse gases in the atmosphere. Yet human activities, including agricultural soil erosion, are rapidly degrading soil faster than it is naturally replenished. At this rate, human security over the next century will be severely threatened by unsustainable soil management practices. Amundson et al. review recent advances in understanding global soil resources, including how carbon stored in soil responds to anthropogenic warming. Translating this knowledge into practice is the biggest challenge remaining. Science , this issue 10.1126/science.1261071

No abstract

Improved quantification of the factors controlling soil organic matter (SOM) stabilization at continental to global scales is needed to inform projections of the largest actively cycling terrestrial carbon pool on Earth, and its response to environmental change. Biogeochemical models rely almost exclusively on clay content to modify rates of SOM turnover and fluxes of climate-active CO 2 to the atmosphere. Emerging conceptual understanding, however, suggests other soil physicochemical properties may predict SOM stabilization better than clay content. We addressed this discrepancy by synthesizing data from over 5,500 soil profiles spanning continental scale environmental gradients. Here, we demonstrate that other physicochemical parameters are much stronger predictors of SOM content, with clay content having relatively little explanatory power. We show that exchangeable calcium strongly predicted SOM content in water-limited, alkaline soils, whereas with increasing moisture availability and acidity, iron-and aluminum-oxyhydroxides emerged as better predictors, demonstrating that the relative importance of SOM stabilization mechanisms scales with climate and acidity. These results highlight the urgent need to modify biogeochemical models to

Estimating carbon (C) balance in erosional and depositional landscapes is complicated by the effects of soil redistribution on both net primary productivity (NPP) and decomposition. Recent studies are contradictory as to whether soil erosion does or does not constitute a C sink. Here we clarify the conceptual basis for how erosion can constitute a C sink. Specifically, the criterion for an erosional C sink is that dynamic replacement of eroded C, and reduced decomposition rates in depositional sites, must together more than compensate for erosional losses. This criterion is in fact met in many erosional settings, and thus erosion and deposition can make a net positive contribution to C sequestration. We show that, in a cultivated Mississippi watershed and a coastal California watershed, the magnitude of the erosion-induced C sink is likely to be on the order of 1% of NPP and 16% of eroded C. Although soil erosion has serious environmental impacts, the annual erosion-induced C sink offsets up to 10% of the global fossil fuel emissions of carbon dioxide for 2005.

No abstract