Soil carbon transition curves: reversal of land degradation through management of soil organic matter for multiple benefits.

Abstract

Abstract Soils provide important ecosystem services at the local, landscape and global level. They provide the basis for crop, livestock and forestry production and help mitigate climate change by storing carbon. With expectations of a growing bioenergy supply to meet global energy demand added to the imperative to feed a global population of 9 billion people by mid-century and beyond, coupled with higher per person food demands than currently provided, the challenges to keep agricultural and rangeland soils healthy and productive are daunting. In this paper, we explore the existence of a common pattern in the use of soils under increasing demand for productivity - here described as a soil carbon transition curve: a rapid decline of soil carbon due to human clearing of natural vegetation for agricultural land use and management practices, followed by a 'crisis' phase of diminished soil fertility and finally by recovery once agricultural practices improve. We test this pattern in its ability to convey the impact of major land-use changes on soils, with examples from arable, grazing and forest land in different parts of the world. The initial stage of the curve represents a trade-off between extractive productivity (growing crops, extracting biomass, excessive burning and grazing) on the one hand and the farm-level, landscape plus global benefits of soil carbon storage on the other hand, based on the loss of an initial endowment of soil organic matter. The second, turnaround stage of the curve often following a crisis tends to be driven by locally relevant loss of ecosystem services, manifest as flooding, pests and crop diseases, erosion and nutrient deficiencies. In the final, recovery stage, local, landscape and global benefits coincide, and synergy between local and global stakeholders' interests dominates the trajectory. In mineral soils used for crop production, recovery generally requires a change in tillage and residue management, crop-livestock integration and/or a return of (agroforestry) trees to agricultural landscapes, plus maintaining adequate nutrient levels. In grasslands, a control of grazing pressure is generally needed in order to recover the vegetation. While global soil carbon storage is linked positively to other ecosystem functions in the recovery stage, the magnitude of the services involved differs substantially between soil types. Incentives for carbon emission reduction and prevention may be drawn towards a small fraction of soils with very high emission potential, especially peatlands, while incentives to maintain or enhance soil carbon storage will be most effective for soil in Stages I and II of the transition curve. Increased carbon storage to mitigate climate change, however, should not be considered the main purpose for improved soil organic carbon management but could be seen as a co-benefit of actions that seek local and watershedlevel benefits from a full set of improved ecosystem services provided by soil organic carbon.