Soil degradation, thresholds and dynamics of long-term cultivation: From landscape biogeochemistry to nanoscale biogeocomplexity

In degrading tropical soils, carbon and nutrient exports represent a significant modification to global biogeochemical cycles. We assessed soil C and nutrient losses on 122 cultivated agricultural fields from three 100-yr chronosequence sites in western Kenya. During cultivation, C stocks and soil nutrients (N, P, K Ca and Mg) were rapidly lost from the surface soil after 15-36 years of continuous cropping. A lag phase was expressed in the half life kinetics between C decline and Ca and Mg rates. For all sites, crop C4--C gains offset between 15 to 34% of the C losses but more than two thirds of the native forest C3--C was lost during 100 years of cropping. Heavy-textured Nandi soils cascaded from high to medium and low C stocks and nutrient equilibria, while medium-textured Kakamega soil, which already had lower nutrient contents instead transitioned from medium to low equilibrium of C stocks. By separating three SOM pools assigned to distinct soil functions as indicators of thresholds, we determined that nearly all C (13.6-24.3 g kg-1) and N (1.5-3.1 g kg -1) contents in the unstable and stable aggregate pools was lost 15 to 36 years after forest conversion. Long-term changes in the unstable and stable aggregate pool were characterized by rapid initial losses that reached equilibrium, a wide C:N ratio (19.3 and 18.3 respectively) and little delta 15N isotopic shift (<1.0‰). In contrast, the stable organomineral pools constituted large C and N contents which sustained only gradual non-equilibrium decay behavior. This large pool had a narrow C:N ratio with a strong enrichment of delta15N (1.7 to 3.5‰). Continuing C and N content decline at equilibrium were linear and the severe loss of stable aggregate C and N was an indicator of low stabilization of organic matter. The long-term cultivation loss of C and soil nutrients was therefore driven by land use changes from C and nutrient-rich tropical rain forest to low equilibria of C and nutrient-poor degraded soil.