Greater soil carbon stocks and faster turnover rates with increasing
agricultural productivity
Abstract. Devising agricultural management schemes that enhance food security and soil carbon levels is a high priority for many nations. However, the coupling between agricultural productivity, soil carbon stocks and organic matter turnover rates is still unclear. Archived soil samples from four decades of a long-term crop rotation trial were analysed for soil organic matter (SOM) cycling relevant properties: C and N content, bulk composition by NMR spectroscopy, amino sugar content, short term C bioavailability assays, and long-term C turnover rates by modeling the incorporation of the bomb-spike in atmospheric 14C into the soil. After > 40 years under consistent management, topsoil carbon stocks ranged from 14 to 33 Mg C ha−1 and were linearly related to the mean productivity of each treatment. Measurements of SOM composition demonstrated increasing amounts of plant- and microbially-derived SOM along the productivity gradient. Under two modelling scenarios, radiocarbon data indicated overall SOM turnover time decreased from 40 to 13 years with increasing productivity; twice the rate of decline predicted from simple steady-state models or static three-pool decay rates of measured C pool distributions. Similarly, the half-life of synthetic root exudates decreased from 30.4 to 21.5 hours with increasing productivity indicating accelerated microbial activity. These findings suggest that there is a direct feedback between accelerated biological activity, carbon cycling rates and rates of carbon stabilization with important implications for how SOM dynamics are represented in models.