Indications for soil carbon saturation in a temperate agroecosystem

The soil C saturation concept postulates that there is an upper limit to the equilibrium soil C level of mineral soils even when soil C input is increased. To test this concept, we analyzed the relationship between steady-state soil C input and soil organic C (SOC) concentration in a temperate corn (Zea mays L.) agroecosystem experiment located in Lexington, KY. In this experiment, a gradient in plant productivity and consequent C input to the soil was produced with four N application rates (0, 84, 168, and 336 kg N ha ^-1 yr^-1) under two disturbance regimes, no-till (NT) and moldboard plowing (MP). We examined whether physical protection of organic matter by soil aggregation and chemical protection by association with silt and clay particles led to SOC saturation. We used wet sieving to physically separate SOC pools that differ in C stabilization potential, and determined the C sequestration in each fraction. Total SOC was positively related to C input, and this was primarily due to C stabilization in small macroaggregates. In both tillage systems, however, microaggregate C and silt-plus-clay C did not increase with greater C input. Within the small macroaggregates, coarse particulate organic matter C and microaggregate C increased with C input, but there was no increase in silt-plus-clay C. Our results indicate that soil fractions with low C stabilization potential exhibit C saturation behavior. Apparent C saturation of some of the fractions indicates that SOC pools have a limited capacity to stabilize added C and that such a limit to C stabilization will constrain the ecosystem services provided by these SOC pools.