High capacity of integrated crop-pasture systems to preserve old stable carbon evaluated in a 60-year-old experiment

Abstract. Integrated crop-pasture rotational systems can store larger amounts of soil organic carbon (SOC) than continuous grain cropping. The aim of this study was to identify if the main determinant for this difference may be the avoidance of old C losses in integrated systems, or the higher rate of new C incorporation associated with higher C input rates. We analyzed the evolution of SOC in two agricultural treatments of different intensity (continuous cropping and crop-pasture rotational system) in a 60-year experiment in Colonia, Uruguay. We incorporated this information into a process of building and parameterizing SOC compartmental dynamical models, including data from SOC physical fractionation (POM > 53 µm > MAOM), radiocarbon in bulk soil and CO₂ incubation efflux. This modeling process provided information about C outflow rates from pools of different stability, C stabilization dynamics, as well as the age distribution and transit times of C. The differences between the two agricultural systems were mainly determined by the dynamics of the stable pool (MAOM). The outflow rate from this compartment was between 3.62 and 5.10 times higher in continuous cropping than in the integrated system, varying according to the historical period of the experiment considered. The avoidance of old C losses in the integrated crop-pasture rotational system determined that only 8.8 % of the MAOM C was incorporated during the experiment period (after 1963) and that more than 85 % was older than 100 years old. Moreover, half of the C inputs to both agricultural systems leave the soil in approximately one year due to high decomposition rates of the POM pool. Our results show that the high capacity to preserve old C of integrated crop-pasture systems is the key for SOC preservation of this sustainable intensification strategy, while their high capacity to incorporate new C into the soil may play a second role.