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

Abstract. Integrated crop–pasture rotational systems can store larger soil organic carbon (SOC) stocks in the topsoil (0–20 cm) 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 temporal changes of 0–20 cm SOC stocks in two agricultural treatments of different intensity (continuous annual grain 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 (particulate organic matter, POM > 53 µm > mineral-associated organic matter, 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, and the age distribution and transit times of C. The differences between the two agricultural systems were mainly determined by the dynamics of the slow-cycling pool (∼MAOM). The outflow rate from this compartment was between 3.68 and 5.19 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 resulted in a mean age of the slow-cycling pool (∼MAOM) of over 600 years, with only 8.8 % of the C in this compartment incorporated during the experiment period (after 1963) and more than 85 % older than 100 years old in this agricultural system. Moreover, half of the C inputs to both agricultural systems leave the soil in approximately 1 year due to high decomposition rates of the fast-cycling pool (∼POM). 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. Maintaining high rates of C inputs and relatively high stocks of labile C appear to be a prerequisite for maintaining low outflow rates of the MAOM pool.