Building Up Organic Matter in a Subtropical Paleudult under Legume Cover-Crop-Based Rotations

The potential of conservation management systems to ameliorate degraded agricultural soils and mitigate global warming is related to their potential for long-term stabilization of soil organic matter (SOM). This study was performed in a 19-yr-old experiment that was set up on a degraded Paleudult (220 g kg(-1) clay) in southern Brazil to (i) evaluate the effect of seven no-till crop rotations (grass- and legume-based cover crop systems) and mineral N fertilization (0 and 145 kg ha(-1) yr(-1)) on soil organic C (SOC) stocks (0-17.5-cm depth) and (ii) estimate rates of SOM dynamics in these systems under subtropical climate conditions. Annual C input (shoot + root) ranged from 2.61 to 7.84 Mg ha(-1), with the highest values in legume-based and N-fertilized cropping systems. The SOC stocks were closely related to C input levels, and a minimum C input of 4.05 Mg ha(-1) yr(-1) was estimated to maintain the original SOC stock of 31.38 Mg ha(-1). Based on the one-compartment model of SOM dynamics, the SOM decomposition rate was estimated to be 1.2% and the humification coefficient was estimated to be 9.6%. After 19 yr, the stock of the original SOC decreased to about 24.78 Mg ha(-1), while accumulation of SOC derived from the crops ranged from 4.26 to 12.79 Mg ha(-1). Our results highlighted the benefits of legume cover crop species in no-till systems for the stabilization of SOC in degraded agricultural soils.