Soil carbon saturation, productivity, and carbon and nitrogen cycling in crop-pasture rotations

Agricultural systems integrating perennial grass-legume pastures in rotation with grain crops sustain high crop yields while preserving soil organic carbon (C _s ) with low nitrogen (N) fertilizer inputs. We hypothesize that C _s saturation in the topsoil may explain the favorable C and N cycling in these systems. We tested this hypothesis by evaluating and simulating three contrasting crop and pasture rotational systems from a 20-year no-till experiment in Treinta y Tres, Uruguay. The systems were: 1) Continuous annual cropping (CC); 2) crop-pasture rotation with two years of crops and four years of pastures (CP); and 3) perennial pasture (PP). Using the Cycles agroecosystems model, we evaluated the inclusion or exclusion of a C _s saturation algorithm. The model simulated forage, soybean, and sorghum grain yields correctly, with low root mean square error (RMSE) of 1.5, 0.7 and 1.0 Mg ha ⁻¹ , respectively. Measurements show C _s accretion and C _s decline for the first and second half of the experiment, respectively. The C _s accretion rate was highest for PP, while the C _s decline was highest for CC (1.3 vs −0.6 Mg ha ⁻¹ y ⁻¹ of C). The model captured this C _s dynamics and performed better when using the C _s saturation algorithm than when excluding it (RMSE 4.7 vs 6.8 Mg C ha ⁻¹ and relative RMSE of 14% and 21% for the top 15-cm). The model with saturation simulated subsoil C _s distribution with depth well, and simulated faster N turnover and greater N availability for the subsequent grain crop in CP vs CC. The results suggest that C _s saturation, and by extension soil organic N saturation, underpin the sustainability of crop-pasture rotations, and that modeling C _s saturation dynamics can be critical to reliably simulate complex crop-pasture rotational systems.