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

Authors: PRAVIA, MARIA - Collaborator; KEMANIA, ARMEN - Pennsylvania State University; TERRA, JOSE - Collaborator; SHI, TUNING - Pennsylvania State University; MACEDO, IGNACIO - Collaborator; Goslee, Sarah

Submitted to: Agriculture, Ecosystems and Environment
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 11/2/2018
Publication Date: 1/12/2019
Citation: Pravia, M.V., Kemania, A.R., Terra, J.A., Shi, T., Macedo, I., Goslee, S.C. 2019. Soil carbon saturation, productivity, and carbon and nitrogen cycling in crop-pasture rotations. Agriculture, Ecosystems and Environment. 171:13-22. https://doi.org/10.1016/j.agsy.2018.11.001.
DOI: https://doi.org/10.1016/j.agsy.2018.11.001

Interpretive Summary: Carbon and nitrogen dynamics are important for both crop production and for environmental effects including soil carbon storage and nutrient losses. Agricultural systems incorporating perennial pasture as part of the crop rotation offer an opportunity for sustainable intensification by increasing soil carbon storage and reducing nitrogen fertilizer inputs needed through biological nitrogen fixation. We used the Cycles model to simulate production, carbon, and nitrogen dynamics in continuous cropping, crop-pasture rotation, and perennial pasture. The model configuration incorporating soil carbon saturation was effective at matching measured biomass and soil carbon on a 20-year cropping systems study in Uruguay. Model results demonstrated that rotations containing pasture had greater nitrogen availability for the grain crops despite lower fertilizer inputs. Continuous pasture retained the greatest amount of soil carbon. Continuous annual cropping stored the least carbon and required the greatest nitrogen inputs. The Cycles model was an effective tool for examining carbon and nitrogen dynamics in these cropping systems.

Technical Abstract: Agricultural systems integrating perennial grass-legume pastures in rotation with grain crops sustain high crop yields while preserving soil organic carbon (Cs) with low nitrogen (N) fertilizer inputs. We hypothesize that Cs saturation in the topsoil may explain the favorable C and N cycling in these systems. We simulated three contrasting crop and pasture rotational systems from a 20-year old 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). We evaluated the inclusion or exclusion of a Cs saturation algorithm using the Cycles agroecosystem model. A new module was integrated in Cycles to simulate mixed grass-legume pastures and inter-seeding. Pasture forage, soybean, and sorghum grain yields were well-simulated with root mean square error (RMSE) of 1.5, 0.7 and 1.0 Mg ha-1, respectively. Both field and simulation data showed a 10-year period of Cs accrual in the top 15-cm of soil for all systems (average 0.85 Mg ha-1 y-1), followed by a 10-year period of C losses for CC and CP (-0.66 Mg ha-1 y-1), while Cs in PP stabilized. Simulations with Cycles captured these management-driven Cs dynamics, although modeled rates of Cs change were less than those observed. The model performed better when using the Cs saturation algorithm than when excluding it (relative RMSE of 14% and 22%, respectively). Subsoil Cs distribution with depth was well simulated with the algorithm including saturation. When Cs saturation was included, the model demonstrated accrual of N during the legume-grass pasture phase, but also faster N turnover and greater availability in the subsequent grain crops. The results of this research suggest that Cs saturation underpins the sustainability of crop-pasture rotations, and that modeling Cs saturation dynamics can be critical to reliable simulation of complex crop-pasture rotational systems.