SOIL CARBON AND NITROGEN SATURATION IN CROP-PASTURE AGRICULTURAL SYSTEMS
Open Access
- Author:
- Pravia Nin, Maria Virginia
- Graduate Program:
- Agronomy
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- July 28, 2017
- Committee Members:
- Armen Ricardo Kemanian, Dissertation Advisor/Co-Advisor
Armen Ricardo Kemanian, Committee Chair/Co-Chair
Douglas Brian Beegle, Committee Member
Gregory Wayne Roth, Committee Member
Jason Philip Kaye, Outside Member
Marvin H Hall, Committee Member - Keywords:
- crop-livestock systems
carbon saturation
nitrogen saturation
agroecosystem modeling
soil organic matter
crop rotations - Abstract:
- Integrated crop-pasture systems can produce high grain yields without large inputs of external nitrogen (N). The extraction of N in grain and animal products, as well as gaseous, leaching and runoff losses should be offset by N fixation during the pasture phase of the integrated crop-pasture systems. The N would be timely delivered in moments of fast crop growth. The design of systems based on these intrinsic synergies offer a pathway for sustainable intensification of agriculture. In this dissertation, I propose that the carbon (C) and N cycling in crop-pasture rotations can be explained by soil C (Cs) saturation theory, and some degree of de-coupling of the C and N cycling. To this end, I developed hypotheses that integrate the concepts of C and N saturation. I tested these hypotheses by combing agroecosystem simulation models and soil incubations using stable isotopes tracing techniques. The systems modeled and the soils for incubations were obtained from long-term crop-pasture rotation experiments at the Instituto Nacional de Investigación Agropecuaria (INIA), Uruguay. Rotation systems that included perennial species showed greater N mineralization and immobilization when simulated with the Cycles agroecosystem model. Soil C stocks evolution and subsoil Cs distribution were more accurately simulated when the model included an algorithm for Cs saturation than when excluding it, indicating that this mechanism can explain Cs cycling. Soil sampling showed greater Cs saturation levels on the topsoil of systems including perennial species. The incubation of soils with residues enriched in 13C and 15N showed that Cs decomposition increased as Cs saturation increased, and Cs humification and N retention decreased. This accelerated turnover with increased Cs saturation underpins N supply in crop-pasture systems. Despite the link observed between Cs and N saturation mechanisms, some degree of decoupling was revealed by a decreasing C:N ratio of the retained or newly formed organic matter as Cs saturation increased. This lowering of the C:N ratio indicates higher N than C retention, buffering N losses in relatively Cs saturated soils. Overall, this dissertation supports soil C saturation theory and the implied reduction in the retention of C and acceleration of turnover as saturation increases, and suggests that C saturation is a fundamental pillar of the integrated crop-pasture systems function and sustainability.