| Optimizing nitrogen (N) management is a critical component of meeting yield potential and increasing global food supply while reducing environmental impacts from agriculture. The objective of this dissertation was to evaluate N management practices in conventional and alternative direct-seeded rice production systems in California in order to enhance rice productivity while minimizing environmental costs, particularly concerning greenhouse gas (GHG) emissions. In Chapter 2, agronomic productivity and N requirements of resource-conserving rice establishment systems utilizing no-till stale seedbed practices for improved weed control were assessed. Without N fertilizer addition, yields were lower for alternative compared to conventional establishment systems, likely as a result of greater soil N losses. Accordingly, economic optimum nitrogen rates based on yield response to N trials indicated an additional 30 to 35 kg N ha-1 was needed to maximize returns to N in water-seeded stale seedbed systems. These results suggest that alternative establishment systems are viable from an agronomic and economic standpoint in California provided N rates are close to optimal and establishment methods are selected to target weed species of concern. In Chapters 3 and 4, the relationship between N management, crop productivity, and GHG emissions was investigated. Integrating climate change and agronomic productivity concerns, global warming potential was assessed on a yield-scaled as well as per unit area basis. It was hypothesized that in response to fertilizer N addition, yield-scaled global warming potential would be minimized at N rates that maximize yields. In a two-year on-farm experiment in water-seeded rice (Chapter 3), N2O emissions remained low regardless of N rate when a permanent flood was maintained but large N 2O fluxes occurred during discrete field drainage periods prior to harvest, particularly at high N rates. In contrast, differences were not observed between N rates for CH4 emissions. Across years CH4 represented 94% of total global warming potential, thus mean annual yield-scaled global warming potential significantly decreased at optimal N rates due to increasing yields. In Chapter 4, these field results were combined with available data from the literature to evaluate yield-scaled global warming potential as a function of yield N surplus, here defined as N application rate minus the N rate at which maximum yield was achieved within each study. At surplus N rates, N2O and yield-scaled N2O emissions increased exponentially. However, CH4 emissions were not impacted by N inputs, hence yield-scaled CH4 emissions decreased with N addition. Overall, yield-scaled global warming potential was minimized at optimal N rates, decreasing by 21% compared to treatments without N addition. Balancing gains in agricultural productivity with environmental concerns, this work supports the concept that high rice yields can be achieved with minimal yield-scaled global warming potential when N inputs are closely matched with crop demand. |
