Date of Award
Doctor of Philosophy
Nitrogen is critical to sustaining the profitability and productivity of agricultural systems. However, plant-available forms of N are highly mobile and cropland N losses have negative environmental consequences. Nitrogen management strategies differ depending on the farming system, with diverse crop rotations (i.e., those including perennial legume crops) and integrated crop-livestock systems relying primarily on biologically-fixed and recycled N, and simple maize (Zea mays L.)-based systems relying primarily on synthetic N fertilizer inputs. The goal of this research was to investigate C and N cycling in farming systems that span a range of N management strategies and to use this knowledge to advance sustainable N management.
Within continuous maize and maize-soybean [Glycine max (L.) Merr.] systems at four Iowa locations, I assessed changes in surface soil organic C (SOC) content over time across a range of N fertilizer rates. I found that N fertilization increased SOC content, with the greatest SOC storage in the optimally-fertilized continuous maize treatment. Using the continuous maize plots at two of these long-term N rate experiments, I investigated legacy impacts of N inputs on fertilizer N use efficiency by tracing isotopically-labeled N fertilizer into crop and soil pools. Fertilizer N recovery was less than 50% at both locations and exhibited a curvilinear response to historical N rate. In my third study, I used three long-term field experiments to determine the impact of crop rotation diversity on SOC content, biochemical composition, and distribution among physical fractions at different depths. Despite greater belowground C inputs in the diverse rotations, crop rotation diversity had inconsistent effects on SOC stocks, and minimal impact on the mechanisms of SOC storage. Finally, I compared the profitability of simple cash grain and integrated crop-livestock systems. The analysis revealed no effect of farming system on profitability, but indicated that more labor is required in the integrated crop-livestock system.
The results indicate that simple maize-based systems receiving agronomically optimum synthetic N inputs can sustain SOC and provide adequate financial returns, but result in high N losses from cropland. The adoption of biologically-based N management would reduce cropland N losses with minimal profit loss.
Hanna Jane Poffenbarger
Poffenbarger, Hanna Jane, "Nitrogen fertilization and crop diversity effects on soil carbon and nitrogen cycling in Iowa cropland" (2017). Graduate Theses and Dissertations. 16197.