Nitrogen fertilization and crop diversity effects on soil carbon and nitrogen cycling in Iowa cropland

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.\nWithin 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.\nThe 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.