The Fate of Organic Nitrogen in Agroecosystems: Drivers and Outcomes

Enhancing soil nitrogen (N) stocks is critical for managing soil health in agroecosystems. Growers apply external N as synthetic fertilizer to increase the amount of available N to crops. However, it is well known that synthetic fertilizer usage in agroecosystems leads to widespread pollution and contributes significantly to climate change. Nevertheless, growers and researchers recognize this N problem as a global problem and are developing solutions via alternative farming methods. Some of these alternative strategies include cover cropping, diversifying crop rotations, organic farming (i.e., no inputs), or combining these methods. Understanding how the different facets of agricultural management influence the biogeochemical processes involved with N cycling is critical for developing long-term, sustainable management plans for practitioners. This dissertation examines how sustainable management strategies to influence N cycling and explores the drivers of different inflection points within the N cycle. With the rare, 15N stable isotope, I conducted studies to assess the impact of agricultural management on N cycling. In Chapter 1, I use 15N as a tracer to understand the fate and transport of N into different soil pools. For Chapter 2, I optimized a pool dilution assay using 15N labeled amino acids to demonstrate how agricultural management acts as a top-down regulator of N cycling (Chapter 2). Finally, I conducted a synthetic analysis of literature that uses 15N pool dilution methods to quantify the effect of N amendments on N transformation rates (Chapter 3). In Chapter 1, I found that organically managed soils facilitate higher levels of microbial activity in comparison to conventionally managed soils, which resulted in rapid breakdown of particulate organic matter and subsequent movement of N into the mineral associated organic matter pool, microbial biomass pool, and total dissolved N pool throughout a year-long incubation. Chapter 2 revealed that complex cropping systems had higher rates of amino acid cycling, but high fertilization rates suppressed amino acid cycling, indicating that synthetic fertilizer may suppress the beneficial impacts that complex cropping systems have on accelerating organic N cycling. The meta-analysis in Chapter 3 showed that external N inputs, regardless of type, accelerated N mineralization rates, but residue inputs facilitated higher immobilization rates relative to inorganic and manure N inputs. Broadly, my work shows that soils under alternative management tend to promote microbial activity, which regulates N cycling and storage in the soil. Together, my findings suggest that increasing organic inputs via alternative management strategies can promote internal N cycling, thus reducing dependence on synthetic fertilizer for bioavailable N