A comprehensive investigation of nitrogen fate in winter wheat-based agroecosystems: cycling, transformations, and nitrous oxide emissions

Abstract

Improving nitrogen (N) use efficiency is essential to sustain crop productivity and quality while reducing fertilizer inputs and minimizing environmental impacts. In particular, agriculture is a major source of nitrous oxide (N₂O), a potent greenhouse gas with a global warming potential nearly 300 times greater than carbon dioxide, accounting for over 75% of global emissions. At the same time, soil processes such as drying–rewetting (DW) cycles play a critical role in regulating carbon (C) and nitrogen (N) dynamics, often triggering pulses of microbial activity and nutrient mineralization. Because cropping systems influence soil organic matter inputs, aggregation, and microbial communities, they can modify both nutrient cycling and greenhouse gas emissions. Therefore, this dissertation investigated N dynamics, greenhouse gas emissions, and soil biogeochemical processes in intensified rainfed wheat-based systems in Ashland Bottoms, Kansas, USA. The first study evaluated nitrous oxide (N₂O) emissions across six cropping systems differing in N management and cropping intensity. Emissions were strongly driven by environmental conditions, with peaks occurring following fertilization and rainfall events, particularly when water-filled pore space exceeded 80% and temperatures were above 25 °C. Intensive systems, regardless of whether N was applied as a single or split application, consistently had lower cumulative and yield-scaled N₂O emissions. In contrast, baseline wheat–fallow and semi-intensive systems had greater emissions, especially in wetter years. Emission factors were generally below 1%, although some treatments exceeded this threshold under favorable conditions. The second study focused on plant N uptake and nitrogen use efficiency using ¹⁵N tracers. While no significant differences were observed between single and split N applications in total grain N recovery, split applications tended to enhance late-season N uptake. Intensive systems consistently ranked highest in overall performance, whereas semi-intensive and baseline systems showed lower N recovery and productivity. These results suggest that optimizing N timing can improve synchronization between N supply and crop demand. The third study examined soil N dynamics, including inorganic and organic ¹⁵N pools and mineralization processes. Inorganic N peaked shortly after fertilization and declined over time, while organic N dynamics were strongly influenced by precipitation. Wetter conditions promoted greater immobilization of N in organic pools, particularly in green manure systems. Additionally, green manure increased potentially mineralizable N, while split N applications maintained N availability later in the growing season. The fourth study assessed the effects of drying–rewetting cycles on soil carbon and nitrogen mineralization, aggregation, and microbial activity. Drying–rewetting cycles induced strong pulses of microbial respiration and N mineralization, while intensive systems exhibited reduced soil aggregation and greater susceptibility to disturbance compared with more diverse systems. Overall, these findings demonstrate that intensified wheat-based systems, particularly those incorporating improved N timing, can enhance nitrogen use efficiency and reduce environmental losses. However, precipitation patterns and soil moisture dynamics remain key drivers of N transformations, emissions, and overall system resilience.