Understanding the Fate and Flow of Nitrogen in Commercial Row Crop Production Systems

Abstract

Nitrogen (N) fertilizers have significantly advanced modern agricultural productivity, ensuring global food security. Despite these benefits, approximately 48% of the applied N globally is recovered by crops, with the remainder lost through leaching, volatilization, denitrification, and surface runoff, raising critical environmental concerns such as groundwater contamination and greenhouse gas emissions. This research aimed to systematically quantify N dynamics and losses across distinct yield-based management zones (MZ1 – stable high yield; MZ2 – stable low yield; MZ3 – unstable yield) within a 190-hectare commercial row crop farm in northern Alabama over four cropping seasons (2021–2024). Utilizing a partial N mass balance approach, inputs (fertilizers, manure, biological fixation, residues, irrigation, and atmospheric deposition) and outputs (crop uptake, residual soil N, and runoff losses) were assessed to estimate unaccounted-for N. The corn system received the highest input yet exhibited considerable inefficiencies, particularly in lower-yielding zones (MZ2 and MZ3), while the wheat system demonstrated substantial unaccounted N due to poor N synchrony and biomass production. Soybean displayed a negative N balance, suggesting soil N depletion. Field runoff monitoring emphasized that MZ3 had significantly higher runoff and cumulative N losses, especially during fallow and wheat cultivation periods, highlighting the necessity for improved N management strategies. Nitrogen transformations within soil systems, particularly mineralization, significantly influence plant-available N. This research further explored spatial variability in potential N mineralization within commercial row crop systems in Alabama, focusing on farms with contrasting cover crop management histories. Results indicated substantial variability in mineralization potential within and between farms, clearly demonstrating higher mineralization rates in farms adopting cover crops and residue retention. These findings validate cover cropping and residue management as pivotal strategies for enhancing soil health and optimizing N cycling, underscoring their critical role in sustainable agriculture. Further, recognizing the logistical complexity of quantifying N losses at a large scale, this study employed the DSSAT Cropping System Model (CSM) model to simulate crop-soil-weather interactions and associated N dynamics of Maize across management zones under varying climatic scenarios (wet, normal, drought). Model calibration and evaluation using field data confirmed CSM’s reliability in predicting crop growth, yield, and N uptake. Simulations identified distinct N loss pathways influenced by zone-specific soil properties and water availability. High-yield zones exhibited greater crop uptake yet higher volatilization and leaching losses, whereas low-yield zones retained more nitrate-N, indicating reduced N efficiency and higher denitrification potential. Weather variability substantially impacted N loss dynamics, underscoring the critical interplay between irrigation management and N dynamics. Additionally, the research further utilized long-term DSSAT CSM-CERES-Maize model simulations to evaluate optimal N rates and timing under diverse irrigation regimes (50% and 30% deficit, rainfed conditions) across two management zones characterized by contrasting yield stability. The findings revealed substantial yield differences influenced by water availability, with denitrification notably higher for the low-yielding zones. A strategic reduction in N application rates significantly decreased environmental losses without compromising yields. Suggesting the effectiveness of integrated N and irrigation strategies tailored to specific zone conditions. The optimal N management scenario included moderate irrigation deficits paired with reduced N rates, effectively balancing productivity and sustainability. Collectively, these comprehensive studies underscore the critical importance of precision agriculture, highlighting the need for site-specific N and irrigation management strategies to enhance nitrogen use efficiency, sustain high productivity, and minimize the environmental impacts of agricultural systems.