Irrigation management and soils as controls on deep percolation and nitrate leaching in agricultural systems

Abstract

Water and nitrogen are critical resources for agricultural production, and managing for efficient water and nitrogen use has benefits for sustainable water resource management and water quality in addition to agricultural productivity. Deep percolation and nitrate leaching limit water and nitrogen use efficiency, and in irrigated systems, sustained periods with high soil water content result in higher risk of deep percolation and leaching losses. In this thesis, I address the question of how soil properties, weather, and irrigation management interact to control risk of deep percolation and leaching. Research was conducted in four fields of cooperating irrigators in western and central Montana. Soil water sensors and suction lysimeters were installed in fields to monitor water content and soil water nitrate concentrations. Modeling work with Hydrus 1D predicted deep percolation for study fields. Findings suggest that structured clay loam soils and sandy loam soils with high sand content had the highest infiltration rates across soil types studied. For clay loam soils, risk of deep percolation was variable throughout a growing season, controlled by the development of soil desiccation cracks. Peak periods of nitrate leaching varied based on how temporal patterns in nitrate concentrations interacted with deep percolation timing controlled by desiccation crack development. Leaching losses of 0.5-3 kg ha -1 were estimated for the clay loam study field in 2023, when nitrate concentrations were low during periods of peak deep percolation. Leaching losses were 5-17 kg ha -1 during 2024, when a slow-release fertilizer application resulted in prolonged periods with high nitrate concentrations. In silt loam soils cultivated for potatoes, infiltration was limited, and leaching risk was low. Runoff may present a larger challenge for water and nitrogen use efficiency than leaching, and fertilizer application timing appears to influence risk of non-growing season nitrate leaching. These results suggest that soil properties including texture and structure as well as irrigation and fertilizer management control water and nitrogen use efficiency. Temporal variability in soil structure is important to consider both for modeling efforts and for management to reduce the risk of deep percolation and leaching.