Soil water and nitrogen dynamics in dryland cropping systems of Washington State, USA

Understanding the fate of soil water and nitrogen (N) is essential for improving crop yield and optimizing the management of water and N in dryland cropping systems. The objective of this study was to evaluate how conventional (CT) and no-till (NT) cropping systems affect soil water and N dynamics. Soil water and N were monitored in 30 cm increments to a depth of 1.5 m for 2 years at growers’ fields in two different agroclimatic zones of Washington State (USA): (1) the annual cropping region with a mean annual precipitation of more than 500 mm (Palouse site) and (2) the grain-fallow cropping region with mean precipitation below 350 mm (Touchet site). In each zone, a CT and a NT cropping system were chosen. All sites had an annual cropping system, except for the CT site in the drier area, which was under a traditional winter wheat/fallow rotation previous to the study. At Palouse, the volumetric water content in the top 1.5 m of the soil throughout the year was about 0.05–0.1 m3 m−3 less under CT as compared to NT, indicating improved seasonal accumulation and distribution of soil water under NT. Cropping systems modeling indicated, that during winter, surface runoff occurred in the CT system, but not under NT. The differences in soil water dynamics between CT and NT were mainly caused by differences in surface residues. Dynamics of NO3−-N at Palouse were similar for NT and CT. At Touchet, differences in soil moisture between NT and CT were less than 0.05 m3 m−3. Under NT, high levels of NO3−-N, up to 92 kg NO3-N ha−1, were found after harvest below the root zone between 1.5 and 2.5 m, and were attributed to inefficient use or over-application of fertilizer. In both climatic zones, grain yield was positively correlated with evapotranspiration.