Biosolids' legacy benefits persist even 7 years after the application in a dryland wheat‐fallow cropping system

Abstract Application of biosolids can be beneficial for low carbon (C) and erosion‐prone soils of the dryland region of inland Pacific Northwest (iPNW). Biosolids have been proven to improve wheat yields and soil health in the iPNW. Although direct agronomic and environmental influences of biosolids with frequent applications have been extensively studied, research‐based information on persistence and long‐term residual or ‘legacy’ effects of biosolids applications have only rarely been reported. Therefore, this study was conducted to understand the longevity of the impacts of biosolids 7 years after the last application. The study included tillage (tandem disk vs. undercutter tillage) as the main‐plot factor and fertilizer type (biosolids vs. synthetic N + S) as the subplot factor in a split‐block experimental design. Biosolids were applied at the rate of 6.5 mt/ha (on dry weight basis to meet N requirements for two wheat crops) in 2012 and 2016. Soil samples were collected from the 0–10 cm depth in 2023 and analysed for an array of soil properties. Results showed that 7 years after the last application of biosolids, compared to the synthetic fertilizer plots, the biosolids plots had increased total C, total N, extractable P, S, Zn, Fe, Mn, and Cu and decreased soil pH, Mg, and Ca. Grain protein, soil EC, NO 3 − N, NH 4 − N, Na and B were not affected by fertilizer type. Conservation undercutter tillage showed increased soil K, Mg, OM, Zn, TN, and TC compared to traditional disk tillage. In addition, biosolids legacy showed 42% higher grain yield, 47% greater straw production, and increases in most grain yield components but 8% lower 1000‐grain weight compared to synthetic fertilizer treatments. Overall, biosolids showed improved nutrient availability, wheat grain yield, and grain yield components in a dryland region 7 years after the last application. Our data show that periodic application of biosolids improves soil health and increases grain yield in dryland wheat systems.