Long-term biosolids applications improve key soil health functions for semi-arid dryland systems

Biosolids are municipal wastewater products (sewage sludge) that have undergone additional treatment to meet EPA requirements to be land applied to crop land as a nutrient and organic matter (OM) source. In semi-arid dryland systems, biosolids may be beneficial for improving soil physical, chemical, and biological attributes. The objective of this study was to determine how agronomically relevant biosolids application rates at two long-term (20+ year) field trials affected soil health properties in semi-arid dryland systems. Soil samples were collected (0-15 cm) from (1) a 26-year trial in central Washington (WA) in a grain-fallow rotation, with three biosolids application rates applied every four years compared to synthetic fertilizer and unfertilized controls, and (2) a 23-year trial in central-eastern Colorado (CO) comparing biosolids to synthetic fertilizer in both a wheat-corn-fallow (WCF) and wheat-fallow (WF) rotation. At the WA site, increasing biosolids applications increased soil carbon (C) and nitrogen (N) pools, potential activity of N-acetyl β-glucosaminidase (NAG) and phosphomonoesterase (PME) enzymes, and available water holding capacity, while decreasing bulk density. At both sites, biosolids increased microbial biomass as measured by phospholipid fatty acids (PLFA) and Mehlich-3 extractable soil P. Overall, biosolids had a greater influence at the WA site, likely due to soil texture and higher cumulative biosolids inputs. In CO, most biosolids effects were seen across both crop rotations but were numerically greater in the WCF rotation, which received greater cumulative biosolids applications than the WF rotation. This study shows that biosolids can improve key soil health functions of C accumulation, nutrient cycling, and water storage in semi-arid dryland cropping systems.