Soil Carbon and Nitrogen Fraction Accumulation with Long‐Term Biosolids Applications

Abstract

Core Ideas Biosolids N contribute to wheat grain and soil N. Applied biosolids contain light fraction and non‐hydrolyzable C and N. Biosolids C efficiently increased soil C at 0.91 Mg C in soil per Mg C in added biosolids. Buildup of non‐hydrolyzable, high C:N light fraction originates in light density biosolids. High total and non‐hydrolyzable C retention rates have implications for greenhouse gas mitigation. Global sustainability depends on the recycling of organic wastes, containing carbon and plant nutrients, back into food production. Biosolids are capable of building total soil organic matter, but their ability to build stable organic C and N fractions is less well understood. The sequestration of stable soil C and replacement of energy‐requiring commercial fertilizers both have implications for greenhouse gas mitigation. Our aim was to assess the effectiveness of biosolids amendments to store labile and stable soil C and N fractions while supplying crop N needs. Three rates of anaerobically digested biosolids were incorporated following wheat harvest every 4 yr in a wheat‐fallow system over 20 yr, and compared to a no‐fertilizer check and a standard commercial fertilizer treatment. Increases in total soil C and N correlated to cumulative application rates of total and acid‐resistant, non‐hydrolyzable (NH) C and N, with 91% of added biosolids C efficiently retained in soil. The soil accumulated light fraction (LF) C and N in biosolids treated plots, whereas the LF pools of the control plots decreased and LF pools in the commercial fertilizer plots stayed relatively constant. Total soil N correlated with cumulative biosolids N, with 35% of added biosolids N retained in the soil of which only 4% was stored in the soil NH N fraction. Anhydrous ammonia increased wheat yields by 27% over the 0‐fertilizer check, without increasing soil N. Biosolids markedly elevated total, stable and LF soil C and N pools in semiarid conditions, while maintaining comparable wheat productivity to commercially fertilized wheat‐fallow.