Plant Available Nitrogen Estimation Tools for a Biosolids‐Amended, Clayey Urban Soil

Abstract

Core Ideas Biosolids produced by heat‐drying and Cambi process had higher plant available N. Biosolids blended with woody byproducts had lower plant available N. Soil nitrate and 7‐d anaerobic incubation were the best indicators of N availability. Soil test reliability may have been reduced by high clay and low C and N of urban soil. Improving mineralizable N estimates from exceptional quality (EQ) biosolids products is important for making more reliable N recommendations to rehabilitate disturbed urban soils for vegetative production. The objectives of this study were to compare the N fertilizer equivalency method and several chemical (NH4–N, NO3–N, total N, and organic N) and biological (7 d anaerobic incubation [7‐AI] and Solvita CO2 Burst) tests for quantifying plant available nitrogen (PAN) and organic N mineralization of EQ biosolids used to grow tall fescue (Schedonorus arundinaceus Schreb.) in a clayey urban degraded soil. The EQ biosolids were products of thermal hydrolysis plus anaerobic digestion (Blue Plains exceptional quality biosolids; BLOOM), blending of BLOOM with woody mulch (BM), blending of BLOOM with sand and sawdust (Blue Pains exceptional quality biosolids + sand + sawdust; BSS), composting (Livingston compost; LBC), and heat drying (OCB; OceanGro thermally dried). The EQ biosolids were applied at agronomic N rates, and the inorganic fertilizer was applied at four N rates in a field study. The N fertilizer equivalency method predicted considerably lower organic N mineralization than calculated for BM (7.1%), BSS (–12%), and LBC (4.6%). Treatments BLOOM, BM, and OCB had the highest 7‐AI organic N mineralization. Correlations between tall fescue N uptake and soil N tests showed that soil NO3–N and 7‐AI were the best indicators of biosolids N availability in our urban soil. However, the relatively low correlations between soil N indicators and tall fescue N uptake was likely due to low residual soil N, high soil clay content, and possible low microbial activity of the low organic matter‐containing anthropogenic soil.