Short‐Term Incubation Assay Effectively Indicates Potentially Available Nitrogen From Organic Residues

This study examines the mineralization of nitrogen (N) in soil treated with anaerobic digestates and spent mushroom substrate. To this end, we used a laboratory aerobic incubation assay over 42 days to determine the maximum NO 3 − ‐N concentration produced. The objective was to test the following organic residues as fertilizers: liquid and solid anaerobic digestate (AN‐LIQ, AN‐SOL); spent substrate from the mushrooms Agaricus and Pleurotus (SMS‐A, SMS‐P); and a mixed (MX) treatment (SMS‐P and AN‐LIQ). Its performace was compared with that of a mineral N fertilizer (MIN) treatment and an unfertilized soil (UN, without N source addition). At 0 and 14 days of incubation, we measured the activity of arylsulfatase (ARYL), ß‐galactosidase (GAL), urease (URE), ß‐glucosidase (GLU), and acid phosphatases (PAC) and alkali (PAK) phosphatases in the AN‐LIQ, SMS‐P, UN, and MIN treatments. Adjustment of the linear‐plateau model and response model indicated that maximum N mineralization occurred between 7 and 28 days of incubation. The SMS‐P treatment immobilized N and showed a lower NO 3 − ‐N content (∼7 mg NO 3 − ‐N kg soil −1 ) than the UN treatment (26.8 mg NO 3 − ‐N kg soil −1 ). The efficiencies of the organic treatments were 59% for AN‐LIQ, 33% for AN‐SOL, 25% for MX, and 9% for SMS‐A, with respect to the mineral. None of the organic residues impaired soil respiration (142–163 mg CO 2 100 g −1 soil/24 h), enzyme activities, or phytotoxicity. These findings indicate that AN‐LIQ and AN‐SOL can serve as partial or complete substitutes for mineral fertilization, depending on crop requirements.