Prediction of potentially mineralizable N from amidohydrolase activities in a manure-applied, corn residue-amended soil

Prediction of potentially mineralizable N as an important N pool from soil amidohydrolases was investigated. Composite soil samples were collected from plots of a field experiment in which 0, 50 and 100 Mg cow manure ha⁻¹ year⁻¹ had been applied for five consecutive years. The soils were treated with corn shoots or roots or remained untreated in a factorial combination with the manure treatments, with three replications. The mineralized inorganic N was measured periodically in 20-week incubations and potentially mineralizable N (N₀) was calculated based on a first-order kinetic model. Urease, l-glutaminase and l-asparaginase activities were measured before and after incubation. The values of N₀ ranged from 208.6 in the controls to 388.4 in soils that had received 50 Mg ha⁻¹ year⁻¹ of cow manure and were amended with corn shoots. Corn residue amendment in the manure treated soils, increased the values of N₀ or changed the N mineralization kinetic pattern from a first-order to a zero-order model. According to a relative sensitivity index, l-asparaginase was the most sensitive enzyme to the treatments. Multiple regression analysis revealed that 92% of N₀ variations can be described by the activities of urease and l-asparaginase and therefore the soil amidohydrolase activities have the potential to evaluate potentially mineralizable N.