Formation of extractable organic nitrogen in an agricultural soil: A 15N labeling study

Few studies have investigated the extractable organic nitrogen (EON) formation mechanisms, and the sources of EON have long been debated. Using 15N labeling, we performed a 120-day laboratory incubation experiment to explore the dynamic contributions of different types of added N (ammonium-N, ryegrass-N and their combination) to soil EON and the role that microorganisms play in N transformation into EON. We show that the 15N abundances and recoveries in soil EON pool were relatively low during the incubation, except the first hours after ryegrass addition in 15N-ryegrass addition treatments. In general, most of the EON during the incubation was soil derived, and both ammonium-N (80 mg kg−1) and ryegrass-N (160 mg kg−1) additions made minor contributions (3-4% and 8-13% during day 1-120) to the soil EON pool. Moreover, along with the decline in 15N recoveries in microbial biomass nitrogen (MBN) pool, the lost MB15N did not enter into the EO15N pool. Our study demonstrates 1) that EON is a stable N pool in agricultural soil and is less affected by exogenous N addition and 2) that microbial N uptake and release processes contribute little to the soil EON pool.