Different responses of the gross nitrification rate to N enrichment in topsoil and subsoil

Abstract

Nitrogen (N) deposition has continually increased since the industrial revolution and its rate varies widely around the world. The dynamic of soil nitrification under N enrichment affects nitrous oxide (N2O) emissions and nitrate (NO3−) leaching which are related to current environmental problems. However, the response of soil nitrification, particularly in subsoil, to a N enrichment gradient and the mechanisms involved have not been well studied. In this study, the gross nitrification rates (GNRs) and relevant abiotic and biotic factors along a N addition gradient in a field were determined at three soil depths using a 15N-labeling method. The GNR in topsoil (0–10 cm) followed a bell-shaped curve as the N addition rate increased. The GNR in subsoil (10–20 and 20–40 cm) linearly decreased as the N addition rate increased. The GNR positively correlated with the ammonia-oxidizing bacteria (AOB) abundance in topsoil (P < 0.05) and the gross mineralization rate in subsoil (P < 0.05). The results indicated that changes in the AOB abundance contributed to the bell-shaped pattern of GNR in the topsoil, and the decrease in mineralization rate contributed to the decrease in GNR with N enrichment by reducing substrate supply in the subsoil. The different responses of soil nitrification to N enrichment at different soil depths and the mechanisms involved should be taken into consideration in biogeochemical models to better predict the soil N cycling in the context of N enrichment.