Responses of nitrification and denitrification to nitrogen and phosphorus fertilization: does the intrinsic soil fertility matter?

Abstract

Nitrogen (N) is not only a major regulator of productivity in terrestrial systems but can also be a pollutant. While the effects of fertilizer addition to soil N cycling processes and the associated soil microbial groups have been extensively studied, little attention has been paid to the interaction of fertilizer amendment and the intrinsic fertility of the soil. We conducted a filed study on the Chinese Loess Plateau with soils of two fertility levels (low fertility and high fertility), amended with or without synthetic fertilizer (N and phosphorus, NP). Soil samples were collected three times in a continuous winter wheat cropping system and analyzed for soil potential nitrification (PNA) and denitrification (DEA) rates, nitrifiers and denitrifiers. Populations of ammonia-oxidizing archaea (AOA) and bacteria (AOB) and the resulting PNA were increased by the NP application regardless of intrinsic soil fertility. The NP application increased DEA by 15–228% in the low fertility soil, but decreased DEA by 18–46% in the high fertility soil. Soil denitrifiers also showed divergent responses to NP application in low and high fertility soils, i.e., the abundance of nirS-nitrite reducers increased in the low fertility soil, while nirS- and nirK-nitrite reducers were unchanged in the high fertility soil. The narG-nitrate reducers, AOB and AOA were more responsive to the NP application in the tillering stage (BBCH 25–26) than in other periods, whereas PNA, DEA and denitrifier gene abundance (nirS and nirK) varied more in the flowering stage (BBCH 65). Our results suggest that the responses of soil nitrification/nitrifiers and denitrification /denitrifiers to NP application varied with intrinsic soil fertility levels. The contrasting responses of nitrification and denitrification to NP application in high fertility soil indicate a decoupling of both processes.