Soil N transformation mechanisms can effectively conserve N in soil under saturated conditions compared to unsaturated conditions in subtropical China

Yushu Zhang,Xiangzhou Zheng,Hong Ding,Jinbo Zhang,Tom Misselbrook,Zucong Cai,Christoph Müller,Alison Carswell

doi:10.1007/s00374-018-1276-7

Abstract

The connection between moisture and nitrogen (N) transformation in soils is key to understanding N losses, particularly nitrate (NO3−) losses, and also provides a theoretical framework for appropriate water management in agricultural systems. Thus, we designed this study to provide a process-based background for management decision. We collected soil samples from the long-term field experiment in subtropical China, which was designed to examine tobacco and rice rotations under a subtropical monsoon climate. The field experiment was established in 2008 with four treatments: (1) no fertilization as control; (2) N, phosphorus (P), and potassium (K) fertilizers applied at recommended rates; (3) N fertilizers applied at rates 50% higher than the recommended amounts and P and K fertilizers applied at recommended rates; and (4) N, P, and K fertilizers applied at recommended rates with straw incorporated (NPKS). Soil samples were collected during the unsaturated tobacco-cropping season and saturated rice-cropping season and were incubated at 60% water holding capacity and under saturated conditions, respectively. Two 15N tracing treatments (15NH4NO3 and NH415NO3) and a numerical modeling method were used to quantify N transformations and gross N dynamics. Autotrophic nitrification was stimulated by N fertilizer both under unsaturated and saturated conditions. The rate of NO3− consumption (via immobilization and denitrification) increased under the NPKS treatment under saturated conditions. Secondly, the rates of processes associated with ammonium (NH4+) cycling, including mineralization of organic N, NH4+ immobilization, and dissimilatory NO3− reduction to NH4+, were all increased under saturated conditions relative to unsaturated conditions, except for autotrophic nitrification. Consequently, NO3−-N and NH4+-N concentrations were significantly lower under saturated conditions relative to unsaturated conditions, which resulted in reduced risks of N losses via runoff or leaching. Our results suggest that under saturated conditions, there is a soil N conservation mechanism which alleviates the potential risk of N losses by runoff or leaching.

Highlights

Nitrogen (N) is one of the key elements required for crop growth
The four treatments examined were (1) no fertilization applied (CK); (2) N, phosphorus (P), and potassium (K) fertilizers applied at the recommended rate (NPK); (3) N fertilizer applied at 50% above the recommended treatment and P and K at the recommended rate (NhPK); and (4) NPK with additional straw (NPKS), where rice straw was chopped and incorporated into soil by plowing at a rate of 3600 kg ha−1
Different lowercase letters indicate significant differences at p < 0.05 between different fertilization treatments under the same soil water conditions DWE dry weight equivalent a CK = no fertilization; NPK = mineral N, P, and K fertilizers applied at recommended rates; NhPK = mineral N fertilizers applied at rates 50% higher than NPK; NPKS = mineral NPK fertilizer applied and straw residue b Nitrate ratio calculated as NO3− /(NO3− + NH4+ )

Summary

Introduction

Nitrogen (N) is one of the key elements required for crop growth. N fertilizer application to agricultural systems has had multiple negative effects on the environment (Sainju et al 2008). In subtropical regions, which is characterized by the high precipitation, NO3− leaching losses occur readily via leaching or runoff, while NH4+ is less mobile (Huygens et al 2007; Rütting and Müller 2007, 2008). Rates of autotrophic nitrification in native soils in the subtropical regions have been reported as low or absent due to low soil pH (Zhao et al 2007), while rates of NO3− immobilization to organic N reported were relatively high (Zhang et al 2013a), especially in low NH4+-N soils (Rice and Tiedje 1989). When native land is converted to agricultural use, microbial immobilization of NO3− is substantially suppressed and nitrification is stimulated (Schimel and Bennett 2004), leading to NO3− dominance in inorganic N in the soil, with associated risks of N loss through runoff, leaching, and denitrification. N retention in native subtropical soils is likely to be reduced when converted to agriculture (Han et al 2012; Yang et al 2010; Zhang et al 2013a)

Methods

Results

Discussion

Conclusion