A 4-year field measurement of N2O emissions from a maize-wheat rotation system as influenced by partial organic substitution for synthetic fertilizer

Abstract

Soil N₂O emissions depend on the status of stoichiometric balance between organic C and inorganic N. As a beneficial management practice to sustain soil fertility and crop productivity, partial substitution of organic fertilizers (OFs) for synthetic fertilizers (SFs) can directly affect this balance status and regulate N₂O emissions. However, no multi-year field studies of N₂O emissions under different ratios of OF_S to SFs have been performed. We conducted a 4-year experiment to measure N₂O emissions in a maize-wheat rotation in central China. Six treatments were included: total SF (TS), total OF, no N fertilizer, and ratios of to SF with 1: 2 (LO), 1: 1 (MO), and 2: 1 (HO), based on N content. Two incubation experiments were performed to further interpret the field data. In the first year, cumulative N₂O emissions (kg N ha^-1) in LO, MO, and HO were 4.59, 4.68, and 3.59, respectively, significantly lower than in TS (6.67). However, from the second year onwards, organic substitution did not reduce N₂O emissions and even significantly enhanced them in the fourth year relative to TS. Soil respiration under OF-amended soils increased over the course of the experiment. From the second year onwards, there was no marked difference in mineral N concentrations between OF- and SF-amended soils. OF caused a drop in soil pH. Cumulative N₂O was negatively correlated with pH. Long-term organic substitution enhanced N₂O emissions produced via denitrification rather than nitrification and resulted in higher temperature sensitivity of N₂O emissions than TS. The enhanced N₂O emissions from the OF-treated soils were mainly attributable to accelerated OF decomposition, increased denitrification-N₂O emissions, and lessened N₂O reduction due to lower pH and greater NO₃^-. These results indicate that OF substitution can reduce N₂O emissions in the first year, but in the long-term it can increase emissions, especially as soils warm.

Keywords: Denitrification; Fertilizer type; Interannual variation; Maize-wheat rotation; N(2)O flux; Temperature fluctuation.