A 4-year field measurement of NO emissions from a maize-wheat rotation system as influenced by partial organic substitution for synthetic fertilizer.

Soil NO 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 NO emissions. However, no multi-year field studies of NO emissions under different ratios of OF to SFs have been performed. We conducted a 4-year experiment to measure NO 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 NO emissions (kg N ha) 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 NO 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 NO was negatively correlated with pH. Long-term organic substitution enhanced NO emissions produced via denitrification rather than nitrification and resulted in higher temperature sensitivity of NO emissions than TS. The enhanced NO emissions from the OF-treated soils were mainly attributable to accelerated OF decomposition, increased denitrification-NO emissions, and lessened NO reduction due to lower pH and greater NO. These results indicate that OF substitution can reduce NO emissions in the first year, but in the long-term it can increase emissions, especially as soils warm.