Effects of different forms of plant-derived organic matter on nitrous oxide emissions

Abstract

To investigate the impact of different forms of plant-derived organic matter on nitrous oxide (N₂O) emissions, an incubation experiment with the same rate of total nitrogen (N) application was carried out at 25 °C for 250 days. Soils were incorporated with maize-derived organic matter (i.e., maize residue-derived dissolved organic matter and maize residues with different C/N ratios) and an inorganic N fertilizer (urea). The pattern and magnitude of nitrous oxide (N₂O) emissions were affected by the form of N applied. Single application of maize-derived organic matter resulted in a higher N₂O emission than single application of the inorganic N fertilizer or combined application of the inorganic N fertilizer and maize-derived organic matter. The positive effect of maize residue-derived dissolved organic matter (DOM) addition on N₂O emissions was relatively short-lived and mainly occurred at the early stage following DOM addition. In contrast, the positive effect induced by maize residue addition was more pronounced and lasted for a longer period. Single application of maize residues resulted in a substantial decrease in soil nitric nitrogen (NO₃⁻–N), but it did not affect the production of N₂O. No significant relationship between N₂O emission and NO₃⁻–N and ammonium nitrogen (NH₄⁺–N) suggested that the availability of soil N was not limiting the production of N₂O in our study. The key factors affecting soil N₂O emission were the soil dissolved organic carbon (DOC) content and metabolism quotient (qCO₂). Both of them could explain 87% of the variation in cumulative N₂O emission. The C/N ratio of maize-derived organic matter was a poor predictor of N₂O emission when the soil was not limited by easily available C and the available N content met the microbial N demands for nitrification and denitrification. The results suggested that the magnitude of N₂O emission was determined by the impact of organic amendments on soil C availability and microbial activity rather than on soil N availability. In agricultural management practices, if the N inputs from organic and inorganic N fertilizers are equivalent, addition of organic N fertilizers that contain high amounts of available C will result in a higher N₂O emission.