How the mass and quality of wheat and vetch mulches affect drivers of soil N2O emissions

Abstract

Crop residue affects nitrous oxide (N2O) emissions in no-tillage systems, but the magnitude of the emissions depends on soil drivers that are directly influenced by crop residue quality and quantity. We conducted a one-year study to investigate how mulch chemical characteristics and mass affect N2O emissions during their decomposition in the field under subtropical conditions. The mulch treatments consisted of vetch and wheat crop residues applied to the soil at quantities of 0, 3, 6 and 9 Mg ha−1 dry matter. We followed the kinetics of mulch carbon (C) and nitrogen (N), soil temperature, moisture and inorganic N, the denitrification end-product ratio [N2O/(N2O + N2)] at days 15 and 30, and N2O and carbon dioxide (CO2) emissions with a static chamber method. Mulch decomposition and C and N dynamics were very rapid for vetch and much more gradual for wheat, reflecting the differences in their initial chemical compositions, but for both residues, the initial mulch mass had no effect on the decomposition rate. The presence of mulches increased soil moisture in the 0–10 cm soil layer, with the results for water-filled pore space by treatment as 0 = 3 < 6 = 9 Mg ha−1. Vetch also significantly increased the inorganic N content in the soil, unlike wheat, which caused net N immobilization. The daily N2O fluxes were strongly and positively correlated with CO2 fluxes (r = 0.83). The highest N2O fluxes were observed in the first 60 days with the vetch residues, and the results increased by treatment as 0 < 3 < 6 = 9 Mg ha−1; for wheat, the mean N2O emitted was 50% lower than that for vetch and was not affected by mulch mass. Although the cumulative N2O emitted was higher in vetch than in wheat, the N2O emission factor was higher for the wheat than for the vetch residues (EF = 4.64% vs 1.75% of N applied). We conclude that while mulch mass and quality strongly influenced soil drivers of N2O emissions, not only the N input but also the nature of residue C and the residue location must be taken into account to understand and predict N2O emissions associated to crop residue management.