N2O emission increases with mulch mass in a fertilized sugarcane cropping system

The nitrous oxide (N2O) emitted from soil was monitored to investigate the effect of sugarcane straw removal on the mechanisms that make mulch a “hot spot” of N2O emissions under subtropical conditions. We conducted a field experiment with the first-ratoon sugarcane with four amounts of straw (0, 4, 8, and 12 Mg ha−1) at the soil surface combined with 0 or 100 kg urea-N ha−1. The urea-N was applied 52 days after straw application. Over the course of 1 year, we measured the N2O and carbon dioxide (CO2) emissions, mineral nitrogen (N), soil moisture and temperature, and remaining straw carbon (C) and N in the mulch. We observed two “hot moments” for N2O emissions: the first one immediately after sugarcane straw application to soil and the second one after fertilizer-N application. High amounts of straw left on the soil led to an increase in the water-filled pore space (WFPS), and both WFPS and straw-C were strongly correlated with N2O fluxes. Cumulative N2O increased from 510 (0 Mg + N) to 1055 (12 Mg + N) g N2O-N ha−1 for the fertilized straw treatments. The N2O emission factors (EFs) of the sugarcane straw N and the fertilizer-N increased linearly with straw quantity, i.e., were not constant but were lower than the IPCC default values. Over 70% of the cumulative N2O emissions measured in straw + fertilizer-N treatments for 1 year were attributed to the presence of straw mulch, which emphasized the importance of the straw layer at the soil surface as a hot spot for N2O emissions.