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 12Mgha(-1)) at the soil surface combined with 0 or 100kg urea-Nha(-1). The urea-N was applied 52days after straw application. Over the course of 1year, 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 (0Mg+N) to 1055 (12Mg+N) g N2O-Nha(-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 1year 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.