Nitrous oxide emissions following the application of wheat residues and fertilizer under conventional-, reduced-, and zero-tillage systems in central Hokkaido, Japan

Nitrous oxide (N2O) is a major greenhouse gas produced by agricultural systems, and has a higher greenhouse effect than carbon dioxide (CO2) and methane (CH4). Cultivation techniques (particularly tillage) and the incorporation of crop residues contribute to N2O release. Our objective was to quantify the rates of N2O emissions from conventional-, reduced-, and zero-tillage systems (CT, RT, and ZT, respectively) with the application of wheat residues and fertilizer. The study included CT, RT, and ZT systems, and a no-fertilizer (NF) treatment without basal and top dressing (CT/NF). N2O flux in each treatment reached the highest value between mid- and late October (for ZT, RT, CT, and CT/NF, 1763, 2640, 1458, and 1620 microg N/square m/hr, respectively). Immediately after snowmelt, large increases in N2O emissions were observed in the ZT, RT, and CT plots, with maximum values of 413, 959, and 439 microg N/square m/hr, respectively. This trend was not apparent in the CT/NF plots, suggesting that basal dressing with nitrogen is responsible for large emissions after snowmelt. No remarkable N2O emission occurred from snowmelt until harvest, indicating that nitrogen application by top dressing is not an important N2O source. Total N2O emissions from the ZT, RT, CT, and CT/NF plots during the sampling period (296 days) were 8.9, 11.1, 9.5, and 8.8 kg N/ha, respectively, and did not differ significantly. Our results suggest that incorporation of wheat straw residues, including surface mulching in the ZT plot, was an important factor that affected large N2O emission. N2O emission before and behind of snowmelt (27 days) accounted for 11% to 18% of total N2O emission during the sampling period. Our results also suggest that full-year investigations are necessary to prevent underestimation of N2O emission due to snowmelt.