Effect of low C/N crop residue input on NO, NO, and CH fluxes from Andosol and Fluvisol fields.

Crop residues are produced from agriculture in large amounts globally. Crop residues are known to be a source of nitrous oxide (NO); however, contrasting results have been reported. Furthermore, the effect of crop residues on nitric oxide (NO) and methane (CH) fluxes has not been well studied. We investigated NO, NO, and CH fluxes after low C/N crop residue (cabbages and potatoes) inputs to lysimeter fields for two years using with automated flux monitoring system. Lysimeters were filled with two contrasting soil types, Andosol (total C: 33.1 g kg; clay: 18%) and Fluvisol (17.7 g kg; 36%). Nitrogen application rates were 250 kg N ha of synthetic fertilizer and 272 kg N ha of cow manure compost for cabbage, and 120 kg N ha of synthetic fertilizer and 136 kg N ha of cow manure compost for potato, respectively. Large NO peaks were observed after crop residues were left on the surface of the soil for 1 to 2 weeks in summer, but not in winter. The annual NO emission factors (EFs) for cabbage residues were 3.02% and 5.37% for Andosol and Fluvisol, respectively. Those for potatoes were 7.51% and 5.10% for Andosol and Fluvisol, respectively. The EFs were much higher than the mean EFs of synthetic fertilizers from Japan's agricultural fields (0.62%). Moreover, the EFs were much higher than the Intergovernmental Panel on Climate Change (IPCC) default NO EFs for synthetic fertilizers and crop residues (1%). The annual NO EFs for potatoes were 1.35% and 2.44% for Andosol and Fluvisol, respectively, while no emission was observed after cabbage residue input. Crop residues did not affect CH uptake by soil. Our results suggest that low C/N crop residue input to soils can create a hotspot of NO emission, when temperature and water conditions are not limiting factors for microbial activity.