Effects of soil aggregate size, moisture content and fertilizer management on nitrous oxide production in a volcanic ash soil

Abstract

A laboratory incubation study was conducted to determine the effects of soil aggregate size, soil moisture content and manure application on nitrous oxide (N2O) production through nitrification and denitrification. In Southern Hokkaido, soil samples were taken from a mineral soil layer (2.5–10 cm) of a grassland to which chemical fertilizer and manure had been applied. The soil aggregates were air-dried and sieved with 4.5 mm and 2 mm sieves, and the soil moisture was adjusted to 60% and 80% of field water capacity (FWC). Immediately after moistening, incubation was initiated and lasted for 9 days at 20°C. Following the start of incubation, a flush of N2O, carbon dioxide (CO2) and nitric oxide (NO) was observed. Production of all gases was higher in larger aggregates from the manure-applied soil. Productions of CO2 and NO were not significantly influenced by soil moisture content, but N2O production was considerably higher in 80% FWC as compared with 60% FWC. Based on the results of the N2O–nitrogen (N)/NO–N ratio, the process of N2O production was mainly due to nitrification in 60% FWC and denitrification in 80% FWC. Soil chemical properties, especially ammonium–N –N), nitrate–N (NO3–N) and water extractable organic C (WEOC) and microbial biomass C (MBC) also changed immediately after moistening. These changes were higher in larger aggregates from the manure-applied soil. Potential denitrification enzyme activity (DEA) was significantly higher in larger aggregates from manure-applied soil with higher moisture content. The N2O production in both 60% and 80% FWC correlated significantly with MBC and DEA. Regardless of soil moisture conditions, MBC correlated significantly with DEA, WEOC consumption and apparent N mineralization. These facts suggest that larger soil aggregates could have quickly developed suitable internal conditions for microbial activity inside the aggregates and consequently stimulated N2O production through nitrification and denitrification.