Effects of environmental factors on temporal variation in annual carbon dioxide and nitrous oxide emissions from an unfertilized bare field on Gray Lowland soil in Mikasa, Hokkaido, Japan

Abstract

Soil is an important source of atmospheric carbon dioxide (CO2) and nitrous oxide (N2O). Studies of CO2 and N2O emissions from bare soil may explain annual changes in carbon (C) in soil organic matter (SOM) and help analyze N2O production from SOM. Therefore, CO2 and N2O emissions associated with the decomposition of SOM from bare soil are important factors for assessing the C budget and N2O emission in agricultural fields. We conducted a study over 7 years to assess the controlling factors of CO2 and N2O emissions from unplanted and unfertilized soil in Mikasa, Hokkaido, Japan. The CO2 flux increased in summer and there were significant positive correlations between the CO2 flux and soil temperature in the first 4 years. However, apparent relationships between CO2 flux and water-filled pore space, soil NH4 and NO3 concentrations were not observed. The slope of monthly CO2 emission against mean monthly temperature was positively correlated with monthly precipitation. These results suggest that the response of CO2 production to increases in soil temperature became more sensitive in wet soils. The average CO2 emission during the study period was 2.53 Mg C ha−1 year−1, and uncertainty in the annual CO2 emission was 24%. Annual precipitation explained the yearly variation (CO2 emission [Mg C ha−1 year−1] = 0.0021 × annual precipitation [mm year−1] −0.0499, R = 0.976, P < 0.001). Nitrous oxide flux increased from July to October and was positively correlated with CO2 flux. Based on the ratio of N2O-N : NO-N of fluxes, N2O appeared to be the main product of denitrification. The average N2O emission over the study period was 4.88 kg N ha−1 year−1, and uncertainty in the annual N2O emission was 58.5%. Strong relationships between the monthly emissions of CO2 and N2O suggest that N2O production by denitrification is strongly affected by SOM decomposition. Unlike CO2 emission, a relationship between N2O emission and precipitation was not observed because of the multiple pathways of nitrification and denitrification for N2O production induced by SOM decomposition.