Evaluation of the soil carbon budget under different upland cropping systems in central Hokkaido, Japan

To evaluate the carbon budget in soils under different cropping systems, the carbon dioxide (CO2) flux from soils was measured in a total of 11 upland crop fields within a small watershed in central Hokkaido over the no snow cover months for 3 years. The CO, flux was measured using a closed chamber method at bare plots established in each field to estimate soil organic matter decomposition. Temporal variation in instantaneous soil CO2 fluxes within the sites was mainly controlled by soil temperature and moisture. Annual mean CO, fluxes and cumulative CO2 emissions had no significant relationship with soil temperature and moisture (P > 0.2). However, there was a significant quadratic relationship between annual mean CO2 flux or cumulative CO, emission and soil clay plus silt content (%) (R-2 = 0.72-0.74, P < 0.0003). According to this relationship, the optimum condition for soil CO, emission is at a clay plus silt content of 63%. The cumulative CO, emission during the no snow cover season within each year varied from 1,159 to 7,349 kg C ha(-1) at the different sires. The amount of crop residue carbon retained in the soils following a cropping season was not enough to offset the CO, emission from soil organic matter decomposition at all sites. As a consequence, the calculation of the soil carbon budget (i.e. the difference between the carbon added as crop residues and compost and the carbon lost as CO, from organic matter decomposition) ranged from -7,349 to -785 kg C ha(-1), except for a wheat site where a positive value of 4,901 kg C ha(-1) was observed because of a large input of organic carbon with compost. The negative values of the soil carbon budget indicate that these cropping systems were net sources of atmospheric CO2.