Seven upland cropping systems in Central Hokkaido, Japan, were investigated during the growing season in 2003 to evaluate the magnitude of N 2 O emission, CH 4 uptake and soil carbon sequestration, and their net effect on the global warming potential (GWP). N 2 O and CH 4 fluxes were measured from field soils planted with crops and CO 2 fluxes were measured from bare soils in attached plots at each site using the closed chamber method. Cumulative N 2 O emissions ranged from 0.02 g N m −2 to 0.62 g N m −2 for different soil types, which accounted for 0.35-4.44% of the applied fertilizer nitrogen. Cumulative CH 4 uptake rates ranged from −0.08 g C m −2 to 0 g C m −2. Soil carbon sequestration, defined as the difference between the net primary production and carbon loss through harvest and soil microbial decomposition, ranged from −410 to −193 g C m −2 , indicating that the carbon loss from soils could not be compensated by the carbon input through plant photosynthesis. The net GWP from the investigated cropping systems ranged from 749 to 1790 g CO 2 equivalents m −2. CO 2 emission contributed to 84-99% of the net GWP and N 2 O contributed 1-16%.

To evaluate the carbon budget in soils under different cropping systems, the carbon dioxide (CO 2 ) 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 2 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 CO 2 fluxes within the sites was mainly controlled by soil temperature and moisture. Annual mean CO 2 fluxes and cumulative CO 2 emissions had no significant relationship with soil temperature and moisture (P > 0.2). However, there was a significant quadratic relationship between annual mean CO 2 flux or cumulative CO 2 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 2 emission is at a clay plus silt content of 63%. The cumulative CO 2 emission during the no snow cover season within each year varied from 1,159 to 7,349 kg C ha -1 at the different sites. The amount of crop residue carbon retained in the soils following a cropping season was not enough to offset the CO 2 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 2 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 CO 2 .

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In order to explore the contributions of wet deposition and runoff formed by rainfall events to the water body mercury burden in Three Gorges Reservoir (TGR), we conducted a 1-year successive study on the deposition fluxes and runoff output characteristics of total mercury (THg) and methylmercury (TMeHg) in a typical small agricultural watershed in TGR areas. The results showed that the annual volume-weighted concentration (VWC) of THg and TMeHg was 18 and 0.23 ng L(-1), respectively. Particulate form was the main form of both THg and TMeHg, accounting for 61 % of THg and 59 % of TMeHg. The annual deposition fluxes of THg and TMeHg in rainfall were 13 ± 2.4 μg m(-2) year(-1) and 174 ± 52 ng m(-2) year(-1), respectively. The VWCs of THg and TMeHg in runoff were 10 ± 6.5 and 0.15 ± 0.15 ng L(-1). The annual output fluxes of THg and TMeHg to TGR from study area were 1.2 ± 0.73 μg m(-2) year(-1) and 17 ± 16 ng m(-2) year(-1), respectively.