Combined emission of CH4 and N2O from a paddy field was reduced by preceding upland crop cultivation

Since crop rotation between paddy rice and upland crops is widely conducted in Japan and other Asian countries, the effect of crop rotation on greenhouse gas emission should be clarified. In this study, methane (CH 4 ) and nitrous oxide (N 2 O) fluxes were simultaneously measured for two years from 2004 to 2005 in a paddy rice field with three different cultivation histories, i.e. consecutive paddy rice cultivation (PR), single cropping of upland rice (UR), and double cropping of soybean and wheat (SW) in the preceding two years from 2002 to 2003. In 2004, the cumulative CH 4 emissions in the UR and SW plots were 511 and 2817 g CH 4 m −2 y −1 , which were 8 and 46%, respectively, of that in the PR plots (6092 g CH 4 m −2 y −1 ). In 2005, the cumulative CH 4 emissions in the UR and SW plots were 5123 and 1331 g CH 4 m −2 y −1 , which were 87 and 23%, respectively, of that in the PR plots (5893 g CH 4 m −2 y −1 ), although the differences were not statistically significant. The soil reduction/oxidation potential (Eh) in the UR plots was higher than that in the PR plots in 2004. However, no distinctive differences in soil Eh among the three cropping systems were found in 2005. In the spring of 2004, the soil iron (Fe) content determined by extraction with dithionite-ethylenediaminetetraacetic acid (EDTA) solution was higher in the UR plots than in the PR and SW plots. However, no significant differences in Fe content among the three cropping systems were found in the spring of 2002 and 2005. The application of a relatively small amount of residue from the upland rice (c. 30% of that from the paddy rice) and the removal of all aboveground crop residues of soybean and wheat before paddy rice cultivation in 2004 could have contributed significantly to the low CH 4 emissions in the UR and SW plots. In addition, change in the form of soil Fe during the preceding periods with upland crop cultivation may also have been related to the decreases in CH 4 emission. The cumulative N 2 O emissions ranged from 39 to 99 mg N m −2 y −1 , and showed no significant difference among the three cropping systems in 2004 and 2005. These results indicate that the combined CH 4 and N 2 O emission from paddy soil is reduced by the introduction of the preceding upland crop cultivation when crop residue from the previous upland crop is small or removed before paddy rice cultivation, although this effect was expected only for one year just after the land use change from upland c