Soil carbon balance after the temporal cessation of cultivation under cool and subtropical humid climate in Japan

The decreasing number of farm households in Japan is beginning to affect soil carbon and nitrogen balance. At both cool and warm sites, the soil C level was the lowest in the second year of the experiment, and thereafter, it gradually increased. The change in soil C concentration was negative at the warmer site; however, the content reached 119 g C m(-2) at the cooler site. Soil N showed a significant increase of 14.3 g N m(-2) from the first year to over 4 years at the cooler site. At the cooler site, less soil decomposition contributed to the retention of soil C and N, and the dominant deciduous woody plants increased the supplemental C and N content. Land-use management strategies involving conventional cultivation, abandonment and abandonment to recultivation did not affect the soil C and N balance for over 6 years. Fallow tillage to recultivation was less effective in retaining the C pool in the fifth year of the experiment, causing significantly lower soil C content (1.97 kg C m(-2)) than abandonment to recultivation (2.20 kg C m(-2)). Whereas the soil C content increased in the subsequent year (i.e. the sixth year), less productivity of recultivation crop (white mustard) contributed to a significant decrease in soil C, and abundant residue of winter wheat could recover the soil C and N in the subsequent year. The results suggest that agricultural abandonment can preserve soil C in cool regions and that the introduction of winter crops can maintain the C pool.