FLUCTUATION AND BALANCE OF ORGANIC CARBON IN HIGH-YIELDING PADDY SOIL WITH WHEAT-RICE- RICE CROPPING SYSTEM AS AFFECTED BY DIFFERENT FERTILIZER MANAGEMENT

With the long-term field experiments, the flutuaticm and balance of organic carbon (OC) in high-yield paddy soils with wheat-rice-rice cropping system in Zhejiang province were studied. It was found that the annual naturally-mineralized amounts of soil OC in plough horizon (0~20) were from 1800 to 2000kg/hm2, and the amounts of OC in root and stubble were closely related to the grain yields with the ratio (amount of OC in root and stubble to grain yield) of 0. 30 (only mineral fertilizers applied or 0.32 (intergrated supply with mineral fertilizers and organic manure as rice straw), which was suitable to estimating the amount of OC in root and stubble kept in plough horizon. The results indicated that OC in crop root and stubble was available for balance of soil OC in plough horizon, but was not enough generally as only mineral fertilizers were supplied. With no fertilizer, the biomass and the amount of OC of crop root and stubble both decreased year after year. It was also obtained that the balance of soil OC and the balanced rate of OC could be assessed with the annual amount of soil OC naturally-mineralized, mineralization facter, grain yield and (or) applied amount of OC. The balanced rate of OC is less than 1600kg/hm2. as rice straw about 4000kg/hm2, which is mostly one third of total biomass of rice straw producted in a year. It can be used as a reference for organic manuring.