Change Of Organic Carbon In Paddy Soils Over The Last Two Decades--A Study At Scale Of County And Village Level

The soil carbon pool is the biggest one in the earth surface system and the soil organic carbon (SOC) plays an important role in global carbon cycling. SOC varies in the temporal and spatial scale. Both natural condition (including climate and parent material, etc) and soil management (including fertilization, tillage, etc) influence the content of SOC in cultivated soils. The SOC data of the 2nd State Soil Survey is usually used to estimate of soil SOC pool in China. Studies on the SOC dynamics of China cultivated soils in the last 20 years have been conducted using SOC data normally obtained through monitoring of the long-term chemical fertilizer trials and GIS data in whole nation. The paddy soil is one of the most important soil resources of China, which is an artificial wetland soil under long-term paddy management. Some data indicate that the soil organic carbon density of paddy soil is higher than that of dry cropland soil in China, and has an increasing tendency over the last 20 years. This paper analyzed the SOC of status of paddy soils in different periods and the key factors influencing the change in last 20 years. It is useful for us to estimate the change and SOC storage of paddy soil.The paddy soils from the Tai Lake region, Jiangsu and these from the Red soil downlands in Jiangxi were used for this study. This SOC dynamic in county level was done by a soil survey of main types of paddy soils of Yinxing and that in village farm was done by soil survey of the Ganyan village, Shangzhu village and Banqiao village in Jiangxi. The main results were obtained as follows:1. Study on the change of SOC of paddy soils in county scale.A. Change of SOC of paddy soils was prominent in last 20 years. Since 1984, the content of SOC has been increased in an exponential function with years, although the SOC was somewhat decreased in 1981-1984. Changes of soil organic carbon density of 4major species soil in last 20 years was as following: Hubaitu( -2.80t.C.hm~2) < Baitu ( 10.36 t.C.hm-2) < Wunitu(13.65 t.C.hm-2)) tillage of rice -fallow system > straw return.C. There was much spatial variability of contents of organic carbon and total nitrogen. The contents of SOC and TN in the soil profiles decreased sharply from 0-30cm depth down. The contents of SOC in different zone in 2002 are: Low hill area > dyke area >Tai Lake and Gehu plain area > hills and valleys > area along the lake, while the contents of TN are: Low hill area > dyke area > hills and valleys >Tai Lake and Gehu plain area > area along the lake.2. Study on the change of SOC of paddy soils in village level.A. Increase of SOC and TN was observed in all the 3 villages. Land ownership and land frag-mentation had profound impact in the content of SOC as shown by the data of Banqiao village. Cropping system also influences the sequestration of SOC. Planting green manure raised SOC. And straw return, which was not popular at the region, was not very obvious in raising SOC in the Red soil region.B. Cropping influenced on the contents of SOC and total nitrogen. Content of SOC and TN in double rice system is higher than that in single rice system, however the content of SOC of paddy soil is significantly higher than that of dry cropland.