Carbon And Nitrogen Cycling In Paddy Soils In The Subtropical Region Of China

Rice cultivation systems are the most important production system in subtropical region of China. The turnover of organic C and N in paddy soils is different from that in the uplands. Studies on the accumulation, driving mechanisms and simulation of soil organic C and N have major significance for the optimal management of paddy soils. With data from typical regions, history data, and long-term field experiments, the regional differences, the changes of soil organic C (SOC) and total N (TN) in the last 30 years and driving mechanisms were discussed. The soil C and N cycling (SCNC) model was validated using data from long-term field experiments, then the model was used to estimate and predict the carbon storage potential in paddy soils. The main results were as follows:(1) There were significant differences in SOC and TN accumulation in paddy soils between different regions. SOC and TN contents of plain region were higher than those of hilly regions. The regional discrepancies of soil microbial biomass C and N (SMBC and SMBN) were different with SOC and TN, Dacai region (hilly) has the highest SMBC and SMBN. Landform was the main factors that that caused the regional discrepancy of SOC and TN. Due to the sensitivity of soil microbes, soil microbial biomass (SMB) could not reflect the effects of stable factors (climate, landform, tillage history) on soil organic C and N cycles. Rice cultivation enhanced the accumulation of SOC and TN significantly (Compared with dryland, increased by 18%～65%). The enhancement was varied with the landform. The difference between paddy soil and dryland in the plain area was lower than that in hilly area. Rice cultivation also increased the SMBC and SMBN, but the enhancement of SMBC was far stronger than SMBN.(2) SOC contents of paddy soils increased by 35.6% from 1979 to 2003 in Pantang region, which are higher than of dryland (27.5%). Compared with 1979, TN content of paddy soils in 2003 increased 12.8%, which are also higher than for dryland (9.8%). Long-term field experiment in Shaoyang also showed that SOC contents in paddy soils increased by 10%～45% in 13 years (1991～2004), whereas TN showed little enhancement, especially after 1994. This shows that in the last 30 years, paddy fields in subtropical China have acted as a "sink" for CO₂. Changes of rotation, addition of fertilizer, new genetype adoption and the enhancement of rice production are the main reasons for the accumulation of SOC and TN in paddy soils.