[Distribution and Dynamics of Cropland Soil Organic Carbon in Jianghan Plain: A Case Study of Qianjiang City].

Taking an example of Qianjiang City in Jianghan Plain, the distribution and dynamics of soil organic carbon (SOC) in croplands was studied in present study. The cropland included both paddy field and dry land. SOC contents were analyzed by taking soil samples of topsoil (0-20 cm) in 2011 according to land uses and soil types, and then compared with the initial SOC conducted in the period of the second soil survey (1983). The results showed that SOC density and storage in 2011 was 30.50 t . hm-2and 452. 82 x 10(4) t, respectively. During the past 28 years, the cropland SOC density was decreased at a rate of 0. 10 t . (hm2.a)-1, and SOC storage was reduced by 9% with the decreasing rate of 1. 53 t.a-1. SOC density and storage in paddy field was about 1. 6 and 1. 3 times over that in dry land in the two selected periods. However, the dynamics of SOC in paddy field and dry land were quite the opposite. In paddy field, SOC was lost by 16% (52. 83 x 10(4) t), with a decreasing rate of 0. 23 t . (hm2.a)-1; whereas in dry land, SOC was increased by 5% (8. 57 x 10(4) t), with an increasing rate of 0. 05 t . (hm2.a)-1. The loss of SOC in paddy field was mainly resulted from gleyed paddy soil, which suffered a fast decrease of SOC density and accounted for 80% of SOC lost in paddy field. In addition, Hydromorphic paddy soil, accounting for 50% of the area of paddy field, tended to loss another 15% of SOC in paddy field. While in dry land, the minor SOC storage increased was dominantly attributed to grey fluvo aquic soil, which accounted for 96% of the area of dry land. Thus, the dynamics of cropland SOC in Jianghan Plain was dominantly controlled by SOC changes in paddy field. Our findings suggest that effective management should be considered to enhance the capacity of SOC accumulation and sequestration in the low-yield paddy field and the types of soils that are large in area.