Land Use On Soil Organic Carbon And Soil Respiration

Land-use change is a crucial factor in regulating soil C processes with a potential to mitigate the green house gases emission by fossil fuel combustion, thus has great influences on global carbon cycling and climatic change. Chongming Island, located at the estuary of Yangtze River, is the largest alluvial island of the world, with a relatively homogeneous soil and short history of soil development. The spatial variation of soil carbon cycling is mainly due to different land uses. It is an ideal place to study the effects of land use on soil carbon cycling and agricultural mitigation.Various sites at the east end of Chongmin Island of different land uses, including farmland (rotation of barley and paddy rice), tree nursery (Cinnamomum camphora and Koelreuteria bipinnata), Metasequoia glyptostroboides forest belt, orchard(Citrus reticutata) and fallow land (reclaimed in 1999 for fish farming then drained as a fallow land since 2006) were selected. All sites were reclaimed from tidal flat during 1960s to 1970s except the fallow land. Soil samples were collected at different depths in October 2007, March, June and September 2008 and analyzed in the laboratory to determine organic carbon components. Soil respiration was measured once a month from November 2007 to November 2008.The average contents of soil organic carbon (SOC), soil microbial biomass carbon (SMBC) and the ratio of SMBC/SOC, at 12.62 g/kg,225.34 mg/kg and 1.18% respectively, were significantly higher in farmland than others. The fallow land had the lowest SOC, SMBC and SMBC/SOC,7.17 g/kg,61.43 mg/kg and 0.90% respectively. There was no significant difference in SOC and SMBC contents between different depths of farmland soil (0-5cm,5-10cm,10-20cm), but declining with depth in other soils.Soil respiration rate varied with land use in the order: C. camphora plantation> fallow land> farmland> C. reticulata orchard> K. bipinnata plantation>M. glyptostroboides shelter belt (averaged at 3.02,2.78,2.77,2.00,1.76 and 1.65 umol·m-2·s-1, respectively). The monthly variation of soil respiration positively related to the change in soil temperature at 5cm depth. However, a low soil respiration rate was observed during June in all sites, reflecting the influence of high soil water content on the decomposition of organic carbon. After the harvest of barley, the land was turned into paddy land and soil respiration decreased dramatically, less than 10% of previous barley rotation.Our preliminary results suggest that agricultural cultivation can increase both soil respiration and SOC stocks in young soils of relatively low SOC, possibly by increasing aboveground productivity and changing the organic C distribution in soil profile and restricting the production and diffusion of CO2 because of irrigation. The emission rate of CO2 from forest soils was significantly smaller than that in cropland, and as well the SOC contents. A possible reason for the low soil respiration and SOC content in forest soils may be attributed to the fast decomposition of litter on the surface of the soil in this area. Reclamation from tidal flat causes the decline of SOC stocks. Arable soil under adequate tillage can increase SOC storage in moist subtropical region and mitigate the effects of human- induced C release.