Studies On Difference Of Carbon Source Or Sink Contribution In Subtropical Cropland Based On Various Soil Types And Land-uses

In this paper, taking two counties (Pucheng located in mid-subtropical and Tong'an situated southernsubtropical) in different geographical locations from Fujian Province in subtropical regions as the study area,the database of1:50000for soil-land use was established according to the measured values of17743samplingpionts at year1982and6596sampling pionts at year2008. The spatio-temporal characteristics of soil organiccarbon density (SOCD) and soil organic carbon storage (SOCS) under soil types (soil groups, soil subgroups,soil genus and soil species) and land-uses (irrigated paddy fields, rain fed paddy fields, vegetable plots and dryland) in past over20years were researched deeply, at the same time, the differences and causes based on thechange of SOCD in mid-subtropical and southern subtropical were also analyzed from the angle of soil typesand land-uses, thereby providing the theoretical foundation for the development of organic carbonmanagement measures and upgrading. The main research conclusions were as follows.1. In1982and2008, SOCD of farmland in Pucheng County was between0.58-6.84kg C m-2and1.19-4.93kg C m-2respectively, and the area-weighted average SOCD were3.37±0.78kg C m-2and3.31±0.45kg C m-2respectively. The SOCD of farmland in Tong'an District was between0.80-4.03kg C m-2and0.57-2.95kg C m-2respectively, and the area-weighted average SOCD were1.79±0.42kg C m-2and1.58±0.32kg C m-2respectively. In terms of soil types, the soils with high curing, poor ventilation or in high elevationzones (yellow soil) had higher SOCD. On the other hand, the soils with low curing or partial sand had lowerSOCD. Seen from the land-use types, the highest SOCD was irrigated paddy field, rain fed paddy fieldfollowed, vegetable plot and dry land lower.2. The SOCD of farmland in Pucheng County and Tong'an District reduced by0.06kg C m-2and0.21kgC m-2respectively, indicating that both of the cropland were carbon sources on the whole, however, the extentof the change about SOCD was different in various soil types and land-uses.The soils along the river(yellowsand soil, black sand soil, etc) increased more in Pucheng County.Yet the soils by dry farming and curingreduced more in the study area.Seen from the land-use types, the SOCD of vegetable plot increased largest by0.71kg C m-2, rain fed paddy field increased slightly,while others reduced and the largest decrease was dry land.The SOCD of all the land-uses types reduced in Tong'an District and the largest decrease was also dryland which reduced by0.28kg C m-2.3. The SOCS in the study area was mainly controled by the area of cropland, during the past over20years,the area of cropland reduced by6530.88hm2and4385.79hm2respectively in Pucheng County and Tong'anDistrict, resulting that the SOCS in Pucheng County reduced by239588.04t from1352528.76t to1112941.05t and the SOCS reduced120722.43t in Tong'an District.The change of SOCS based on various soil types andland-uses was also mostly influenced by the area of cropland, having an increasing or decreasing in differentdegrees.4. The SOCD in Pucheng County was obviously higher than Tong'an District, but the decomposing rateof soil organic matter in Tong'an District was faster, which was related to climate conditions and croplandmanagement measures.With Pucheng representing mid-subtropical and Tong'an representing southernsubtropical by up-scaling, the changes of SOCD in mid-subtropical and southern subtropical were comparedfrom the angle of soil types and land-uses. The results indicated that the change was significantly different invarious climatic zones: the soils with partial sand or low curing (percogenic paddy soil, submergenic paddysoil and bleached paddy soil) or located on either side of river (fluvo-aquic soil and grey sand fieid), vegetableplot and rain fed paddy field were carbon sink in mid-subtropical, while those soils were carbon source insouthern subtropical. These soils had larger differences of carbon source or sink contribution in varioussubtropical cropland, nevertheless, the soils with partial viscosity or high curing, irrigated paddy field and dryland had smaller differences.