Spatial Heterogeneity Of Paddy Soil Organic Carbon And Their Affecting Factors In Taihu Lake Region

Taihu Lake region is one of the main rice production regions in China,and the dynamic of soil organic carbon(SOC) for paddy soils is closely related to the global climatic change. The research on spatial heterogeneity of soil fertility indexes and SOC storage change as well as their affecting factors at different scales is very important to improve field management and water and fertilizer efficiency.Also,it will provide the theory basis for evaluating properly atmosphere carbon cycling.Based on a newly compiled 1:50,000 soil database,spatial heterogeneity of soil nutrient,the distribution pattern of SOC storage and SOC density as well as their affecting factors at different scales(the whole Taihu Lake region,cities and counties) and soil types were studied in this paper.1:50,000 soil database in the study area was constructed through integrating spatial and attribute data by Pedological Knowledge Based method(PKB).Soil spatial data was obtained from the maps over the 37 counties which were digitalized and edge matched.Soil attribute data was obtained from the Soil Species published in a series of monographs over the 37 counties in the Second National Soil Survey of China,which included soil texture (Sand,Silt and Clay),pH,soil organic matter(SOM),total nitrogen(TN),total phosphorus (TP),total potassium(TK),available phosphorus(AvP) and available potassium(AvK) etc. The database includes 12 soil groups,48 subgroups,and 1670 typical soil profiles.It contains 71763 polygons,in which paddy soil has the highest number polygons(52034), and accounted for 73%of the total polygons,followed by Chao soil and red soil,which accounted for 14%and 4%of the total polygons respectively.Based on the 1:50,000 database,the results are shown as follows:The range of AvP in Taihu Lake region is nearly 100 mg kg^-1,and the coefficient of variation is more than 100%,which belongs to the strong variability.The coefficients of variation for soil pH,SOM,TN,TP,TK and AvK are in the range of 10-100%,which belongs to the medium variability,pH mostly ranges from 5.5 to 7.5.Generally,SOM,TN, TP,TK and AvK in the plain area are fertile,while the soil TP,AvP and AvK in the hilly area are relatively low,which are under the deficient level in general.SOC storage of paddy soils at the depth of 0-100 cm and 0-20 cm are 251.66 Tg and 80.96 Tg,respectively,with the mean SOC density of 127.1 t C ha^-1 and 38.5 t C ha^-1.The SOC storage at the depth of 0-20 cm accounts for one third of that at the depth of 0-100 cm. Further,SOC density varies with different soil subgroups of paddy soils.For the 0-100 cm, Gleyed subgroup has the highest SOC density(200.0 t C ha^-1),followed by that in Degleyed subgroup(186.4 t C ha^-1),and Bleached subgroup has the lowest SOC density (73.9 t C ha^-1).As for the other soil subgroups,the mean SOC density ranged from 75 t C ha^-1 to 105 t C ha^-1.For the 0-20 cm,the highest mean SOC density is found in Degleyed subgroup(48.3 t C ha^-1),and the lowest value(25.8 t C ha^-1) is found in Bleached subgroup. The mean SOC density varies from 27 to 40 t C ha^-1 for the other subgroups.In terms of the impact of soil properties and environmental factors on SOC for paddy soils,soil clay can explain 12.7%and 10.2%of the variation of SOC density at the depth of 0-100 cm and 0-20 cm.Soil pH has a weakly correlated with the SOC density of paddy soil. SOC is most strongly correlated with TN(P<0.05),but no evidence shows that there is a significant relationship between TP or TK and SOC.Mean Annual Precipitation(MAP) and Mean Annual Temperature(MAT) can explain 4.2%and 2.4%of the variation of SOC density in the profile and the surface layer.But the smaller the region is,the less the effect of climate on SOC density variation is.However,effects of parent material and topography on SOC density variation vary in different soil subgroups of paddy soils.Soil texture,pH,TN,TP,TK,MAP,MAT,topography and parent material can explain 28.8%and 72.4%of the variation of SOC density in the profile and surface layer, respectively.The affecting factor of SOC density variation in the soil profile is in the order of TN>clay>topography>parent material>TK>pH value,while that in the surface layer is in the order of TN>parent material>topography>TK>clay.With the shrink of spatial scale,the numbers of the affecting factors on SOC density variation is decreasing,and vice versa.This means that the factors affecting on SOC density variation at a small scale is relatively simple due to the weak variation of environmental factors such as topography,parent material and climate condition.In contrast,with the enlargement of spatial scale,the affecting factors on SOC density variation become complex in the number,and the interaction between factors become stronger,but the impact of single factor on SOC density variation becomes weaker.