Distribution Of Particulate And Mineral-associated Organic Carbon In Cropland Soils And Its Influencing Factors In The Sichuan Basin Of China

The soil organic carbon（SOC）plays an important role in global C cycling and C-climate feedbacks.Cropland soils are considered to have huge potential to sequester carbon.However,the controls regulating SOC dynamics at different scales are not well understood,leading to high uncertainty regarding the estimation of future SOC stock changes in different soils and ecosystems under global climate change.In this study,we measured cropland topsoil two SOC fractions（particulate and mineral-associated soil organic carbon,POC and MAOC）content and spatial distribution characteristics using 871soil samples collected across the Sichuan Basin,which is one of the four major basins and the largest paddy and rapeseed producing area in China,with abundant arable land resources.We firstly assessed the effects of cropland type,mean annual temperature,mean annual precipitation,net primary productivity（NPP）,soil texture,topographical factors and chemical fertilizer amount on soil POC and MAOC by quantifying their relative importance,and then explored the association of the two SOC fractions with these variables within different cropland types.By these analyses,we aim to identify the dominant control of cropland soil POC and MAOC and to elucidate how environmental combined with anthropogenic factors affect cropland SOC fractions over broad geographic scales.The results show that:（1）The mean content of SOC in the topsoil layer is 14.79g kg^-1.The mean content of soil POC is 5.01 g kg^-1,while the mean content of soil MAOC is 9.79 g kg^-1.The ratio of POC to total SOC is 32.54%,indicating that the main component of SOC of cropland is MAOC.Both POC and MAOC correlate significantly with the total SOC and increase with total SOC content（p<0.01）.Soils with high SOC contents have larger proportions of POC.This suggests that developing effective measures to preserve and augment POC is important for sequestering C in cropland.（2）The sampling sites with higher POC(>6 g kg^-1)and MAOC(>12 g kg^-1)content are mainly distributed in the peripheral area of the basin,while the sampling sites with lower POC and MAOC content are concentrated in the middle of the basin.The sampling sites with high POC content are mainly distributed in the western part of the basin in Mianzhu County and An County,the southwest Ya’an City and Mingshan County,and the southern Junlian County and Xuyong County.The low-value zone(<3 g kg^-1)is mainly distributed in the central and northeastern parts of the basin.The content of MAOC is relatively high in the western Chengdu Plain,the southwest Muchuan County and Qianwei County,and the southern Junlian County and Xuyong County.The low-value area(<6 g kg^-1)is mainly distributed in the middle of the basin,and a small number of samples were distributed in the eastern part.There are also POC and MAOC high-value sample points in Guangyuan City and Wangcang County in the northeast of the basin.（3）The correlation between SOC fractions and natural and anthropogenic factors varies depending on the cropland type in the Sichuan Basin.The two SOC fractions in dryland and dryland-paddy rotation land are more vulnerable to external environmental variables.Mean annual temperature and NPP significantly affecte dryland soil POC and MAOC（p<0.01）,and there are significant correlations between them and mean annual precipitation,topographic factors,soil texture and chemical fertilizer amount（p<0.05）.For dryland-paddy rotation land,POC and MAOC are significantly correlated with mean annual temperature,slope,sand content,silt content and NPP（p<0.05）,and POC is also significantly correlated with chemical fertilizer amount（p<0.05）,MAOC is also significantly correlated with clay content（p<0.05）.While for paddy,SOC fractions are less affected by external environmental factors,and only correlate significantly with soil texture and NPP（p<0.05）.（4）Cropland type is the dominant control of the two SOC fractions in the Sichuan Basin.Cropland type,mean annual temperature and NPP are the main factors affecting cropland soil POC in Sichuan Basin,with relative importance of 62.80%,31.71% and 23.71%,respectively.The main influencing factors of MAOC are cropland type,mean annual temperature and sand content,and the relative importance is 58.79%,39.90%and35.18%,respectively.In contrast,soil texture has a greater influence on MAOC than POC.In summary,to accurately predicting future cropland SOC change and reliably evaluate the potential contribution of cropland soil C sequestration,fraction-specific control mechanisms related to cropland type should be sufficiently considered in carbon models.