Vertical Distributions Of Soil Carbon And Water Contents And Their Controls Along Toposequences At Different Slope Aspects In The Typical Loess Region

The Loess Plateau,located in arid and semi-arid regions in northern China,is well known for having the deepest and largest area of loess deposit in the world.Vegetation rehabilitation in the Loess Plateau has greatly altered the rates of carbon sequestration and emissions,as well as accelerated the process of soil desiccation.These changes exert an important influence on the global carbon and water cycle as well as the climate change.Slope is the fundamental geomorphic unit of a watershed,a better understanding of the vertical distributions and the controls of soil carbon and water contents at different slope aspects is critical to evaluate the ecological benefits of“Grain for Green”Program.In this study,we investigated the profile distributions of soil carbon and water contents and their influencing factors in two toposequences at different slope aspects?i.e.,NE sequence in the northeast facing slope and W sequence in the west facing slope?with vegetation varying with altitudes in the Liudaogou watershed.The main results and conclusions were as follows:1 The measured soil properties were differentiated into four groups according to the coefficient of variations?CVs?within the 0-200 cm profiles:?1?the first behaving relatively conservatively?pH and bulk density?with CVs<10%,?2?the second showed low to moderate variation?silt,capillary porosity,total porosity,saturated moisture and magnetic susceptibility?with CVs varying from 4%to 29%,?3?the third exhibited moderate variation?clay,sand,non-capillary porosity and soil mass water content?with CVs ranging from 13%to 60%,and?4?the last showed moderate to high variation?saturated hydraulic conductivity?with CVs ranging from 48%to 168%.There are discrepancies of profile distributions of soil properties due to the effects of plant types and slope aspects.Soil bulk density?BD?,non-capillary porosity?NCP?and clay content increased with soil depth,while capillary porosity?CP?,total porosity?TP?,saturated hydraulic conductivity?K_s?and saturated moisture?SM?decreased with soil depth.Soil pH and the soil mass water content?SWC_m?increased initially and then decreased with soil depth,silt and sand contents showed the opposite trends and both fluctuated with soil depth.Soil magnetic susceptibility?MS?decreased in the surface layer and then fluctuated within the profile.Soil bulk density and porosity correlated well with most of the other physical and chemical properties?p<0.05 or p<0.01?,indicating that soil bulk density and porosity were the main indicators of soil physical properties.2 Soil organic carbon?SOC?contents along toposequences in Liudaogou watershed decreased with depth within 0-50 cm soil layer and then remained relatively constant below 50cm.The weight mean SOC contents in the 0-50 cm soil layer were significantly higher than that in the 50-200 cm soil layer?p<0.05?.However,there were no significant differences of the weighted mean SOC in the same depth?0-50 cm,50-200 cm?under either forestlands or grasslands among different slopes?p<0.05?.Compared with SOC,soil inorganic carbon?SIC?contents in different slopes were relatively higher and enriched in the different soil depths below50 cm.The weighted mean SIC contents in the 0-50 cm soil layer of W sequence were significantly higher than that in the 50-200 cm soil layer?p<0.05?.The weighted mean SIC contents in the same depth?0-50 cm,50-200 cm?in NE sequence were comparable in the forestlands and the grasslands?p>0.05?.In contrast,the weighted mean inorganic carbon contents in the W sequence were significantly higher in the forestlands than that in the grasslands?p<0.05?.The weighted mean inorganic carbon contents in the 0-50 cm under grassland was higher in the NE sequence than that in the W sequence?p<0.05?,while the weighted mean inorganic carbon contents in the 0-50 cm and 50-200 cm soil layer under the forestlands were lower in NE sequence than that in W sequence?p<0.05?.SOC contents under different vegetation types were negatively correlated with soil pH and BD,but positively correlated with K_s?p<0.05?.SIC contents were only positively correlated with clay content?p<0.05?.Soil carbon density within 0-2 m profile were 15.20⁴7.38 kg m^-2 and 18.29⁵1.28kg m^-2 in the NE sequence and W sequence,respectively,in which inorganic carbon density accounts for 78%⁹4%.Total carbon stocks within 1-2 m soil layer were 35%⁷4%of the stocks in 0-2 m soil profile.Our study indicates that neglecting inorganic carbon or neglecting deep soil carbon?1-2 m?would lead to underestimation of soil carbon stock by 88%and 51%respectively.A simultaneous consideration of organic and inorganic carbon in both shallow and deep loess profiles in the future study would enhance our understanding of carbon cycle on the Loess Plateau and improve the estimation accuracy of terrestrial carbon pools.3 Soil water contents?SWCs?in the 0-500 cm profile along toposequences at different slope aspects could be divided into three layers based on the standard deviation?SD?and coefficient of variation?CV?of SWCs:active layer?0-1 m?,sub-active layer?1-2 m?and relatively stable layer?2-5 m?.SWCs increased gradually with depth in the active layer with CV>20%,then decreased slowly in the sub-active layer with CV ranging from 9%to 17%,and finally kept stable in the relatively stable layer with CV<12%.The mean SWCs in the sub-active layer under grassland in both NE and W sequence were significantly higher than that in the active and relatively stable layer?p<0.05?.There were no significant differences of the mean SWCs in the active and sub-active layer under forestland in both NE and W sequence?p>0.05?,which were higher than that in the relatively stable layer?p<0.05?.The mean SWCs in the sub-active layer and relatively stable layer were higher under grassland than that under forestland?p<0.05?,while there were no significant differences of the mean SWCs in the active layer between grassland and forestland?p>0.05?.The mean SWCs in diverse soil layers for both grassland and forestland in the west-facing slope were significantly higher than that in the northeast-facing slope?p<0.05?,except for the SWCs in the sub-active layer?1-2 m?for the grassland?p>0.05?.SWCs in the upper soil layers?the active and sub-active layer?showed evident temporal variations during the study period,while the temporal changes of SWCs in the deeper soil layers?relatively stable layer?were limited.However,the SWCs in the deeper soil layers could also be affected by the strong rainfall.SWCs were negatively correlated with and BD,while they showed positive correlation with K_s?p<0.05?.CVs of SWCs correlated well with BD,NCP,sand content and magnetic susceptibility?MS??p<0.05?.These results indicated that the temporal variation of SWCs was not only affected by precipitation and evapotranspiration,but also affected by the basic soil properties.