Spatial Variation Of Soil Organic Carbon Sequestration Capacity Under The Condition Of Slope In The Hilly Region Of Loess Plateau

[Introduction]Loess Plateau in the Northwest of China covers an area of some640,000km~2. It issuffering from severe soil erosion and soil degradation. Conversions of cropland intowoodland or grassland are known to improve soil organic carbon (SOC) stocks and soilerosion. Little is known about SOC spatial variations and dynamics under conversion ofcropland into woodland or grassland. To understand large-scale ecological and policyimplications of cropland abandonment, it is critical to understand the spatial pattern andvariations of SOC and their relationship with climate, vegetation and soil erosion, as well asdevelops effective approaches to estimate SOC over large landscapes and regions.[Objectives]The goal of this study was to improve our understanding of the spatial pattern of SOCand their controlling factors in loess landscapes and develop methods for estimating the soil Csequestration under cropland abandonment. Specific objectives of this study were to:(1)Characterize SOC spatial distribution;(2) Quantify the spatial variations in soil Csequestration; and (3) Assess the influence of climate, vegetation and erosion modulus factorson soil C sequestration in the hilly region of Loess Plateau.[Material and methods]In the hilly region of the Loess Plateau, three regions (i. the loess ridges and wide gully(RWG); ii. the loess rounded knolls and loess ridges (RKR); and iii. the sand-covered loessrounded knolls (SRK)) were examined. Six small watersheds with different patterns ofagriculture abandonment, varying soil erosion, and differing climates were selected in thethree geomorphologic regions. We surveyed a diverse set of sites under different land use onsloping land at each watershed to examine changes and spatial controls on soil carbon poolsand soil C sequestration at a regional scale. Totally,695soil samples taken from woodland,grassland and cropland were analyzed for SOC by the wet oxidation technique,Walkley-Black method. Soil organic carbon density (SOCD) was calculated usingbulk-density relationships. The SOC increment percentage was estimated by the differencebetween woodland or grassland and cropland divided by the cropland average. The SOCD increment rate was calculated by the difference between the woodland or grassland and thecropland. The data about climate and soil erosion modulus were collected from the stations ofChinese Ecosystem Research Network.[Results]Across the hilly region of Loess Plateau, SOC in sloping land ranged from1.67g/kg to29.55g/kg with an average of7.39g/kg and a coefficient of variability (CV) of56%. Thehighest variability of SOC was in the woodlands (CV=66%), the next in the grasslands (CV=55%) and the lowest in the croplands (CV=42%). For the croplands, SOC in RWG, RKR,and SRK area was7.04g/kg,4.44g/kg, and4.03g/kg, respectively. For the grasslands, SOCin RWG, RKR, and SRK area was11.65g/kg,6.57g/kg, and4.41g/kg, respectively. Forwoodlands, SOC in RWG, RKR, and SRK area was10.68g/kg,9.25g/kg, and4.51g/kg,respectively.There was high variability in the SOC increment percentage and SOCD increment rateacross the hilly region. For the conversion of cropland into woodland, the SOC incrementpercentage was108.3%,49.5%, and11.5%in the RKR area, RWG area and SRK area,respectively, and the SOCD increment rate1.61kg/m~2,0.57kg/m~2and0.45kg/m~2,respectively. For the conversion of cropland into grassland, the SOC increment percentagewas66.3%,48.0%, and9.9%in the RKR area, RWG area and SRK area, respectively, and theSOCD increment rate1.45kg/m~2,0.53kg/m~2and0.08kg/m~2, respectively.The annual precipitation contributes little to SOC spatial pattern across the hilly region.There were no significant relationship of annual precipitations to the SOC incrementpercentage and the SOCD rate across the hilly region. Temperature gradient significantlyinfluences SOC spatial distribution across the hilly region. Under the cropland abandonment,there was a negative relation of annual temperature to the SOC increment percentage and theSOCD increment rate. However, with the increasing annual temperature, the decreases in theSOC increment percentage and the SOCD increment rate under the conversion of croplandinto woodland were lower than that under the conversion of cropland into grassland. Therewas a negative relation of soil erosion modulus to the SOC increment percentage and theSOCD increment rate. The decreases in the SOC increment percentage and the SOCDincrement rate under the conversion of cropland into woodland were higher than that underthe conversion of cropland into grassland.These results suggest that fundamental spatial differences exist between the soils that areuseful for agricultural purposes and the land that was converted into grassland or woodlandacross the three geomorphologic regions across in the hilly region of the Loess Plateau. Themean annual temperature, soil erosion modulus and vegetation strongly influence the spatial differences.