Study On Spatial Variation Of Soil Quality And Its Influencing Factors In Small Eroded Watershed In Mollisols Region Of Northeast China

Soil erosion and cultivation are the main factors driving the degradation of Mollisols of Northeast China.However,the current understandings of the dynamic mechanism of Mollisols fertility components and their interactions as affected by erosion and cultivation are still unclear,hindering the improvement of degraded Mollisols.We focus on the key scientific issues that the effects of soil erosion and cultivation on soil quality and the spatial distribution characteristics of soil quality in eroded small watersheds.Herein,we investigated the variations in soil properties as affected by different land-use types,cultivation time,ridge-tillage practices and the intensities of soil erosion at a slope scale.Additionally,we examined the spatial distribution characteristics of soil properties in a small watershed in the Hebei watershed of Heshan Farm of Heilongjiang Province.Forestland,shrubland and cropland are selected to address the effects of land use type on soil properties.Four fields,which included a forestland as control and three croplands that were converted from forestland 41,50 and 65 years ago,are selected to analyze the effects of cultivation time on soil properties.Contour farming and downslope tillage were selected to address the impacts of ridge-tillage practices on soil properties.We investigated the variations in soil properties at different slope positions that subjected to various intensities of soil erosion（non-erosion,light erosion,moderate erosion,and heavy erosion）and deposition positions along a maize field in the agricultural region.We investigated the influences of land use,cultivation time,ridge-tillage practices and soil erosion on spatial heterogeneity of soil physical and nutrient properties at the 0-100 cm soil depth and clarified the influence mechanism of soil erosion and cultivation on the degradation of Mollisols.We addressed the spatial distribution characteristics of soil physical and nutrient properties and soil quality at depth of the 0-30 cm in a small watershed.The main results and conclusions are listed as bellows:1.With increasing soil depth,in spite of the trend of soil total porosity,water content（SWC）,capillary moisture capacity（CMC）,field capacity（FC）along the 0-100 cm soil profile in croplands after different time of cultivation,soil bulk density（BD）increased but other soil indices and soil fertility indices（i.e.,soil nutrient fertility index（NFI）,environmental fertilizer index（EFI）and the integrated fertility quality（IFI））significantly decreased as affected by the different land use types,ridge-tillage practices,soil erosion and uncultivated forest at the slope scale.In a small watershed,soil clay and silt content,the proportions of>0.25 mm aggregates,mean weight diameter（MWD）and geometric mean diameter（GMD）were greater,but SWC,the proportions of<0.25 mm aggregates,soil structural stability index（St）,p H and the concentrations of soil organic carbon（OC）and nutrients were lower in deep soils（15-30 cm）than top soils（0-15 cm）.Furthermore,the spatial variability of SWC,p H,St and the concentrations of OC,total nitrogen（TN）,total phosphorus（TP）,NO₃^--N were also greater but the spatial variability of each aggregate and the concentrations of available phosphorus（AP）,available potassium（AK）and NH₄⁺-N were lower in deep soils than top soils.2.The SWC,BD and proportions of<0.25 mm aggregates were higher,but the total porosity,FC,CMC,K_s and St were lower along the 0-100 cm soil profile in cropland,compared with forestland and shrubland.The proportions of>0.25 mm aggregates were higher at the 0-50 cm soil depth but were lower at the 50-100 cm soil depth in cropland than in forestland and shrubland.Land use types significantly affected the concentrations and stocks of OC and nutrients and St regardless of the concentration of TP.The concentrations and stocks of OC and TN and St were higher,but the concentrations and stocks of AK,AP,NH₄⁺-N and NO₃^--N were lower in forestland and shrubland than those in cropland.The concentrations and stocks of soil OC,TN,AP,AK,NH₄⁺-N and NO₃^--N and St did not differ from forestland and shrubland.Additionally,land use type did not significantly influence the concentrations of OC and TN associated with aggregates,but significantly affected the stocks of OC and TN associated with aggregate,probably due to the contrasting responses of the soil aggregate proportions.3.Soil porosity,CMC,FC and K_s decreased by 5.1%,3.9%,14.2%and 40.9%at 0-50 cm soil depth as affected by the conversion of forest to cropland,respectively.However,the conversion of forest to cropland improved soil porosity,CMC,FC and K_s at 50-100 cm soil depth,and the improvement was enhanced with the cultivation time.The responses of BD to the cultivation time were opposite with soil porosity.Converting forest to cropland decreased the soil aggregate stability along 0-100 cm soil profile,as suggested by the decreases in MWD and GMD,which the decreases also enhanced with the cultivation time.Converting forest to cropland could increase the concentrations and stocks of AP,AK,NH₄⁺-N and NO₃^--N along such soil profile.The SWC,St and the concentrations and stocks of OC,TN and TP from 0-15 cm soils decreased with cultivation time after the conversion of forest to cropland within the last 50 years.Converting forest to cropland ameliorated subsoil（15-100 cm）moisture and St and increased the concentrations and stocks of OC,TN and TP from subsoil,especially for croplands with relatively longer time of cultivation.4.At 0-50 cm soil depth,soil porosity,CMC,FC and soil aggregates stability were higher in cropland using downslope tillage than those in cropland with contour farming on both the south-and north-facing slopes.However,soil BD had opposite responses to ridge-tillage practices and slopes in terms of total porosity.Compared with cropland using downslope tillage,the SWC in cropland with contour farming at 0-15 and 50-100 cm soil depths were higher on the south-facing slope with smaller slope,but the SWC and total porosity in cropland with contour farming along 0-100 cm soil profiles were lower on the north-facing slope with larger slope.However,at 0-100 cm soil depth,the St,K_s and the concentrations and stocks of soil OC and nutrients in cropland using downslope tillage were higher than those in cropland with contour farming on both the south-and north-facing slopes.5.Soil erosion could significantly reduce the SWC,FC,CMC,the proportions of<0.25mm aggregates,K_s,St and the concentrations and stocks of soil OC,TN,TP and AK,but increase the soil BD,the proportions of>0.25 mm aggregate and the concentrations and stocks of NH₄⁺-N and NO₃^--N.Furthermore,the effects generally enhanced with the intensities of soil erosion.Additionally,soil erosion also resulted in significant decreases of the concentrations and stocks of OC and TN associated with aggregates,and the decreases in the concentrations of OC and TN associated with aggregates dominated the decreases in their stocks.The effects of soil erosion on the concentrations of OC and nutrients in bulk soils and the concentrations of OC and TN associated with aggregates decreased with increasing soil depth.6.In the small watershed,the spatial distribution of the soil physical and nutrients properties at the depths of 0-15 and 15-30 cm were generally consistent with the spatial distribution of land use types and soil erosion.However,the area of spatial distribution of each soil index at the depths of 0-15 and 15-30 cm was correlated with the spatial variability of such soil property at each soil depth.The SWC was lower in heavy erosion area in cropland,forestland and shrubland than that in light erosion sites in cropland and depositional sites.The proportions of>0.25 mm aggregate and aggregates stability were higher in forestland,grassland,heavy erosion sites in cropland and depositional sites than other lands.Soil from heavy erosion sites in cropland and shrubland had the lower St and concentrations of soil OC,TN,TP and NH₄⁺-N,compared with forestland,grassland and depositional sites.On the small watershed scale,in spite of the positive correlation between the amount of soil erosion and>0.25 mm aggregates and between the amount of soil erosion and soil aggregates stability,there were the negative relationships of the amount of soil erosion to SWC,silt contents,the proportions of<0.25 mm aggregates,and the concentrations of soil OC and nutrients,respectively.The results indicated that soil erosion was main driving factor leading to the decrease in soil fertility in a small watershed.7.The principal component analysis and correlation analysis methods were used to evaluate the soil qualities at the slope scale and the small watershed scale.The NFI and IFI at 0-100 cm soil depth were 18.3%and 17.5%higher,respectively,and the EFI at 50-100 cm soil depth were 12.6%higher in cropland than those mean values in forestland and shrubland.However,the EFI at 0-50 cm depth in cropland was 3.6%lower,compared with forestland and shrubland.The NFI and IFI at 0-100 cm soil depth and the EFI at 0-50 cm soil depth were highest in cropland after 65 years of cultivation,were followed by those in forest and cropland after 41 years of cultivation,but were lowest in cropland after 50 years of cultivation.However,the EFI at depth of 50-100 cm increased with cultivation time.On both the south-and north-facing slopes,the NFI and IFI at 0-100 cm soil depth were 40%and 64.5%higher,and the EFI at 0-50 cm soil depth was 13.6%higher in cropland with downslope tillage,respectively,but EFI at 50-100 cm soil depth was 5.3%lower in cropland with downslope tillage,compared with cropland with contour farming.Soil erosion could significantly decrease the NFI,EFI and IFI,and the effects increased with the intensities of soil erosion.The NFI,EFI and IFI decreased by 33.3%,26.9%and 50%as affected by soil erosion,respectively,At the small watershed scale,the NFI,EFI and IFI at depths of 0-15and 15-30 cm decreased with increasing the amount of soil erosion.In this study,we addressed the effects of land use types,cultivation time,ridge-tillage practices and soil erosion on the physical and hydraulic properties and nutrient properties of Mollisols at the slope scale,and investigated the spatial distribution of the soil physical and nutrient properties in a small watershed.In addition,we analyzed the variability of soil quality at slope scale and small watershed scale,respectively.The results indicated that cultivation could deteriorate soil physical environment,but the input of fertilizer improved the soil nutrient fertility in cropland,thereby increasing the integrated fertility quality in cropland.The responses of soil quality to cultivation varied with the cultivation time.The soil quality at the 0-50 cm soil depth decreased with the increase of the cultivation time after the conversion of forest to cropland within the last 50 years.The effects of ridge-tillage practices on soil quality varied with slope position in sloping cropland.Although contour farming could decrease the losses of soil and water and nutrients,contour farming did not improve soil quality at upper slope position suffering from heavy erosion.However,soil erosion directly resulted in decrease in soil quality.According to the effects of such factors on soil quality at the slope scale and the relationship of the amount of soil erosion to soil quality at the small watershed scale,we concluded that soil erosion is the direct driving factor leading to the reduction of soil quality in a small watershed.We addressed the spatial variations of soil quality and its influencing factors at different size scales（slope and small watershed）and in different situations（land use types,cultivation time,ridge-tillage practices and erosion）in a eroded watershed in Mollisols region of Northeast China.The conclusions will provide scientific basis for the improvement of Mollisols fertility.