Assessing Impacts Of Soil Erosion On Soil Quality In Sloping Farmland Of Chinese Mollisol Region

Revealing the impacts of soil erosion-deposition on soil quality can provide an important guidance for Chinese Mollisol protection and utilization.However,the impacts of soil erosion on soil degradation degree in farmland remain unclear,so it’s difficult to regulate the issue of soil degradation caused by soil erosion.Thus,this study,taking two typical sloping croplands with a hundred years of cultivation in Keshan County of Heilongjiang Province as a case study,used combined methods of field survey,¹³⁷Cs and ²¹⁰Pb_ex tracer techniques,soil properties analysis and statistical analysis to evaluate the impacts of soil erosion-deposition on soil quality.The specific objectives of this study were to determine the slope spatial distribution characteristics of soil erosion-deposition and classify soil erosion intensity grades,to analyze the effects of slope erosion-deposition on soil nutrients,enzyme activities and microbial properties,and to assess the impacts of soil erosion-deposition on soil quality in farmland.The research results can deepen the understanding of the mechanism of soil erosion degradation,and provide scientific basis for improving soil quality in Chinese Mollisol region.The main conclusions were as follows:（1）The slope spatial distribution characteristics of soil erosion-deposition were determined,and soil erosion intensity grades were classified.¹³⁷Cs tracing result showed that slope erosion-deposition rates ranged from-1563.2 to 7916.1 t·km^-2·a^-1（+represents erosion rates and-represents deposition rates）,with the average erosion rates of 2871.4 and 2432.8t·km^-2·a^-1 in the two study sites,respectively.²¹⁰Pb_ex tracing result showed that slope erosion-deposition rates ranged from-994.8 to 10914.5 t·km^-2·a^-1,with the average erosion rates of3507.6 and 3274.2 t·km^-2·a^-1,respectively.Both ¹³⁷Cs and ²¹⁰Pb_ex tracing results indicated that soil erosion intensity in the study area was mainly light to moderate grade,with the proportion of sample points being 54.3%-73.3%in this study.The slope distribution characteristics of soil erosion-deposition were obvious,which displayed that the light and moderate erosion intensity grades were located at the upper slope position,the intense and severe erosion intensity grades occurred at the middle slope position,and deposition mainly took place at the slope toe.In addition,there was a significant positive correlation between soil erosion-deposition rates estimated by ¹³⁷Cs and ²¹⁰Pb_ex tracer techniques（P<0.05）,but soil erosion rates estimated by²¹⁰Pb_ex tracer technique were higher than those estimated by ¹³⁷Cs tracer technique,reflecting the increasing trend of average soil erosion rates in the last 20 a.（2）The effects of slope erosion-deposition on the spatial distributions of soil nutrients were analyzed.In general,the slope spatial distributions of soil nutrient contents and their stoichiometric ratios showed a decrease with the increase of soil erosion intensity grade.Among them,the spatial patterns of soil organic carbon and total nitrogen contents were opposite to soil erosion-deposition rates.Soil organic carbon contents decreased by 11.2%to11.6%in slope severe erosion positions compared to light erosion positions,and significantly decreased by 16.7%to 18.3%compared to deposition positions（P<0.05）.Soil total nitrogen contents decreased by 4.3%to 10%in slope severe erosion positions compared to light erosion positions,and significantly decreased by 12%to 18.2%compared to deposition positions（P<0.05）.Soil nutrient contents and their stoichiometric ratios presented a decreasing trend with an increase in soil erosion rates or a decrease in soil deposition rates.Especially,the contents of soil organic carbon and total nitrogen as well as the ratios of carbon to nitrogen showed a significant negative correlation with soil erosion-deposition rates（P<0.05）.In addition,different soil nutrients had different sensitivity ranges in response to soil erosion-deposition rates.The sensitive ranges of soil organic carbon and total nitrogen contents to soil deposition rates were-1000 to-200 t·km^-2·a^-1,and the sensitive ranges of both to soil erosion rates were 500 to 1600 and 8000 to 8800 t·km^-2·a^-1.（3）The responses of soil microbial biomass and enzyme activities to slope erosion-deposition were clarified.Overall,soil microbial biomass carbon andβ-1,4-N-acetylglucosaminidase activities decreased by 7%and 11.9%in slope intense erosion positions compared to light erosion positions,and significantly decreased by 11.2%and 30.6%compared to deposition positions,respectively（P<0.05）.Soil microbial biomass nitrogen and the ratios of enzyme carbon to phosphorus and enzyme nitrogen to phosphorus decreased by22.9%,2.4%and 1.9%in slope severe erosion positions compared to light erosion positions,and significantly decreased by 27.6%,7%and 10.5%compared to deposition positions,respectively（P<0.05）.On the contrary,soil acid phosphatase activities increased respectively by 5.4%and 15.8%in slope severe erosion positions compared to light erosion and deposition positions.Otherwise,soil microbial biomass carbon,the ratios of enzyme carbon to phosphorus and enzyme nitrogen to phosphorus showed a significant negative correlation with soil erosion-deposition rates（P<0.05）,and their sensitivity ranges in response to soil erosion-deposition rates were different.（4）Soil microbial nutrient limitations and their responses to slope erosion-deposition were identified.The vector model of enzymatic stoichiometry indicated that soil microorganisms were strongly limited by relative carbon and phosphorus in farmland of the study area.The vector length showed a decrease with an increase in soil erosion rates or a decrease in soil deposition rates,and its sensitive ranges to soil erosion rates were 500 to 1300,1600 to 2500,3300 to 4200 and 5200 to 8000 t·km^-2·a^-1.However,the vector angle was significantly positively correlated with soil erosion-deposition rates（P<0.05）,and its sensitive ranges to soil erosion rates were 1500 to 2500,3300 to 4200 and 8000 to 8800 t·km^-2·a^-1.（5）The impacts of soil erosion-deposition on soil quality were assessed.The total data set for soil quality evaluation contained sixteen soil property indicators.For the minimum data set,seven soil property indicators were selected,including soil organic carbon,total nitrogen,β-1,4-glucosidase,mineral nitrogen,available phosphorus,p H and clay.For the optimal data set,three soil property indicators were selected,including soil organic carbon,available phosphorus andβ-1,4-glucosidase.The soil quality index results obtained from three soil quality evaluation data sets were generally consistent,and their spatial distributions were opposite to soil erosion-deposition.Totally,the soil quality index results obtained from three soil quality evaluation methods decreased respectively by 2%to 26.5%,1.9%to 34.6%and6.3%to 33.3%in slope moderate,intense and severe erosion positions compared to light erosion positions;however,they increased significantly by 63.9%,79.4%and 84.4%in slope deposition positions compared to severe erosion positions,respectively（P<0.05）.Soil quality index results decreased significantly with an increase in soil erosion rates or a decrease in soil deposition rates（P<0.01）.The sensitive ranges of soil quality index to soil deposition rates were-1000 to-150 t·km^-2·a^-1,and the sensitive ranges of soil quality index to soil erosion rates were 2600 to 4000 and 5700 to 8800 t·km^-2·a^-1.In conclusion,soil erosion was an important factor causing soil degradation in sloping farmland of the study area.