| Karst rocky desertification is a kind of land desertification associated with fragile ecological geological background and human activities in karst region of Guizhou Province, China. The core of karst rocky desertification is human irrational land utilization which leads to land quality decreased, even deserted. The karst soil physical and chemical properties changes and their association with karst rocky desertification are the basic and important segment of the study on the rule and dynamic mechanism of rocky desertification. The Maolan national nature reserve of Libo County and Houzhai subterranean stream basin of Puding County with the typical characteristics of rocky desertification in Guizhou Province were chosen as the study areas. The soils in different rocky desertification process were studied. The characteristics of soil moisture characteristics, soil organic carbon (SOC), soil total nitrogen (N), soil organic carbon fractions, soil low-molecular-weight (LMW) organic acids and pollution element-fluorine (F) were analyzed, and the effects of rocky desertification on soil quality attributes were discussed. The substantive characteristics of karst rocky desertification from the angle of iron oxide were studied. The purpose of this study is to provide scientific basis for ecological system recovery of rocky desertification and ecological restoration of degraded soil environment quality.The results were as follows:(1) Soil moisture characteristics were influenced by rocky desertification in karst region. With intensifying rocky desertification, soil saturated hydraulic conductivity, soil water content and soil water capacity trended towards decrease. Soil water characteristic curve in study areas showed steep in the beginning and slower in the end. The field water capacity ranged from 341.5 g·kg-1 to 587.8 g·kg-1, soil available water capacity ranged from 270.6 g·kg-1 to 435.8 g·kg-1, and soil saturated hydraulic conductivity ranged from 27.2×10-4cm·s-1 to 50.8×10-4cm·s-1. With intensifying rocky desertification, soil field water capacity and available water capacity trended towards decrease, and soil water content under different soil water suction decreased in the study areas. Karst rocky desertification led to the decline of soil water-holding capacity and soil moisture transport capacity. With intensifying rocky desertification, soil moisture total capacity, water reserving capacity and available water capacity decreased. The main impact factors on soil moisture characteristics were soil organic carbon content, soil particle composition, soil bulk density and soil porosity.(2) Soil organic carbon and nitrogen contents declined with intensifying rocky desertification in karst region. The SOC content was ranged from 14.89 g·kg-1 to 67.30 g·kg-1, with an average value of 35.49 g·kg-1. With intensifying rocky desertification, the content of SOC, the content of light fraction organic carbon (LFOC) and the content of heavy fraction organic carbon (HFOC) trended downwards, and the renewal of soil carbon pool reduced. The variability ranges of SOC in the phase of non-degraded and potential rocky desertification were higher than those in light rocky desertification and moderate rocky desertification. The SOC refresh rates in the phase of non-degraded and potential rocky desertification were 19% and 20% respectively and those in light rocky desertification and moderate rocky desertification were 11% and 14% respectively. The soil total N content in karst region was ranged from 1.82 g·kg-1 to 5.42 g·kg-1, with an average value of 3.26 g·kg-1. With intensifying rocky desertification, the content of total N decreased, and The temporal variabilities of soil total N reduced. The contents of soil total N, SOC in summer and autumn were lower than those in winter as to high temperature and humidity in the study areas.(3) Soil organic carbon fractions were significantly influenced by rocky desertification in karst region. With intensifying rocky desertification, the contents of soil organic carbon fractions decreased. There was significant influence of karst rocky desertification on. The sequence of the contents of soil organic carbon in karst region was humin (HM) carbon> fulvic acid (FA) carbon> humic acid (HA) carbon.The value of FA/HA was currently less than 0.4. The fulvic acid soils in karst soils were low humification degree. With the development of rocky desertification, HM carbon, FA carbon and HA carbon in soil were decreased. But HA/FA showed increasing in the beginning and decreasing in the end with the development of rocky desertification. The stability of organic carbon in the light rocky desertification was maintained by humification. Soil organic carbon content and soil texture were the important impact factors to the soil organic carbon fractions in karst region.(4) Soil LMW organic acids were significantly influenced by rocky desertification in karst region. With intensifying rocky desertification, the concentrations of soil LMW organic acids decreased. The concentration of total LMW organic acids in topsoil solution ranges from 0.358μmol·g-1 to 1.823/μmol·g-1, with an average value of 0.912μmol·g-1. The concentration in acetate and formate among different types of LMW organic acids were the highest. The lactate, oxalate and malate were followed respectively. The mean concentrations (μmol·g-1) of lactate, acetate, formate, malate, and oxalate were 0.212±0.089, 0.302±0.228,0.301±0.214,0.014±0.018 and 0.086±0.118, respectively. Significant positive correlations were also observed among individual LMW organic acids in soil solution, and between them and soil available P, available K, exchangeable Ca, inorganic anions (chlorides, nitrates, and sulfates) in karst topsoil solution, respectively. Soil LMW organic acids play important roles in the corrosion process of limestone and the cycling of nutrients in karst regions. Soil organic carbon content was the dominated factors to the soil LMW organic acids concentration in karst soil solution.(5) Soil iron oxide was influenced by rocky desertification in karst region. With intensifying rocky desertification, the content of total iron, the content of free iron oxide and the degree of free iron trended increasing, and the degree of active iron was decreased.The soil total iron content in karst region was ranged from 38.9 Fe2O3 g·kg-1 to 53.9 Fe2CO3 g·kg-1. The free iron oxide content was ranged from 18.0 Fe2CO3 g-kg-1 to 26.7 Fe2O3 g·kg-1. The degree of free iron was ranged from 44.1% to 73.4%, and the degree of active iron was ranged from 7.2% to 11.4%. With the increase of soil profile depth, in karst non-degraded soil with primeval forests, the content of total iron trended increasing, but the content of free iron oxide and the degree of free iron trended decreasing. The rocky desertification process in natural natural eco-system showed obvious water erosion features on surface layer. With the increase of soil profile depth, in the light rocky desertification soil in agroecological system, the total iron and free iron oxide were the richest in subsoil layer, and showed obvious leaching and deposition features, but the degree of free iron trended increasing. The rocky desertification process in agroecological system showed obvious leaching and deposition features, and was "soil loss" process.(6) Soil F content was influenced by rocky desertification in karst region. With intensifying rocky desertification, the content of T-F trended increasing in the initial rocky desertification phase and then the content of T-F trended decreasing. The content of Ws-F generally trended decreasing with intensifying rocky desertification.The content of soil total F (T-F) in karst region was ranged from 45.2 mg-kg·1 to 97.7 mg·kg-1, with an average value of 65.6 mg·kg-1, and the content of Ws-F was ranged from 0.38 mg·kg-1 to 3.12 mg·kg-1, with an average value of 1.44 mg·kg-1. According to critical level of soil water soluble F (Ws-F), the studied soils in Puding County were polluted, and those in Libo County were unpolluted. The soil F showed obvious leaching features in soil profile, and the contents of soil F in bottom layer were higher. The soil and water erosion process in surface layer could increase fluoride pollution. The soil parent material, biological action and surface charge characteristics of inorganic colloids were the major impact factors to the content of soil T-F. The surface charge characteristic of inorganic colloids was the major impact factor to the soil Ws-F.(7) Guizhou Province is in a subtropical humid monsoon climate, and has a favorable climate. But unreasonable land use had led soil quality degradation, decreased soil moisture, soil physical properties and soil fertility, worsened vegetation survival foundation environment, and hindered the regional ecological restoration. Soil is an important medium between living and non-living. The first question of rocky desertification administration is to improve soil quality and to provide basis material conditions for vegetation restoration by decreasing soil erosion to increase soil amount and to improve soil moisture conditions and soil fertility with strengthen management. |
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