The Characteristics And Assessment On Soil Quality Of Dam Land And Terrace In Loess Plateau

Dam land and terrace are basic farmland in hilly-gullied and gullied of Loess region and considered as life farmland. Good and bad on soil quality of them is the decisive factor of regional agriculture and economic sustainable development. This research aimed at the soil quality problem of dam land and terrace, took the Nianzhuanggou watershed of Yan'an City (dam land and terrace) and Nihegou wateshed of Chunhua county(terrace), Shaanxi province, as the research object, took the collection of materials and data, field investigation, laboratory analysis on soil physical, chemical and biological indicators as a breakthrough point, used methods of variance analysis, correlation analysis, factor analysis and stepwise regression analysis, and systematic research on the basic feature, spatial and temporal distribution and evolvement, correlation, indicator system and comprehensive assessment of soil quality were finished. The main research results were as follows:1. Coarse silt content is the most(about 60%) and coarse sand content is the least(about 5%) in mechanical composition. The microaggregate content of 0.01～0.25mm and <0.01mm accounts for more than 80% and about 10%, respectively, and the content of clay, CaCO₃ and organic matter(OM) had significant infiuence(p<0.01) on microaggregate content. The total content of water-stable aggregate is low(less than 12%) and their water stability is bad, and influenced significantly by organic matter and physical clay(p<0.01). As a whole, texture coarseness, aggregation rate and mean weight diameter(MWD) of Chunhua terrace(CT) was the best, and that of Yan'an dam land(YD) and Yan'an terrace(YT) was very small difference. The content of aggregation rate, total aggregate and MWD showed decrease of top-down trend in vertical profile, but had no significant differences in profile. Soil water content YD>CT>YT, and it was influenced significantly by physical clay(p<0.01). Dam land possessed better moisture condition that ensured crops to provide stable and high yields. Cyclothemic hiberarchy of profile in dam land was extremely favorable to preserve soil water and fertility and to increase erosion resistance, this structure illustrated ecological change footprint of particle coarsening and erosion resistance weakening.2. The content of various soil nutrients in topsoil had great differences in study area. The content of total P(TP) and various soil K only was rich in study area, and other content of soil nutrients belonged to "middle" or "low" level, among them, available N(AN) was lack only in CT, and they all lack in YD and YT. High total quantity of nutrient did not mean enough effective nutrients. CEC content of CT was one grade(high), and that of YD and YT was three grade(low). CaCO₃ content all belonged to six grade(high). pH value was alkaline in YD and slightly alkaline in CT and YT.The content of OM, various nitrogens(except NO₃^--N), available K(AK), readily available K(RAK), available P(AP) and CEC in topsoil was highest and their significant differences were found between topsoil and subsoil, but there was no obvious differences in subsoil, the content of other indicator had no significant differences in profile. In inter-area, the content of each indicator(except AN) of terrace in Chunhua was the highest. In intra-area, the content of each indicator of dam land in Yan'an, except AP, CEC, slowly available K (SAK) and CaCO₃, was larger than that of terrace in Yan'an. All chemical properties(except pH) were obviously assembled in fine particles. Water and soil loss meant loss of nutrient.3. Effected by habitat condition and own sensitiveness of biological properties, differences of biological indicator in inter-area had no consistent law. The content of enzyme activities, microorganism quantities, respiration intensity, microbial biomass C(MBC), MBN and MBP showed decrease of top-down trend in vertical profile, among them, the quantities of three major microorganism dropped suddenly and showed extreme sensitiveness of habitat change. 5 kinds of enzyme activity, MBC, MBN are closely related to OM. Microorganism quantities showed bacterium(10⁷)>actinomyces(10⁴)>fungi(10³), among three, bacterium's quantities accounted for more than 99% and it played a decisive role in the decomposition of OM. Although there was poor content of MBC, MBN and MBP, their biological validity was very obvious. Bacterium's quantities and MBC's content can better reflected erosion process.4. The overwhelming majority of soil physical, chemical and biological quality indicators increased incipiently and decreased subsequently with the increasing of dam land and terrace use years. They have been degenerate state and degradation will be lasted. As a whole, The critical year of each indicator of terrace in Chunhua hung behind that of dam land in Yan'an, and dam land in Yan'an hung behind terrace in Chunhua.5. Significant or very significant correlations were showed among 10 soil physical indicators(except bulk density), especially physical clay. The whole correlations of 17 soil chemical indicators were better, especially OM. The results of path analysis of chemical indicators showed that TN was mainly determined by OM and C/N, AP was mainly determined by CEC, TP and OM, the order of direct influence on RAK was OM>CEC>SAK>TN, and CEC was mainly influenced by OM, AP, TK and SAK.Protease had antagonistic effect on other enzyme, significant correlations were found among other 4 enzyme activities, especially invertase activity. There were better correlations between bacterium, fungi and urease, alkaline Phosphatase, invertase, respectively. Catalase and protease are closely related to actinomyces. The influence degree of biological indicators on MBC was MBN>invertase>protease>bacterium, the influence order on MBN was MBO actinomyces>invertase>protease, and the influence order on MBP was fungi>urease>alkaline Phosphatase. The results of path analysis on chemical and biological indicators showed that the order of direct influence on OM was invertase>catalase>actinomyces, the direct influence degree of invertase on AN was largest, the direct influence on AP was MBN>urease> invertase, and the direct influence degree of protease on RAK was largest.6. 13 comprehensive assessment indicators of soil quality were screened out in study area and these indicators included physical clay, texture coarseness, water content, TK, OM, AN, TP, electrical conductivity, CaCO₃, protease, alkaline Phosphatase, bacterium and fungi. Soil quality of CT belonged to "higher" level and was grade two, and that of YD and YT belonged to "middle" level and was grade three. In Yan'an study area, soil quality of dam land was better than that of terrace slightly. Regression computation model and evolutional model changing over time for soil quality comprehensive assessment were established. Comprehensive soil quality have been degenerate state and degradation will be continued, and the critical year was about 33 years in CT and 40 years in YD and 22 years in YT.