Spatial Variation Of Soil Physical-chemical Properties And Stoichiometric Characteristics In Benggang System

Designated as "ecological ulcer" in eroded red soil region, the term of "Benggang", originated a Chinese pictograph of geomorphy, has been known to describe a kind of severely eroded ecosystem that gully head collapsed, transported and developed a deep-cut shape, concave-deforming deep-seated failure landform, eventually evolved higher stage of hilly and gully erosion, which consisted of representative subsystems, such as upper catchment, collapsing wall, colluvial deposit, erosional gully and alluvial. Mostly located in hilly area of Yangtze River Basin, Pearl River Basin and Southeast Coastal Rivers Basin, the total amount of Benggang of which collapsing area exceeded 60 square meters reached 2.39×105, and the total erosion area were also 1.22×103 km2 along with the average erosion modulus surpassing 5.90×105 t·km-2-yr-1 at national scale. With such characteristics of loose poor surface soil, strong acidity, sparse vegetation and a failure to afforestation, Benggang erosion together with debris flow and landside, the most main three hazards in mountainous region of China, results in retrogressing soil productivity, silting the waters, and causing deterioration of regional environment as well as restricting severely the regional sustainable development.Thus, Benggang was applicable to become a representative case of degraded terrestrial ecosystem. Soil stoichiometry played an important role in modulating ecological strategies of trophic structure, nutrient limitation and biogeochemical coupling of soil and plant combined system in terrestrial ecosystems. However, few studies investigated spatial variation of soil nutrients and their stoichiometric mechanism on the process of ecological restoration of degraded ecosystem in China, especially in Benggang system.Therefore, in this study, three typical Benggangs with vegetation coverage of 2%, 20% and 95%, respectively, in each Benggang seven systemic positions including upper catchment, collapsing wall ?top, middle and bottom part?, colluvial deposit ?upper and lower part? and channel outlet, respectively, were selected to investigate and measure soil properties including surface ?0—10 cm? soil bulk density and water content, practical composition, pH, soil organic carbon ?SOC?, total nitrogen ?TN?, total phosphorus ?TP?, total potassium ?TK?, ammonium nitrogen ?NHU+-N?, nitrate nitrogen ?NO₃^--N?, available phosphorus ?AP? and available potassium ?AK? in three soil depth ?ie.0—10 cm,10—20 cm, and 20—30 cm, repectively? in Huangniken Benggang group in Zhuotian Town, Changting County of southwestern Fujian. Besides, Soil stoichiometric characteristics in three Benggangs with different depth and locations, and spatial variation of soil physical-chemical properties and ecological stoichiometry characteristics were also analyzed.The results indicated that within Benggang system sand content?more than 50%?were of significant majority, while both of silt and clay content were quite low ?less than 30%?. Value of pH ranging from 4.56 to 5.73, showed middle-strong acidity. Values of soil bulk density at the depth of 0—10 cm were high, water content on the same layer were opposite, which the averages of bulk density and water content were 1.38 g/cm3 and 8.43%. Furthermore, variation coefficients of soil physical characteristics were lower than 36%, belonging of "light-middle level". In general, soil physical structure in Benggang erosion region was terrifically poor.The content of SOC, TN and TP remained rather low within Benggang system, their average values were 2.42 g/kg,0.33 g/kg and 0.06 g/kg, respectively, but the content of TK kept relative high with the mean value 4.85 g/kg. The diminishing order of content of soil available nutrients was AK>NH₄⁺-N>NO₃^--N>AP, and the average values were 25.07 mg/kg,7.75 mg/kg,1.03 mg/kg and 0.53 mg/kg, respectively.On the other hand, the sequence in stoichiometric relationship of soil nutrients was C:P ?mean value 34.76, the same below?>AN/AP ?21.02?>C/N ?5.50?>N/P ?5.26?>AN/AK ?0.49?>C/K ?0.47?>N/K ?0.07?>AP/AK ?0.03?>P/K ?0.01?. In contrast, variation coefficients of all indices of soil nutrients and stoichiometric ratios were higher than 36%, being of scope with "strong level", suggesting that there were remarkable spatial heterogeneity.As a gradual raise of vegetation coverage under three experimental Benggangs, in principle, composition of silt and clay particle, SOC, TN, TK, C/N, C/P, C/K, N/P and N/K increased, but bulk density, water content, sand particle, NFL4+-N, NO₃^--N, AP and AK decreasd while AN/AP and AN/AK decreased firstly and increased later assiocated with TP, P/K and AP/AK hardly being changed. Moreover, with increment of soil depth, the content of SOC, TN, NH₄⁺-N, NO₃^--N,AP, C/N, C/P, C/K, N/P showed a decrease tendency except for AK, all of soil physical properties were kept relative consistent. On the other hand, along erosion pathway from upper catchment collapsing wall, colluvial deposit, till channel outlet, bulk density, sand composition, pH, AP had a decrease tendency, and content of AK decreased firstly and increased later which changes on stoichiometry ratios of soil available nutrients with different locations were just opposite. However, C/N, C/P, C/K, N/P and N/K showed a fluctuating trend in which decreased first, then increased and decreased finally with different erosion locations.There existed significant correlation relationship among soil physical-chemical properties within Benggang system.Moreover, C/N, C/P, C/K, N/P and N/K were closely related to clay particle, SOC, TN and TK while P:K were regulated dominantly by sand and AP content; meanwhile, AN/AP and AN/AK mainly related with silt, clay, NH₄⁺-N, NO₃^--N, and there was non-significant scarely liner correlation between AP/AK ratio and soil property.