Evolution Of Soil Fertility During Vegetation Restoration In Hilly Region Of Purple Soil Of Central-south Hunan Province,China

As the main driving force of vegetation restoration,the improvement of soil fertility is conducive to the growth and development of successive species and the formation of stable plant community.Due to the bad ecological environment,vegetation restoration in hilly region of central-south Hunan Province purple soil(referred to as hilly region of central-south Hunan purple soil)was extremely difficult.Hilly region of central-south Hunan purple soil was one of the key areas of ecological environment management in both Hunan Province and the whole country.At present,the related studies in purple soil mainly focused on the effects of vegetation restoration on runoff and sediment,nutrient loss and techniques of vegetation restoration.However,studies on the effects of vegetation restoration on soil carbon,nitrogen,and phosphorus stoichiometric ratios and soil comprehensive fertility are rarely reported.By using the method of space-for-time substitution,bare land(BL)and three types of natural vegetation communities at different vegetation restoration stages:Arundinella hirta-Paspalum orbiculare-Prunella vulgaris grassland(APP),Vitex negundoet-A belia Chinensis-Pyracantha fortuneana shurbbery(VAP),and Liquidambar formosana-Quercus fabri-Quercus acutissima deciduous broad-leaved forest(LQQ)were selected in hilly region of central-south Hunan purple soil.BL was used as a control to study soil carbon,nitrogen,and phosphorus stoichiometric ratios and the evolution of soil comprehensive fertility during vegetation restoration.Our objectives were to:(1)reveal the evolution mechanism of soil comprehensive fertility in hilly region of central-south Hunan Province purple soil;(2)provide scientific bases for vegetation restoration and reconstruction,soil matrix improvement and effective nutrient management in hilly region of central-south Hunan Province purple soil.The main results were summarized as following:(1)Along vegetation restoration,soil bulk density decreased generally,and in APP,VAP and LQQ were significantly lower than in BL.Soil moisture showed an increasing trend with vegetation restoration,and in APP,VAP and LQQ were significantly higher than in BL.Sand percentage at each soil depth decreased,while silt and clay percentage increased.Sand was still the dominant particle in BL,APP and VAP soil,while in LQQ silt was the dominant particle.Soil pH value ranged from 4.53 to 8.68.The contents of soil organic carbon(SOC),total nitrogen(TN),total phosphorus(TP),total potassium(TK),available nitrogen(AN),available phosphorus(AP),available potassium(AK)and activities of Alkaline phosphatase(ALP),Catalase(CAT),Urease(URE),Invertase(INV)increased during vegetation restoration.Moreover,the greatest increasing range was shown in LQQ or VAP,suggesting that soil physicochemical properties and enzyme activities can be improved significantly by vegetation restoration.Referring to the national second soil survey nutrient standards,soil nutrient of BL is at a very low level,that of APP and VAP are at a low to intermediate level,that of LQQ is at a more than middle level and that of the study area is generally at a medium level.(2)Soil bulk density was significantly and negatively correlated with soil moisture,INV and ACT.Soil moisture and texture were significantly correlated with pH value,SOC,TN,TP(except moisture),AN(except texture),AP,AK and INV(except texture).Except TK,there were significant correlations among soil pH value,SOC,TN,TP,AN,AP and AK.There were significant positive correlations among SOC,TN,AN,AK and INV,and between AN and ACP.URE was significantly and positively correlated with ALP,INV and CAT.The results indicate that soil physicochemical properties and enzyme activities have a relationship of mutual promotion.In addition,the improvement of soil physical properties is conducive to increase soil nutrients and enzyme activities,then the improvement of soil chemical properties and enzyme activities facilitate the improvement of soil physical properties in return.(3)Soil fertility of BL and VAP were limited by SOC,AN and AP,but APP and LQQ were limited by ALP and URE.Therefore,in order to promote vegetation restoration,for BL and APP,SOC,AN and AP were increased by soil matrix improvement,but for VAP and LQQ,soil fertility was improved by improving soil ALP and URE activities.Soil comprehensive fertility evaluation results of factor analysis method which based on full index,soil comprehensive fertility index method and grey relational degree method which based on minimum data set were consistent.The results were following the order:LQQ>VAP>APP>BL,and there were significant differences among different vegetation restoration stages.Significant correlations with each other among the three soil comprehensive fertility evaluation methods were detected,indicating that it is comparable and feasible to evaluate soil comprehensive fertility by using the three evaluation methods,and the minimum data set can well reflect the synthesis of all the indexes in this study.Soil comprehensive fertility was significantly and positively linear correlated with community biomass,while not significantly correlated with soil Shannon-Wiener index,suggesting that soil comprehensive fertility was significantly affected by community biomass.Therefore,promoting vegetation restoration,improving community structure and increasing community biomass are important ways to improve purple soil comprehensive fertility.(4)With vegetation restoration,soil C:N and C:P ratios of each soil depth increased dramatically,while the variations of N:P ratio at different soil depths were different,but different vegetation restoration stages were not significant.The values of soil C:N,C:P and N:P ratios in the study area were 3.95-10.74,11.21-14.52 and 1.46-3.94,respectively,which were lower than the national average level and forestland in red soil region of central Hunan Province.The results suggest that purple soil of central-south Hunan is characterized with fast mineralization,moreover,soil P availability is high,and N is relatively lack,thus,plant growth is mainly restricted by N.Therefore,in order to alleviate soil N stress and promote vegetation restoration,organic fertilizer can be applied timely and appropriately during vegetation restoration in purple soil of central-south Hunan.(5)Soil C:N and C:P ratios were strongly and negatively correlated with bulk density and sand percentage(except C:P ratio),but significantly and positively correlated with soil moisture,silt percentage(except C:P ratio),clay percentage(except C:P ratio),SOC,TN,TP(except C:P ratio),AN,AP,AK(except C:P ratio),ACP(except C:N ratio),URE(except C:P ratio),INV and CAT;N:P ratio was strongly and negatively correlated with silt percentage and TP,but strongly and positively correlated with soil moisture,sand percentage,AN,ACP,and INV,indicating that the changes of C:N,C:P and N:P ratios with vegetation restoration closely related to soil physicochemical properties and enzyme activities.