The Effect And Mechanism Of Root Functional Traits On Soil Conservation Following The Restoration Of Eroded Land

Vegetation restoration is the main eco-engineering measure to control soil erosion and restore the degraded soils.However,long-term severe erosion can constrain plant growth and vegetation succession,and consequently limits the efficiency of erosion mitigation and soil restoration by vegetation.Recently,scientists have proposed the biological measures for soil and water conservation in China should be oriented to enhance ecological function,and combine the specific ecological problems and social economic factors to set the ecosystem service as the major goal of the ecological restoration.To achieve those goals,managers require clear“concepts,models,methodologies and tools”.Noticeably,advances in functional ecology,especially the linkages between multiple plant functional traits and ecosystem services,have leaded to the trait-based approaches became a powerful tool to explore how plant respond to environmental change and how plant influence ecosystem processes.Unfortunately,limited study has reported the impact of erosion stress on plant traits,and the linkages between plant functional traits and soil conservation have yet to be studied in detail.With this context,the main objective of this study was to reveal the mechanism of how plant functional traits affect soil conservation following the restoration of eroded land.We therefore selected Wulongchi small catchment as the study area,which had a long-term erosion history and was a typical demonstration of vegetation restoration around the Danjiangkou reservoir.Then we performed field erosion and vegetation survey,large scale of vegetation and soil sampling,plant traits and soil properties analysis,simulated concentrated flow erosion experiment and literature search.First,we investigated the characteristics of plant traits along an erosional degradation gradient(moderate,severe and very severe)at community level,and then clarified how trait variability response to erosion stress.Second,based on a“trait-based response-and-effects framework”(which provides restoration ecology for a robust scaffold on how plant traits respond to environment change and how those changed traits affect ecosystem function),we linked aboveground plant characteristics and root functional traits to a set of soil biochemical properties,physical structure and erodibility which are key to soil conservation.The main results are as follows:1)Long-term soil erosion had resulted in soil loses and profile configuration destructive,leading to the entire degradation of soil quality and quantity,including thinning soil layers,increasing surface soil bulk density and coarser soil particles,reducing water retention and soil nutrient(nitrogen and phosphorus content).Moreover,vegetation coverage and plant form size decreased with erosion acceleration,suggesting that long-term soil erosion can stress plant growth and distribution.However,no significant effect was found between erosion degree and species diversity(p>0.05),this may due to the intense competition of resources(e.g.,light and nutrients)in the high coverage slope under moderate erosion degree.2)The present study is the first to report the effects of erosion stress on root functional traits.Erosion degree and vegetation type significantly affected vegetation coverage and the distribution of root diameter(p<0.05),but had no significant effect on the diameter range(mainly 0-2 mm).In particular,fine roots(0.2 mm<diameter<2 mm)accounted for51-93%of total root length in topsoil.Moreover,soil available nitrogen had positive effects on vegetation coverage,root mass density and coarse root length density;soil available phosphorus,water content and soil thickness had positive effects on vegetation coverage and litter mass,but had negative effects on fine root length density;with respect to soil texture,the enrichment of fine particles(silt and clay)reduced vegetation coverage,root mass density,fine root length density and coarse root length density,the enrichment of coarse particles(sand)improved the very fine root length density but reduced coarse root length density;furthermore,slope gradient had only negative effects on vegetation coverage(p<0.01),and bulk density had only negative effects on root mass density(p<0.01).Overall,our results indicated that in order to adapt soil degradation by erosion,plant are prone to choice high efficient-quick return investment strategy.Namely,plant functional traits,which facilitate resource acquisition,increased with soil nutrient decreased.In contrast,vegetation cover,plant litter and coarse roots favoured resource-rich environments.In addition,given that root length densities(62.76 m/dm~3)in the study slopes were far less than the values of shrub and grass community which were observed by other studies,the current vegetation restoration strategy in Wulongchi catchment may have great potential in improving root content in the top soil layer.3)Vegetation restoration improved multiple soil biochemical properties and physical structure key to soil functions(p<0.001),and plant functional traits played a substantial role in those relationships.Soil bacterial and fungal abundance,soil organic matter,glomalin-related soil protein,aggregated stability,non-capillary porosity and shear strength followed the order of woodland>shrubland>grassland>bare land.Moreover,soil p H and shear strength were also significantly affected by erosion degree(p<0.001).Except for p H,all of the above soil properties varied in a coordinated way with soil organic matter,and were significantly and positively correlated with vegetation coverage,litter mass,root mass density,fine root length density,coarse root length density and mean root diameter.Importantly,results of redundancy analysis(RDA)showed that plant key traits explained 73%of the variations of the above-mentioned soil properties,while erosion factors also explained 65%,indicating that plant traits and erosion environment jointly shaped the ecological processes and functions under the restoration condition.Covariance analysis further demonstrated that the positive effects of roots and litter on soil aggregation were mainly through the increasing effects on soil microbial abundance,glomalin-related soil protein and organic matter.Notably,the effects of plant traits and soil biochemical properties on aggregate stability were not linear,but involved clearly thresholds.However,evidences in previous researches had shown that the response of aggregate stability to vegetation degradation was negatively and immediately,suggesting that maintenance of high aggregate stability requires the continuous input of high-quality roots and litter.Therefore,we concluded that maintaining ecosystem functions require maintaining high-intensity input of soil organic carbon,and continuous monitoring and long-term vegetation management should be considered when to restore degraded ecosystems through vegetation measures.4)Our results indicated that the fine root length density and soil shear strength were the most powerful variables explaining soil detachment capacity among root traits and soil structure,respectively.With this context,we built a prediction model for soil detachment capacity based on flow shear stress,root functional traits and soil properties(R~2=0.88,NSE=0.85,n=360).Since the trait-based response-and-effect restoration framework involved formative indicators,and the sample number in this study was on 50,partial least squares-structural equation model(PLS-SEM)is quite appropriate for exploring the complex relationships here.The PLS-SEM results showed that plant key traits were the main drivers of soil erodibility mitigation during the vegetation restoration.In particular,the effects of plant traits on soil erodibility were mediated via their effects on a range of plant-induced soil properties(e.g.,soil organic matter,aggregate stability and shear strength),however,those effects were limited by severe erosion degree due to the erosion stress on plant traits.In addition,our study found that vegetation can still be efficient in maintaining and restoring soil functions,even on severely degraded land.5)Soil conservation should be the key priority for maintaining soil carbon sequestration in an eroded environment.Soil total carbon density and inorganic carbon density had a linear relationship with soil thickness,but had a logarithmic relationship with organic carbon.Those results suggested that soil carbon storage was very sensitive to erosion.In particular,while there was a clear threshold for the enhancing soil organic carbon sequestration through the vegetation biochemistry effects,there was no limit for the contribution of soil conservation effect to soil inorganic carbon sequestration.We therefore concluded the mechanism of how vegetation restoration improved soil carbon storage under the eroded land,that is,(i)vegetation restoration improved soil organic carbon storage through biochemical carbon fixation,(ii)vegetation could control erosion and consequently reduce carbon emissions caused by the erosion process,and(iii)vegetation increased both soil inorganic carbon and organic carbon storage through soil conservation effect.Based on the current findings,we proposed a trait-based response-and-effect framework for ecological restoration on eroded land.That is,plant traits should be selected according to target restored ecosystem service(e.g.,soil conservation,hydrological regulation and carbon storage).First,the environmental stress factors and species interactions of the restoration site should be clearly identified.Second,selected traits should not only affect the target ecosystem functions,but also not influenced by environmental(or inter-species interaction)stress or have stress tolerance.We believe that this framework could extend the current problem-oriented erosion-related research ideas and practices,and have important practical implications for vegetation restoration which target on improving ecosystem services.