Study On The Characteristics Of Plant-litter-soil Nutrient Stoichiometry In Different Restoration Stages Of Difficult Sites In Yimeng Mountains

Yimeng Mountain Area is the main ecologically fragile area in Shandong Province,and it is also a national key control area of soil and water erosion.Strengthening the understanding of nutrient distribution,restriction and balance in difficult sites in Yimeng mountain area will help to provide a scientific basis for accelerating the process of local ecological restoration.Based on the theory of ecological chemometrics,this paper selection of bare land(BLS),grassland(GS)and irrigated grass(SS)for natural restoration process,and artificially planted coniferous forests(CS),mixed forests(MS)and broad-leaved forests(BS)in Yimeng mountain area as the research objects,respectively at the level of plant organs(leaves,branches,stems and roots)At the plant community level(tree layer,shrub layer and herb layer)and on the plant litter soil continuum,investigate its eco stoichiometric characteristics,clarify the distribution and limitation of nutrients in the study area,reveal the adaptation mechanism of plants to environmental changes and the circulation process and influencing factors of nutrients in the plant litter soil continuum,in order to increase restrictive nutrients Regulating community structure and other means to promote its ecological restoration process,and provide reference for vegetation restoration of other similar difficult sitesThe main results are as follows:1.Plant stoichiometry characteristics and limiting factors at different restoration stagesIntegrating the nutrient characteristics at the plant organ level,community level and overall level,in different restoration stages,coniferous forests have higher C content,and need to synthesize more organic matter to maintain metabolism due to their own growth characteristics,while N,P and K content show an increasing trend with restoration,and C:N,C:P and C:K decrease with restoration,and the structure and function of their ecosystems are more mature in the late restoration BS,and plants In the late restored BS,its ecosystem structure and function are more mature,plants have higher growth rate,interspecific competition is weakened,and they are in a fast growth state,and plants change from resource conservative strategy to resource rapid acquisition strategy;by comparing the N:P,N:K and P:K of plants,combined with their threshold ranges,it is concluded that plants in this study area are mainly restricted by N,and the degree of N restriction gradually decreases in the restoration process,in addition,shrub layer plants are less restricted by N and have a In addition,the variability of N is stable and small at all plant scales,reflecting the view that plants in the study area are Nlimited,which is consistent with the limiting nutrient element stability hypothesis.2.Nutrient stoichiometry characteristics of apoplastic sediment at different restoration stagesThe C and N contents of apoplankton gradually increased and the P content gradually decreased with the restoration process,and the K content changed with strong fluctuation.Based on the contents of N and P in the apoplankton,it can be concluded that the reabsorption of apoplankton by plants in the early stage of restoration was dominated by N elements,while in the later stage,it was dominated by P elements;in addition,the C:N values decreased with restoration,indicating that the apoplankton in MS and BS had a faster decomposition rate;in addition,the N:P in apoplankton was <25 and the P content was >0.22 g-kg-1,indicating that the decomposition of apoplankton in the study area was not limited by P The decomposition of apoplast in the study area was not limited by P.3.Characteristics of soil nutrient stoichiometrys at different restoration stagesThe soils in the study area showed an overall nutrient distribution pattern of low N and low P.The soil-forming parent material was rich in P and K,but the loss was serious.Soil C and N contents increase with restoration,while P and K contents are significantly higher at BLS and then tend to decrease,indicating that the restoration process is conducive to the accumulation of soil organic matter with the improvement of ecosystem structure and the increase of plant diversity;in addition,soil P and K contents mainly originate from rock weathering and other soil formation processes,but in the restoration process,in addition to being absorbed and transformed by plants,they are also affected by surface runoff and human activities,resulting in losses.In addition,the soil P and K contents are mainly derived from soil formation processes such as rock weathering.From the characteristics of C:N,C:P and N:P ratios in the study area,the C:N value is relatively stable and at a high level,while C:P is on an increasing trend and N:P is at a low level,which indicates that the soil conditions are poor,the soil fertility is low,the N element in the soil is not saturated,and the effectiveness of P becomes low.4.Nutrient distribution mechanism among plant organsThe nutrient contents of tree layer,shrub layer and herb layer were larger in leaves or roots,and the values of C:N,C:P and C:K were lower.The nutrient distribution among organs showed high similarity in different restoration stages and growth types,indicating that the nutrient distribution among plant organs was not influenced by the environment but more by their own genetic characteristics,reflecting the distribution strategy among plant organs for nutrients."Resource acquisition type i.e.the more active organs are allocated more nutrients",which is consistent with the element allocation hypothesis.In addition,the coefficients of variation of all indicators of plant roots are at a high level and are influenced by soil nutrients,which is consistent with the elemental plasticity hypothesis.5.Nutrient utilization mechanism among plant communitiesThe higher N and P contents and smaller C:N,C:P,and C:K contents of herbaceous and shrub layer plants compared with those of tree layer plants reflect that shrub and herbaceous layer plants have more efficient N,P,and C:K contents during growth and development compared with tree plants due to their short growth cycle.This is consistent with the species composition hypothesis.In addition,the C:N of apoplankton has the greatest influence on the nutrients of each plant community,indicating that the nutrients of plants in the study area mainly come from the return of apoplankton,and it is important to accelerate the nutrient decomposition rate of apoplankton to improve the nutrient conditions in the study area.6.Influencing factors of nutrients in the plant-apoplankton-soil continuumThe restoration stage,apoplastic P and K content,slope,Shannon-Wiener index,Pielou's evenness index,the amount of explanation of Simpson's index,and soil thickness exerted important effects on the stoichiometry characteristics of plants,and the restoration stage,soil N and K content,and plant N content exerted important effects on the stoichiometry characteristics of apoplastic,and the Shannon-Wiener index,richness,and apoplastic P content had important effects on soil nutrients.This indicates that changes in plant species composition and diversity directly affect the nutrient conditions of plants,apoplankton and soils,and that a low N content in soils encourages plants to increase their reabsorption of N and affects the content of apoplankton N.Plants adapt to environmental changes by regulating their own nutrient uptake and reabsorption,and apoplankton returns nutrients to the soil through its own decomposition.The cycling process of nutrients in the plant-apoplankton-soil continuum accelerates the improvement of environmental conditions in the study area,which is a virtuous cycle.To sum up,the plants in the study area are mainly limited by N,and the contents of soil N and P are relatively low,while N is more deficient than P.in the later stage,the soil conditions can be improved by applying N fertilizer.In addition,accelerating the decomposition of litter is also expected to promote the ecological restoration of the study area;Herbaceous and shrub plants have higher utilization efficiency of N and P elements.In the early stage of restoration,shrub and grass plants can be planted more.Grass plants can consider gramineous and composite plants,and shrubs can better adapt to the nutrient poor soil conditions in the early stage of restoration.However,the diversity of shrub layer in the study area is relatively simple,so it can be considered to introduce Leguminous Shrub plants to improve the soil environment and accelerate the process of ecological restoration;Broad leaved tree species have a higher growth rate than coniferous tree species,and the decomposition rate of broad-leaved tree litter is faster,which leads to the rapid accumulation of soil nutrients.In sites with a certain soil foundation,broad-leaved tree species can be given priority as the preferred tree species for restoration.Our research is expected to provide theoretical guidance for the ecological restoration of the study area and other difficult sites and accelerate the restoration process.