The Evolutionary Characteristics Of Robinia Pseudoacacia Nutrient Acquisition Strategies Under Vegetation Rehabilitation On The Loess Hilly Region

The project of returning farmland to forest is important for reconstructing degraded ecosystems in the Loess Plateau.It has the function of improving soil physical and chemical properties,especially increasing nutrient availability.The availability of nitrogen（N）and phosphorus（P）not only regulates the vegetation productivity in the terrestrial biosphere,but also change the energy flow and nutrient cycle by affecting plant-soil-microorganism in the ecosystem.Plant resorption and soil mineralization are key processes in biogeochemical models,which play an important role in plant restoration and ecosystem stability.This thesis is based on such background,taking abandoned farmland and restoring 15a,21a,31a,46a Robinia pseudoacacia plantation forests since farmland abandonment as the research object,the experiment using through space instead of time method to investigate soil mineralization,plant nutrient resorption and their interactions during vegetation restoration.We further analysis evolutionary characteristics of Robinia pseudoacacia nutrient acquisition strategies under vegetation rehabilitation.And then revealed the balance between plant nutrient acquisition and production in the process of vegetation restoration.The researchful results are not only helpful to understand the nutrient cycle of the soil-plant system in the process of returning farmland to forest in ecologically fragile areas,but also provide a scientific basis for guiding the management of soil nutrients and the sustainable development of ecosystems in plantations on loess hilly region.（1）Evolution characteristics of soil net N,P mineralization rates at different Robinia pseudoacacia plantation restoration ages since farmland abandonmentSoil net N and P mineralization rate increased with the vegetation restoration.Furthermore,soil net mineralization rates were negatively correlated with p H,positively correlated with soil moisture content,enzyme activity and microbial biomass,but not significantly correlated with microbial biomass stoichiometric characteristics.The soil physical,chemical and biological properties explained 89.8%of the variation on net mineralization rate,and the interaction between biological and non-biological factors explained the highest rate（51.7%）.Partial least squares path model showed that the imbalance of N and P input caused by vegetation restoration would change the soil C,N and P contents and their stoichiometric characteristics,affecting MBN,MBP and MBN:MBP,changing the investment of microorganisms in extracellular enzymes,and further affecting soil net N and P mineralization rate.Soil enzyme activity was the key factor to regulate net mineralization in the whole restoration process.In addition,the vector angle of enzyme stoichiometry indicated that the shift from N to P of microbial metabolic restriction.Due to the homeostasis characteristics of microorganisms,the corresponding extracellular enzymes would be secreted according to the limitation of nutrients,resulting in a significant increase in the relative mineralization rate(P_min:N_min).（2）Evolutionary characteristics of plants N,P resorption efficiency at different Robinia pseudoacacia plantation restoration ages since farmland abandonmentPlant N and P resorption efficiency decreased with the vegetation restoration.There was a significant negative correlation between plant nutrient resorption efficiency and soil nutrient availability,but no significant correlation between plant nutrient resorption efficiency and mature leaves nutrients.The content of C,N,and P in soil and plants and their stoichiometric characteristics explained 69.3%of the variation in plant nutrient resorption efficiency,and the explanation rate of soil factors was the highest（29.2%）.Partial least squares path model showed that the imbalance of N and P input caused by vegetation restoration would change the soil C,N and P contents and their stoichiometric characteristics,affect the soil N and P availability,and lead to the decrease of net N and P resorption efficiency.During the whole vegetation recovery process,soil nutrients available are the key factors to regulate plant nutrients resorption.In addition,Robinia pseudoacacia leaves N:P indicated that plant growth was limited by P in this study area.However,plant relative resorption efficiency（PRE:NRE）decreased significantly with the restoration of plantation forests since farmland abandonment.This was caused by the fact that soil net mineralization changed N,P relative availability.（3）Evolutionary characteristics of plants nutrient acquisition strategies at different Robinia pseudoacacia plantation restoration ages since farmland abandonment.Plant N,P demand increased with the vegetation restoration.Partial least squares path model shows that soil mineralization and plant nutrient resorption have a positive effect on the Robinia pseudoacacia N,P storage,which are two major ways for plants to obtain nutrients.In addition,Soil net mineralization can indirectly inhibit plant resorption by improving the availability of nutrients,but there is no direct relationship between them.Moreover,the variation of plant nutrient demand and acquisition indicated that the nutrient acquisition pathway of Robinia pseudoacacia gradually changed from a conservative plant resorption pathway to soil mineralization process dominated.These results indicate that plant resorption and soil net mineralization strongly affect nutrient cycling in the plantation ecosystems.Moreover,the reconstruction and restoration of vegetation is an effective and sustainable ecological management measure for restoring the Loess Plateau.