Nutrient Cycling Of Major Plantation Ecosystems In The Longzhong Loess Plateau

The plantation ecosystem is an important part of the forest ecosystem in drylands,which can alleviate and improve the ecological environment problems in drylands.The nutrient cycling of plantations can regulate and affect the material cycle,information transmission and energy flow of ecosystems,and is the key for maintaining the ecological service function and sustainable development of dryland plantation ecosystems.However,the current understanding of the processes and mechanisms of nutrient cycling in dryland plantation ecosystems is still unclear.Therefore,this study took the major plantation ecosystems in the Longzhong Loess Plateau as the research object,including six common afforestation tree species,namely C.korshinskii,P.orientalis,A.sibirica,P.tabuliformis,P.hopeiensis and T.chinensis,and three afforestation years of 13-years,35-years and ≥50-years.Through field investigation,indoor measurement and other technical methods,firstly,the nutrient cycling characteristics of different components of major plantation ecosystems in the Longzhong Loess Plateau were clarified,and the effects of litter chemical properties and soil enzyme activities of different tree species on soil nutrients in ≥50-years plantations were analyzed.Finally,the differences of soil microbial communities in C.korshinskii plantations and P.orientalis plantations and their interrelationships with plant community characteristics and soil properties were mainly explored,and the nutrient cycling process and its underlying mechanisms of major plantation ecosystems in the Longzhong Loess Plateau were revealed.The main results were as follows:(1)Tree species(C.korshinskii,P.orientalis,A.sibirica,P.tabuliformis,P.hopeiensis and T.chinensis)and afforestation years significantly affected the organic carbon(C),total nitrogen(N),total phosphorus(P),total potassium(K)contents and their ratios of the green leaves,fine roots,leaf litter and surface soil,and leaf nutrient reabsorption efficiency in the major plantation ecosystems of the Longzhong Loess Plateau,and they also differed significantly among different components(green leaves,fine roots,leaf litter and surface soil).Among different components,N,P and K contents of green leaves were significantly higher than those of fine roots,leaf litter and surface soil,and fine roots had the highest C contents,while the C and N contents of surface soil and the P and K contents of fine roots were the lowest,resulting in their average C:N:P:K ratios in order: fine roots > leaf litter > green leaves > surface soil.And the C:N:P ratios of different components also followed this pattern.The reabsorption efficiencies of C,N,P and K in leaves were 21.83%,44.34%,52.68%and 55.90%,respectively.The 13-years P.orientalis plantation and 50-years P.tabulaeformis plantation were jointly restricted by N and P,while other plantations were only restricted by P.The leaf P reabsorption efficiency was significantly negatively correlated with the P of different components,and the green leaves of plants showed a “homeostatic” to soil available phosphorus(AP),indicating that trees adapted to the P-limiting growth environment by taking strategies that combined improving the leaf P reabsorption efficiency and maintaining the green leaf P homeostasis.(2)In ≥50-years plantations,soil nutrient contents of C.korshinskii,P.orientalis,A.sibirica,P.tabuliformis and P.hopeiensis were significantly different.Soil organic carbon(OC),total nitrogen(TN),available nitrogen(AN)and AP of the C.caragana plantation were significantly higher than those of the P.hopeiensis,A.sibirica,P.tabuliformis and P.orientalis plantations;and soil TN,total phosphorus(TP),total potassium(TK)and available potassium(AK)of the P.orientalis plantation were significantly lower than those of P.hopeiensis and A.sibirica plantations,and its soil AP was significantly lower than that of P.tabuliformis and A.sibirica plantations.Compared with the natural grassland,soil total carbon(TC),OC,TN,AN and AK in the C.caragana plantation increased significantly by 28.42%,56.08%,57.41%,107.25% and 10.29%,respectively;while soil TN,TP,AP,TK and AK in the P.orientalis plantation decreased significantly by 38.89%,30.58%,76.39%,8.25% and8.33%,respectively.Compared with the P.orientalis plantation,the leaf litter of the C.korshinskii plantation had significantly lower C content and higher N,P and K contents,and its soil urease and alkaline phosphatase(ALP)activities were also significantly higher than those of the P.orientalis plantation.Overall,the leaf litter quality and soil enzyme activities contributed 62.20% to total variability of soil nutrients,and the litter phosphorus(LP)and soil ALP more significantly and positively affected soil nutrients,indicating that tree species,LP and ALP were the key factors driving soil nutrient change in major plantations of the Longzhong Loess Plateau.In addition,the significantly coupling relationship between N and P in the litter,soil enzyme and soil nutrients revealed that the P cycle in the plantation ecosystem could actively respond to the increasing changes in nitrogen levels.Especially in legume plantations,C.korshinskii improved the P status in the ecosystem by increasing soil N content to alleviate the limitation of P on plant growth.(3)In C.korshinskii and P.orientalis plantations,their plant community characteristics,soil properties and soil microbial community characteristics showed large differences.Except for soil bacterial Shannon index and soil fungal Simpson index,plant community coverage,plant diversities,soil nutrient contents and soil microbial diversities of C.korshinskii plantations were higher than those of P.orientalis plantations.Compared with P.orientalis plantations,the contribution of the deterministic process in soil bacterial and fungal community assembly of C.korshinskii plantations was greater,whose proportion in soil bacterial community assembly reached 91.43%;while the proportion of the stochastic process in soil bacterial and fungal community assembly of P.orientalis plantations were 66.67%and 100%,respectively.Meanwhile,soil bacterial and fungal co-occurrence networks of C.korshinskii plantations were characterized by: higher diversity and richness,shorter average path lengths and network diameters,more key species,interrelations and positive relationships,greater connectivity and graph density;while P.orientalis plantations only showed the larger average clustering coefficients,which further proved that C.korshinskii legume plantations shaped a more complex and stable soil microbial community,forming the positive feedback of “plant-soil-microbes” to enhance the stability and sustainable development potential of legume plantations.In summary,the tree species dominate the nutrient cycling of major plantation ecosystems in the Longzhong Loess Plateau,the cycling of C,N,P and K among green leaves,fine roots,leaf litter and surface soil are obviously heterogeneous,and plantations can take some strategies,such as raising plant P reabsorption efficiency,maintaining plant P element homeostasis,increasing soil alkaline phosphatase to induce more P release from leaf litter,and improving the N level to promote P cycling,to adapt to P-deficient environments.Meanwhile,the positive feedback between plant-soil-microbes is the key to nutrient cycling and the important basis for sustainable and stable development of plantation ecosystems.This study not only deepens the understanding of the process and mechanism of the nutrient cycling of dryland plantation ecosystems,but also provides the important scientific support for the healthy management and sustainable development of plantation ecosystems in drylands.