Key Processes Of Nitrogen And Phosphorus Cycle And Their Coupling Coordination Of Different Vegetation Restoration Periods In The Subtropical Region Of China

Nitrogen(N)and phosphorus(P)are the main nutrient factors affecting ecosystem productivity,which co-regulate key ecological processes such as plant growth and development,soil nutrient cycling and soil and water conservation.At present,the studies on the key processes of N and P cycle has concentrated mainly on the evolution characteristics of single nutrient or single process with vegetation restoration.However,it is a key problem to explore whether vegetation restoration can change N and P accumulation,transformation,and their coupling and coordination from the perspective of plant—litterfall—soil system.According to the restoration gradient,four fixed plots were respectively established in four distinct plant communities:4-5 years scrub-grassland,10-12 years shrubbery,45-46 years Pinus massoniana and broadleaf tree mixed forest,and>90 years evergreen broad-leaf forest by using the space—for—time substitution method.The samples of vegetation layer,litter layer and soil layer were collected.Based on the 3 processes of plant absorption and utilization,litter decomposition and releases,and soil accumulation and transformation,the coupling coordination model of N and P was constructed.Our objective was to:(1)reveal the evolution characteristics of N and P coupling intensity and coordinated development relationship in different key processes of N and P cycle during vegetation restoration;and(2)provide a scientific basis for promoting vegetation restoration,improving nutrient fixation potential of ecosystem,and maintaining efficient and coordinated utilization of nutrients.The main results are summarized as follows:(1)Vegetation restoration significantly increased the total N and P storage of ecosystem and its components(p<0.05).The vertical distribution pattern of N and P storage in ecosystem varied with vegetation restoration,among which the contribution rate of N and P storage in vegetation layer increased by 9.28%and 3.12%respectively,but that in soil layer decreased by by 9.53%and 3.19%respectively,and that in litter layer changed little.(2)During vegetation restoration,litter carbon(C),litter C:P,litter N:P,soil moisture content,microbial biomass C(MBC)and microbial biomass P(MBP)were the main factors affecting soil N content,and the common explanation rate was 98%.Litter C and N,litter C:N,fine root P,soil bulk density,percentage of soil clay content,soil organic C,MBC and microbial biomass N(MBN)were the main regulatory factors for the increase of soil P content,and the common explanation rate was 80%..(3)As litter decomposition,the trend of N release different restoration periods exhibited basic unanimously trends:leaching—enrichment—release,while the trends of P release in different restoration periods were different:4-5 years scrub-grassland and 45-46 years Pinus massoniana and broadleaf tree mixed forest showed the enrichment patterns,but 10-12 years shrubbery and>90 years evergreen broad-leaf forest were enrichment first and then release.The N and P annual loss rates increased synergistically with the linear slope of 1.29,and the N and P annual return amounts of changed asynchronously(the linear slope of 0.02)with vegetation restoration.Litter decomposition rate,N and P annual release rates and annual return amounts were significant positively affected by litter initial carbon(C)content and C:P,and significant negative affected by litter initial N:P,C:N,and P content.(4)The contents of soil N and P forms increased with vegetation restoration and the ranges were higher than 117%and 36%,respectively.Soil N forms(or most soil P forms)were significantly positively correlated with each other(p<0.05).The contents of NH4+-N,NO3--N,HON,SON and MBN were remarkably positively correlated with the contents of NaHCO3-Pi,NaHCO3-Po and MBP,the contents of NO3--N and MBN had significantly positively correlation with HCl-Pi content,and the contents of NH4+-N,HON and SON were positively correlated with NaOH-Po content remarkably(p<0.05).(5)N(NO3--N,and SON)and P(NaHCO3-Pi,NaHCO3-Po,HCl-Pi,and MBP)forms with substantial changing had a strongly significant influence on the variation of fine roots N and P contents(p<0.05).During vegetation restoration,leaf N and P had significantly allometric growth relationship(p<0.05).The use efficiency of N and P nutrients in leaves showed decreasing trends with decreasing rate of 41%and 43%respectively,and reabsorption efficiency showed increasing trends with increasing rate of 102%and 62%respectively.The leaf N content had weak homeostasis,and leaf P content had strong homeostasis.There were significant positive correlations between fine root N and P content,between leaf N and P use efficiency and between leaf N and P reabsorption efficiency(p<0.05).Soil N:P had the greatest direct effect on leaf N and P use efficiency and reabsorption efficiency.The contents of fine root N and P were significantly promoted by soil available N:P(p<0.05).(6)The coupling coordination relationship of N and P in the absorption and utilization process of plant and the decomposition process of litter basically maintained at the level of primary balanced.In soil accumulation and transformation process,the coupling coordination relationship of N and P increased significantly from seriously imbalanced to superiorly balanced level during vegetation restoration.The N and P coupling coordination relationship in the key process of ecosystem nutrient cycle showed a positive trend from slightly imbalanced to superiorly balanced level with N and P synchronization.(7)The changes in plant community factors(species diversity,community biomass)and soil fertility factors(percentage of soil clay content,soil moisture content,soil organic C,total N,total P,total potassium,alkali hydrolyzable N,available P contents)were strongly influenced by the interaction between N and P comprehensive indexs,with an explanation rate of 50.7%—99.9%.Therefore,reasonable management measures can be adopted to promote vegetation restoration,improve the biomass of vegetation layer,and make use of the coupling coordination relationship of N and P,further enhance the nutrient fixation potential of ecosystem,facilitate the efficient and coordinated utilization of nutrients,accelerate the formation and restoration of N and P cycles,and improve the productivity and stability of the ecosystem.