Phosphorus Release Dynamics And Influencing Factors During Litter Decomposition Of Abies Fabri Forests In Gongga Mountain

The return and release of phosphorus(P)from litter are important processes in the P cycle of forest ecosystems and substantially affect the P nutrient status of plants.Subalpine forest ecosystems are sensitive to global changes.The litter layer in subalpine ecosystems is relatively developed and densely distributed with plant roots and microbial biomass.Therefore,clarifying the dynamics and influencing factors of P release in litter may help predict the successional trend and carbon sequestration of subalpine ecosystems under the background of global changes.Current research on litter decomposition focuses on its short-term decomposition processes but lacks the combination of short-term and long-term processes.Moreover,attention has been paid to litter decomposition itself,but without linking to the P cycle on the ecosystem level.Therefore,further research on the P release and its influencing factors in the litter layer of subalpine forests is needed.In A.fabri forests of the eastern slope of Gongga mountain(altitude 2800 m-3500m),four sampling sites at different altitudes were selected,with altitude gradient representing different climatic characteristics and different litter layers representing different stages of long-term litter decomposition.Through field investigation and sampling,litter bag experiment and lab incubation experiment,we analyzed the phosphorus forms,enzyme activities and microbial community structures in the litter layer,and also quantified the litter P return at different altitudes,the stocks and turnover fluxes of litter P,and the short-term and long-term litter P releases and their influencing factors.The main results and conclusions are as follows:(1)Conifers needles dominated the litters in A.fabri forests of different altitudes.Physical and chemical properties,carbon(C)quality and elemental ratio did not change along the altitudinal gradient but were significantly affected by the litter decomposition stage.The natural abundances of ¹³C and ¹⁵N increased with the decomposition degree of litter,reflecting the increased portion of microbial products as litter decomposition proceeded.The C quality(expressed as the ratio of cellulase and oxidoreductase activity)decreased with litter decomposition.Microbial biomass,enzyme activity and the C:P ratio of enzyme activity also showed a decreasing trend with litter decomposition.The microbial community structure shifted significantly with litter decomposition,and symbiotic fungi replaced saprophytic fungi to become the dominant functional group.(2)Microbes played important roles litter in the P stocks and fluxes of the litter layer,which was vital to the P cycling of the subalpine forests.The annual return of P in A.fabri forests at different altitudes was between 0.189±0.017 g/m² and 0.389±0.027 g/m²,and was higher in the non-growing season than in the growing season.The stocks of P in the litter layer ranged from 8.0±0.5 g/m² to 18.3±3.3 g/m² with the turnover time ranging from 20.6 to 80.6 years.The stocks of labile P in litter layer were significantly different among different altitudes(2.4±0.1 g/m² to 5.1±0.5 g/m²).They were significantly higher than the annual P demand of the A.fabri forests(0.11 g m^-2yr^-1 to 0.31 g m^-2 yr^-1),indicating that the P supply of A.fabri forests in the eastern slope of Gongga Mountain was relatively sufficient.The stocks of microbial biomass P in litter layer ranged from 1.5±0.2 g/m² to 11.0±0.7 g/m²,accounting for 12%-85%of the plant P stocks.There was a positive correlation between the microbial biomass P and the labile inorganic P content,indicating that the rapid microbial biomass P turnover did not lead to competition for P between microorganisms and plants.(3)Field litter bag experiment and lab incubation experiment revealed the net P release rates in different altitudes and in different decomposition stages.Based on the field litter bag decomposition experiment(30 months),the estimated half release time of P in A.fabri needles was 2.1 to 3.0 years,and the 95%release time was 10.6 to 15.2years.However,this short-term experiment may overestimate the release rate of P in litter.Lab incubation experiments showed that the net release rate of P in litter was between 0.03 mg kg^-1 day^-1 and 2.66 mg kg^-1 day^-1,which was higher in the early stage(L layer and F layer)than in the late stage(H layer)of litter decomposition.The decrease of microbial biomass P during the lab incubation was significantly related to the net P release rate(R²=0.34,p<0.001),indicating that the decrease of microbial biomass P,caused by the depletion of labile organic C,was the source of the net P release.Based on our lab incubation experiment,the net mineralization rates of nitrogen(N)and P were not directly coupled with each other.(4)Temperature and litter decomposition stage were major factors influencing the net P release rate in the litter,while the constitutive expression of the phosphatase gene regulated the potential P release rate.The lab incubation experiment showed that the net P release rate of litter was positively correlated with temperature(R²=0.15,p=0.002).However,the net P release rate obtained from litter bag experiment did not show significant altitudinal difference,possibly due to the differences in moisture content,winter snow cover and microbial community composition of the litter under field conditions.Litter C quality was positively correlated with the P net release rate(R²=0.47,p<0.001),while C:P ratio was not the main factor affecting the net P release rate.Redundancy analysis showed that the potential release rate(expressed by phosphatase activity)of P in the litter layer was slightly affected by phosphorus forms,but was closely related to the physicochemical properties(total carbon,p H)and the microbial biomass of the litter.Based on gene function prediction and path analysis,we propose that the constitutive expression of acid phosphatase gene largely determined the phosphatase activity in the litter layer,which further regulated the potential P release rate.This study may help reveal the mechanisms of P cycling in the subalpine ecosystems and may provide data and theoretical support for predicting the successional trend and carbon sequestration potential of the subalpine ecosystems under the background of global changes.The results of this study may also contribute to the proper protection and management of the subalpine forests.