Nitrogen And Phosphorus Return And Resorption Characteristics During The Senescence Of Constructive Tree Leaves In Several Subtropical Forests

Phosphorus（P）and nitrogen（N）are the limiting nutrients for plant development in terrestrial ecosystems.As a major component of forest litter,the return of nitrogen and phosphorus from senescing leaves is of critical importance to the material cycle of forest ecosystems.Before the leaves wither,plants will transfer nitrogen and phosphorus to achieve nutrient conservation and reuse.This process can reduce the dependence of plants on soil nutrients and profoundly affect key material cycling processes,such as litter nutrient return and decomposition.At the same time,plant leaf nitrogen and phosphorus stoichiometric ratios can reveal the carbon（C）accumulation dynamics and nitrogen and phosphorus limitation patterns of the ecosystem.Therefore,a comprehensive analysis of leaf litter N and P return,leaf resorption,and stoichiometric characteristics can help to understand the internal and external cycling processes of nitrogen and phosphorus in forest ecosystems.However,due to the differences in forest structure and species characteristics,there are obvious differences in nutrient return and resorption processes during leaf senescence of constructive trees species in different forests.However,the seasonal dynamics of leaf litter nutrient return,leaf resorption,and stoichiometric ratios and their synergistic relationships still lack a unified understanding and need to be studied in depth.Therefore,we investigated the leaf N and P content,return,resorption efficiencies,and stoichiometric ratios in a typical subtropical Castanopsis carlesii natural forest,C.carlesii secondary forest,C.carlesii plantation,and Cunninghamia lanceolata plantation from May to October 2021（the proposed 1-year cycle of sampling was difficult to complete due to the Covid-19）,and analyzed N and P use and return strategies of the constructive trees.The findings showed that:（1）The leaf litter production dynamics of the constructive trees in different types of forests differed significantly.The leaf litter production of C.carlesii natural forest,C.carlesii secondary forest,and C.carlesii plantation all increased in May-June,decreased during July-August,and continued to increase during August-October.The leaf litter production of C.lanceolata plantation decreased during May-July,rebounded during July-August,and then changed gently.All forests reached the peak of leaf litter production in October.In general,the leaf litter production of C.carlesii secondary forest and C.carlesii plantation ranged from 155.96 kg·ha^-1 to 277.07kg·ha^-1 and 134.85 kg·ha^-1 to 259.86 kg·ha^-1 respectively,which was higher than that of C.carlesii natural forest(67.89 kg·ha^-1-247.71 kg·ha^-1)and C.lanceolata plantation(20.70 kg·ha^-1-170.93 kg·ha^-1),and reached a significant difference in the early growing season.（2）The trends of C,N,and P returns of leaf litter in different forests were generally consistent with the changes of their leaf litter production.The amount of C,N,and P returned by leaf litter was higher in C.carlesii secondary forest,and C.carlesii plantation than in C.carlesii natural forests and C.lanceolata plantation,and the significant difference was mostly reached in the early growing season（May-July）.The monthly N return of leaf litter in the natural forest,secondary forest,and plantation of C.carlesii ranged from 0.85 kg·ha^-1to 3.35 kg·ha^-1,which was higher than their N resorption yield(0.18 kg·ha^-1-1.49 kg·ha^-1),and the monthly N return of leaf litter(0.014 kg·ha^-1-0.062 kg·ha^-1)was lower than the P resorption yield(0.016kg·ha^-1-0.109 kg·ha^-1).Both N and P returns from the C.lanceolata plantation were lower than the P resorption yield in the late growing season.（3）The dynamics of N and P resorption in leaves of constructive trees differed significantly among forest types.The N and P resorption efficiencies of C.lanceolata plantation was lowest in July and highest in September.The N and P resorption efficiencies of natural forest,secondary forest,and plantation of C.carlesii were lower in May,June,and August.The leaf N and P resorption efficiencies of C.lanceolata plantation were not significantly related to temperature and precipitation,while the nutrient resorption efficiencies of the other three forests were significantly negatively related to temperature and precipitation.Overall,the N resorption efficiency of the C.lanceolata plantation ranged from 26.85%to 56.28%,which was higher than those of other forests（11.25%-34.23%）.And its P resorption efficiency was 38.60%-54.85%,which was lower than that of other forests（49.22%-64.28%）.Meanwhile,the resorption proficiency was consistent with the pattern of resorption efficiency and varied more between species.（4）The N:P of leaves of natural forest,secondary forest,and plantation of C.carlesii were all higher than 20 during the growing season,and the growth was strongly limited by P.The leaf N:P of C.lanceolata plantation all ranged from 10 to20,indicating that growth of C.lanceolata plantation was limited by both N and P.The C:P of leaves in C.carlesii secondary and natural forests ranged from 894.89-1144.49 and 855.31-1062.35,respectively,which were higher than those of C.carlesii plantation（833.99-1022.54）and C.lanceolata plantation（753.09-990.83）,indicating that the leaf P use efficiency of C.carlesii natural forest and the secondary forest was higher.This may be related to their higher P resorption efficiency.The C:N of C.lanceolata plantation was significantly higher than that of other forests,indicating that the leaf N use efficiency was higher,which might be related to its higher N resorption efficiency.In addition,the senescent leaves of C.lanceolata had lower N content compared with other forests.In conclusion,compared with the C.lanceolata plantation,the leaf litter production and C,N,and P returns were higher in the natural forests,secondary forests,and plantation of C.carlesii,and were more significant in the early growing season.The P resorption efficiency and use efficiency of leaves in the three types of C.carlesii forests were also significantly higher than those in the C.lanceolata plantation.And the lower N resorption efficiency made the senescent leaves of these three forest types have higher N content,lower C:N,and higher quality of leaf litter.In contrast,the lower leaf litter production and element content of C.lanceolata plantation resulted in lower carbon,nitrogen,and phosphorus returns from the leaf litter.The higher N resorption efficiency resulted in lower N content of senescent leaves and higher C:N,which may lead to lower decomposition rate and nutrient release efficiency of leaf litter.These results indicated that compared with C.lanceolata plantation,the material cycling process by leaves in natural forest,secondary forest,and plantation of C.carlesii was more efficient and beneficial to the maintenance and improvement of soil fertility in the understory.This provides a scientific basis for the understanding of N and P nutrient cycling in different forests in the subtropics,which is of significance for the sustainable management of subtropical forests.