Biological Element Transfer And Its Short-term Response To Nitrogen Addition In Typical Mixed Plantations Of The Rainy Area Of Western China

The return of litter biological elements and the transfer and resorption of biological elements in plants are the key biochemistry processes in forest ecosystems,and also an important mechanism for plants to adapt to environmental changes.Thus,studying the biological element transfer of different plantations and their responses to environmental changes can provide a vital scientific basis for the sustainable management of plantations.Additionally,atmospheric nitrogen（N）emissions are constantly increasing the global atmospheric N deposits.However,the effects of N addition on the processes of litter element transfer and resorption in forest ecosystems remain unclear.China is the country with the largest area of planted forests and the fastest growth rates in the world,and these plantations play an important role in mitigating global climate change and supporting national ecological civilization.However,the ecological problems of planted forests（such as the degradation of soil fertility,biodiversity,ecosystem resistance and stability）caused by the single composition of plantations and the simple structure of forest stands have become increasingly prominent.Thus,the construction of mixed plantations has become a vital way to optimize the structure of plantations and enhance the services and functions of forest ecosystems.The current scientific questions are:（1）Is there a significance difference in the transfer of biological elements in mixed plantations composed of different tree species?（2）What are the effects of atmospheric N deposition on the processes of biological elements transfer in mixed plantations?Therefore,we conducted a two-year（from November 2016 to October 2018）continuous field monitoring experiment in the rainy area of Western China,where rainfall is abundant,atmospheric N deposition is relatively large,planted forest resources are abundant,and ecological status is important.Three typical mixed plantations,including P.zhennan and C.acuminate mixed plantation（PCP）,Q.acutissima and C.acuminate mixed plantation（QCP）,multi-species mixed plantation（MSP）were selected to compare the forest litter production and litter element return from organs to communities in different mixed plantations,and to investigate the resorption of leaf elements in different functional groups,and to estimate the ecological stoichiometry of carbon（C）,N and phosphorus（P）in plant,litter,and soil.Meanwhile,to better understand the effects of N addition on the biological elements transfer of mixed plantations,we set up three N addition treatments(0 kg·N·hm^-2·a^-1,20 kg·N·hm^-2·a^-1,40 kg·N·hm^-2·a^-1)in these mixed plantations based on the background of regional atmospheric N deposition.The results showed that:（1）The annual litter production ranged from 5249.8-7494.4 kg hm^-2 a^-1 in three typical mixed plantations.The monthly dynamics were bimodal,with the highest peak occurring in November and December,and the lowest peak occurring in February and March.Compared with different litter components,leaf litter production was significantly higher than the other litter components,and played a dominant role in total litter production and its monthly dynamics.There was a significant difference in litter production among the three mixed plantations,and the highest and lowest values were found in QCP and PCP,respectively.Compared with different functional groups,the litter productions of deciduous trees were significantly higher than that of evergreen trees in all mixed plantations,and the litter production of deciduous trees was the highest in QCP,while the litter production of evergreen trees was the largest in MSP.Compared with different tree species,the litter production and annual contribution rate of C.Acuminata（PCP）,Q.acutissima（QCP）and P.stenoptera（MSP）were higher than the other dominant species in corresponding mixed plantations.In addition,short-term N addition had no significant effect on litter production in different organ components,species,functional groups,communities and their monthly dynamics in the three mixed plantations.These results suggest that anthropogenic management（especially the selection and matching of tree species）play a dominant role in the litter production and its dynamics of mixed plantations,while short-term simulated N deposition experiments limited the impacts on litter production in the mixed plantations.（2）The overall order rankings（from the highest）of biological elements returned to the surface by litter from mixed plantations were C,Calcium（Ca）,N,Potassium（K）,Magnesium（Mg）,P,Iron（Fe）,Aluminum（Al）,Manganese（Mn）.Compared with different phenological stages,the order rankings of litter element return in four key phenological stages were:senesced leaf stage>frondent leaf stage>leaf expanding stage>leaf sprouting stage.Compared with different litter components,the return amount of each element in litter leaves was significantly higher than that in the other litter components,and played a dominant role in litter element return and its dynamic characteristics.Additionally,there was a significant difference in litter element return among the three mixed plantations.The return of C,N,P,K,Fe,Al in litter of QCP and MSP were higher than those of PCP,and the return of Ca,Mg in litter of PCP and MSP were greater than those of QCP,while the litter Mn return was highest in QCP.Compared with different functional groups,the litter element return of deciduous trees was significantly higher than that of evergreen trees,and there were some differences among the three mixed forests,varying with the elements.Compared with different tree species,the litter element return of C.Acuminata（PCP）,Q.acutissima（QCP）and P.stenoptera（MSP）were larger than the other dominant species in corresponding mixed plantations.Moreover,short-term N addition had no significant effect on litter element return at different scales（from organs to communities）and their dynamic of key phenological stages in the three mixed planted forests.These results indicate that the composition of tree species may determine the litter element return and its dynamic characteristics in mixed plantations,while short-term N addition slightly affected litter element return in the three mixed plantations.（3）The overall order rankings（from the highest）of biological elements resorption efficiency of dominant plant leaves in the three mixed plantations were:potassium resorption efficiency（KRE,62.1%）>nitrogen resorption efficiency（NRE,52.8%）>phosphorus resorption efficiency（PRE,50.2%）>magnesium resorption efficiency（Mg RE,28.7%）>calcium resorption efficiency（Ca RE,-25.4%）>manganese resorption efficiency（Mn RE,-38.9%）>aluminum resorption efficiency（Al RE,-41.5%）>iron resorption efficiency（Fe RE,-42.2%）.In addition,there were significant differences in the element resorption efficiencies of different species and of different functional groups.The NRE,PRE,Ca RE,and Mg RE in herbaceous plants were generally larger than those in woody plants,but the opposite results were found in Fe RE and Mn RE.However,N addition significantly decreased the NRE and increased the PRE,KRE and Mg RE;whereas no significant effect on the resorption efficiency of other elements.Meanwhile,the macronutrient element resorption efficiency（except Ca）of understory plants was much more sensitive response to N addition than tree plants.For example,the effects of N addition on the element resorption efficiency of tree plants were not significant,but N addition had significant effects on some shrubs or herbaceous plants.These results indicate that the transfer and resorption of elements in plants have genetic capacities,which is a balanced mechanism for plant growth and reproduction redistribution of nutrients.However,the effects of short-term N addition on the element resorption efficiency greatly varied with elements,and was regulated by functional groups and species,thereby being hierarchical.（4）There were significant differences in the stoichiometry of C,N and P of plant leaves and litter leaves of different species and different functional groups.The average N and P concentrations of leaves and litter leaves of understory plants were higher than that of tree plants,while the opposite results were true for the concentration of C and the stoichiometry of C,N and P.Meanwhile,the concentrations of C,N and P in soil varied significantly with the three mixed plantations.The soil C,N and P concentrations of PCP and MSP were higher those of QCP,but the opposite results were found in the ratios of C and P.Compared with plant leaves,litter leaves and soil,the concentrations of N and P and their ratios were highest in the plant leaves,and the C concentrations,the ratios of C to N,and C to P were the greatest in the litter leaves,whereas the lowest in the soil.N addition significantly increased the N concentrations,C to P ratios,and N to P ratios of plant leaves and litter leaves,and significantly decreased the P concentrations and C to N ratios,but had no significant effect on C concentrations.Moreover,different functional groups significantly affected the stoichiometry of C,N and P of plant leaves and litter leaves in responses to N addition.The effects of N addition on the stoichiometry of C,N and P in tree plants were not significant,but profoundly changed the C,N and P stoichiometry of understory plants.N addition did not significantly affected the soil C,N and P stoichiometry of the three mixed planted forests.In brief,the stoichiometry of C,N and P varied with plant leaves,litter leaves and soil,and the response of the stoichiometry of C,N and P to N addition were regulated by different species and different functional groups.These results suggest that under increasing N deposition,the soil may have a higher stoichiometric homeostasis relative to plants whereas the plants are more predictive and indicative compared with the soil.In conclusion,the composition of tree species determines the litter production and litter elements return of mixed plantation.There was a significant difference in litter production and litter element return among different mixed planted forests,but their dynamic characteristics were consistent.The litter production and litter element return of deciduous tree species were significantly higher than that of evergreen trees,and there were significant differences among the three mixed plantations.Meanwhile,N addition did not significantly affected the litter production and litter element return,and their dynamics at different scales of the three mixed plantations.In addition,the NRE,PRE,Ca RE,and Mg RE in herbaceous plants were generally larger than those in woody plants,but the opposite results were found in Fe RE and Mn RE.However,N addition significantly decreased the NRE,increased the PRE,KRE and Mg RE,and the macronutrient element resorption efficiency（except Ca）of understory plants was much more sensitive to N addition than tree plants.Moreover,the concentrations of C,N and P,and their ratios in plant leaves and litter leaves were greater than in soil,and N addition significantly affected the C,N and P stoichiometry of understory plants,but had slightly effects on the stoichiometry of C,N and P in soil.These results enrich the research content of biological element transfer in mixed plantation,and thereby advancing our understanding concerning the effects of N addition on material cycling and energy flow of forest ecosystem,and providing theoretical data to accurately improve the quality of plantation and maintain soil fertility under global changes.