Effects Of Interspecific Competition Of Tree Species On Plant And Soil Carbon And Nitrogen Contents In Subtropical Forests

Soil carbon(C)and nitrogen(N)play important roles for ecosystem functions,which were affected by complementarity and competition between plant species.However,plant competition can impose species-specific effects on the soil C and N dynamics(i.e.,different species have different performances in the same environment).Therefore,quantifying the interactions between plant species is crucial for deepening the understanding of the relationship between plant diversity and ecosystem functions.Forests contain largest C and N pools in terrestrial ecosystems.It is important to quantify the effects of plant competition on soil C and N dynamics in forest ecosystems for a better understanding of global C and N cycling.Based on this,our study took tree species of subtropical forests(including deciduous and evergreen species)as the research subjects.Seedlings(one year old)and saplings(ten years old),as two main development stages of trees,were selected to conduct the competition control experiments.In this study,the relative interaction intensity(RII)was calculated from the changes in plant biomass.The plant C and N contents,soil C and N pools active components,as well as soil C and N mineralization rates,were measured to clarify the impacts of interspecific competition on soil C and N dynamics and its regulatory mechanisms.The main results were shown as follows:(1)The results of investigation into the impacts of interspecific competition on plant C and N contents for seedlings showed that plant interaction were in competition in both eight mixed treatments.These results also found that interspecific competition had species-specific effects on plant C and N contents for seedlings in different mixed treatments.However,interspecific competition significantly decreased plant C and N contents with increasing RII for seedlings.(2)The results of investigating soil C and N dynamics response to interspecific competition in seedlings have shown that interspecific plant competition increased soil labile organic C and N contents,whereas it tended to decrease NH₄⁺-N and soil net N mineralization rates.Interspecific plant competition significantly increased soil labile organic C and N contents with increasing RII.The results indicated that a strong nutrient competition may occur between plants and soil microorganisms during seedlings stages,which decreased soil microbial activity and soil N mineralization rates,and result in increasing soil labile organic C and N contents.(3)The results of investigation into the impacts of interspecific competition on plant C and N contents for saplings showed that only Quercus serrata–Castanopsis sclerophylla had facilitative effects while others imposed interspecific competition in mixed forests.These results also found that interspecific competition had species-specific effects on plant C and N contents for saplings in different mixed forests.However,interspecific competition significantly decreased plant C and N contents with increasing RII,which was similar with previous results at seedlings..(4)Effects of interspecific competition on soil C and N cycling for saplings showed that Quercus serrata–Castanopsis sclerophylla treatment increased soil total C contents,soil C/N ratio,and soil net N mineralization rates.After classifying the communities into different functional types,interspecific competition only increased soil C and N contents,and labile organic C and N contents under deciduous–evergreen species mixtures.This study also found that the soil total C and N contents,labile organic C and N,as well as soil net N mineralization rates were significantly negatively correlated with RII.These results indicated that the facilitative effects could provide more soluble organic matter and mineral N for plants and microorganisms in saplings,which may effectively promote plant growth and soil C storage.On the contrary,intensely interspecific competition restricted plant growth,which will reduce the input of soil organic matter and reduce the availability of soil N.Taken together,interspecific competition had species-specific effects on plant and soil C and N pools in mixed treatments.Interspecific competition significantly decreased plant C and N contents with increasing RII,both at seedlings and saplings.However,interspecific competition had different influences on soil C and N pools active components(i.e.,soil labile organic C and N)at different development stages,indicating that there was a trade-off between plants and soil microorganisms for the utilization of soil N.Overall,these findings quantify the effects of interspecific competition on soil C and N pools for seedlings and saplings,and reveals the relationships between plant and soil under plant competition.The results will help us better understand the soil C and N cycling of forest ecosystems and provide theoretical support for forest ecological management.