| Phyllostachys pubescens is a typical invasive bamboo species,and its expansion into adjacent forest is a widespread phenomenon in south China.This ecolocgial processs has seriously threatened the ecosystem services including biodiversity,production,biomass accumulation and hydrological functions of massive secondary evergreen broadleaved forests.Nitrogen（N）and phosphorous（P）are essential elements for plants growth.Ecosystem N and P biogeochemistry cycling and their ecological stoichiometry concern the formation and maintenance of forest ecosystem functions,which can easily be affected by species composition,community structure and community environment.However,little is known about the effects and its mechanisms of bamboo expansion exerting on N and P distribution pattern and processes of evergreen broadleaved forests.A field experiment was conducted by continuously sampling P.pubescens forest（PPF）,bamboo-broad-leaved mixed forest（BMF）and evergreen broadleaved forest（EBF）perpendicular to a bamboo-broad-leaved forest interface on Dagang Mountain,Jiangxi Province,China.Firstly,I inversigated the community composition and structure of three forests.Then,I compared the rhizosphere effects of N（P）for P.pubescens and main broadleaved trees,and their leaf N（P）resorption rates along the process of P.pubescens expansion.The spatial and temperal patterns of N（P）in bamboo body were also monitored according to the rhythm of clonal reproduction.Additionally,I mesured the indexes including net primary production（NPP）,biomass accumulaton and major pools and fluxes of N（P）between EBF and PPF,aiming to quantify the changes of N（P）pools and fluxes after bamboo expansion.The results were as follows:The rhizosphere effects of NH4+-N and available P for P.pubescens increased by 73%and 25%,but those of other eight trees decreased by 17%31%and 11%26%in the process of P.pubescens expansion,indicating higher ability of nutrient excavating for P.pubescens.The N and P resorption rates for P.pubescens were 39.7%and 70.6%,significantly higher than those of broadleaved tress（N:15.5%28.0%,P:11.3%41.1%）,resulting in litter N and P content decreasing by 20%and 28%,and litter C/N and C/P increasing by 19%and 35%.Moreover,P.pubescens had distrinct capacity of physiological integration,with the transfer of N and P to support the growth of new ramets involving adult six generations.The conversion of EBF to PPF altered the distribution pattern of community biomass,with the ratio of aboveground biomass to belowground biomass(MA/MB)decreasing from4.1 in EBF to 2.1 in PPF.The NPP increased by 52%because P.pubescens had higher fine root procution than trees.The plant N and P pools for the PPF were 38%and 32%larger than those of the EBF,respectively,because of higher N and P content in P.pubescens tissues.Whereas the soil available N and P pools significantly decreased by27%and 40%after bamboo expansion,respectively.Moreover,the plant annual N and P uptake for PPF slightly exceeded by 8%and 16%than those for EBF,respectively,however,the litter N and P return to soil in PPF were 14%and 26%less than those in EBF.Thereby,the N and P cyling rate（the ratio of return to uptake）decreased by22%and 36%from EBF to PPF,and the N and P cycle period increased by 60.1%and78.5%,respectively.These results indicate that invasive P.pubescens not only has high capability of capturing for N and P,but also possesses a relative integral biochemistry system to reabsorb and integrate N and P.In other words,this bamboo is a species charactered by“absorbing more but returing less N and P”.These advantages could enhance the N and P biochemistry cycling,but reduce the soil nutrient availability and N（P）biogeochemisty cycling rate of evergreen broadleaved forest,paiticularly for P cycling.The findings not only have great additional information for the assessment on ecological consequences of bamboo expansion,but also enrich our understanding of mechinisms for bamboo expansion. |
