Effects Of Nitrogen And Phosphorus Additions On Growth And Biodiversity In Subtropical Evergreen Broad-leaved Forests Of Different Forest Ages

The rapid increase in global nitrogen deposition has significantly altered the global carboncycle and the functioning of forest ecosystems.Subtropical evergreen broad-leaved forest,an important part of our country and the world forest ecosystem,is widely distributed in Eastern China.This study,based on the long-term experiment of nitrogen（N）and phosphorus（P）additions for over nine years,aims to explore N and P inputs on ecosystem functioning and stability of the subtropical evergreen broad-leaved forest,and to reveal the responsive mechanisms of this subtropical forest to N and P addition.The main findings are as follows.（1）Effects of N and P additions on forest growth at different forest ages.Tree growth rate（RGR₉）was significantly affected by the treatment（P<0.01）.The responses of tree growth rate to the additions of N and P were different in different aged forests.In the middle-aged and old-growth stand,the tree growth rate was significantly reduced after the 9-year addition of N.The results indicate that the continued addition of N exceeds the demand for N in forest ecosystems.At the same time,P addition showed a significant promoting effect in middle-aged stand and mature stand,It suggests that the tree growth in the study area was limited by P,and P addition may alleviate the negative effect of high N addition on tree growth.（2）Effects of N and P additions on understory vegetation diversity at different aged forests.The results showed that forest age was an important factor affecting the understory Shannon-Wiener indices,and the Shannon-Wiener index,Simpson index and Pielou index were significantly decreased by HN addition for the understory vegetation in the middle-aged stand（P<0.05）.However,the undergrowth shrub diversity indices of mature stand and old-growth stand had no significant response to N addition.In addition,forest age was also an important factor affecting herbaceous diversity（P<0.01）.The older the stand age was,the lower the herbaceous diversity was.The understory herbaceous diversity of the old-growth stand was significantly decreased by the addition of N and P（P<0.05）.Meanwhile,the understory species diversity indices of the three forest ages was decreased after the addition of N,which indicated that the addition of N might reduce the understory species diversity in the study area.The response of understory shrub and herbaceous diversities to N and P additions was different,which might be caused by the different species composition.The diversity of undergrowth herbaceous vegetation was significantly positively correlated with soil p H,soil C/N ratio（P<0.01）,and negatively correlated with soil microbial biomass carbon（MBC）and microbial biomass nitrogen（MBN）,indicating that the addition of N could affect the changes of soil physical and chemical properties,and may change the species composition,thus resulting in reduction of the undergrowth vegetation diversity.（3）Effects of N and P addition on functional diversity of different aged forests.Forest age had a significant effect on functional diversity（P<0.01）,and there was a negative correlation between forest age and functional diversity.That is,the older the stand,the lower the functional diversity,and the more unstable the ecosystem.The effects of N and P addition on functional diversity in mid-maturation forest performed significantly（P<0.05）.By adding HN,the internal functions of the middle-aged stand diversity declined significantly.At the same time,in the mature stand and the old-growth stand,by adding nitrogen,functional diversity are declined。It is show that persistent N add may have a negative effect on functional diversity.Species diversity was positively correlated with functional diversity（P<0.01）.Functional diversity was positively correlated with soil p H and soil C/N ratio（P<0.05）,and negatively correlated with microbial biomass nitrogen（MBN）and total nitrogen TN（P<0.05）,which was consistent with the results of the relationship between species diversity and environmental factors.It can be inferred that N and P addition indirectly changed the species diversity of subtropical evergreen broad-leaved forest ecosystem by changing the physical and chemical properties of soil.With the decrease of species diversity caused by N addition,functional diversity also decreased.