Effects Of Simulated Nitrogen Deposition On Leaf Traits And Their Trade-offs Of Evergreen Broad-leaved Forest In Rainy Area Of West China

With the increasing of nitrogen deposition,nitrogen deposition has become one of the important environmental problems around the world.Forest is the main body of terrestrial ecosystems and also an essential part of the nitrogen cycle of terrestrial ecosystems.It is of great significance to study the response of forest ecosystems to nitrogen deposition.Leaf traits are the main expression form of forest ecosystem functions,and forests adapt to environmental changes by altering traits and the relationships between traits.However,studies on the impact of nitrogen deposition on the relationship between leaf traits are limited.Based on the above background,the evergreen broad-leaved forest（Michelia wilsonii secondary forest）in Rainy Area of West China,was selected as the research object to explore the response of leaf traits to simulated nitrogen deposition.Nine leaf traits of plants with different life forms and seasons were investigated,they were leaf chemical traits:leaf carbon（LCC）,nitrogen（LNC）and phosphorus（LPC）,potassium（LKC）,calcium（LCa C）,magnesium concentration（LMg C）,and leaf p H（p H）and structural traits:leaf dry matter content（LDMC）and specific leaf area（SLA）.The differences in response of leaf traits and the relationship between them to different simulated nitrogen deposition levels(0 kg hm^-2a^-1,20 kg hm^-2a^-1,40 kg hm^-2a^-1)were studied.The results were as follows:（1）LCC and leaf p H varied less among treatments and species,and the coefficient of variation was less than 25%,while other nutrients（LNC,LPC,LKC,LCa C,LMg C）and leaf structural traits（LDMC and SLA）varied relatively larger,with a coefficient of variation of 25%-50%.The addition of nitrogen caused an increase or decrease in plant traits values,but it was not significant.（2）Plants in different life forms respond inconsistently to nitrogen addition.The LNC and LKC of tree was sensitive to nitrogen addition:the LNC of the LN treatment was significantly higher than that of HN（P<0.05）;the LKC of LN and HN treatment was significantly lower than that of CK.For shrub,LKC and LMg C were significantly different among treatments:Shrubs under the HN treatment had significantly lower LKC than other treatments,LN treated shrubs LMg C was significantly higher than other treatments.Leaf traits of herb did not respond significantly to nitrogen addition.Compared different life forms in the same nitrogen treatment,nitrogen addition reduced the difference in LNC and SLA between shrubs and herbs（P>0.05）.However,the significance of the differences and the relative magnitude between traits of different life forms did not change with nitrogen addition.（3）Different nitrogen treatments had different effects on the seasonal dynamics of leaf functional traits.Under CK and LN treatment,LCC increased and then decreased with time series,while under HN treatment,LCC increased with time series.There was no significant difference between LCC in November and April（P>0.05）,and nitrogen addition strengthened the difference between the two months（P<0.05）.LPC and LKC showed a downward trend with time series,LPC in April was significantly higher than in November under LN treatment,but there was no significant difference between other treatments among seasons.LKC under LN and HN treatments in April was significantly higher than in November,while there was no significant difference between CK treatment among seasons.Under CK treatment,LCa C decreased significantly with time series,while there was no significant difference between the other nitrogen addition treatments for three seasons.The results of different nitrogen addition treatments in the same season showed that nitrogen application significantly reduced LKC in November,while there were no significant differences in the remaining leaf traits among treatments in the same season.（4）There was a close relationship between leaf functional traits and which was significantly affected by simulated nitrogen deposition.There was a significantly positive correlation between leaf p H with LNC,LPC,SLA and LMg C with LNC,LPC,LKC,LCa C.Leaf p H had a significantly negative correlation with LCC,LDMC,and LDMC with LNC,LPC,SLA,LKC,LCa C,LMg C,too.The slopes of LPC and LNC,SLA and LNC under LN treatment were significantly higher than those of CK.The slopes of LKC and LNC first increased and then decreased with the increase of nitrogen addition,and LN was significantly higher than that of CK and HN.The slopes of LMg C and LDMC decreased gradually with nitrogen addition,and the slope（LMg C-LDMC）under HN treatment was significantly lower than that of CK and LN.Simulated nitrogen deposition had no significant effect on the relationship between other leaf traits.In conclusion,nitrogen addition treatment had effect on the functional traits of plant leaves,but these effects did not reach the significant level in the community.There were differences in the response of plants in different life forms to simulated nitrogen deposition,but the significance of the differences and the relative magnitude between traits of different life forms did not change with nitrogen addition.Nitrogen addition had different seasonal dynamic effects on different leaf traits,and nitrogen addition increased the differences between LCC,LPC and LKC in April and November,and eliminated the difference of LCa C among seasons.In addition,there was a close relationship between leaf traits,and nitrogen addition had a significant effect on the relationships.These results enrich the study of the influence of nitrogen addition on plant leaf traits and their relationships,which is conducive to deepen the understanding of the response of forest ecosystems to nitrogen deposition,and can provide data support and theoretical basis for the study of leaf economic spectrum under the background of global change.