| Since the industrial revolution,the burning of fossil fuels,the production and use of nitrogen（N）fertilizer and the development of animal husbandry have led to the rapid increase of the active n content in the atmosphere.N deposition has caused many negative effects on forest ecosystem,which has become one of the most serious problems in the current global change.However,how does long-term N deposition affect trees in a northern subtropical forest ecosystem in the transitional zone between temperate and subtropical remains unclear.Long-term high N deposition in the tropics usually increases the phosphorus（P）limitation of plants,but it is still unclear whether long-term P addition can alleviate the physiological stress of mature trees caused by excessive N input.In order to answer these questions,this paper selected Castanopsis sclerophylla,a common dominant tree species with high economic value,as the research object by N-addition(100kg N ha-1 year-1),P-addition(50 kg P ha-1 year-1)and NP interaction(100 kg N ha-1 year-1+50 kg P ha-1 year-1)for up to 8 years,the effects of long-term N and P addition and their interaction on the growth,photosynthesis and leaf chemistry of Castanopsis sclerophylla in different seasons（dry and wet）were studied.The main conclusions of this study are as follows:（1）Long-term N and P addition promoted the growth of Castanopsis sclerophylla,and the interaction between N and P was significant（p<0.05）.Long-term N addition significantly decreased soil p H（p<0.05）,accelerated soil acidification,but compared to N addition,P addition and NP addition alleviated soil acidification.N addition,P addition and NP addition significantly increased soil TN（p<0.05）,but had no significant effect on soil TP（p>0.05）.（2）N addition in wet season and dry season significantly increased leaf N contents（p<0.05）,and leaf N contents in dry season was significantly higher than that in wet season.Compared with N/P of leaves in wet season,N addition in dry season further aggravated the N,P nutrient imbalance caused by N deposition in plant tissues（leaf N/P=24.69 in wet season,leaf N/P=40.67 in dry season）.Compared with N addition,P addition and NP addition in wet season and dry season significantly increased leaf P contents（p<0.05）,which led to the decrease of leaf N/P and alleviated the imbalance of N and P.The mature Castanopsis sclerophylla in dry season treated with N addition,P addition and NP addition might be able to cope with the dry environment by maintaining a lower specific leaf area（SLA）,that is,by adopting a more conservative resource utilization strategy.（3）In general,N addition in wet season and dry season had little effect on Chlorophyll a,Chlorophyll b and Carotenoid,but P addition and NP addition in wet season and dry season had significant effect on leaf pigment contents（p<0.05）,the interaction of N and P has significant effect on pigment.In addition,there was a dynamic change in pigment content in each treatment.The contents of Chlorophyll a and Chlorophyll b in wet season were significantly higher than those in dry season,while the contents of Chlorophyll a and Chlorophyll b in dry season were opposite in Carotenoid（4）N addition,P addition and NP addition in wet season and dry season significantly increased leaf net photosynthetic rate（Pn）（p<0.05）,but Pn in wet season was higher than that in dry season.Pn in wet season was more sensitive to P addition than N addition and NP addition,and Pn in dry season was more sensitive to N addition and NP addition than P addition.In addition,there was significant positive correlation between leaf Pn and leaf relative Chlorophyll content（SPAD）in both wet and dry seasons.Compared with N addition and P addition,NP addition significantly affected leaf photosynthetic nitrogen use efficiency（PNUE）in both wet and dry seasons（p<0.05）.（5）N addition,P addition or NP addition in wet and dry seasons promoted the utilization of N by Castanopsis sclerophylla,and increased the ability of N assimilation in leaves.N addition and P addition in wet season significantly increased the soluble protein content in leaves（p<0.05）,and only NP addition in dry season significantly increased the soluble protein content in leaves（p<0.05）.P addition in wet season significantly decreased the content of free amino acid,while NP addition in wet season significantly increased the content of free amino acid（p<0.05）.N addition,P addition and NP addition had no significant effect on leaf free amino acids in dry season（p>0.05）.（6）N addition,P addition and NP addition in wet season and dry season significantly increased the contents of soluble sugar and non-structural carbohydrate（NSCs）in leaves（p<0.05）,and the contents of soluble sugar and NSCs showed a dynamic change in wet season and dry season,their contents in dry season was higher than that in wet season.Compared with N addition and NP addition,soluble sugar and NSCs were more sensitive to P addition in wet season.Compared with P addition and NP addition,leaf soluble sugar and NSCs were more sensitive to N addition in dry season.In dry season,only NP significantly increased the content of starch in leaves（p<0.05）.Similar to the change of soluble sugar and NSCs content in leaves,the contents of starch in Castanopsis sclerophylla leaves also showed a dynamic change in wet and dry seasons.（7）The growth characteristics of Castanopsis sclerophylla were greatly influenced by P element in wet season,and that of Castanopsis sclerophylla was greatly influenced by N element in dry season.Because the leaf pigment,Pn,soluble sugar and NSCs were significantly positively correlated with leaf P in wet season,but significantly positively correlated with leaf N in dry season.（8）For the subtropical Castanopsis sclerophylla secondary forest,the addition of P at a reasonable concentration can positively promote the physiological performance of mature Castanopsis sclerophylla,however,long-term P addition may result in the transfer of restrictive elements from P to N in the future. |
