Effects of simulated N deposition on the photosynthetic and physiologic characteristics of dominant understorey plants in Dinghushan Mountain of subtropical China

Recent increase of nitrogen(N) deposition in forest ecosystem has shown a variety of effects on plants,including elemental imbalance,growth disturbance, foliar accumulation of free amino acids,etc.The purpose of this study is to determine the effects of simulated N deposition on the photosynthetic physiology of three dominant understorey plants(Randia canthioides,Cryptocarya concinna,and C.chinensis) in a monsoon evergreen broadleaved forest of subtropical China.The experimental design was a randomized complete block with three replications(plots) of four treatments,consisting of the control(0 kg/(hm²·year)),low N(50 kg/(hm²·year)),medium N(100 kg/(hm²·year)) and high N(150 kg/(hm²·year)).Each plot covered an area of 10 m×20 m,surrounded by a 10 m wide buffer zone.From July 2003,NH₄NO₃ solution,whose concentrations were about 0,0.3,0.6 and 0.9 mol/L(nitrogen) for the control,low N,medium N and high N treatments respectively,was sprayed onto the floor of these plots once every month.Here,the authors reported the results after being treated for 21 months.The foliar N content increased greatly with increasing N deposition,but there were no significant changes for foliar P content among the treatments.Generally,mediate N deposition significantly enhanced the maximum net photosynthetic rate(P_max),light saturation point(LSP) and specific leaf weight(SLW) for R.canthioides and C.chinensis.But the P_max and LSP reached the highest values in low N treatment for C.concinna.This may imply that the responses of SLW to N deposition lagged behind the photosynthetic parameters. However,these physiologic parameters greatly and negatively responsed to high N treatment.The chlorophyll content of C.concinna increased significantly with N addition,but not significantly for the other two.The responses of apparent quantum yield(AQY) and light compensation point(LCP) for the three plants to the simulated N deposition were not significant either.But AQY of Randia canthioides and Cryptocarya chinensis exhibited the trends that lower dose of nitrogen deposition may be propitious to utilizing light and higher dose of nitrogen deposition may be not.The results above indicate that the three plants could still self-regulate and adapt to the N deposition environment,though the monsoon evergreen broadleaved forest has been N saturated. C.concinna is more sensitive to N deposition than the other two plants, and low N treatment is in favor of its growth.However,Randia canthioides and Cryptocarya chinensis are still growing well in medium N treatment.So lower dose of N deposition is in favor of plant growth in the monsoon evergreen broadleaved forest in a short term,but higher deposition may exceed the sustainability,disturb physiological process and restrain the plant growth.