大气氮沉降对海北高寒草甸优势种叶片光合作用过程的影响

为研究大气氮沉降对海北高寒草甸光合作用过程的影响,对不同施氮水平和氮沉降类型〔KNO₃、(NH₄)₂SO₄和NH₄Cl〕下5个群落优势种叶片w(TN)、异针茅叶片光合参数和冠层LAI(叶面积指数)进行测定和分析,并利用Farquhar植物光合作用模型估算光合参数变化对GPP(总初级生产力)的可能影响,其中,氮沉降设置4个水平,即对照〔0 kg/(hm2·a)〕、低氮〔10 kg/(hm2·a)〕、中氮〔20 kg/(hm2·a)〕和高氮〔40 kg/(hm2·a)〕,均以N计.结果表明:除了施加(NH₄)₂SO₄、低氮处理下甘肃棘豆叶片w(TN)显著增加9.4%以外,其他施氮水平和施氮类型对5个优势种叶片w(TN)均无显著影响.高氮处理下V_cmax25(25 ℃时最大羧化速率)、J₂₅(25 ℃时电子传输速率)、TPU₂₅(25 ℃时丙糖磷酸传输速率)和R_d25(25 ℃时暗呼吸速率)分别较对照处理高出38.6%、30.4%、27.8%和164.3%.氮添加后,R_d25的增加抵消了因V_cmax25增加引起的光合作用增强效果,使得整体上GPP的增加趋势不显著.不同施氮水平和施氮类型下海北高寒草甸群落优势种的LAI均未发生显著变化.研究显示,当大气氮沉降量低于40 kg/(hm2·a)时,氮沉降量的增加会促进海北高寒草甸植物叶片光合作用的反应速率,但对植物叶片w(TN)、LAI和GPP的影响并不显著. 

Effects of Atmospheric Nitrogen Deposition on Photosynthesis of Dominant Plant Species in the Haibei Alpine Meadow Ecosystem

To quantify the effects of nitrogen deposition on photosynthetic processes in the Haibei alpine meadow, we analyzed the differences in observed leaf nitrogen content(w(TN)) of five dominant species, leaf photosynthetic parameters of Stipa aliena and canopy leaf area index(LAI) measured in simulated atmospheric nitrogen deposition treatments with various nitrogen levels(control of 0 kg/(hm2·a), low nitrogen treatment of 10 kg/(hm2·a), medium nitrogen treatment of 20 kg/(hm2·a) and high nitrogen treatment of 40 kg/(hm2·a)) and forms(KNO₃,(NH₄)₂SO₄ and NH₄Cl). We also used a photosynthesis model to estimate the possible effects of photosynthetic parameters on gross primary productivity(GPP). The results showed that nitrogen levels and forms had no significant effects on leaf nitrogen content for five dominant species, except for Oxytropis kansuensis, the leaf nitrogen content of which increased by 9.4% significantly in the low level nitrogen treatment when using(NH₄)₂SO₄ fertilizer. Values of the maximum carboxylation rate at 25 ℃(V_cmax25), electron transfer rate at 25 ℃(J₂₅), triose phosphate transfer rate at 25 ℃(TPU₂₅), and dark respiration rate at 25 ℃(R_d25) in the high level nitrogen treatment were 38.6%, 30.4%,27.8% and 164.3% higher than those in the control.Increasing R_d25 offset the increase in GPP induced by increasing V_cmax25 after nitrogen addition, which caused no significant change in GPP. Observed LAI had no significant responses to nitrogen levels and forms. These results suggest that nitrogen input within the level of 40 kg/(hm2·a) accelerated the reaction rates in leaf photosynthesis, but did not significantly affect leaf nitrogen content, LAI, and GPP.