Aims Elevated nitrogen(N)deposition in tropical regions may accelerate ecosystem phosphorus(P)limitation.However,it is not explicitly addressed that how changes in soil N and P availability affect foliar nutrients and photosynthesis of plants in tropical forests.In this study,we examined the effects of N and P additions on foliar nutrients and net photosynthesis of two dominant understory species,Randia can-thioides(R.canthioides)and Cryptocarya concinna(C.concinna)in an N-saturated old-growth tropical forest(>400-year-old)in south-ern China.Methods A full factorial NP addition experiment(2×2)was established in 2007 and continued through August 2010.Four treatments,including control,N addition(150 kg N ha^(−1) year^(−1)),P addi-tion(150 kg P ha^(−1) year^(−1))and NP addition(150 kg N ha^(−1) year^(−1) plus 150 kg P ha^(−1) year^(−1))were set up in this experiment.Photosynthetic traits maximum photosynthetic CO_(2) assimilation(Amax),stomatal conductance(gs),leaf transpiration(E),light satu-rating point,concentrations of chlorophyll a/b and foliar nutri-ents(N and P)of the two species were measured with standard methods.Important Findings Three years of N addition had no significant effects on any measured photosynthetic parameter of either species.However,N addition significantly elevated foliar N and P concentrations of one species(R.canthioides),resulting in lower photosynthetic nitrogen use effi-ciency(PNUE).N treatments decreased foliar P concentration of the other(C.concinna),resulting in increased photosynthetic phos-phorus use efficiency,which was potentially related to N-induced P shortage.In contrast,positive effects of P treatments on gs of R.can-thioides,Amax and chlorophyll a+b of C.concinna were observed.P treatments also elevated foliar P and PNUE of both species,imply-ing P induced more efficient use of N.Our results suggested a more important role of P than N on influencing photosynthetic traits of these two understory species.Alleviation of P shortage through P addition may enhan
施肥是退化草地初期恢复常用的管理措施,被用于改良草地群落结构和提高牧草生产力.本文以藏北轻度和重度退化高寒草甸为研究对象,设置低氮(50 kg N·hm-2·a-1,LN)、高氮(100 kg N·hm-2·a-1,HN)施肥水平,及其与磷配施组合试验(50 kg N·hm-2·a-1+50 kg P·hm-2·a-1,LN+P;100 kg N·hm-2·a-1+50 kg P·hm-2·a-1,HN+P),研究施肥梯度与施肥类型对两种退化程度高寒草甸群落结构、物种丰富度与多样性、群落生物量及其分配的影响.结果表明:单独施氮对轻度和重度退化高寒草甸群落盖度和生物量均无显著影响,而氮磷配施显著提高群落盖度及地上、地下生物量,其中LN+P处理的促进效果最显著.这表明本研究区地上生产力受到氮和磷的共同限制.施肥处理对重度退化样地总生物量无显著影响,但显著降低了根冠比.HN与HN+P处理均显著降低轻度退化群落的物种丰富度和均匀度,说明HN处理不利于轻度退化草地物种多样性和稳定性的维持.在轻度退化样地,施氮(尤其是HN处理)主要提高了禾草植物的重要值和生物量,而氮磷配施则有利于莎草类植物的生长.施肥对重度退化群落植物的重要值影响较小,主要提高了杂草生物量.说明轻度退化高寒草甸可选择低氮配施磷肥的措施,而对重度退化草地可能需要结合围栏和补播牧草等其他管理措施进行改良.
高寒草甸是氮素匮乏的生态系统,氮添加和放牧都会显著改变土壤养分的有效性及高寒植物对养分的利用方式,从而影响群落生产力。为了认识氮沉降和放牧干扰对青藏高原高寒草甸的影响,在西藏当雄县高山嵩草草甸开展了氮添加及模拟放牧实验。在模拟放牧样地(G+N)和不放牧样地(NG+N)分别设置4个氮添加处理:0、10、20和40 kg N.hm-2.a-1,在生长季采用原位封顶埋管法测定净氮矿化速率,同时分析群落生物量分配与植物氮利用的相互关系。研究表明:氮添加与模拟放牧对植物的分配模式有不同的影响。在无放牧压力下,氮添加主要刺激植物地上部分的生长;在放牧压力下,氮添加更倾向于促进植物地下部分的生长。虽然剪草移走了15%～20%的地上生物量,但由于氮添加促进了植物的补偿生长,模拟放牧与不放牧处理的总生物量无显著差异,甚至前者高于后者。高寒草甸植物生物量分配对氮添加和模拟放牧的响应也体现在土壤供氮潜力的作用模式上,土壤净氮矿化速率在NG+N样地与植物地上生物量呈显著负相关,而在G+N样地与植物地下生物量呈显著负相关。这表明高寒草甸植物可以通过改变自身光合产物的分配模式来响应土壤养分状况和放牧干扰,在有效养分匮乏的高寒草甸添加氮素能够促进植物的补偿性生长。
