华南热带森林两种林下植物氮磷比对氮磷添加的响应

Nitrogen to phosphorus ratios of two understory plant species in response to nitrogen and phosphorus addition in tropical forest of southern China

基于华南热带次生林长达3年的氮(N)磷(P)添加实验,测定两种分布较广的林下乡土树种大青(Clerodendrum cyrtophyllum)和紫玉盘(Uvaria microcarpa)地上部(叶片和枝条)的N、P含量,试图阐明该热带森林的养分限制问题.结果显示:(1)两种植物的N、P含量差异极显著(P<0.001);同一植物中不同组织的N、P含量也存在差异;N、P添加对枝条的N、P含量影响最大,其次为新叶,对老叶的影响较小;(2)+N处理对大青和紫玉盘的N、P含量均无显著影响,但显著地降低了N:P(P=0.001),降低幅度分别为9%和50%;(3)+P处理对植物组织的N含量影响不显著,但极显著地提高了植物的P含量(P<0.001),分别提高了大青和紫玉盘P含量的54%和88%;+P处理极显著地降低了植物的N:P(P<0.001),其值分别为28%和60%;(4)+N或+P处理下,两种植物组织的P变化量与N:P变化量呈极显著负相关关系(P<0.001),可见植物P含量的变化是驱动N:P变化的主要因子.本研究表明,N、P添加对两种植物不同组织的N、P含量均有一定程度的影响,且+P处理的影响要大于+N处理,枝条比新叶、老叶对N、P添加的响应更敏感.因此,P素有效性可能是该热带地区植物生长的限制因子.

The secondary tropical forests in southern China have suffered from frequent human disturbance and increasing high N deposition.In order to explore the nutrient limitation status in secondary tropical forests of South China,this 3-year field experiment of nitrogen（＋N） and phosphorus（＋P） addition investigated nitrogen（N） and phosphorus（P） concentrations of the aboveground tissue（leaf and branch） of two widely distributed understory native species Clerodendrum cyrtophyllum and Uvaria microcarpa in a secondary tropical forest of South China.The results showed that：1） the N and P concentrations of the two species were significantly different（P〈 0.001）;N and P concentrations of different tissues in the same species were different;NP addition greatly affected N and P concentrations in branch rather than new leaf and older leaf.2） ＋N treatment had no significant effect on N or P concentrations of either species,but significantly decreased N：P ratios（P = 0.001）,at the level of 9%for C.cyrtophyllum and 50%for U.microcarpa,respectively.3） ＋P treatment had no significant effect on tissue N concentrations,but significantly increased plant P concentrations（P〈 0.001）,at 54%for C.cyrtophyllum and 88%for U.microcarpa,respectively;＋P treatment significantly decreased plant N：P ratios（P〈 0.001）,at 28%and 60%,respectively.4） The alterations of P concentrations of two species had significantly negative correlations with N：P alterations under ＋N/＋P treatment（P〈 0.001）,suggesting that the alteration of P concentrations in plant tissue was the major driver for N：P alteration.Our results show that N and P addition would affect tissue N and P concentrations of the two species,with ＋P treatment having relatively greater effect on nutrient concentrations than ＋N treatment;the branch is more sensitive than new or older leaf in response to nutrient addition.Therefore,P availability may be the limiting factor for plant growth in the tropical forests.