关键词: 土壤氮添加, 氮利用效率, 氮利用策略, 优势树种, 典型森林群落, 关帝山林区

Abstract: Objective To study the differential responses of nitrogen contents（NCs） of new branches and leaves (NBL), above-ground biomass increment (AGBI), and nitrogen use efficiency (NUE) of the 3 dominant species in 3 typical forest communities of Larix principis-rupprechtii forest , Betula platyphylla forest and Hippophae rhamnoides forest to different soil nitrogen addition gradients in Guandishan Forest Region. The objectives are: elucidate the different functional characteristics and nitrogen use strategies, and improve the growth of the forests and precision forest management with the increasing global nitrogen deposition. Method Five levels of soil nitrogen addition (0, 9, 18, 27, 36 g·m^?2a^?1) experimentation were implemented in field to compare the responses of nitrogen contents of new branches and leaves, above-ground biomass increment, and nitrogen use efficiency to the different soil nitrogen supplements, and linear models were used to simulated the changes of above-ground biomass increment, nitrogen contents and nitrogen use efficiency with the increment of the soil nitrogen additions. The effective pathway of soil nitrogen addition to above-ground biomass increment in the 3 forest communities was explored by constructional equation model analysis.Result The nitrogen contents of new branches and leaves of the 3 dominant species were totally presented as L. principis-rupprechtii > B. platyphylla > H. rhamnoides. The nitrogen contents of new branches and leaves in July were higher than those in September, and were all increase with nitrogen addition gradient. The differences sequence of the nitrogen contents of new branches and leaves in 3 dominant species between the two growth stages are B. platyphylla>L. principis-rupprechtii > H. rhamnoides. The sequence of the saturated soil nitrogen addition and saturated soil nitrogen contents for the nitrogen contents of new branches and leaves of the 3 forest communities are all L. principis-rupprechtii <B. platyphylla < H. rhamnoides. The new biomass of the 3 forest communities were all increased with increasing soil nitrogen addition, but the response patterns and increasing rates were significantly different as L. principis-rupprechtii > H. rhamnoides > B. platyphylla. The nitrogen use efficiency of the 3 forest communities decreased with increasing soil nitrogen addition, and the decrease sequence was L. principis-rupprechtii> B. platyphylla > H. rhamnoides. And there also decreased with the increase of nitrogen contents in branches and leaves in the 3 forest communities by the decreasing sequence of B. platyphylla > L. principis-rupprechtii??????? > H. rhamnoides. Dominant species and community type determined by community composition directly effected the nitrogen storage in new biomass and community nitrogen use efficiency, also secondly effected by the soil nitrogen additions, and furthermore positively driving the new biomass accumulation in the community. Conclusion The new biomass increment of the three forest communities was mainly determined by the community type and its dominant species but directly affected by the nitrogen use efficiency of the dominant species and the nitrogen contents of the new branches and leaves both regulated by the soil nitrogen supply. Differential nitrogen use strategies of the three communities could be indicated by nitrogen requirement, nitrogen use efficiency, nitrogen sensitivity, and saturated nitrogen supply comprehensively. The three nitrogen use strategy types are in turn: “low demand - high efficiency-high sensitivity - low saturation”, “low demand - low efficiency- high sensitivity - low saturation” and “nitrogen fixing - high demand -low efficiency-low sensitivity-high saturation”. There could provide the principles for the dominant tree species selection, composition regulation and stand site cultivation in precise forest management under different nitrogen deposition scenario or soil nitrogen contents.

Key words: soil nitrogen addition, nitrogen use efficiency, nitrogen use strategy, dominant species, typical forest community, Guandishan Forest Region