模拟氮沉降对杉木幼苗细根的生理生态影响

Ecophysiological effects of simulated nitrogen deposition on fine roots of Chinese fir( Cunninghamia lanceolata) seedlings

细根对氮沉降的生理生态响应将显著影响森林生态系统的生产力和碳吸存。为了揭示氮沉降对杉木细根的生理生态影响,对一年生杉木(Cunninghamia lanceolata)幼苗进行了模拟氮沉降试验,并测定施氮1年后杉木幼苗细根生物量、细根形态学特征(比根长、比表面积)、元素化学计量学指标(C、N、P、C/N、C/P、N/P)、细根代谢特征(细根比呼吸速率、非结构性碳水化合物)。结果表明:(1)杉木细根生物量随氮添加水平的升高而显著降低,尤其是0—1 mm细根生物量;细根比根长和比表面积随氮添加水平升高而显著增大。(2)氮添加后杉木细根C含量、C/N、C/P显著降低,高氮添加导致1—2 mm细根N含量和N/P显著升高,而低氮添加导致1—2 mm细根P含量显著升高、N/P显著降低,而0—1 mm细根的N、P含量则保持相对稳定。(3)氮添加后杉木细根比呼吸速率无显著变化,细根可溶性糖含量随氮添加增加而显著增加,而淀粉含量和NSC显著降低。综合以上结果表明:氮添加后用于细根形态构建的碳分配减少,这可能会减少土壤中有机碳的保留,0—1 mm细根的形态更易发生变化,但是其内部N、P养分含量相对更稳定以维持生理活动,细根NSC对氮添加的响应表明施氮可能导致细根受光合产物的限制。

The ecophysiological responses of fine roots to nitrogen deposition will significantly influence the productivity and carbon sequestration of forest ecosystems. Currently, there is large quantity of researches on the responses of fine roots to nitrogen deposition, but there are still some controversies. In the present study,a simulated nitrogen deposition experiment was conducted to investigate the impact of nitrogen deposition on fine root ecophysiological traits of 1-year-old Chinese fir （Cunninghamia lanceolata） seedlings. According to the ambient nitrogen deposition, three nitrogen addition levels were set：control （CT, 0 kg N hm-2 a-1）, low nitrogen （LN, 40 kg N hm-2 a-1）, and high nitrogen （HN, 80 kg N hm-2 a-1）. Ingrowth core method and soil core method were applied to measure fine root biomass, morphology （specific root length and specific root surface area, root tissue density）, stoichiometry （C, N, P, C/N, C/P, N/P）, and metabolic characteristics （specific root respiration rate, soluble sugar, starch, nonstructural carbohydrates） after one year of simulated nitrogen deposition. The results showed that：（1） Nitrogen addition significantly reduced Chinese fir fine root biomass, but significantly increased specific root length and specific root surface area, especially for roots of the 0-1 mm diameter class. While nitrogen addition had no significant effect on fine root tissue density, HN treatment did decrease fine root tissue density of the 0-1 mm roots. Compared with the 1-2 mm roots, the 0-1 mm roots were more flexible to change in terms of root biomass, specific root length, specific root surface area. （2） Nitrogen addition significantly reduced C concentration and decreased C/N and C/P in fine roots, N concentration and the N/P ratio in fine roots of 1-2 mm were significantly increased in HN treatment, while in LN treatment, P concentration was significantly increased and N/P significantly decreased in fine root of 1-2 mm. There seemed an interesting phenomenon that P absorption of fine roots was improved only under low level nitrogen addition. Compared with the 1-2 mm roots, the 0-1 mm roots had more conservative concentrations of both N and P.（3）Under nitrogen addition, specific fine root respiration rate had no significant change. Soluble sugar concentration in fine roots increased significantly with nitrogen addition, but the concentrations of starch and nonstructural carbohydrate were significantly reduced. In conclusion, the results showed that the C allocation to build fine roots decreased with nitrogen addition and it might decrease soil C sequestration through fine root turnover. Compared with the 1-2 mm roots, the 0-1 mm roots seemed more flexible to alter their morphology （specific length and surface area）, while more conservative in N and P concentrations in order to maintain the ecophysiological activities in fine roots. Under nitrogen addition, specific root respiration was unchanged, but the total nonstructural carbohydrate in fine roots significantly decreased and more starch was converted into soluble sugar, which implied that fine roots under nitrogen addition might subject to carbohydrate limitation.