氮对固氮和非固氮树种细根寿命的调控机制研究

Fine root longevity play very importand roles both in the forest carbon cycling and forest productivity.The forest fine root longevity were affected by the following factors: soil nitrogen(N) content, the root structure, C/N ratios in fine root and so on. An intensive management forest with fertilizer and atmospheric N deposition will lead directly to increace the N content of forest soils. How will the forest fine roots longevity respond to the case? The mechanism of the effect of N concentration on fine root longevity has become the core issue of the forest carbon cycling belowground. So, with both N-fixation and non N-fixation tree species and the experiment of fertilization treatments to increase soil N content, a reseach will be carried out in forest fine root longevity by means of root screen and minirhizotron methods. Meanwhile, the soil N content, fine root morphology, anatomy structure and C, N content also will be tested. Based on these data, the different respose of N-fixation and non N-fixation tree species to soil N content will be found; the mechanisms of the effect of the root morphological, anatomical structure and chemical composition of different fine root orders of N-fixation species and non N-fixation species on fine root longevity will be discussed. Which factor, the soil N content or fine root N concentration, will affects the fine root longevity will be clarified. Throuth solving above problems, not only can the mechnism of the effect of N content on fine root longevity be revealed, but also be two types of species response to N and their roles in the forest ecosystems predicted.

林木细根寿命的长短影响森林碳循环和森林生产力，细根寿命受土壤氮（N）供应、根系结构、根系C/N组成等控制。以施N肥为主的森林集约经营以及大气N沉降将直接导致森林土壤N含量增加。在此背景下，林木细根的寿命如何响应是森林地下C循环研究的核心问题。为此，本项目拟以氮利用方式不同的固氮和非固氮树种为研究对象，通过施肥增加林地土壤N含量，并采用盆栽法控制对苗木的供N水平,结合根网法和微根管法定位观测野外和温室盆栽条件下林木细根寿命，并测定土壤N含量，细根形态、解剖结构及C、N含量等。目的是比较固氮和非固氮树种细根寿命的差异及其对N的响应；探讨两类树种不同根序细根的形态、解剖结构和化学组成对细根寿命的影响机制；弄清细根寿命到底是受土壤N控制还是受根组织N控制。通过以上问题的解决，不仅可以揭示N对细根寿命的影响机理，而且为预测两类树种对增N的响应及其在生态系统中的地位提供重要理论依据。

林木细根寿命与森林碳循环和森林生产力密切相关。以施N肥为主的森林集约经营以及大气N沉降将直接导致森林土壤N含量增加。在此背景下，林木细根的寿命如何响应是森林地下C循环研究的核心问题。以固氮树种（合欢、刺槐）与非固氮树种（马尾松、杉木）为研究对象，在施氮肥处理下，采用微根管技术对细根的年生长动态进行了原位观察与分析，研究结果表明：（1）野外林分细根有明显的季节波动，即在7至9月份出现峰值，之后逐渐下降，12月份最低，次年1月份起开始逐渐回升；而温室盆栽条件下，各树种细根的各生长指标整体呈上升趋势。（2）无论是固氮还是非固氮树种，施氮肥均降低了野外林分的细根总根长、总表面积生产量，而施肥导致温室盆栽苗木的细根生产增加。（3）野外林分土壤中N的有效性与两类树种的细根年生产量之间均负相关。与固氮树种相比，非固氮树种的细根生产量对施肥的响应差异较大。（4）施氮肥显著延长了不同林分春季和初夏（3-6月）时节出生的细根寿命（P<0.05）。以上研究结果不仅揭示N对细根寿命的影响机理，而且为预测两类树种对增N的响应及其在生态系统中的地位提供重要理论依据。.森林土壤呼吸是全球大气温室气体的重要来源，即使土壤呼吸发生较小的变化也将会对大气组成及全球气候产生较大的影响。森林遭受破坏后，研究林分抚育措施对土壤碳库及温室气体释放的影响如何，对于指导灾后森林的管理具有重要意义。感染松材线虫的马尾松林进行不同抚育措施（施肥、砍伐），结果表明：（1）增施氮肥的干扰活动会显著加快土壤N2O的排放速率，会显著增加CO2在湿季的排放速率；（2）砍伐处理的N2O排放速率与对照样地没有显著性差异，但提高CO2湿季排放速率。为了保证森林健康可持续的生态环境，合理的营林措施显得至关重要。