基于根际微生物过程的杨树混交氮素效应机制研究

Microbial diversity and functional compositions play crucial rules on regulating soil nitrogen transformation and supply. This effect is especially important in rhizosphere. Some processes, such as root exudation, are persistently changing the rhizospheric environment, adjusting microbial compositions and activity, consequently altering soil N dynamic and availability, and finally, giving feedback to N assimilation and tree growth. However, due to the method restriction, there is a lack of information on such issues in wood plantations. Poplar is an important timber tree species suitable for planting in the flood plain of central China. Its fast-growth performance depends greatly on soil N supply. By now, site degradation in poplar plantation has become serious due to the continuous cultivation with mono-species. Natural forests can generally self-fertilize from little return and rhizospheric processes. Based on this conception, the project focuses on the rhizospheric processes in mixed-poplar plantations with the combination use of in situ and greenhouse rhizobox experiments. The status of root exudation of organic carbon and their interactions with rhizospheric microbial composition and N dynamic will be studied in both the mature mixed-poplar plantations and the simulated rhizobox mixed-cultivation of poplar with different tree species. Their feedback of interactions on soil N availability and tree growth will also be evaluated. The results can give great insights into the N nutrition efficiency of mixed-poplar plantations from the rhizospheric-microbial interaction aspects, and will provide great significance for maintaining sustainable productivity of poplar plantations.

土壤微生物多样性和功能群的组成对氮的转化过程和供应起着至关重要的作用，是目前林业生态系统研究的重点和难点。特别是在林木根际，其根际沉降等过程可以改变根际土壤环境条件，调节根际土壤微生物的群体特征和分解活性，从而改变氮的转化和供应水平，对林木的养分吸收和生长起着直接的反馈作用。由于研究方法的限制，目前有关该方面研究的报道甚少。杨树是重要的速生用材树种，其速生性与土壤氮的供应有着直接的联系。由于长期的纯林经营和多代连作，地力衰退现象已非常严重。本项目依据林分自我培肥的生态学原理，以根际微生物过程为切入点，结合野外原位根箱和室内根箱模拟试验，研究杨树混交林分根系有机碳分泌及其与根际土壤微生物群体特征和氮转化过程的相互作用，并分析其对土壤氮供应水平和林木生长的反馈，阐明杨树不同混交组合的氮素营养机制和适应性，对实现杨树人工林可持续经营具有重要的理论价值和实践意义。

土壤氮素的供给状况是限制杨树人工林生长的主要因子之一。与固氮树种的混交可以通过根系有机碳的分泌作用影响根际土壤微生物的群落结构、功能特征及土壤养分转化过程，从而影响土壤氮的供应。根际土壤微生物多样性和功能群的组成对氮的转化过程和供应起着至关重要的作用，是评价杨树混交林生态系统混交效应的重点。本项目以杨树-桤木混交林为对象，以根际微生物过程为切入点，研究杨树混交林分根系有机碳分泌及其与根际土壤微生物群体特征和氮转化过程的相互作用，阐明杨树混交林的氮素营养机制和适应性。研究结果表明，杨-桤混交林林地和根际土壤pH值高于杨树纯林，杨树纯林根系有机酸大量分泌，根际土壤有机碳富集明显，表现出明显的根际酸化现象。混交林林地土壤具有较高的氮供应水平，桤木根际氮富集明显。混交林根际土壤碳、氮转化相关酶活性显著高于纯林，有较高的微生物分解活性。混交林土壤微生物对碳源的利用能力高于纯林，且对更多种碳源表现出显著的根际效应。相较于纯林，混交林、特别是杨树根际有更高的细菌、真菌种类和Chao1指数，根际效应明显，表明根际弱势种群的增加。混交林和纯林的细菌β多样性差异不大，但混交林根际固氮菌类群优势明显，体现出了固氮树种的混交效应；而纯林的担子菌门丰度显著高于混交林，表现出以菌根共生为养分获取策略。混交林和纯林以及根际和非根际在土壤微生物群落组成上表现出显著的差异，其中根际与非根际土壤细菌、真菌多样性的差异尤为显著。根系低分子量有机碳的分泌显著提高了根际土壤的微生物分解活性，特别是低分子量有机酸多样性的提高对土壤氮的总矿化有一定程度的促进效果。混交林不同树种根系有机碳的分泌特征反映了其对养分的利用策略。项目资助发表SCI收录论文2篇，已接受中文核心期刊论文1篇，待发表论文4篇。培养研究生7名，其中3名已取得硕士学位，4名在读。项目经费的支出与预算基本相符，剩余经费计划用于本项目研究的后续支出。