根系分泌物对毛竹扩张引起阔叶林土壤“增铵抑硝”效应的解析

Bamboo expansion into adjacent forests has imposed serious ecological threats on ecosystem processes and functions. More insights into the underlying mechanisms are essential for understanding the negative influences of bamboo expansion. Our previous studies indicated that the expansion of Phyllostachys edulis significantly altered the N biogeochemisty cycling of adjacent secondary evergreen broad-leaved forest, with the most prominent of effect being the enhanced soil ammonification but inhibited nitrification. We speculated that the root exudation might be the main force driving for this pattern. .To test this hypothesis, we plan to perform an experiment field study in the previous plots of Ph. edulis expansion into evergreen broad-leaved forest, in Dagang Mountain National Forest Ecological Station of Jiangxi Province, China. We would take Ph. edulis, Castanopsis fargesii and Schima superba (two dominate species of broadleaved forest) as experimental objects, and then to determine the root exudation release rates and chemical compositions of these three plants. The soil microorganism community composition, structure and function as well as soil enzyme activities would be measured for the rhizosphere and non-rhizosphere soil of three plants. Further, a laboratory incubation experiment would be also conducted to quantify the effects root exudation on the activities of microorganism communities, ammonification and nitrification rates. Finally, we would try to build an intrinsic correlation of root exudation and soil microorganism and soil N transformation processes to elucidate how root exudation drives the N transformation in the processes of Ph. edulis expansion into evergreen broadleaved forest. .The research achievements would not only shed light on the internal mechanisms for the effects of Ph. edulis expansion on N biogeochemistry cycling of adjacent ecosystems, but also be beneficial to N management fertilizer of Ph. edulis forest.

竹子扩张已严重影响到邻近森林生态系统的过程与功能，解析扩张效应及机制，是认识竹子扩张危害的基础。前期研究发现，毛竹向邻近次生常绿阔叶林扩张，增强了土壤N素氨化作用，抑制了硝化作用，即“增铵抑硝”效应，并推测根系分泌物是诱发这一效应的重要原因。为此本项目通过野外调查采样，重点研究：毛竹与阔叶树(丝栗栲、木荷)根系分泌物数量与化学成分；毛竹与阔叶树根际和非根际土N转化相关的微生物组成、结构与功能及土壤酶活性特征。室内培养试验分析根系分泌物对微生物活性及N转化速率的影响，探讨根系分泌物-微生物-N转化速率的关联性，解析毛竹扩张过程中“增铵抑硝”效应的发生机制。研究成果可深入认识毛竹扩张对周边森林N素生物地球化学循环的影响，同时对毛竹林N肥管理也有一定指导作用。

竹子扩张已严重影响到邻近森林生态系统的过程与功能，解析扩张效应及机制，是认识竹子扩张危害的基础。前期研究发现，毛竹向邻近次生常绿阔叶林扩张，增强了土壤N素氨化作用，抑制了硝化作用，即“增铵抑硝”效应，并推测毛竹与阔叶树之间根际微生物及根系分泌物的差异性是诱发这一效应发生的关键原因。.为验证这一推测，2019-2021年在江西省官山国家自然保护区采集毛竹与阔叶树丝栗栲(Castanopsis fargesii)、红楠(Machilus thunbergii)及木荷(Schima superba)根际土壤，通过常规方法检测土壤养分含量，微生物高通量测序分析氮转化的微生物群落的组成及功能；气质联用仪（GC-MS）测定植物根系分泌物成分及相对丰度，解析毛竹扩张过程中土壤氮素转化改变的根系分泌物机制。结果表明：.(1) 毛竹与阔叶树根系分泌物组成存在显著的差异。毛竹以醇类物质为主，而丝栗栲、红楠与木荷则以酯类物质为主，尤其是十六酸乙酯在木荷、丝栗栲中占比最高。.(2) 毛竹根系分泌物对3种阔叶树根际土壤土壤氨化作用具有促进作用，而对硝化作用一定的抑制作用。.(3) 毛竹根际氨化细菌丰度土壤氨化细菌序列数量高于红楠和丝栗栲，但氨化细菌的多样性指数与均匀度指数显著低于3种阔叶树。相反，毛竹根际土壤亚硝化细菌与古菌丰度均低于阔叶树，尤其是亚硝化古菌群落主要受毛竹高土壤pH的影响，其丰度下降更明显。.(4) 毛竹氨化代谢通路蛋白的丰度高于红楠、丝栗栲，但低于木荷；相反，毛竹亚硝化代谢通路蛋白的相对丰度显著低于阔叶树，其根际土壤未发现氨单加氧酶，而木荷的氨单加氧酶的相对丰度最高。.以上结果表明，由根系分泌物引起的土壤氮化细菌和硝化微生物和酶活性的变化是毛竹扩张引发土壤“增氨抑硝”效应的的关键原因。研究成果可深入认识毛竹扩张对周边常绿阔叶林氮素生物地球化学循环的影响，同时对毛竹林氮素微生物肥料的开发有一定启发作用。

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