Dissertation Research: Linking N cycling bacterial community composition and function along a mycorrhizal gradient

在所有生态系统中，土壤细菌在氮的循环中起着核心作用，植物的生产力往往受到氮的可用性的限制。某些类型的细菌能够从大气中捕获氮，并通过固氮过程使其为植物所利用。其他细菌群利用它们的新陈代谢将氮返回到大气中。土壤中的植物根系和真菌可以影响细菌，但这些相互作用对氮循环的影响尚不清楚。该项目的目的是(1)表征n循环反应与不同细菌群之间的关系，以及(2)探索这些关系如何受到植物根和真菌的影响。了解土壤因子如何控制植物氮素有效性对农业和林业具有重要意义。该博士论文改进资助项目将加强研究生的论文研究，为本科生提供发展生物地球化学领域和实验室技术的机会，并将整合到波士顿大学生物地球科学项目组织的外展活动中。植物根系和菌根真菌可以影响土壤中无机氮的形态和有效性，从而潜在地影响氮循环细菌功能群的组成和活性。本研究将验证三个假设：(1)在以外生菌根真菌（ECM）为主的林分中，相对于以丛枝菌根真菌（AM）为主的林分，植物和土壤微生物对N的竞争加剧，它们会选择固氮剂，并选择硝化和反硝化剂；(2)根际土壤对无机氮的高需要量将对重氮营养菌产生偏好，对硝化和反硝化菌产生排斥；(3)根际效应随ECM丰度的增加而增大。为了验证这些假设，将在马萨诸塞州彼得舍姆的哈佛森林LTER站点建立菌根梯度，对根际土壤样品中与n固定物、硝化物和反硝化物相关的基因拷贝（DNA）和基因转录物（RNA）的丰度进行量化。稳定同位素方法将用于测量固氮、硝化和反硝化的速率。这些配对分析将阐明微生物群落组成和功能之间的关系，并确定这些关系如何根据n循环细菌、菌根真菌和植物根系之间的生物相互作用而变化。

Soil bacteria play a central role in the cycling of nitrogen (N) in all ecosystems, where plant productivity is often limited by the availability of N. Certain types of bacteria are able to capture N from the atmosphere and make it available for plants through the process of N-fixation. Other bacterial groups use their metabolism to return N to the atmosphere. Plant roots and fungi in soil can influence bacteria, but the effect of these interactions on N cycling is unclear. The aims of this project are to (1) characterize relationships between N-cycle reactions and different groups of bacteria, and (2) to explore how these relationships are influenced by plant roots and fungi. Understanding how factors in soil control N-availability for plants is important for agriculture and forestry. This Doctoral Dissertation Improvement Grant project will enhance the graduate student's dissertation research, provide opportunities for undergraduate students to develop field and laboratory techniques in biogeochemistry, and will be integrated in outreach activities organized through the Boston University Biogeoscience program. Plant roots and mycorrhizal fungi can influence the form and availability of inorganic N in soil, potentially impacting the composition and activity of N-cycling bacterial functional groups. This research will test three hypotheses: (1) enhanced competition for N between plants and soil microbes in forest stands dominated by ectomycorrhizal (ECM) fungi relative to stands dominated by arbuscular mycorrhizal (AM) fungi will select for N-fixers and select against nitrifiers and denitrifiers; (2) high plant demand for inorganic N in rhizosphere soils relative to bulk soils will select for diazotrophs and select against nitrifiers and denitrifiers; and (3) the magnitude of the rhizosphere effect will increase as ECM abundance increases. To test these hypotheses, the abundance of gene copies (DNA) and gene transcripts (RNA) associated with N-fixers, nitrifiers, and denitrifiers will be quantified in rhizosphere soil samples across a mycorrhizal gradient established at the Harvard Forest LTER site in Petersham, MA. Stable isotope approaches will be used to measure rates of N-fixation, nitrification and denitrification. These paired analyses will elucidate relationships between microbial community composition and function and identify how these relationships vary based on biotic interactions between N-cycling bacteria, mycorrhizal fungi and plant roots.