Defining the Roles of Microbial Taxa in Soil Nitrogen Turnover

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Soil microorganisms, especially bacteria and fungi, are essential players in organic matter decomposition. Their activities both release and sequester plant-available nutrients, such as nitrogen. Although thousands of different species of bacteria and fungi participate in nitrogen cycling, little is known about which species most actively participate or their preferences for different forms of nitrogen. This research will use a new, state-of-the-art method to identify microbial species that use different forms of nitrogen and how this changes across a range of natural and managed soils. The information gained by this research has the potential to deepen our understanding of how microorganisms process nitrogen in soils, which has implications for our ability to forecast plant and ecosystem responses to environmental change. For example, if different types of fungi and bacteria process nitrogen in the same way, then current models of nitrogen cycling are robust to changes in species composition; however, if there is specialization, then models would be substantially improved by incorporating these differences. Ultimately, this information will lead to better understanding of how nitrogen is cycled and has the potential for improving how nitrogen is managed to sustain the productivity of terrestrial ecosystems.The researchers will couple a new, high-sensitivity stable isotope probing method (Chip-SIP), with the well-established method of 15N isotope pool dilution used to measure gross N cycling rates in soil. Chip-SIP uses NanoSIMS isotopic imaging to quantitatively measure incorporation of 15N substrates into microbial rRNA hybridized to a phylogenetic microarray. The objective is to identify the dominant microbial taxa assimilating organic and inorganic forms of N in soils with histories of high and low N inputs. Soils from three sites with a 30-year+ history of high and low N inputs will be used to answer two questions: 1) Which microbial taxa dominate the assimilation of organic N vs. inorganic N sources, and does it depend on N input history? 2) How does the ratio of NH4+ assimilation/nitrification relate to the types and diversity of dominant taxa, and is this relationship influenced by N input history? To address these questions, N assimilation will be determined by adding tracer levels (low concentrations, high enrichments) of 15N-labeled substrates, and using the rate of isotope dilution to calculate gross N process rates, and Chip-SIP to identify microorganisms actively assimilating N. The data generated will provide insights into microbial structure-function relationships for key biogeochemical processes in the soil N cycle, which are integral to productivity and sustainability of natural and managed terrestrial ecosystems. This novel application of Chip-SIP combined with isotope pool dilution in soils will identify the dominant microbial taxa assimilating organic vs. inorganic N under conditions of low and high N inputs. By correlating these taxa with process rates, and measuring how N processing rates and active taxa change when the activities of different functional groups are suppressed, this research will provide insights into the nature of their interactions and establish if N inputs affect competitive and facilitative behaviors in N cycle processes. Finally, incorporation of taxa-specific N assimilation parameters into N cycle models holds promise for improvement of their predictive capability.