Midori Yano

Nitrate is an important nitrogen source for Arctic tundra plants

Significance How terrestrial plants use N and respond to soil N loading is central to evaluating and predicting changing ecosystem structure and function with climate warming and N pollution. Here, evidence from NO3− in plant tissues has uncovered the uptake and assimilation of soil NO3− by Arctic tundra plants, which has long been assumed negligible. Soil NO3− contributed about one-third of the bulk N used by tundra plants of northern Alaska. Accordingly, the importance of soil NO3− for tundra plants should be considered in future studies on N and C cycling in Arctic ecosystems where C sequestration is strongly determined by N availability. Plant nitrogen (N) use is a key component of the N cycle in terrestrial ecosystems. The supply of N to plants affects community species composition and ecosystem processes such as photosynthesis and carbon (C) accumulation. However, the availabilities and relative importance of different N forms to plants are not well understood. While nitrate (NO3−) is a major N form used by plants worldwide, it is discounted as a N source for Arctic tundra plants because of extremely low NO3− concentrations in Arctic tundra soils, undetectable soil nitrification, and plant-tissue NO3− that is typically below detection limits. Here we reexamine NO3− use by tundra plants using a sensitive denitrifier method to analyze plant-tissue NO3−. Soil-derived NO3− was detected in tundra plant tissues, and tundra plants took up soil NO3− at comparable rates to plants from relatively NO3−-rich ecosystems in other biomes. Nitrate assimilation determined by 15N enrichments of leaf NO3− relative to soil NO3− accounted for 4 to 52% (as estimated by a Bayesian isotope-mixing model) of species-specific total leaf N of Alaskan tundra plants. Our finding that in situ soil NO3− availability for tundra plants is high has important implications for Arctic ecosystems, not only in determining species compositions, but also in determining the loss of N from soils via leaching and denitrification. Plant N uptake and soil N losses can strongly influence C uptake and accumulation in tundra soils. Accordingly, this evidence of NO3− availability in tundra soils is crucial for predicting C storage in tundra.

N2O production by denitrification in an urban river: evidence from isotopes, functional genes, and dissolved organic matter

Rivers are important sources of N2O emissions into the atmosphere. Nevertheless, N2O production processes in rivers are not well identified. We measured concentrations and isotopic ratios of N2O, NH4+, NO2−, and NO3− in surface water to identify the microbial processes of N2O production along the Tama River in Japan. We also measured the functional gene abundance of nitrifiers and denitrifiers (amoA-bacteria, nirK, nirS, nosZ clade I, nosZ clade II) together with concentrations of dissolved organic carbon (DOC) and fluorescence intensities of protein and humic components of dissolved organic matter (DOM) to support the elucidation of N2O production processes. The observed nitrogen (δ15N) and oxygen (δ18O) of N2O were within the expected isotopic range of N2O produced by nitrate reduction, indicating that N2O was dominantly produced by denitrification. The positive significant correlation between N2ONet concentration and nirK gene abundance implied that nitrifiers and denitrifiers are contributors to N2O production. Fluorescence intensities of protein and humic components of DOM and concentrations of DOC did not show significant correlations with N2O concentrations, which suggests that DOC and abundance of DOM components do not control dissolved N2O. Measurement of isotope ratios of N2O and its substrates was found to be a useful tool to obtain evidence of denitrification as the main source of N2O production along the Tama River.