Interactions between Nitrogen Fixation and Methane Cycling in Northern Minnesota Peat Bogs

Abstract:

Peatlands cover only 3% of the Earth’s surface, yet store a third of soil carbon. Increasing global temperatures have the potential to change peatlands from a net sink to a net source of atmospheric carbon. N is a limiting nutrient in oligotrophic Sphagnum-dominated peatlands and biological N₂ fixation likely supplies a significant but unknown fraction of N inputs. Moreover, environmental controls on diazotrophic community composition in N-limited peatlands are poorly constrained. Thus, improved understanding of feedbacks between the CH₄ and N cycles is critical for predicting future changes to CH₄ flux from peat bogs. We coupled measurements of N₂ fixation activity measured by the acetylene (C₂H₂) reduction assay (ARA) with molecular analyses of expression and diversity of nifH genes encoding the molybdenum (Mo)-containing nitrogenase from two peat bogs in the Marcell Experimental Forest, Minnesota, USA. The top 10 cm of peat was sampled from the high CH₄ flux S1 bog and the low CH₄ flux Zim bog in April and June 2014. Despite similar N concentrations in the top 10 cm of both bogs (0.5-1.0 µM NO₂^-+NO₃^- and 2-3 µM NH₄⁺), the S1 bog displayed variable ARA activity (1-100 nmol C₂H₄ h^-1 g^-1) whereas the Zim bog had consistently low ARA activity (<1 nmol C₂H₄ h^-1 g^-1). Highest ARA activity was measured in June from S1 bog hollows with higher moisture content incubated without O₂ in the light (20-100 nmol C₂H₄ h^-1 g^-1). Dissolved Fe (1-25 µM) was higher in hollow vs. hummock samples, and at S1 vs. Zim bog, while dissolved V (4-14 nM) was consistently higher than Mo (1-4 nM), suggesting that alternative V or Fe-containing nitrogenases might be present in these bogs. In contrast, Cu, an essential micronutrient for aerobic methanotrophs, was higher in hummocks (25-48 nM) than hollows (6-17 nM). The facultative methanotroph Methylocella was the dominant diazotroph in the S1 bog based on high throughput next generation sequencing of nifH cDNA amplicons. Given previous reports of C₂H₂ inhibition of methanotrophy, we measured CH₄ consumption in the presence or absence of 1% C₂H₂. Preliminary results suggest minimal effect of C₂H₂ on CH₄ oxidation. Future measurements of ¹⁵N₂ incorporation coupled to molecular analysis will elucidate whether methanotroph diazotrophy was suppressed by C₂H₂ in ARA incubations.