Soil pH Controls on Anaerobic Microbial Activity and Organic Carbon Transformations in Arctic Polygon Tundra

Soil acidity directly affects pore water biogeochemical speciation and microbial community composition and interactions. Organic carbon-rich Arctic soils and permafrost vary significantly in pH, which is expected to influence the thermodynamics of dominant anaerobic processes of soil organic matter (SOM) hydrolysis and fermentation, methanogenesis, and anaerobic respiration through iron reduction. While ecosystem models usually consider pH as a fixed parameter, hydrologic changes and biogeochemical activities can cause changes in pH gradients through the soil column. We measured the buffering capacity of polygon tundra soil and permafrost from Utqiaġvik (formerly Barrow), Alaska, which was significantly lower than the values reported from soils in temperate regions. Predominant quartz minerals identified in Arctic soils by X-ray diffraction have minimal contribution to buffering capacity or cation exchange. Iron (Fe) K-edge X-ray absorption spectroscopy and extended X-ray absorption fine structure analyses identified Fe(II) and Fe(III) in the soils as organic-bound or poorly crystalline iron (oxyhydr)oxides, which contributed to buffering capacity at low pH. We developed a simple representation of proton-binding characteristics of SOM using a selective range of proton binding constants from organic acids and lignin derivatives commonly found in soil, which explained a large portion of pH buffering capacity in these soils. Substantial pools of iron (oxyhydr)oxides also buffered pH via simulated dissolution/precipitation reactions and formation of complexes. Anoxic incubation experiments using soils buffered at acidic and alkaline pH identified changes in methanogenesis and iron reduction that illustrate differences in pH response factors for anaerobic processes and their potential feedbacks on soil pH. Molecular analyses of microbial community composition changes in these incubations describe organismal feedbacks on key biogeochemical processes. Separate parameterizations of pH response functions for fermentation, iron reduction and methanogenesis in a biogeochemical kinetic model improved the simulation of pH evolution, including the initial pH drop due to organic acid accumulation caused by fermentation and then a pH increase due to iron reduction and methanogenesis.