STIMULATION OF ANAEROBIC ORGANIC MATTER DEGRADATION BY NITROGEN ADDITION IN TUNDRA SOILS

Nitrogen (N) availability in arctic soils is increasing due to accelerated mineralization of organic N from soil organic matter, and from the increasing abundance of shrubs like alder that harbor N-fixing symbionts. This will relieve N limitation and stimulate growth of both plants (increasing C inputs to the soil via photosynthesis) and microbes (increasing C losses via organic matter decomposition). We investigated the effects of N addition on anaerobic organic matter degradation through both field and lab-based experiments. In the field experiment, we injected a solution of ¹³C- and ¹⁵N-labeled glutamate 35 cm below the soil surface in a tundra soil near Nome, Alaska, and observed the resulting changes in pore water geochemistry and dissolved greenhouse gas concentrations. In the lab experiments, we added either a single pulse or a continuous injection of glutamate to an anoxic column filled with soil collected from the same field site. In both experiments, the added N was quickly assimilated or immobilized within hours of injection, consistent with microbial N limitation. We also consistently observed increased concentrations of dissolved CH₄ and Fe(II), indicating stimulation of methanogenesis and Fe(III) reduction. Accumulation of low molecular weight organic acids such as acetate and propionate suggests that N addition stimulated the decomposition and fermentation of more complex organic compounds, as a result of relieved substrate limitation for anaerobic respiration. However, the resulting organic matter degradation appeared selective: while the total dissolved organic carbon (DOC) concentration declined by as much as 50%, there was no change in the size of the aromatic DOC fraction as indicated by UV-Vis absorbance. Together, these results indicate that increasing N availability in anoxic arctic soils could accelerate warming-induced CO₂ and CH₄ production by relieving N limitation of fermenting microorganisms.