Paper No. 33-1
Presentation Time: 1:35 PM
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.