Summer warming accelerates sub-arctic peatland nitrogen cycling without changing enzyme pools or microbial community structure

The balance of primary production and decomposition in northern peatlands may shift due to climate change, with potential feedbacks to atmospheric CO2 concentrations. Nitrogen availability will modulate this shift, but little is known about the drivers of soil nitrogen dynamics in these environments. We used a long-term (9 years) open top chamber (OTC) experiment in an ombrotrophic Sphagnum peat bog in sub-arctic Sweden, to test for the interactive effects of spring warming, summer warming and winter snow addition on soil nitrogen fluxes, potential activities of nitrogen cycle enzymes, and soil microbial community composition. These simultaneous measurements allowed us to identify the level of organization at which climate change impacts are apparent, an important requirement for developing truly mechanistic understanding. Organic-N pools and fluxes were an order of magnitude higher than inorganic-N pools and fluxes. Summer warming approximately doubled fluxes of soil organic nitrogen and ammonia over the growing season. Such a large increase under 1 degrees C warming is unlikely to be due to kinetic effects, and we propose that it is linked to an observed seasonal decrease in microbial biomass, suggesting that N flux is driven by a substantial late-season dieback of microbes. This change in N cycle dynamics was not reflected in any of the measured potential peptidase activities. Moreover, the soil microbial community structure was apparently stable across treatments, suggesting a non-specific microbial dieback. Our results show that in these widespread peat bogs, where many plant species are capable of organic-N uptake, organic soil N dynamics are quantitatively far more important than the commonly studied inorganic-N dynamics. Understanding of climate change effects on organic soil N cycling in this system will be advanced by closer investigation of the seasonal dynamics of the microbial biomass and the input of substrates that maintain it.