Temperature sensitivity of peatland C and N cycling: Does substrate supply play a role?

Northern peatlands constitute an important component of the global carbon (C) cycle due to their long-term accumulation of soil organic matter. This function as a carbon sink is partly dependent on low temperatures limiting decomposition and nutrient cycling, so global warming has the potential to alter the C balance of these systems and feedback to climate change. Field observations have shown that peatland organic matter decomposition, ecosystem respiration and nitrogen cycling are closely related processes that show a large degree of temperature sensitivity. In the current study, we investigated whether seasonal dynamics of substrate input may be an indirect mechanism accounting for this observed sensitivity. We carried out a 60-day mesocosm incubation experiment with sub-arctic peat soil to compare the direct effects of temperature increase with the indirect effects of increased microbial- or plant-derived organic matter input on key soil C and N cycling processes and substrate pools. Additions of dead microbial cells led to an 83% increase in organic N pool sizes, 16-64% increases in the potential activities of most soil enzymes, a transient increase in the relative abundance of beta-proteobacteria, and a decrease in the relative abundance of alpha-proteo-, Actino- and Acido-bacteria. Neither the addition of plant root litter, nor a 5 degrees C alteration in incubation temperatures, had comparable effects on these parameters. Peat respiration was positively affected by both substrate addition (20-46% increase) and higher incubation temperatures (34-38% increase), but the temperature-only effect was not sufficient to account for the increases in respiration observed in field experiments. Thus, it appears that warming effects on C and N cycle processes can potentially be driven by indirect effects, with alterations to the seasonal flux of microbe-derived organic matter a particularly potent mechanism. The high temperature sensitivity of decomposition and respiration may therefore be largely a result of warming-induced changes in substrate supply rates. We propose that climate change models of soil carbon and nitrogen cycling should seek to incorporate realistic microbial biomass dynamics. (C) 2013 Elsevier Ltd. All rights reserved.