Whole-ecosystem warming increases plant-available nitrogen and phosphorus in the SPRUCE bog

Colleen Iversen, Jana R. Phillips and Paul J. Hanson, Climate Change Science Institute and Environmental Sciences Division, Oak Ridge National Laboratory, Oak Ridge, TN, John Latimer and Keith Olehieser, XCEL Engineering, Oak Ridge, TN, Deanne J. Brice, Joanne Childs, Natalie A. Griffiths and Verity G. Salmon, Environmental Sciences Division and Climate Change Science Institute, Oak Ridge National Laboratory, Oak Ridge, TN, Holly M. Vander Stel, Kellogg Biological Station, Michigan State University, Hickory Corners, MI, Avni Malhotra, Department of Earth System Science, Stanford University, Stanford, CA, Jake Graham, Boise State University, Richard Norby, Ecology and Evolutionary Biology, University of Tennessee Knoxville, Knoxville, TN, Stephen D. Sebestyen, Northern Research Station, USDA Forest Service Research, Grand Rapids, MN

Background/Question/Methods
Warming is expected to increase the release of carbon from highly-organic peatland soils, potentially leading to a positive feedback to future warming. This response is expected to be mediated by the response of peatland vegetation to rising atmospheric [CO2], as well as the effects of warming on plant-available nutrients and water. We quantified the effects of a range of whole-ecosystem warming (from +0°C to +9°C), as well as elevated [CO2], on plant-available nutrients in the SPRUCE (Spruce and Peatland Responses Under Changing Environments; mnspruce.ornl.gov) experiment in a nutrient-limited ombrotrophic bog in northern Minnesota, USA. Two questions guided our research: (1) Does a gradient of warming, as mediated by elevated [CO2] and changing water table levels, increase plant-available nutrients? (2) Does the availability of nitrogen (N) and phosphorus (P) increase at differing rates in response to warming? We further considered whether these responses to warming varied across microtopographic positions and peat depth and over time, focusing in particular on the rooting zone. Also, because bogs are unique ecosystems bathed in porewater that is a potential source of nutrients for plant acquisition, we compared resin-available nutrient dynamics to the dynamics of inorganic and organic porewater nutrients.
Results/Conclusions
NH4-N was by far the most available N source, with NO3-N making up a negligible fraction; PO4-P availability was intermediate. Resin-available NH4-N and PO4-P increased with peat depth and tended to be greater in depressed hollow compared with raised hummock microtopography. We found that: (1) Warming increased resin-available NH4-N and PO4-P, but that the magnitude of the response increased over time and varied across the highly heterogeneous bog surface and with peat depth. These dynamics were generally unaffected by elevated [CO2] or the relatively minor changes in water table depth with warming. (2) We did not find any significant differences in the relative responses of resin-available NH4-N and PO4-P to warming, though the increasing NP ratio with peat depth indicated increasing P limitation. Resin-available nutrient dynamics were somewhat correlated with inorganic and organic porewater nutrients, but these methodologies likely represent different nutrient pools. Furthermore, we observed complex interactions with the declining uptake of nutrients by ubiquitous non-vascular Sphagnum mosses with warming. Predictions of peatland nutrient availability under climate change scenarios must account for microtopography, peat depth, and the interplay between vegetation nutrient acquisition and microbial activity.