Sources of Respired Carbon in a Northern Minnesota Ombrotrophic Spruce Bog: Preliminary 14C Results from the SPRUCE Site.

Abstract:

A significant uncertainty in future land-surface carbon budgets is the response of wetlands to climate change. A corollary and related question is the future net climate (radiative) forcing impact from wetlands. Active wetlands emit both CO₂ and CH₄ to the atmosphere. CH₄ is, over a few decades, a much more potent greenhouse gas than CO₂. CO₂ has a longer atmospheric lifetime and a longer 'tail' to its radiative influence. Whether wetlands are a net source or sink of atmospheric carbon under future climate change will depend on ecosystem response to rising temperatures and elevated CO2. The largest uncertainty in future wetland C-budgets, and their climate forcing is the stability of the large below-ground carbon stocks, often in the form of peat, and the partitioning of CO₂ and CH₄ released via ecosystem respiration.

In advance of a long-term experimental warming and elevated CO₂ manipulation at the DOE Spruce and Peatland Responses Under Climatic and Environmental Change (SPRUCE) site in the Marcell Experimental Forest, we have characterized the source of respired carbon used for both the production of CO₂ and CH₄. Samples were collected in early June, late July, and will be collected in early September from three large (~1.1 m², ~0.5m³) chambers from the control plot, and two of the experimental plots selected for heating (+9°C, +4.5°C).

Early June fluxes from the three chambers were ~5500 mgC-m^-2-d^-1 and ~16 mgC-m^-2-d^-1 for CO₂ and CH₄ respectively. Radiocarbon analysis of CO₂ and CH₄ indicate that the source for the respired carbon is for the most part recent, with most ¹⁴C values between 30 and 40‰ – i.e., carbon that was photosynthetically fixed in the last few years. In concert with rising air and ground temperatures fluxes in late July increased to ~6500 mgC-m^-2-d^-1 and ~86 mgC-m^-2-d^-1. Although deep-heating was initiated in mid to late June we hypothesize that the July respiration signal is dominated by the regular seasonal cycle of natural warming.

We will present additional flux (September) and isotope (July, September) data and place the results in the context of next year’s experimental manipulations.

This work was performed under the auspices of the U.S. Department of Energy by Lawrence Livermore National Laboratory under Contract DE-AC52-07NA27344.