Biogeochemical and vegetation controls on carbon storage and GHG losses in southeastern pocosin peatlands: Do drought and drainage really matter?

Background/Question/Methods

Natural peatlands are threatened by projected increases in prolonged droughts and increased temperatures according to recent IPCC models. Millions of hectares of former peatlands have also been drained and converted to agriculture and forestry in the U.S.  However, while drought and drainage promotes the decomposition of organic matter in soil, leading to accelerated soil subsidence, and severe carbon losses in northern sphagnum peatlands, shrub dominated SE peatlands seemingly defy accelerated loss of carbon. We postulate that some different controlling factors or substrates—directly from drought or indirectly from drought induced species changes—rather than anoxia alone must exist in the seasonally unsaturated SE peatlands and activate during drought, allowing these peat soils to resist further peat decomposition and CO₂ emission. To test this hypothesis we did comparative field studies of GHG fluxes and C storage in 2010-2011 in replicated drained, restored and reference pocosin peatlands. These pocosins (shrub bogs) are characterized by a very dense growth of mostly broadleaf evergreen shrubs with scattered Pinus serotina. The typically thick layer of peat soils 1-3 m (Histosols) underlying pocosins store nearly 300 Mt of carbon in NC alone, a potential major source of atmospheric carbon input if fully decomposed. 

Results/Conclusions

Comparative field studies of GHG fluxes in replicated drained, restored and reference sites indicate that losses were greatest for CO₂, and lowest for N₂O and CH₄.  Comparison of treatments found more CO₂ release under dryer soils but only immediately after drainage, further suggesting that climate induced drought or drainage does not results in continued massive losses of CO_2. Importantly, regression analysis showed that polyphenol was the main factor inhibiting CO₂emission. Pocosin peats were found to contain substantial amounts of polyphenol, derived from high phenolics in leaves and high lignin in pocosin shrub leaves and stems. Seasonal drought enhanced the release of polyphenol in soil with reduced phenol oxidase activity. However, most studies in saturated boreal peatlands show that a drop in water level introduced more oxygen and accelerated phenol oxidase activity, decomposing more polyphenol. The differing results between our seasonally unsaturated and saturated northern peatlands suggest that phenol oxidase activity may have a peak optimal moisture level threshold. Above this threshold, anoxia is the key factor inhibiting phenol oxidase activity in waterlogged peat soil. Below it, reduced soil moisture limits enzyme activity. Consequently, more polyphenol will exist under climate induced moderate drought helping shrub formed peat to resist decomposition.