Dual controls on carbon loss during drought in peatlands

Abstract

Peatlands store one-third of global soil carbon 1 . Drought/drainage coupled with climate warming present the main threat to these stores 1-4 . Hence, understanding drought e ects and inherent feedbacks related to peat decomposition has been a primary global challenge 5,6 . However, widely divergent results concerning drought in recent studies 3,7-11 challenge the accepted paradigm that waterlogging and associated anoxia are the overarching controls locking up carbon stored in peat. Here, by linking field and microcosm experiments, we show how previously unrecognized mechanisms regulate the build-up of phenolics, which protects stored carbon directly by reducing phenol oxidase activity during short-term drought and, indirectly, through a shift from low-phenolic Sphagnum/herbs to high-phenolic shrubs after long-term moderate drought. We demonstrate that shrub expansion induced by drought/warming 2,6,10,12,13 in boreal peatlands might be a long-term self-adaptive mechanism not only increasing carbon sequestration but also potentially protecting historic soil carbon. We therefore propose that the projected 'positive feedback loop' between carbon emission and drought in peatlands 2,3,14,15 may not occur in the long term.Peatlands, covering only 3% of Earth's land area, store about 445 Pg of carbon 1 . These stores result from a small imbalance between production and decomposition over millennia under predominantly waterlogged conditions. However, drought and drainage coupled with warming have been substantially lowering water levels for decades and have resulted in a degradation of more than 11% of global peatlands 1 . These hydrologic shifts often threaten carbon stores, changing peatlands from a carbon sink to a carbon source by increasing decomposition 2,3,[14][15][16] ; concomitantly, the crucial peat-forming Sphagnum mosses are replaced by shrubs/trees 2,10,12 with possibly substantial feedbacks on climate change 6,10 . However, recent evidence showed that in some peatlands drought had little impact or even decreased CO 2 emission and increased carbon accumulation [7][8][9][10][11][17][18][19] (Supplementary Table 1). These contrasting results raise uncertainty on the future fate of peat carbon and question the conventional theory that anoxia is the key to sustaining peat carbon.Detailed comparisons of these drought studies (for example, refs 2,3,7-11,14,15,17,18) show that the initial water level and dominant species varied before drought manipulation (Supplementary Table 1), indicating that these peatlands were in different successional stages 6 . In Sphagnum peatlands, where water levels were mostly above or near the ground surface, drought increased CO 2 emission. However, drought seemed to have less impact in unsaturated and shrub/tree-dominated peatlands. Phenolic inhibitory e ect is defined as a negative value of Pearson's r between soil respiration and soluble phenolics in the surface soil. The grey square is one masked value (removed from the regression) in a drained site where samples were collected a...

Tipping point in plant–fungal interactions under severe drought causes abrupt rise in peatland ecosystem respiration

“…We further observed that the spectrophotometric slope coefficient S 250‐465 of WEOM, an index inversely related to the molecular weight and aromaticity of dissolved organic matter (Hansen et al., ), was greater under low water level and high graminoid cover. In accordance with other studies (Dieleman, Branfireun, & Lindo, ; Robroek et al., ; Wang, Richardson, & Ho, ), we interpret these results as evidence that the quality of WEOM was controlled by vascular plants, and especially by graminoids which released less aromatic C compounds presumably as root exudates (Crow & Wieder, ). Root exudates are indeed predominantly composed of simple labile C compounds, such as sugars (Gunina & Kuzyakov, ), easily available for microorganisms and easily respired as CO 2 (Werth & Kuzyakov, ).…”

Estimation and Uncertainty of Recent Carbon Accumulation and Vertical Accretion in Drained and Undrained Forested Peatlands of the Southeastern USA

“…An additional explanation, which may be at least as important, involves the ubiquitous presence of shrubs throughout southeastern swamps. As mentioned above, the high polyphenol content of these shrubs and their litter, which becomes incorporated into peat, has been shown to retard decomposition under drought (Wang et al, ), which is fundamentally similar to drained conditions. These two factors, increased productivity and decreased decomposition, likely are responsible for carbon accumulation rates in CDR being similar to those of undisturbed forested peatland systems.…”

Drained coastal peatlands: A potential nitrogen source to marine ecosystems under prolonged drought and heavy storm events—A microcosm experiment

“…Most previously studied drought incubation experiments were relatively short-term manipulations, between several days and 1-2 months (Estop-Aragonés and Blodau, 2012; Fierer and Schimel, 2002;Venterink et al, 2002). In this experiment, we employed a 14-month drought, which is a rare but realistic event in our field situation, thus conducive to detecting an effect of long-term drought durations (Wang et al 2015). The strong temporal variations of NMR (Fig.…”

“…Generally, the lowest and highest water levels occur in summer and winter, respectively. Throughout the year, groundwater levels rarely rise above ground surface and often reach 20 cm below the surface, but fall to over 100 cm in depth in the summer (Wang et al, 2015).…”