Persistent high temperature and low precipitation reduce peat carbon accumulation.

Abstract

The ongoing climate change is expected to modify the atmospheric carbon (C) sink function of peatlands, which store about 450 Gt of carbon accumulated over centuries and millennia (Fenner & Freeman, 2011). Indeed, the simultaneous rise of temperature and decrease in water level favors soil oxygenation, organic matter (OM) decomposition and, in turn, the release of large amounts of CO2 into the atmosphere. Recently, Bragazza et al. (2016) suggested that climate extreme events reduce carbon accumulation in peatlands by up to 30% owing to the decline of Sphagnum productivity and the rise of microbial decomposition of OM. However, investigating the impact of climate extreme events on OM decomposition requires that indirect effects, such as vegetation changes and water physicochemical properties, did not falsify interpretation of the changes in the microbial activities (Delarue et al., 2014, 2015). By investigating the chemical composition of peat, Bragazza et al. (2016) argued that climate extreme events lead to a ‘rapid increase in OM decomposition due to enhanced microbial metabolism’. This assumption was solely based on the disappearance of the chemical distinction between the above- and belowground layers of the peat through extreme climate events, which is hard to evaluate because Fourier transform infrared spectroscopy (FTIR) spectra, calculation modes and error bars are lacking in their study. Such an observation can be alternatively linked to the changes in the plant cover or in OM decomposition. After 3 years of experiment, Bragazza et al. (2016) demonstrated that the cover of vascular plant species increased from ca. 26% to 39%, whereas that of Sphagnum fallax decreased by ca. 50%. S. fallax, and Eriophorum vaginatum leaves share equivalent FTIR spectra revealing, in turn, no differences at the scale of the structure of OM (Fig. 1a). An effect of the aromatic-enriched root biomass of E. vaginatum (IR absorption band at 1600 cm−1; see Fig. 1a) cannot be excluded as it represents a substantial part of the aboveground biomass. As pointed out by Bragazza et al. (2016), OM decomposition may then be the main driver of the chemical composition of peat, although it may be promoted by the addition of new vegetation litter (Gogo et al., 2014). As suggested by the study of two adjacent areas distinguished by slight changes in vegetation and water content (Delarue et al., 2011; Fig. 1b, c), the observed chemical shift is more likely attributed to preferential decomposition of labile polysaccharides rather than decomposition of ‘complex OM’ as suggested by Bragazza et al. (2016). As a result, the aboveground layer became more and more aromatized due to the increase in microbial activity associated with a preferential decay of polysaccharides. This process did not occur in the belowground layer (initially depleted in readily available polysaccharides), which remained dominated by aromatic moieties, leading, after 3 years of experiment, to the lack of chemical distinction between these two layers. Following the example illustrated in Fig. 1b, c, the increase in polysaccharide decomposition seems more likely driven by the combined effect of the decrease in water content and of litter quality change rather than temperature. The effect of climate extremes on peat decomposition was also studied through dissolved organic carbon (DOC) production and aromatization index (Bragazza et al., 2016). DOC is a transient compartment within the soil C cycling (Schimel & Weintraub, 2003), whose apparent value depends on the imbalance between its production (peat decomposition) and consumption (mineralization). The value of DOC can also change through dilution/concentration effects determined through water content measurements. Bragazza et al. (2016) did not consider this issue for the pore-water chemistry. Evaluated on the basis of their figures, DOC, SUVA256, HIX and POA rise by up to ca. 40%, 15%, 165% and 15%, respectively. These parameters are expressed as a function of the water sampling volume (error bars were not provided) rather than the real water content, which decreased through climate extreme events (by ca. 28%). Hence, a large part of the observed increase in DOC, SUVA256 and POA values is explained by a simple concentration effect caused by the reduction in water content. The HIX index is the only parameter for which variations seem disconnected to the water content decrease. However, the value of the HIX index is highly sensitive to the DOC concentration, then requiring an additional correction (Ohno, 2002) that was not considered. Hence, both the effect of water content and DOC concentration can invalidate the value of the HIX index. Considering these pitfalls, dissolved OM does not provide compelling evidence for the enhancement of microbial activities. These conclusions shed light on the current pitfalls in understanding the effect of climate extreme events on peat microbial degradation through peat and pore-water chemistries. This work was funded by the ANR PEATWARM project grant (ANR-07-VUL-010).