Effects of short-term ecosystem experimental warming on water-extractable organic matter in an ombrotrophic Sphagnum peatland (Le Forbonnet, France)

Abstract

In a future warmer world, peatlands may change from a carbon sink function to a carbon source function. This study tracks changes in water-extractable organic matter (WEOM) after 1 year of in situ experimental warming using open top chambers (OTCs). WEOM was studied in the upper peat layers (0–10 cm) through analysis of water-extractable organic carbon (WEOC), stable C isotopic composition (δ¹³C), specific UV absorbance at 280 nm and sugar composition of cores taken from an open bog (DRY sites) and a transitional poor fen (WET sites). At the DRY sites, the impact of OTCs was weak with respect to WEOM parameters, whereas at the WET sites, the air warming treatment led to a decrease in peat water content, suggesting that the supply of heat by OTCs was used mainly for evapotranspiration. OTCs at the WET sites also induced a relative enrichment at the surface (0–5 cm depth) of aliphatic and/or aromatic compounds with concomitant decrease in WEOC, as a result of decomposition. On the contrary, WEOC and sugar content increased in the deeper peat layer (7.5–10 cm depth) probably as a result of increased leaching of phenolic compounds by roots, which then inhibits microbial activity. The different response to experimental warming at DRY and WET sites suggests that the spatial variability of moisture is critical for understanding of the impact of global warming on the fate of OM and the carbon cycle in peatlands.

Introduction

Owing to an imbalance between primary production and organic matter (OM) decay, northern peatlands – which contain about 455 Pg C – currently act as an important C sink (Clymo, 1983, Gorham, 1991). Peatland sink function is mainly the result of the interaction of several factors, such as being water logged, anoxia, acidity and low temperature, which limit OM decomposition (Moore and Knowles, 1990, Laiho, 2006). Peatlands are predominantly abundant in continental boreal and subarctic regions where a greater temperature increase is expected over the next century (Immirzi and Maltby, 1992, IPCC, 2007). If environmental constraints which favour C sequestration change (Davidson and Janssens, 2006), peatlands may switch from a C sink function to a C source function (Oechel et al., 1995, Waddington and Roulet, 1996).

To determine the response of peatland ecosystems to climate change, in situ warming experiments are now commonly performed, e.g. with open-top chambers (OTCs). Most studies of the impact of OTCs on peatland functioning have dealt with changes in plant communities and primary production (Dorrepaal et al., 2003, Aerts et al., 2006, Sullivan et al., 2008) or with CO₂–CH₄ balance (Welker et al., 2004, Chivers et al., 2009, Dorrepaal et al., 2009). Recently, on the basis of litter bag experiments, Dabros and Fyles (2010) studied the impact of OTCs on soil OM decomposition, including nutrient supply and acidity. In contrast to the study of Dorrepaal et al. (2009) on C respired, Dabros and Fyles (2010) showed that higher air temperature induced by 14 months’ OTC treatment (i) reduced the temperature of the soil as a result of increased evapotranspiration (the paradox of “colder soils in a warmer world”; Groffman et al., 2001) and (ii) had no effect on the decomposition rate of Sphagnum and spruce litter.

The biogeochemical processes in early peat OM decomposition remain poorly understood and thus constitute a limiting factor in understanding the fate of C pools in peatlands (Limpens et al., 2008, Zaccone et al., 2008). Furthermore, experimental in situ air warming studies have more often focussed on ecosystem responses such as gas exchange at the surface and seldom take into account organic C pools where various reactions to increased temperature may be expected (Davidson et al., 2000, Kirschbaum, 2000, Knorr et al., 2005).

Water-extractable OM (WEOM) reflects OM decomposition (Said-Pullicino et al., 2007) and can therefore be a suitable indicator of the consequences of experimental warming. WEOM consists of a heterogeneous mixture of more or less labile organic compounds soluble in water (Balesdent, 1996, Zsolnay, 2003) and is provided by both freshly decomposed litter and products of microbial metabolic activity (Charman, 2002, Zaccone et al., 2009). Within such pools, the most labile OM has been studied mainly through the analysis of sugars, which are considered readily degradable constituents used preferentially by microorganisms (Haider, 1992, Volk et al., 1997). In investigating sugar composition and its link to microbial activity, Medeiros et al. (2006) showed that some sugars such as mannitol, a polyol or reduced sugar, can also be seen as an indicator of osmotic stress. On the other hand, the less labile OM of the WEOM is often investigated by way of specific ultraviolet absorbance at 280 nm (SUVA₂₈₀), which provides an estimate of aromaticity of the WEOM (Traina et al., 1990, Kalbitz et al., 2003, Weishaar et al., 2003).

The aim of the present work was to investigate the impact of in situ experimental air warming on WEOM properties in the upper 10 cm of peat, where most of the labile OM is decomposed. We hypothesized that peatland warming has detectable consequences on WEOM properties and that some biogeochemical parameters can be used as early indicators of change. We considered the impact of OTCs on the peat temperature recorded at 7 cm depth. First, we used the dry mass/wet mass (DM/WM) ratio and the mannitol content for assessing environmental conditions related to water table depth and/or soil humidity, and second we inferred the fate of labile and recalcitrant OM in relation to changes in decomposition processes, using water-extractable organic carbon (WEOC), isotopic composition (δ¹³C), SUVA₂₈₀ and sugar composition (neutral monosaccharides, neutral disaccharides and polyols). The study was performed at the undisturbed Sphagnum-dominated “Le Forbonnet” peatland, using a transitional poor fen site (‘WET’) and an open bog site (‘DRY’).