Drained, dug, burned, peatlands have, in the past, rarely been valued for their own sake. The recognition of climate change as a global threat, and the role peatlands play in the story, has markedly changed that perception. Covering only a few percent of the Earth’s land surface, peatlands hold approximately one third of the world’s soil organic carbon (C) – an amount equivalent to nearly half the C held in the atmosphere as CO2 (Gorham, 1991; IPCC, 2007). Their unique function derives from the acidic and water-logged conditions that slow decomposition rates, and the recalcitrant nature of Sphagnum, the main peat-forming moss. No other terrestrial biome is so efficient at slowly packing away and storing carbon, year after year, century after century (Fig. 1). However, as Limpens et al. report in this issue of New Phytologist (pp. 496–507), just as we are understanding the value of preserving these unique ecosystems for their capacity to sequester C, this potential is under threat by climate change itself, and by the deposition of pollutants, particularly reactive nitrogen (N).
‘No other terrestrial biome is so efficient at slowly packing away and storing carbon, year after year, century after century.’
Most peatland C is stored in high latitudes, where climate warming is most pronounced, and where a more active hydrologic cycle is generally predicted, with wetter overall conditions but more frequent extreme wet or dry spells (IPCC, 2007). Warmer temperatures, particularly combined with summer drought, can accelerate decomposition, enhance erosion, promote fire, and alter species composition toward non-peat-forming graminoids and shrubs, threatening peatland integrity and long-term C sequestration. Reactive N, originating from agricultural emissions and fossil fuel combustion, is elevated in deposition across most of the developed world and is an increasing concern for many developing countries (Galloway et al., 2004). As a limiting nutrient, N supports the long-term establishment of vascular plants, and further enhances peat decomposition. High levels of N in deposition can also trigger nutrient imbalances and toxicity reactions in bryophytes such as Sphagnum, which readily absorbs nutrients (as well as pollutants) due to the lack of a well-developed cuticle (Bates, 2002).
These stressors clearly have the potential to shift impacted peatlands from a C sink to a source (Dorrepaal et al., 2009). But, how can we quantify the magnitude, direction, and nature of these impacts, both individually and collectively? Limpens et al. combined experimental and correlative approaches to answer this question. They analysed the results of N-addition experiments conducted within 29 different studies across North America, Europe and Asia, assessing the response of Sphagnum moss production, height growth, and N concentration in each of the experiments. Using these as response variables, the authors then statistically evaluated the importance of a range of potential explanatory variables, including the presence or absence (by experimental removal) of vascular plants, the addition of phosphorus (P), and a number of site variables including the mean annual precipitation, background N deposition, summer temperature, long-term water table position, and dominant Sphagnum species.
The analysis revealed a complex picture, with the magnitude and direction of response to N deposition depending upon both the instantaneous levels of other drivers, and the long-term environmental conditions at the site. If the experimental input of N was low, and the peatland was in a region of low N deposition, Sphagnum production was either promoted or remained unaffected by increased N loading. In most other cases, enhanced N deposition reduced Sphagnum production. Sites with higher mean summer temperatures or receiving higher annual precipitation showed an exacerbated negative impact. That is, the ‘cross-over’ point between the beneficial and harmful effects of the N treatment was shifted toward peatlands receiving lower long-term N deposition. Adding P or removing vascular plants lessened the negative response, suggesting that both nutrient imbalances and competition with higher plants play important roles in the Sphagnum response to N deposition. Sphagnum height growth responded similarly to production, and tissue N concentration closely followed N deposition levels.
Combining experimental and correlative analyses benefits from the strengths, but also suffers from the weaknesses, of both approaches. One major criticism of manipulation experiments such as those collated in this study is that the level of the experimental treatment (in this case, N dose and concentration) is often substantially higher than that received by even highly impacted ecosystems, leading to treatment artefacts (Pearce & Van der Wal, 2008). Limpens et al. addressed this issue by showing that the increase in Sphagnum tissue N concentration with N load across the experiments was indistinguishable from that measured across a natural gradient of N deposition in a previous study (Bragazza et al., 2005). In addition, excluding experiments that used unrealistically high N application rates did not change the outcome of their analysis. The authors also highlighted that correlation does not prove causation, and that the interactions between climate and N identified through the statistical meta-analysis need experimental underpinning. In particular, experiments to determine the processes by which temperature, precipitation and N input, individually and combined, affect Sphagnum physiology, rates of peat decomposition, and competition with vascular plants, are essential.
The message sounded by Limpens et al. is that, even if nitrogen deposition remains at current levels, rising temperatures and precipitation can increase the sensitivity of Spahgnum to this pollutant, seriously compromising the capacity of peatlands to sequester carbon. Generally, interactions between air pollution and climate remain poorly understood (RoTAP, 2011), and this study provides a clear warning of the difficulties of predicting the impact of one without understanding the influence of the other.