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- Materials and Methods
Northern bogs are characterized by a sustained imbalance between rates of plant production and decay, which results in the accumulation of partially decomposed organic matter (i.e. peat) (Gorham, 1991; Tolonen & Turunen, 1996). Most of the northern bogs represent long-term sinks for atmospheric CO2 (Turunen et al., 2002), although carbon (C) flux can vary from a sink to a source in individual years, mostly owing to weather fluctuations (Roulet et al., 2007). Despite rather low CO2 uptake rates through photosynthesis (Frolking et al., 1998), emission rates of CO2 from northern peatlands are much slower than photosynthetic CO2 fixation. Harsh environment, including acidic pH, cold soil temperatures and frequent substrate anoxia, account for slow decay and, hence, low rates of CO2 emission from peatland surface. However, organic matter decomposition in peatlands is also regulated by litter quality, the latter depending upon intrinsic features of bog plants (Moore & Basiliko, 2006). Bogs are hydrologically ombrotrophic, that is, they are fed exclusively by precipitation, which historically has very low concentrations of dissolved nutrients. As a consequence, bog vegetation is formed of species adapted to cope with extreme nutrient poorness (Aerts et al., 1999).
The vegetation of ombrotrophic bogs consists of two functionally distinct layers, as plants in the two layers use different sources of nutrients. On one hand the moss layer, mostly comprising Sphagnum species, depends on nutrients derived from the atmosphere through dry and wet deposition. On the other hand the field layer, including rooting vascular species of various growth forms, absorbs nutrients derived from the mineralization of the litter (Malmer et al., 1994). Bog plant litter, in general, breaks down slowly because of the low nutrient content in plant tissues. However, rates of bog litter decomposition vary greatly in relation to growth form. In particular, decomposition rates of Sphagnum litter always are much slower than those of vascular litter (Bragazza et al., 2007), as Sphagnum tissues are rich in recalcitrant phenolics (Verhoeven & Liefveld, 1997; Freeman et al., 2004).
Environmental changes associated with human activities, including both climate warming and increased atmospheric nutrient deposition, are likely to affect the C balance of northern peatlands. However, variations in individual environmental factors can have different, and in some cases contrasting, effects on C incorporation in, and C loss from northern peatlands. For example, higher temperatures are likely to raise net plant productivity but they are also expected to enhance litter decay (Dioumaeva et al., 2002) in bogs. Increased drainage and aeration of peatland surface, as a possible consequence of higher evapotranspiration rates, can reduce both photosynthetic C incorporation and respiratory C loss from living bog plants, especially Sphagnum mosses (Gerdol et al., 2007), but they likely promote C loss from the underlying peat layers (Scanlon & Moore, 2000). Increased atmospheric deposition of nutrients, especially nitrogen (N) reduces the capacity of the Sphagnum layer to retain N from aerial deposition, thus increasing N availability for vascular root absorption (Heijmans et al., 2002). Consequently, a higher nutrient status in the rooting layer will promote growth of vascular plants, thus altering the competitive balance between Sphagnum mosses and vascular plants and, possibly, among vascular species of different growth forms. In particular, higher vascular plant cover can be detrimental to Sphagnum because of shading (Hayward & Clymo, 1983). This may eventually lead to changes in the floristic composition of bog ecosystems (Berendse et al., 2001). If the accumulation of recalcitrant Sphagnum litter decreases with respect to the accumulation of more easily decomposable vascular litter, this can further increase rates of organic matter decay. However, the effects of increased nutrient input on peat decay are still unclear, with extant studies reporting enhanced (Coulson & Butterfield, 1978), reduced (Rochefort et al., 1990), or unchanged (Tomassen et al., 2004) peat decomposition rates after experimental N addition.
In 2002 we set up an experiment for N and phosphorus (P) addition in an ombrotrophic bog in the south-eastern Alps (North Italy). This bog was chosen for this experiment because total atmospheric N deposition in this region (c. 0.8 g m−2 yr−1; see Gerdol et al., 2007) probably is close to the threshold at which the N retention capacity of the Sphagnum layer is saturated (Gunnarsson & Rydin, 2000; Bragazza et al., 2004). The main goal of the experiment was to test whether the Sphagnum layer in the bog still is able to absorb the amended N. If not, the N not retained by Sphagnum will be absorbed by vascular plants, thus ameliorating the nutrient status in their tissues. Since previous studies have reported stoichiometric imbalance between N and P in Sphagnum tissues as possibly hindering absorption of additional N by Sphagnum plants (Bragazza et al., 2004; Limpens et al., 2004), a reduced ability of Sphagnum plants to take up N may be counteracted if P is amended together with N.
Our main hypothesis was that increased growth of vascular plants owing to fertilization will, in the short term, enhance rates of CO2 fixation since these rates are positively correlated with vascular cover (Humphreys et al., 2006; Wilson et al., 2007). We also hypothesized that in the long term the accumulation of vascular litter will accelerate C loss through enhanced heterotrophic respiration, thus counteracting and possibly reversing the short-term trend. We report here on the short-term results of this experiment, corresponding to the years 2002–2005, when the experiment was strongly affected by an unprecedented heatwave in summer 2003. The frequency of extreme climate events, particularly heatwaves, is expected to increase in the alpine region as climate continues to warm (Beniston, 2007). Therefore, the 2003 anomaly represented a sort of natural experiment, which allowed us to explore interactions between nutrient amendment and climate.