Nitrogen deposition interacts with climate in affecting production and decomposition rates in Sphagnum mosses

Abstract

Increasing rates of atmospheric nitrogen (N) deposition may reduce growth and accelerate decomposition of Sphagnum mosses in bogs. Sphagnum growth and rates of Sphagnum litter decomposition may also vary because of climate change as both processes are controlled by climatic factors. The initial purpose of this study was to assess if growth and litter decomposition of hummock and lawn Sphagnum species varied with increasing N input in a factorial mid-term (2002)(2003)(2004)(2005) experiment of N and phosphorus (P) addition, in a bog on the southern Alps of Italy. However, as the experimental period was characterized by an exceptional heat wave in summer 2003, we also explored the interacting effects of fertilization and strongly varying climate on growth and decomposition rates of Sphagnum. The heat wave implied strong dehydration of the upper Sphagnum layer even if precipitation in summer 2003 did not differ appreciably from the overall mean. Sphagnum production was somewhat depressed by high levels (3 g m À2 yr À1 ) of N addition without concomitant addition of P presumably because of nutrient imbalance in the tissues, but production rates were much lower than the overall means in 2003, when no effect of nutrient addition could be observed. Adding N at high level also increased the potential decay of Sphagnum litter. Higher CO 2 emission from N-fertilized litter was due to amelioration of litter chemistry showing lower C/N quotients in the N-fertilized treatments. Rates of CO 2 emission from incubated litter also were more strongly affected by water content than by nutrient status, with practically no CO 2 emission detected when litter was dry. We conclude that higher rates of atmospheric N availability input may depress Sphagnum growth because of P, and presumably potassium, (co-)limitation. Higher N availability is also expected to promote potential decay of Sphagnum litter by ameliorating litter chemistry. However, both effects are less pronounced if the growing Sphagnum apex and the underlying senescing tissues dry out.

Response of Sphagnum species mixtures to increased temperature and nitrogen availability

“…This proved to be the case, including the small positive effect of high temperature on height increment of S. fuscum. Negative effects of increased temperature have been found in other studies too (Gerdol et al 2007;Weltzin et al 2001) but have usually been ascribed to desiccation. In our experiment desiccation could not have been the cause of the negative effect of temperature, because we saw no desiccated capitula.…”

“…The effect of increased N availability is less straight-forward because a small increase in N can enhance Sphagnum production at locations where N is a limiting nutrient (Turunen et al 2004). However, when N concentrations exceed a critical threshold value, Sphagnum production and cover are reduced (Berendse et al 2001;Gerdol et al 2007;Gunnarsson and Rydin 2000;Lamers et al 2000). Yet any positive effect of increased temperature and N on carbon sequestration in bogs may be counteracted by the positive effect of temperature (Hobbie 1996) and N (Franzén 2006) on decomposition rates.…”

“…Although the effects of temperature (Asada et al 2003;Dorrepaal et al 2003;Gerdol 1995) and N (Aerts et al 2001;Gerdol et al 2007;Limpens and Berendse 2003a) on growth of different Sphagnum species have been studied, and a few studies have examined their separate effects on mixtures of Sphagnum species (Breeuwer et al 2008a;Gunnarsson and Rydin 2000;Robroek et al 2007a), only one study (Wiedermann et al 2007) has examined the combined effects of temperature and N on Sphagnum cover. Since global warming can indirectly cause increased nutrient availability through increased decomposition and N mineralisation in bogs (Hobbie 1996;Rustad et al 2001), it is even more important to study the effects of increased temperature and N availability together in order to predict the effect of global change on Sphagnum vegetation.…”

Plant Litter Decomposition and Nutrient Release in Peatlands

“…Sources are Aerts and De Caluwe [1997], Scheffer et al [2001], Thormann et al [2001], Bridgham and Richardson [2003], Aerts et al [2006], Bubier et al [2007], Breeuwer et al [2008], Turetsky et al [2008], Bragazza, unpublished data [2008]. so that below a certain level of litter moisture, decomposition initially slows down and then stops completely [Crow and Wieder, 2005;Aerts, 2006;Gerdol et al, 2007]. For vascular plants, increasing N availability had variable species-specific effects on mass loss depending on the chemistry of initial litter Breeuwer et al, 2008].…”

“…Indeed, increasing N availability can be expected to enhance short-term Sphagnum litter decomposition through an increase of N content [Williams et al, 1999;Bragazza et al, 2006;Gerdol et al, 2007] and a decrease of (soluble) phenolics [Bragazza and Freeman, 2007], thereafter reducing the phenolics/N and the C/nutrient quotients. On the other hand, Sphagnum litter decomposition is strongly affected by its water content, and Sphagnum litter (solid circles) based on studies in which mass loss was monitored for more than 1 year.…”