Exposure of wood in floodplains affects its chemical quality and its subsequent breakdown in streams
Graphical abstract
Introduction
In most forested streams, leaf litter is the most important fraction of allochthonous organic matter inputs (Benfield, 1997, Wallace et al., 1995), and is thus an essential energy source for ecosystems (Fisher and Likens, 1973, Kuehn, 2015, Tank et al., 2010). Additionally, wood litter can represent an important energy source for aquatic heterotrophs and also a relevant structural support for microorganisms that grow on its surface (i.e. Golladay and Sinsabaugh, 1991, Rabeni and Hoel, 2000). Whereas leaves are rapidly flushed away, wood litter is a long-lasting resource that increases the pool of nutrients (Romero et al., 2005) and stored carbon and significantly contributes to the energy flux in streams (i.e. Elosegi et al., 2007). In arid and semiarid regions, where deciduous vegetation is scarce, or even absent, and riparian vegetation is dominated by perennial woody shrubs (Bruno et al., 2014, Salinas and Casas, 2007), wood litter may represent an essential energy resource for freshwater ecosystems functioning (Jacobson et al., 1999).
The dynamics and breakdown of organic matter in streams are affected by climate, flow regimes and the riparian vegetation structure (Larned, 2000, Schade and Fisher, 1997). In dryland streams, before entering the stream channels, most coarse organic matter from terrestrial and riparian vegetation is usually transported by floods and retained in floodplains (Jacobson et al., 1999), where its preconditioning may have a substantial effect on its subsequent use by aquatic organisms (Fellman et al., 2013, Pu et al., 2014). These dynamics differ from those occurring in more humid regions, where the bulk of leaf litter inputs from riparian trees tend to enter the stream channel directly (Pozo et al., 1997, Winterbourn, 1976). Today arid regions occupy almost 40% of the land surface (Safriel et al., 2005), but are likely to increase as a result of ongoing climate change (Döll and Schmied, 2012, Reynolds et al., 2007). Therefore, it is important to improve the knowledge on the effects of preconditioning on breakdown.
When exposed in floodplains, organic matter may be affected by abiotic (solar radiation, wind, rain and soil burial) and biotic factors (microbial colonization and grazing by invertebrates). However in dry areas, abiotic factors usually have a stronger influence on organic matter breakdown than biotic ones due to water scarcity (Austin, 2011, King et al., 2012, Whitford and Wade, 2002). In these environments, photodegradation can be an important process in organic matter breakdown as it can affect mass loss and chemical composition (Austin and Ballaré, 2010, Brandt et al., 2010, Day et al., 2007, Gallo et al., 2009). For instance, photodegradation can degrade recalcitrant compounds, such as lignin or phenolic compounds, which absorb solar radiation (Austin and Ballaré, 2010, Gallo et al., 2009). This fact has been proved to increase the leaching of soluble phenolic compounds (Fellman et al., 2013, Gallo et al., 2009) and to facilitate access of microorganisms to labile C resources (Foereid et al., 2010, Henry et al., 2008, Pu et al., 2014). Nevertheless, photodegradation may also reduce organic matter quality by promoting the leaching of nutrients and labile C compounds (Dieter et al., 2013). Hence, it is well know the positive linear relationship between organic matter decomposition rates and N and P concentration (e.g. Enríquez et al., 1993). As the chemical composition of organic matter is a key factor in determining its decomposition and use by microorganisms (Webster and Benfield, 1986, Zhang et al., 2008), any change in organic matter quality that occurs during its exposure in floodplains is expected to have implications on its subsequent aquatic decomposition. Previous studies found contrasting effects of terrestrial preconditioning of leaf litter on its aquatic decomposition. Some showed increased decomposition rates (Fellman et al., 2013, Pu et al., 2014), whereas others reported the opposite or no effects (Dieter et al., 2011, Dieter et al., 2013, Mora, 2014). Given the potential significance of organic matter preconditioning in floodplains, which could also extend to more humid streams in the near future, this study aimed to analyze the effect of a long exposure period of wood in a stream floodplain on both its chemical quality and its subsequent aquatic decomposition. Special attention was paid to leaching and microbial activity in streams. For this purpose, we compared changes in chemical quality, aquatic decomposition and microbial activity between preconditioned and non-preconditioned wood. We hypothesized preconditioning to affect wood chemical composition and to increase aquatic breakdown rate due to greater microbial activity.
Section snippets
Experimental approach
To carry out this study, we designed an experiment divided into two phases, firstly we simulated wood exposure in a stream floodplain during a summer drought (terrestrial phase); then we simulated its arrival at streams (aquatic phase). In both phases, we analyzed changes in wood quality, breakdown rates and microbial activity. For this experiment we used standard wooden sticks (untreated tongue depressors made of Betula sp.), hereafter sticks. Standard wooden sticks have proved to perform
Climatic and environmental variables
Rambla de la Parra floodplain received during the terrestrial phase an average UV radiation of 71,160 J m− 2 and an average global solar radiation of 520,425 kJ m− 2. Daily average air and soil temperatures were 21.13 °C and 24.58 °C, respectively, whereas average air relative humidity was 58.92%. Total precipitation was 174 mm and was concentrated on a few days during the last two months (AEMET). With regard to the aquatic phase, the main differences between streams were in flow discharge and water
Discussion
As it was expected, floodplain exposure greatly affected the chemical composition of wood. Some soluble nutrients were lost and, according to the C:N and lignin:N ratios, the wood chemical quality improved. Yet, contrary to what we initially hypothesized, preconditioning did not increase stick breakdown rate in streams, although it accelerated microbial activity on the first days of immersion.
Conclusions
Floodplains preconditioning could turn wood from a long-lasting resource for freshwater microorganisms into a short-term resource. Soluble elements were mostly leached in floodplains as results of summer rainfalls, among other factors, whereas microbial activity appeared to contribute, probably together with photodegradation and leaching, to the C and lignin loss. Such as changes, as well as the presence of an active terrestrial fungal community, indeed controlled aquatic breakdown during the
Acknowledgments
We thank Gonzalo González for his advice on data analysis and Hellen Warbuton for revising English. We also thank the anonymous reviewers for providing helpful comments on earlier versions of this manuscript. This research was funded by the European Regional Development Fund (FEDER) and by the Spanish Ministry of Economy and Competitiveness through Project CGL2010-21458. R. del Campo was funded by a PhD contract (FPU R-269/2014) from the University of Murcia.
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