Interactive effects of warming, soil humidity and plant diversity on litter decomposition and microbial activity

Abstract

Human activity has induced a multitude of global changes that are likely to affect the functioning of ecosystems. Although these changes act in concert, studies on interactive effects are scarce. Here, we conducted a laboratory microcosm experiment to explore the impacts of temperature (9, 12 and 15 °C), changes in soil humidity (moist, dry) and plant diversity (1, 4, 16 species) on soil microbial activity and litter decomposition.

We found that changes in litter decomposition did not mirror impacts on microbial measures indicating that the duration of the experiment (22 weeks) may not have been sufficient to determine the full magnitude of global change effects. However and notably, changes in temperature, humidity and plant litter diversity/composition affected in a non-additive way the microbial parameters investigated. For instance, microbial metabolic efficiency increased with plant diversity in the high moisture treatment but remained unaffected in low moisture treatment suggesting that climate changes may mask beneficial effects of biodiversity on ecosystem functioning. Moreover, litter decomposition was unaffected by plant litter diversity/composition but increased with increasing temperature in the high moisture treatment, and decreased with increasing temperature in the low moisture treatment.

We conclude that it is inevitable to perform complex experiments considering multiple global change agents in order to realistically predict future changes in ecosystem functioning. Non-additive interactions highlight the context-dependency of impacts of single global change agents.

Introduction

Mankind is currently facing multiple global environmental changes, such as rising temperature, altered precipitation regimes and depletion in biological diversity. Increasing concentrations of atmospheric greenhouse gases are expected to raise global mean surface temperature by 1.1–6.4 °C by the end of this century (IPCC, 2007). Projections of future precipitation regimes are more uncertain, however, there is little doubt that they will change at the global and regional scale (IPCC, 2007). One scenario expects little changes in the total amount of precipitation, but increases of inter- and intra-annual variability of precipitation associated with higher risk of extreme precipitation events (IPCC, 2007). Both temperature and precipitation are key factors regulating ecosystem carbon cycling, so that projected alterations of the global climate will most likely affect ecosystem carbon balance. The decomposition of leaf litter is one of the major processes of the ecosystem carbon balance (Bardgett, 2005, Cadisch and Giller, 1997, Wardle, 2002;) and it has been estimated that the decomposition of plant material contributes approximately 70% of the total annual carbon flux. Thus any change in litter decomposition dynamics will have important consequences for the global carbon budget (Couteaux et al., 1995, Raich and Schlesinger, 1992).

Temperature and precipitation affect the biomass, activity and community composition of saprotrophic microorganisms (Davidson et al., 2000, Liu et al., 2009, Schimel et al., 1999, Wan et al., 2007, Zhou et al., 2007). Since soil microorganisms contribute up to 100% to total decomposition (van der Heijden et al., 2008), changes on the microbial level may in turn strongly affect ecosystem carbon balance. Beside climate change, many ecosystems experience massive losses in biodiversity caused by anthropogenic activity (Loreau et al., 2001, Sala et al., 2000). The main cause of species extinctions are overexploitation, habitat loss, degradation and fragmentation (Gaston and Spicer, 2004, Sala et al., 2000, Tilman et al., 2001). Anthropogenic changes of global climate, however, also directly affect plant species diversity. For example, there is evidence that rising temperature and altered precipitation regimes affect the distribution, physiology and phenology of plant species, with major consequences for plant species and ecosystem diversity (Bakkenes et al., 2002, Thuiller et al., 2005). Changes on the producer-level, however, are likely to affect ecosystem functions via the microbial pathway. For example, litter of different plant species or functional groups varies markedly in chemical and physical traits which in turn affects microbial community and decomposition dynamics (Cornelissen et al., 1999, Cornwell et al., 2008, Pérez-Harguindeguy et al., 2000). In addition, decomposition dynamics in litter mixtures often deviate from that in single species litter, with effects of litter species identity overriding effects of diversity (Gartner and Cardon, 2004, Hättenschwiler et al., 2005). As a consequence, shifts of plant species community composition likely affect decomposition dynamics by changing the diversity, quality and quantity of plant litter entering the microbial decomposer system.

In general, increasing temperature, altered precipitation regime and plant litter diversity, all strongly affect microbial biomass, activity and community composition, with potentially interactive effects on the ecosystem carbon balance. However, most previous studies focussed on the effect of plant species diversity (Ball et al., 2008, Gartner and Cardon, 2004, Hoorens et al., 2003, Hoorens et al., 2010), increased temperature (Fierer et al., 2005, Hobbie, 1996, Liski et al., 2003), altered precipitation (Lensing and Wise, 2007) or the combined effect of increased temperature and altered precipitation on litter decomposition dynamics (Bontti et al., 2009), but to date no study explored the combined effect of all three. For instance, based on a meta-analysis of grassland decomposition Bontti et al. (2009) suggested that carbon flux from root decomposition in grasslands would increase, as result of increasing temperature, only if precipitation is not limiting. However, where precipitation is limiting, increased temperature would decrease root decomposition. In addition, they found that litter quality essentially drove decomposition processes; however, their approach did not allow investigating interactive effects with global environmental changes. Given the projected simultaneous changes in temperature, precipitation and plant species diversity, it is important to understand how the interaction of all three factors will affect litter decomposition dynamics and microbial activity.

Using a microcosm study we investigated the interactive effect of increased temperature (9, 12 and 15 °C), humidity (moist, dry), plant species (monocultures, 4-species and 16-species combinations) and functional group richness (1, 2, 3 and 4) and presence of certain plant functional groups (grasses, small herbs, tall herbs and legumes) on grassland leaf litter mass loss and microbial biomass and activity. Assuming that litter quality controls microbial decomposition dynamics, we hypothesized that (1) leaf litter mass loss differs between mixtures due to presence and absence of specific functional groups. Moreover, we hypothesized that (2) moderate experimental warming and increased water availability facilitates microbial biomass, activity and leaf litter mass loss, whereas litter mass loss is decelerated by increasing temperature when water is limiting. The temperature-sensitivity of microbial decomposition processes have been shown to depend on organic matter quality (Bosatta and Ågren, 1998) and it has been stated that the decomposition of high-quality litter is less sensitive to temperature than low-quality litter (Fierer et al., 2005). This is likely modified by moisture conditions, since the diffusion of substrates, nutrients, inhibitory compounds and enzymes are controlled by water availability. Therefore, we hypothesized that (3) litter mass loss of low- (grasses) and high-quality (legumes) plant functional groups differently respond to temperature and humidity regimes. Moreover, processes on the plant functional group level are expected to propagate into that in leaf litter mixtures due to synergistic and antagonistic interactions between litter species (Hättenschwiler et al., 2005).