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Drought has the potential to lead to severe imbalances in the terrestrial carbon (C) cycle, by affecting the production of organic matter by plants and decomposition by microorganisms (Ciais et al., 2005; Schwalm et al., 2010; Reichstein et al., 2013). Drought directly alters the physical environment for soil microorganisms and plants: it decreases soil water content and concomitantly increases the proportion of oxygen-filled soil pores (Schimel et al., 2007; Manzoni et al., 2012a; Moyano et al., 2013); it reduces the mobility of nutrients in the soil, thereby disconnecting organisms from substrates, while nutrient concentrations in the remaining soil water increase (Schjønning et al., 2003; Schimel et al., 2007). Thus, drought has a strong but equivocal effect on nutrient availability.
The response of a microbial community to drought depends on the physiological tolerance and metabolic flexibility of the constituent microbes (Allison & Martiny, 2008). Generally, fungi are thought to be more tolerant to dry periods than bacteria (Schimel et al., 2007; Strickland & Rousk, 2010; Manzoni et al., 2012a). They are able to create large hyphal networks that facilitate nutrient and water transfer over long distances, and to explore water-filled soil pores not accessible for plant roots (Allen, 2007; Joergensen & Wichern, 2008), and they have lower nutrient requirements than bacteria (Strickland & Rousk, 2010). Mycorrhizal fungi are directly connected to plant roots from which they obtain recently assimilated C (10–30% of the net primary production; Allen, 2007; Jones et al., 2009; de Deyn et al., 2011). Mycorrhizas can even enhance water supply for plants during drought by taking up water from smaller soil pores (Wardle et al., 2004; Allen, 2007). Soil ecosystems dominated by fungi may therefore be considered to be less sensitive to drought than bacteria-dominated soils (Yuste et al., 2011; de Vries et al., 2012).
Bacteria, however, are expected to inhabit smaller soil pores, and may be protected for longer from desiccation (Moyano et al., 2013); nonetheless, they have to balance the increasing osmotic potential of the soil solution (Schimel et al., 2007). Alternatively, they can endure unfavourable conditions by shifting to dormancy or producing cysts (Schimel et al., 2007; Lennon & Jones, 2011). Gram-positive bacteria appear to be more resistant to drought than Gram-negative bacteria, because of their thicker peptidoglycan cell wall layer (Schimel et al., 2007; Manzoni et al., 2012a). Genera of Gram-positive bacteria (e.g. Firmicutes) have been termed ‘drought-adapted generalists’ (Lennon et al., 2012). Overall, drought may select for more resistant microbial groups, which can result in the shift of an existing microbial community (Castro et al., 2010), and affect microbial-driven ecosystem functions by changing their activities (Schimel et al., 2007; Allison & Martiny, 2008; Lennon & Jones, 2011; Wallenstein & Hall, 2012).
In addition to direct physical effects, drought can affect soil microbes by altering the input of plant C into the rhizosphere (Bardgett et al., 2008). On the one hand, drought may alter fine-root turnover (Chaves et al., 2003), and on the other hand drought may affect the input of root exudates (including secretions, lysates from border cells and mucilage; see Jones et al., 2009). Root exudates represent an important source of organic C for soil microorganisms in the rhizosphere and enhance soil organic matter decomposition (i.e. the ‘priming effect’; e.g. Fontaine et al., 2003; Kuzyakov, 2010), which in turn makes nutrients accessible for microbial as well as for plant uptake. During drought periods, plants may alter belowground C allocation (Chaves et al., 2003; Ruehr et al., 2009; Albert et al., 2011; McDowell, 2011; Manzoni et al., 2012b). This may severely affect the quantity and quality of C available for soil microbes in the rhizosphere, such as fungi and Gram-negative bacteria, which seem to be tightly connected to recently assimilated plant C (Denef et al., 2009; de Deyn et al., 2011; Bahn et al., 2013). Nevertheless, it is not clear how drought affects processes at the root–soil interface (Bardgett et al., 2008; Compant et al., 2010; Sanaullah et al., 2011). Moreover, it is poorly understood how drought affects belowground C allocation in usually well-watered ecosystems, such as grasslands in the European Alps (Wieser et al., 2008), which are predicted to experience more frequent drought periods in the near future (Schär et al., 2004; IPCC, 2007; Seneviratne et al., 2010).
In many European grasslands, mowing, that is, clipping and subsequent harvesting of aboveground plant biomass, is a common management practice and shapes plant and microbial communities, as well as nutrient composition in the soil (Bardgett et al., 2001; Klumpp et al., 2011; de Vries et al., 2012; Meyer et al., 2012; Shahzad et al., 2012). In contrast to drought, mowing is an immediate disturbance, abruptly changing the soil microclimate (Bahn et al., 2006) and C input, as plant roots release a pulse of low-molecular-weight compounds (Paterson & Sim, 1999; Hamilton et al., 2008; Henry et al., 2008). This has been shown to induce a transient increase in microbial nitrogen (N) mineralization, allowing the higher N demand of plants to be met to rebuild biomass (Paterson & Sim, 1999; Hamilton et al., 2008; Henry et al., 2008; Cheng et al., 2011; Shahzad et al., 2012). It is, however, still unexplored whether drought alters such effects of mowing, but it may be speculated that plants under drought conditions may have lower nonstructural C reserves, potentially decreasing the intensity of the C pulse.
In this study, we therefore aimed to assess the direct and indirect plant-mediated effects of a severe summer drought on microbial processes and community composition in a mountain meadow. We asked the questions of how drought under field conditions affects the biomass of plants and the abundance of microbial groups, how it affects the transfer of C from plants to microbes, and how it alters short-term C turnover in grasslands after mowing. Specifically, we hypothesized that drought decreases plant C pools, thereby reducing the availability of recently fixed plant C for microbes in the rhizosphere; and that, overall, drought decreases microbial biomass, reducing the abundance of bacteria more strongly than that of fungi. Finally, we hypothesized that drought reduces the pulse of recently assimilated C to the soil after mowing compared with controls. We experimentally simulated an extended drought period in a mountain meadow in the Austrian Central Alps. Soil microbial biomass and community composition were determined using phospholipid fatty acids (PLFAs). To investigate the allocation of recently assimilated C to microbial biomass, we pulse-labelled plants with 13CO2 and traced labelled C from plants via the extractable organic C (EOC) pool to microbial PLFAs. Both controls and drought plots were mown towards the end of experimental drought and after labelling.