Bacterial and fungal growth on different plant litter in Mediterranean soils: Effects of C/N ratio and soil pH
Introduction
In the Mediterranean region, forests cover an area of over 85 million ha (estimated in 2010; FAO, 2010). Trees affect the soil environment in several ways through litter fall, labile C input, rhizodeposition, root turnover and effects on soil microclimate (Eviner and Chapin, 2003). Especially the quality of litter associated with different tree species influences the microbial community (Thoms et al., 2010, Aponte et al., 2014), since leaf litter is the main energy and nutrient source for soil microorganisms. Different microbial communities were also reported in soil under different tree species (Hackl et al., 2005, Thoms et al., 2010, Schweitzer et al., 2011).
Bacteria and fungi are the main decomposer groups involved in the recycle of soil organic matter. The environmental factors determining the importance of these two groups during decomposition processes are not completely understood, although for example the canonical effect of pH has been studied recently (Rousk and Bååth, 2011), with low pH being more conducive for fungal growth. The chemical composition of the substrate (e.g. the C/N ratio) is also predicted to be of importance, with higher C/N ratio of the litter being more conducive for fungal growth due to fungal hyphae having a higher C/N ratio than bacterial cells (Paustian and Schnürer, 1987, Bakken, 1985, Wallander et al., 2003) and the potential to translocate N to overcome limitation (Frey et al., 2003). However, N availability in itself is not always enough to explain differential growth of fungi and bacteria on plant litter, as shown by Rousk and Bååth (2007) after adding extra N to litter with originally different C/N content, suggesting that other chemical and physical conditions of different litter types will be of importance. Growth of fungi and bacteria during decomposition has mostly been studied on fairly easily available substrates, like glucose (Meidute e al., 2008; Reischke et al., 2014), manure (Maienza et al., 2014) or alfalfa and straw (Rousk and Bååth, 2007). Few studies have been focused on comparing different leaf litter (Rousk and Bååth, 2011).
The aim of our study was to investigate the influences of different litter types on fungal and bacterial growth in two forest soils, differing in pH. For this purpose, leaf litters belonging to three different Mediterranean forest systems (beech, holm oak and turkey oak forests) were added to two soils (beech and holm oak) from the same mountain area. Straw and alfalfa were also included as litter treatments having very different C/N ratios. Bacterial and fungal growth rate was measured over time using radioactive tracer incorporation techniques (leucine incorporation and acetate into ergosterol incorporation for bacteria and fungi, respectively) and compared with total activity (respiration) and changes in total biomass (SIR). We hypothesized that fungal growth would be relatively more important than bacterial growth in the soil with lower pH, as well on litter types with higher C/N ratio.
Section snippets
Soils
Soil was sampled during summer 2013 from a beech (Fagus sylvatica L.) and a holm oak (Quercus ilex L.) stand, using the top layer (0–5 cm) after removing litter. Both stands are located in the Matese mountain area (Apennines district, southern Italy). A more detailed description of the two forest stands is reported in Grosso et al. (2014). Soil cores, randomly collected at each stand, were pooled to obtain a representative sample. Soil samples were sieved (<2 mm) and stored at 4 °C until the
Respiration rate and microbial growth
The respiration rate increased after all litter additions in both soils (Fig. 1). The respiration was highest after alfalfa addition and lowest after beech litter amendment in both soils. Respiration usually peaked, except for straw additions, after 3 days (the first measurement occasion), and then decreased over time or was similar over the 27 days incubation. Straw additions resulted in highest respiration after 6 days (holm oak soil, Fig. 1B) or between 10 and 18 days (beech soil, Fig. 1A).
Effect of litter types
Our main finding is the importance of the C/N ratio of the different litter types in determining the balance of fungal to bacterial growth during early decomposition, with fungal growth being more important in litter low in N (high C/N ratios). This was earlier reported for litter with C/N of 19.7 and 108 (Bossuyt et al., 2001) and when comparing alfalfa (C/N 15) and straw (C/N 75) addition (Rousk and Bååth, 2007). We got the same results for the latter two litter types, with the leaf litter
Acknowledgements
We are grateful to Dr. Johannes Rousk (Lund University) for the precious assistance in the laboratory, and to Dr. Daniela Baldantoni (University of Salerno) for the analyses of C and N in the litters.
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