Elsevier

Applied Soil Ecology

Volume 108, December 2016, Pages 1-7
Applied Soil Ecology

Bacterial and fungal growth on different plant litter in Mediterranean soils: Effects of C/N ratio and soil pH

https://doi.org/10.1016/j.apsoil.2016.07.020Get rights and content

Highlights

  • C/N ratio of different plant litter affected the fungal to bacterial growth ratio.

  • Litter with high C/N ratio favoured fungal growth.

  • Decomposition in low pH soil further shifted the growth balance to fungi.

  • Fungal to bacterial growth ratios did not correlate to decomposition rates.

Abstract

Plant litter represents an important source of nutrients and energy for soil microorganisms, but will also selectively affect which organism group, fungi or bacteria, that will be favoured during decomposition. The balance of fungal to bacterial growth will furthermore be affected by soil chemistry like pH. A laboratory experiment was carried out using two different Mediterranean forest soils differing in pH, adding five types of litter varying in C/N ratio from 15 to 75, including the major litter type from the two soils. Growth of bacteria (using the leucine incorporation technique) and fungi (using the acetate into ergosterol incorporation technique) was then followed during 6 weeks. The balance of fungal to bacterial growth was positively affected by litter with increasing C/N ratio, while the C availability, as judged by evolved CO2, did not have any influence. Furthermore, low pH in the soil further favoured fungal growth, irrespective of the litter type. Despite differences in fungal to bacterial growth this appeared to have little influence on respiration rates from the added litter, suggesting functional redundancy. Our results highlight how both initial soil conditions (pH) and litter composition (C/N ratio) independently affects fungal and bacterial growth during decomposition.

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.

References (36)

Cited by (101)

View all citing articles on Scopus
View full text