Is microbial community composition in boreal forest soils determined by pH, C-to-N ratio, the trees, or all three?

Abstract

In Fennoscandian boreal forests, soil pH and N supply generally increase downhill as a result of water transport of base cations and N, respectively. Simultaneously, forest productivity increases, the understory changes from ericaceous dwarf shrubs to tall herbs; in the soil, fungi decrease whereas bacteria increase. The composition of the soil microbial community is mainly thought to be controlled by the pH and C-to-N ratio of the substrate. However, the latter also determines the N supply to plants, the plant community composition, and should also affect plant allocation of C below ground to roots and a major functional group of microbes, mycorrhizal fungi. We used phospholipid fatty acids (PLFAs) to analyze the potential importance of mycorrhizal fungi by comparing the microbial community composition in a tree-girdling experiment, where tree belowground C allocation was terminated, and in a long-term (34 years) N loading experiment, with the shifts across a natural pH and N supply gradient. Both tree girdling and N loading caused a decline of ca. 45% of the fungal biomarker PLFA 18:2omega6,9, suggesting a common mechanism, i.e., that N loading caused a decrease in the C supply to ectomycorrhizal fungi just as tree girdling did. The total abundance of bacterial PLFAs did not respond to tree girdling or to N loading, in which cases the pH (of the mor layer) did not change appreciably, but bacterial PLFAs increased considerably when pH increased across the natural gradient. Fungal biomass was high only in acid soil (pH < 4.1) with a high C-to-N ratio (>38). According to a principal component analysis, the soil C-to-N ratio was as good as predictor of microbial community structure as pH. Our study thus indicated the soil C-to-N ratio, and the response of trees to this ratio, as important factors that together with soil pH influence soil microbial community composition.

Relationships between soil water repellency and microbial community composition under different plant species in a Mediterranean semiarid forest

“…This is different to a number of studies that identified soil pH as one of the main environmental factors driving soil microorganisms functions and structure (Bääth and Anderson 2003;Hackl et al, 2005;Högberg et al, 2007). However, the range of the pH values in our soils is very low (7.8-8.2) which is probably the main reason for the lack of the significant correlations.…”

Biochar Impacts Soil Microbial Community Composition and Nitrogen Cycling in an Acidic Soil Planted with Rape

“…Hogberg et al 50 have reported that the C/N ratio was as good as pH in predicting the soil microbial community. However, in this study the C/N ratio was increased somewhat artificially by the addition of C in the biochar, which was unavailable to microbes in a short incubation period, unlike the natural system investigated by Hogberg et al 50 Thus, in envfit test, C/N ratio was found to be significant in shaping the soil microbial community composition, which could be due to its high correlation with soil pH. Rape planting also showed some influence on the soil microbial community composition but only in treatments with biochar.…”

Long-term dynamics of aboveground fungal communities in a subalpine Norway spruce forest under elevated nitrogen input

“…The immediate response of the ectomycorrhizal community to the N supply has been seen in several N-fertilization experiments and studies of N gradients and is widely accepted to be caused by a reduced supply of plant C to the fungal symbionts (e.g. Högberg et al 2007). Whereas the nutritional conditions for ectomycorrhizal species changed rapidly, the conditions for saprobic species probably changed during the course of the experiment by altered soil biogeochemistry as mentioned in the previous section.…”

“…Ectomycorrhizal fungi are directly dependent on the C supply from trees. When N availability in the soil is high, trees reduce C allocation to the roots and reduce investment of C in mycorrhizal fungi (Demoling et al 2008;Högberg et al 2007). However, there are few ectomycorrhizal species, such as Paxillus involutus and Lactarius rufus, and species of the Thelephoraceae and Corticiaceae which have been reported to remain unaffected, or even to increase their production of sporocarps in forest soils to which N has been added (Laiho 1970;Ohenoja 1978;Wästerlund 1982;Peter et al 2001a).…”