Environmental variables
pH values were consistent between the soil locations (depth1p (d1p) = 6.33 ± 0.06, depth2p (d2p) = 6.35 ± 0.20, depth3i (d3i) = 5.56 ± 0.88, depth4t (d4t) = 6.40 ± 0.16, depth5c (c1) = 6.07 ± 0.29, depth6c (c2) = 6.58 ± 0.47). Kruskal-Wallis tests showed no significant difference in pH value (χ2 = 46.38, df = 45, p = 0.42; supplementary date Fig. 1) between soil locations and significant differences in moisture content (χ2 = 53, df = 17, p < 0.001; supplementary date Fig. 1). Nemenyi tests showed that d1p and d2p had significantly more moisture than c1 and c2, but all other comparisons were not significantly different.
Elemental analysis obtained from pXRF analysis showed significant changes in elemental composition between the differing locations. D1p and d2p, the sites of best preservation, displayed different elemental composition with respect to phosphorus, sulphur and iron (P < 0.001). Pairwise comparisons using Dwass-Steele-Critchlow-Fligner (DSCF) showed that the interaction between phosphorus, sulphur and iron in d1p and d2p was not significantly different to each other, but were significantly different to d3i, d4t, c1 and c2. Elemental composition for major and minor elements are provided in Fig. 1. Phosphorus has a range of Max = 1.094; Min = 0.512; Range = 0.582; Mean = 0.813; Median = 0.806 in d1p and d2p.
Microbial community profile
There is a shift in the microbial community profile between the preservation samples at d1p and d2p and interface layer d3i when compared with other locations (Figure 2). Spearman’s rank correlation analysis showed significant positive correlations with depth and Acidobacteria (ρ = 0.825, P = 2.52 x10-5), Actinobacteria (ρ = 0.887, P = 9,189 x10-7), Chloroflexi (ρ = 0.724, P = 6.78 x10-4) and Planctomyces (ρ = 0.699, P = 1.24 x10-3) and negative correlations with Firmicutes (ρ = -0.937, P = 9.76 x10-9), Bacteroidetes (ρ = -0.687, P = 1.65 x10-3) and Spirochaetes (ρ = -0.901, P = 3.347 x10-7). There was no significant correlation linking pH values to relative abundance of any phyla.
At OTU and genera level there were marked differences between the soil locations clearly separating samples from occupation layers with good preservation and interface samples from others (Figure 3). Specifically, identifiable dominating genera within preservation and interface layers were Sulfuricurvum, Flavobacterium and Methylophilus, with Ktedonobacter, Geobacter and Methylobacter dominating in non-preservation layers. Although the composition of the community changes this was not reflected in a shift in diversity with both Shannon diversity and Evenness measures showing little difference between the locations: d1p H’ 5.724 ± 0.14, Evenness 0.747 ± 0.001; d2p H’ 6.340 ± 0.12, Evenness 0.773 ± 0.006; d3i H’ 5.286 ± 0.10, Evenness 0.688 ± 0.011, d4t H’ 6.741 ± 0.03, Evenness 0.825 ± 0.000; c1 H’ 5.936 ± 0.09, Evenness 0.7778 ± 0.002; c2 H’ 6.120 ± 0.09, Evenness 0.779 ± 0.009.
Microorganisms indicative of preservation layers
LEfSe analysis of the microbial community revealed specific community structure associated with preservation layers characterised by Firmicutes, Epsilonproteobacteria, BRC1 and Chloroflexi with no phyla indicative of non-preservation layers due to varied communities. Tentative organisms which may be indicative of preservation layers are identified as: the Firmicute Turicibacter; AlphaproteobacteriaFilomicrobium and Sphingobium; Betaproteobacteria Pusillimonas and Vogesella; Epsilonproteobacteria Arcobacter; and Gammaproteobacteria Thiovirga (Figure 4)
Composition of samples that contain vivianite.
When compared with soils within 10 cm, soils with vivianite formation showed comparable Shannon diversity (visible vivianite = 3.211 ± 1.65; no visible vivianite = 3.636 ± 1.97) and evenness (visible vivianite = 0.598 ± 0.21; no visible vivianite = 0.618 ± 0.23) at phyla level. Adjacent soils profiles were not dissimilar to the preservation layers shown in Figure 2 (Figure 5). However, sections with vivianite were dominated by a single genus (50.3% of the total OTU within one of the samples) Methylophilus compared with sections without (P=0.0162) (Figure 5).