Statistical Analysis
Proportional representation of OTUs across experimental groups was calculated by means of two-way contingency tables for each comparison of interest. Because DNA sequences derived from mid-succession control soils were systematically shorter than sequences from all other groups, these soils were excluded from our cross-succession comparison. This left us with four comparisons of interest: 1) early vs. late succession, 2) alder vs. non-alder soils in late succession, 3&4) mid-succession control vs. mid-succession fertilized soils (both N and P). Each comparison was made using a separate contingency table, with statistical significance of each bivariate relationship determined via likelihood ratio tests using the ‘contingency analysis’ platform in JMP Pro v. 16.0. Statistical comparison of proportional differences in OTU representation across comparison groups was conducted using the ‘categorical analysis’ platform in JMP Pro v. 16.0. Briefly, this platform allows a user to define a set of pair-wise comparisons between levels of a categorical explanatory variable. Differences in levels of the categorical response variable are calculated, and statistical significance determined, via likelihood ratio, Pearson chi-square, and Fisher’s exact tests (JMP 2021). We based our comparisons on the latter, as it consistently provided the most conservative results.