Acid deposition: Reduced base saturation, available phosphorous, and changed fungal community composition in an old-growth forest

Background/Question/Methods

Human activities have increased acid deposition to many types of terrestrial ecosystems in eastern North America. Acid deposition can alter soils by increasing mobilized Al and the leaching of base cations (e.g., Ca). Consequently, many eastern forests exposed to chronic acid deposition may be approaching a pedogenic threshold where Al, not Ca, dominates surface soil exchange reactions. We hypothesize that crossing this threshold would reduce the availability of P and increase the influence of fungi that are specialized for P uptake under highly acidic conditions. We tested this hypothesis at Dysart Woods, an old-growth mixed mesophytic forest. We describe soil quality by measuring pH, exchangeable Al, base cations, cation exchange capacity (CEC), and P on a north and south aspect stand. Because we have soil pH from 1971, we were able to use pH as a proxy for past soil conditions. PCR and terminal restriction fragment length polymorphism (TRF) were used to determine the occurrence and distribution of soil fungi. TRF peak areas were used as a measure of the proportional abundance of each fungal operational taxonomic unit (OTU) within each soil core.

Results/Conclusions

Surface soil pH has decreased from above 5.0 in 1971 to 4.8 ± 0.1 SE (north stand) and 4.3 ± 0.1 (south stand) in 2007. The CEC was similar between the two stands at 11.6 ± 0.4 (cmol_c kg^-1); however % base saturation (BS) was 82 ± 4 in the north stand, but 52 ± 7 in the south stand. The north stand had 3 times as much inorganic P than the south stand, which can be explained by the negative correlation between P and exchangeable Al (r = -0.46, p < 0.01). Fungal communities were different (p < 0.05) between the two stands and some fungal OTUs were negatively correlated (p < 0.05) with BS and P, whereas other OTUs were positively correlated (p < 0.05). Because soil pH was highly correlated with %BS (r = 0.95, p < 0.01), it is likely that surface soil BS has decreased as much as 50% since 1971. This suggests that acid deposition is rapidly diminishing soil quality where the importance of Al and P mediating soil processes is increasing. These results indicate that fungi respond to changes in soil chemistry, and these data are consistent with our hypothesis that increased P limitation will lead to altered soil fungal communities.