Forested ecosystems across the Northeast U.S. remain impaired by the legacies of acid precipitation, despite recent declines in deposition. Recent evidence suggests that ecosystem recovery from acid precipitation may induce dramatic changes to soil organic matter (SOM) dynamics, including decreased SOM stabilization, enhanced soil respiration, and increased dissolved organic carbon fluxes to receiving waters. Ecosystem recovery from acid precipitation is predicted to occur over timescales of decades to centuries, and will likely alter a suite of soil geochemical properties, including soil pH, calcium fertility, aluminum solubility, and SOM complexation with metals. We sought to understand how shifts in these geochemical properties might drive changes to SOM dynamics in hardwood forests recovering from acid deposition.
A whole-watershed acid mitigation experiment at Hubbard Brook Experimental Forest elevated soil pH and Ca fertility, causing an increase in forest biomass but a dramatic decrease in standing stocks of SOM. To determine if geochemical indicators of recovery from acid precipitation were correlated with rates of SOM turnover, we conducted field surveys in the acid-mitigated and adjacent reference watersheds at Hubbard Brook. To determine SOM responses to changes in individual geochemical indicators, we also performed laboratory manipulations on acid-affected soils and measured soil C and N cycling rates. We are continuing greenhouse plant-soil mesocosm experiments to examine plant-mediated SOM responses to alterations to soil pH and Ca status.
These experiments show that elevated soil pH and increased Ca fertility may substantially alter SOM dynamics. In field soil surveys, we found that soil exchangeable calcium was positively correlated with microbial respiration (r2=.54) and net nitrogen mineralization (r2=.57) in forest floor soils. Respiration and net nitrogen mineralization were uncorrelated with soil pH. By contrast, in laboratory long-term C mineralization assays of manipulated soils, we found that a one-unit increase of soil pH enhanced soil respiration by 7-15%, but a two-fold increase soil exchangeable Ca suppressed respiration by 13-50%. These results suggest that individual geochemical changes predicted under ecosystem recovery from acid rain may have varying and possibly antagonistic effects on SOM cycling, and the net effect of these changes remains unresolved.