Experimental increases in pH and P availability exert long-term impacts on decomposition in forests

Forests are an important sink for atmospheric carbon (C), but global change phenomena threaten to alter their C storage capacity. One such global change driver is acid deposition, which can alter rates of decomposition by changing the composition of microbial communities as well as the quality of litter inputs, and ultimately nutrient cycling. The impacts of acid deposition in forests can potentially be ameliorated by increasing soil pH and addressing concurrent nutrient deficiencies. Here, we explore the capacity for experimental acid deposition reversal-induced shifts in the soil microbial community and litter quality to drive changes in rates of C cycling relative to ambient, acidic conditions. Specifically, we conducted a laboratory-based microcosm incubation experiment using soil and litter collected from a 7-year field-based acid deposition reversal experiment (through addition of Hi-Ca limestone to raise pH and triple super phosphate to increase phosphorus [P] availability) to explore the impacts of long-term acid deposition on rates of litter decomposition. We combined litter and soil from untreated (ambient, acidic) and treated (elevated pH + P) plots in a reciprocal design and incubated them in the lab, measuring rates of CO₂ evolution and microbial extracellular enzyme activity during litter decomposition. Decomposition was generally depressed on soils from plots with elevated pH and P availability relative to soils from ambient conditions early in the experiment, while decomposition was greater on litter from elevated pH and P plots late in the experiment. There were no effects of treatment on cumulative CO₂ evolution over the course of the experiment. Greater P content in litter from elevated pH + P plots led to reduced phosphatase activity in treated plots. We also saw greater β-glucosidase activity in elevated pH + P plots, likely due to greater nutrient availability in these plots, allowing microbes to direct foraging toward more labile forms of C. These results suggest that acid deposition reversal can depress decomposition by altering microbial community functioning, relative to ambient, acidic conditions where we saw accelerated rates of decomposition. Our results emphasize the importance of considering the potential for shifts in microbial communities under acidified conditions to alter C storage and ecosystem functioning.