Soil pH is known to structure microbial community composition from continental to field scales, yet few studies have directly manipulated soil pH to understand its impact on microbial community structure and function. Here, we examine how increasing soil pH through watershed liming has changed fungal and bacterial community structure and decomposition processes 25 years after lime application. Our study was conducted at Woods Lake, in the Adirondack region of New York, where watershed liming doubled organic matter stocks in the forest floor soil horizons two decades later. We hypothesized that liming had reduced the potential activities of extracellular enzymes thereby accounting for the accumulation of organic matter, and predicted that increasing soil pH had increased microbial diversity. We measured microbial biomass, carbon mineralization, extracellular enzyme activities (a-glucosidase, β-xylosidase, β-glucosidase, cellobiohydrolase, N-acetyl glucosaminidase, leucine aminopeptidase, acid phosphatase, polyphenol oxidase, and peroxidase), and used the Illumina MiSeq platform to sequence soil bacterial and fungal DNA.
Liming increased soil pH from 4.5 to 5.5 and significantly reduced the activities of seven decomposition enzymes in the Oa horizon, where the largest accumulation of organic matter was observed. This indicates that the long-term impacts of liming may be different from previously reported short-term stimulation of microbial activity. As hypothesized, soil pH positively correlated with bacterial and fungal diversity, and liming significantly altered bacterial and fungal communities in the Oa horizon. Overall, liming impacted bacterial communities; of those OTUs that responded to liming, 75-80% increased in relative abundance in Oa and Oe horizon, respectively. In contrast, liming did not significantly impact fungal communities in the Oe horizon, but reduced the relative abundance of 78% of responding fungal OTUs in the Oa horizon. The differences between bacterial and fungal responses combined with lower enzyme activities in the Oa horizon, suggest that liming may have reduced the relative abundance of fungi that play an important role in organic matter decomposition. This study demonstrates that liming can have a long-term impact on soil pH, and is associated with structural shifts in the bacterial and fungal communities with functional consequences for nutrient cycling and carbon storage in soils at Woods Lake .