Forest organic matter removal leads to long-term reductions in bacterial and fungal abundance

Abstract

Intensive organic matter removal (OMR) associated with forest harvest has been shown to significantly affect soil physical, chemical, and biological properties; however, the influence on microbial abundance has been neglected, especially at depth and over time. We used quantitative PCR to assess the long-term impact of two different intensities of OMR, soil depth, seasonality, and their interactions on bacterial and fungal abundances 18 years post-harvest, relative to an unharvested control. Both bacterial and fungal community sizes were significantly reduced by intensive OMR, and these effects were largest in surface soil (0–10 and 10–30 cm), and largest in summer and spring. The response ratios for bacteria and fungi were not statistically different, indicating that both taxonomic groups were susceptible to long-term changes in soil properties induced by intensive OMR. Furthermore, we found that root biomass was significantly correlated to bacterial and fungal abundance in unharvested, low-intensity harvest, and high-intensity harvest stands throughout time and across soil depths, suggesting that the input of organic matter through root exudation and turnover are key to controlling microbial community size. These reductions in bacterial and fungal community sizes suggest that microbial community functions related to biogeochemical processes may be altered for decades post-harvest, with potential implications for forest productivity and ecosystem function during subsequent rotations.