Sustained effects of chronic N addition on the capacity of fungi to degrade lignin
The plant polymer lignin is degraded primarily by white-rot fungi using extracellular enzymes. The production of these enzymes is affected by soil nitrogen (N) levels. While previous studies have shown shifts in enzyme activity levels in response to soil N enrichment, it is unclear whether these shifts are due to changes in fungal community composition or enzyme production/activity of individual species. The Harvard Forest Long-Term Ecological Research (LTER) site in Petersham, MA provides a unique opportunity to study white-rot fungi in a chronically N-amended mixed hardwood forest. The site consists of three N treatment plots, where inorganic N is added at annual rates of 0, 5, or 15 g N m-2. Fungal taxa were isolated from each of the three field treatments and inoculated onto simulated N environments containing a common oak litter and nutrient media with N concentrations matching those found in the field treatments. All isolates were incubated in each the three environments, and after seven weeks, litter mass loss and the activities of lignin-degrading enzymes were measured. Additionally, isolates were exposed to the described laboratory conditions for longer time periods to assess whether differences in decomposition rates could be reversed by prolonged exposure to reciprocal N environments.
Litter mass loss and bleaching of the litter indicated that fungal isolates were strong lignin degraders. Fungal isolates from the highest N field treatment that were transplanted onto an unamended (no added N) environment exhibited a lower capacity to decompose litter when compared to the same species isolated from the control field treatment (no added N). This finding, evidenced both by mass loss and enzyme activity measurements, suggests that chronic N addition affects the capacity of individual fungal species to break down lignin even in the absence of excess N. If this is the case, rising global N deposition rates could be altering the long-term capacity of soil fungi to produce enzymes that degrade lignin. These results are consistent with previous studies showing a suppression of plant litter decay and soil organic matter decomposition at field sites exposed to long-term soil N enrichment.