Sustained effects of chronic nitrogen addition on the capacity of fungi to degrade lignin

Background/Question/Methods

The plant polymer lignin is degraded slowly and primarily by white-rot fungi. This reaction is dependent on peroxidases and other extracellular enzymes produced by these fungi which are able to penetrate the lignin's complex structure. The production of these enzymes is affected by soil nitrogen (N) levels. Increasing rates of anthropogenic N deposition have increased available N worldwide, affecting rates of decomposition and soil nutrient cycling. At the Harvard Forest Long-Term Ecological Research site in Petersham, Massachusetts, a 25 year N fertilization experiment provides a unique opportunity to study decomposition rates by white-rot fungi in chronically N-amended forests. The study site, a mixed-hardwood forest, consists of three N treatment plots: high N, low N and control (unamended). Fungal isolates from each of these field treatments, representing abundant species, were inoculated onto laboratory “environments” representing the three N field treatments. The environments consisted of plates containing media with concentrations of N matching those found in the three field treatments, as well as oak litter from those treatments. Using a full factorial design, the fungal isolates were plated onto the three environments, and after seven weeks incubation, litter mass loss and the activities of lignin-degrading enzymes were measured.

Results/Conclusions

One fungal species, from the white-rot genus Irpex, isolated from the high N field treatment exhibited a decreased capacity to decompose leaf litter compared to the same Irpex species isolated from the unamended field treatment. Interestingly, these Irpex isolates from the high N field treatment that were transplanted onto an unamended (no added N) environment still exhibited a lower capacity to decompose litter when compared to the same species isolated from unamended field treatment. This finding, evidenced both by mass loss and enzyme activity measurements, suggests that chronic N addition affects the capacity of some fungi to break down lignin even when high N conditions are removed. If this is the case, rising global N deposition rates could be altering the long-term capacity of fungi to produce enzymes that degrade lignin. Our results warrant further investigation into the long-term effects of N addition on lignin-degrading fungi.