Background/Question/Methods
Climate warming and associated increases in nutrient mineralization may increase the availability of nitrogen (N) in high latitude ecosystems, such as boreal forests. These changes in N availability could feed back to affect the decomposition of litter and organic matter by soil microbes. Since fungi are important decomposers in boreal forest ecosystems, we conducted a 69-day incubation study to examine N constraints on fungal decomposition of organic substrates common in boreal forest soils, including cellulose, lignin, spruce wood, spruce needle litter, and moss litter. We added 0, 19.4, or 194 μg N (as ammonium nitrate) to vials containing 200 mg substrate in factorial combination with five fungal species isolated from boreal soil, including an ascomycete, a zygomycete, and three basidiomycetes. We hypothesized (1) that N addition would increase CO₂ mineralization from the substrates, and (2) that a greater capacity for degradative enzyme production would make the basidiomycetes more effective decomposers than the other fungi.
Results/Conclusions
In support of the first hypothesis, cumulative CO₂ mineralization increased by 26% across all fungal species and substrates in response to 19.4 μg added N; however, there was no significant increase at the highest level of N addition. We also observed clear differences in the substrate preferences of the fungal species. Consistent with our second hypothesis, the zygomycete mineralized little CO₂ from any of the substrates, while the basidiomycetes mineralized all of the substrates except spruce needles. The ascomycete (Penicillium) was surprisingly efficient at mineralizing spruce wood and was the only species that substantially mineralized spruce litter. There was no interaction between N levels and fungal species, although there was some evidence that N addition had a stronger positive effect on the decomposition of substrates with high carbon:N ratios, such as spruce wood. Overall, our results suggest that fungal species may respond similarly to N addition, despite specialization on different organic substrates. Furthermore, the response to N addition is non-linear, with the greatest substrate mineralization at intermediate N levels.