COS 114-4
Chronic nitrogen addition increases carbohydrolase and oxidase production by ectomycorrhizal fungi
Anthropogenic nitrogen (N) deposition impacts biogeochemical processes and soil microbial communities in forest ecosystems. Nitrogen enrichment acts in opposing directions on microbial processing of plant detritus, hastening breakdown of labile components such as cellulose, but slowing turnover of more resistant polymers such as lignin. Emerging evidence suggests that, via extracellular enzymes, ectomycorrhizal fungi may catalyze soil carbon transformations; however, whether this function depends on N availability is unknown. We sought to determine 1) how ectomycorrhizal enzyme production interacts with seasonality and N enrichment and 2) the relatedness between enzyme production by ectomycorrhizal fungi and the whole microbial community in bulk soil across an experimental N deposition gradient. On five sampling dates between May and December, we assessed activity of carbohydrolytic and oxidative enzymes on the surface of ectomycorrhizal root tips and in soils from a temperate mixed-hardwood forest (Harvard Forest, Petersham, MA) exposed to ambient (< 1 g m-2 y-1), low (5 g m-2 y-1), and high (15 g m-2 y-1) rates of long-term (25 y) N deposition.
Results/Conclusions
Ectomycorrhizal fungal carbohydrolase production peaked at leaf emergence in late May and after leaf senescence in early December and remained at one-third of peak levels while the tree canopy was present in July, August, and October. In organic horizon soil, high N deposition increased carbohydrolase (ß-glucosidase, cellobiohydrolase, and chitinase) production relative to ambient N deposition on all sampling dates. Under low N deposition, ectomycorrhizal carbohydrolase production was intermediate to ambient and high N rates but increased relative to ambient N rates at early- and late-season samplings. N-induced increases in ectomycorrhizal carbohydrolases were attenuated in the mineral soil horizon, indicating soil substrate composition is an important factor determining ectomycorrhizal carbohydrolase responses to N enrichment. Ectomycorrhizal fungal oxidase production varied by two-fold across sampling dates, did not follow the same temporal pattern as carbohydrolases, and was highest under high N deposition in both organic and mineral soil horizons. Of enzymes under study, cellobiohydrolase and peroxidase produced by ectomycorrhizal fungi were most strongly correlated with respective enzyme pools in organic horizon soil, with positive correlations for cellobiohydrolase and negative correlations for peroxidase observed on four of five sampling dates. These correlations were driven by higher ectomycorrhizal enzyme production in N-enriched soils. The contribution of ectomycorrhizal fungi to soil carbon hydrolysis and oxidation appear to be strongest in long-term N-enriched soils and, for carbohydrolases, when the plant host is dormant.
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