Towards an improved understanding of forest ecosystem responses to N deposition: Do mycorrhizal associations matter?

Background/Question/Methods

Chronic nitrogen (N) deposition has been linked to changes in carbon (C) and N retention and loss in temperate forest ecosystems, but the magnitude and direction of such responses are highly variable.  Identifying the drivers of this variation is critical for predicting ecosystem-scale feedbacks to climate change.  We hypothesized that the mycorrhizal association of the dominant tree species is an important variable mediating how forests respond to N deposition.  Arbuscular mycorrhizal (AM) trees are associated with labile litter, exist in high N soils, and depend on soil microbes to access available N.  In contrast, ectomycorrhizal (ECM) trees have more recalcitrant litter, persist in N-poor soils, and can access organic forms of N through the enzymatic activities of their mycorrhizal associates.  Hence, we hypothesized that ECM forests will saturate more rapidly than AM forests resulting in greater nitrate leaching. 

We performed a meta-analysis of 31 N-enrichment experiments in temperate forests.  To exclude potentially confounding effects of leaf habit, only hardwood forests were included in the analysis.  Additionally, we excluded one-time N fertilization experiments as well as those conducted in young forests (<10 years old).  Stands were categorized as ECM-dominated, mixed, and AM-dominated.  Concentrations of nitrate in tension lysimeters below the rooting zone were used as analogs to nitrate leaching from forests.  

Results/Conclusions

On average, nitrate leaching was more than 5 times higher in N-enriched plots than control plots.  We found that mycorrhizal type alone explained 26% of the variation in nitrate leaching among sites.  With the addition of percent clay and total N addition, 58% of the variation in nitrate leaching was accounted for by this model.  Nitrate leaching was 1.3 times higher in ECM-dominated forests than AM-dominated forests, although these differences were not significant (p=0.79).  Our results suggest that dominant mycorrhizal associations may drive variations in N retention in forest communities exposed to N deposition.