Recent evidence suggests that ecosystems dominated by ectomycorrhizal (ECM) fungi (e.g. boreal forests) store more soil carbon than ecosystems dominated by arbuscular mycorrhizal (AM) fungi (e.g., grasslands) due to the wider C:N ratios of ECM soils. Given that ECM-associated and AM-associated plants often co-occur in the same ecosystem (e.g. in deciduous forests), we investigated the extent to which ECM or AM dominance of trees could be used to predict soil C and C:N ratios at plot and landscape scales. Accordingly, our objectives were to investigate the fine-scale relationship between mycorrhizal dominance and soil C and N in deciduous forests, and to determine whether the relationships between the overstory tree community and soil C and N pools change with soil depth. We quantified C and N stocks in soils (Oe/Oa and 0-10, 10-20, 20-50 and >50cm depth) in forest plots spanning a gradient of AM-dominated to ECM-dominated trees (by basal area). We hypothesized that ECM-dominance would be positively associated with soil C and soil C:N in surface soils (0-10cm) due to both the high C:N of the ECM detrital inputs and exploitation of N by ECM fungi, but that the relationship would attenuate with depth concordant with declines in root biomass.
Results/Conclusions
We found a strong positive relationship between the dominance of ECM-associated trees and O horizon C stocks (R2=0.62; P<0.01) and upper surface soil C:N (R2=0.77; P<0.01). However, contrary to our hypothesis, the relationship between ECM dominance and soil C:N generally persisted throughout the soil profile, whereas the relationship between ECM dominance and soil C did not. At 50cm below the surface, for example, the percentage of ECM trees still explained 21% of the variation in soil C:N. Notably, variations in C:N were driven by a decrease in soil N rather than an increase in soil C. In fact, we observed a slight decrease in soil C under ECM trees at depth. Taken together, our results suggest that although ECM-associated trees may store more C per unit N, this does not equate to greater total soil C storage when the complete vertical distribution of organic matter is considered. However, to the extent that these findings apply in other forests, our results suggest that ECM and AM trees affect the vertical distribution and chemistry of soil organic matter. These differences may fundamentally affect the long-term stability of soil carbon and, as such, should be considered in ecosystem and global C models.