Tree mycorrhizal association, largely ectomycorrhizal (ECM) or arbuscular mycorrhizal (AM), is hypothesized to integrate plant and soil properties and provide a tool for predicting carbon (C) and nitrogen (N) cycling. Specifically, ECM-associated trees may initiate more conservative C and N cycling than AM-associated trees through production of lower quality leaf litter and the unique capacity of ECM to utilize organic N. In temperate mixed mycorrhizal forests, these hypothesized pathways are confounded. However, when considering high species diversity at a global scale, mycorrhizal association has no consistent effect on foliar chemistry. We investigated a Panamanian tropical forest where ECM litter quality is near the community average for AM species, allowing us to assess the effects of mycorrhizal association on C and N cycling in isolation. We quantified C and N stocks of ECM- and AM-dominate stands at four sites established on unique parent materials: rhyolite, granodiorite, dacite and andesite. To accomplish this, we sampled organic soils from a 0.25m by 0.25m square area of soil surface and mineral soil at 0-10cm and 10-20cm depths using a 10 cm diameter soil core and tested for the effects of parent material, stand mycorrhizal association and depth on C and N stocks.
Results/Conclusions
We show significantly higher C:N ratios in ECM versus AM organic (P<0.008) and mineral soils (0-10cm (P<0.0001), 10-20cm (P<0.0001)). However, the effect of stand mycorrhizal association on soil C:N varied by parent material; C:N was significantly higher in AM versus ECM stands established on Inceptisols derived from dacite and andesite (P=0.0084, P=0.0001 respectively) but did not differ between ECM and AM stands established on Ultisols derived from rhyolite and granodiorite. Although C stocks did not differ between ECM and AM organic or mineral soils, N stocks were significantly greater in AM compared with ECM mineral layers (P=0.041, P=0.036 respectively). We therefore conclude that ECM soils may have higher C:N than AM soils despite similar quality of leaf litter inputs suggesting direct uptake of organic N by ECM may contribute to high soil C:N in ECM systems. Our results provide further support for this hypothesis, showing that differences in N, not C stocks are responsible for C:N variation between ECM and AM soils. We suggest that multiple pathways contribute to mycorrhizal mediation of C and N cycling and that resulting differences in C:N between ECM and AM soils are mediated by variation between sites, such as shifts in parent material.