Mycorrhizal mediation of nitrogen deposition effects on extracellular enzyme activity and soil carbon pools
Recent studies provide compelling evidence that plant-mycorrhizal associations could play a major role in mediating the response of NPP and soil organic matter decomposition to N deposition. Increased inorganic N availability from N deposition can release free-living decomposers from competition for organic N with EMF, leading to increases in soil respiration and decreases in soil C. Our study was conducted in an N fertilization experiment established in the Fortuna Forest Reserve in western Panama in 2007 (n = 4 plots). We composited three soil cores (0-10 cm depth) collected from under focal tree species associated with arbuscular mycorrhizal fungi (AMF) (Micropholis melinoniana and Eschweilera panamensis) and EMF (Oreomunnea mexicana); three subreplicate trees per species were sampled in each replicate field plot. We measured activity of five hydrolytic enzymes and performed density fractionation of the bulk C pool into the free light, occluded light, and heavy fractions. Bacteria and saprotrophic fungi use hydrolases to decompose labile forms of C whereas saprophytic fungi can use and EMF exclusively use oxidative enzymes to decompose recalcitrant forms of C. Thus, we expected to observe an increase in hydrolytic enzyme activity and a decrease in the labile C pool with N fertilization under EMF-associated trees.
In the control plots, activities of β-1,4-glucosidase and N-acetyglucosaminidase were higher under O. mexicana than M. melinoniana (p = 0.06 and p = 0.08, respectively). No significant difference was observed among species for Xylosidase activity; data for alkaline phosphatase and Leucine-amino-peptidase are currently being analyzed.
We observed an average 13 % increase in activity of the hydrolytic enzyme, β-1,4-glucosidase, under O. mexicana, an EMF-associated species, and did not affect activity of this enzyme under E. panamensis or M. melinoniana, the AMF-associated species. We are currently analyzing the data to determine N fertilization effects on activities of the other hydrolytic enzymes as well as the soil C fractions. Our preliminary data suggest that soil processes under focal trees reflect their mycorrhizal association, and provide some support for our hypothesis that hydrolytic enzyme activity will increase with N fertilization under EMF-associated trees but not AMF-associated trees.