PS 12-116
Mycorrhizal mediation of nitrogen deposition effects on extracellular enzyme activity and soil carbon pools

Monday, August 10, 2015
Exhibit Hall, Baltimore Convention Center
Nate C. Lawrence, Natural Resources and Environmental Sciences, University of Illinois Urbana-Champaign, Urbana, IL
Wendy H. Yang, Departments of Plant Biology and Geology, University of Illinois at Urbana-Champaign
Alex H. Krichels, Plant Biology, University of Illinois Urbana-Champaign

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.