Ectomycorrhizas (ECM) as switch-hitters?: Balancing resources from plant and soil?

Background/Question/Methods
Forest ecosystem productivity is maintained by nutrient acquisition, a central ecosystem function. Nutrient limitation constrains fertility of many forests and soils worldwide, and plants depend critically on their mycorrhizal roots to meet their nutritional demands.  Mycorrhizas balance fungal and plant nutrient demands with soil nutrient availability, which may impact their ability to act as pure mutualists with higher plants. Our objective was to determine whether ectomycorrhizal (ECM) fungi switch from acquisition of one resource to another in response to changing availabilities in resources from plants (C) and soils (N, P). We hypothesized that ectomycorrhizas will switch extracellular enzyme activity from phosphatases (P acquisition) to aminopeptidases (organic N acquisition) to glycosidases (C acquisition), depending on availability of P, N and C.  We tested our hypotheses in a California pine-oak (Pinus sabiniana and Quercus douglasii) woodland. In winter and spring, we harvested pine and oak ectomycorrhizal root tips from pine and oak saplings, sorted the tips by mycorrhizal morphotypes and determined the activity profile for extracellular enzymes that breakdown soil organic matter and release P, N and C.  Examples of enzymes include and acid phosphatase, leucine-aminopeptidase and glucoronidase.

Results/Conclusions
The enzymatic profiles differed with treatment and with ectomycorrhizal morphotype.  In winter, soil C degradation activity (glucoronidase) was greater in oak roots than in pine, when the deciduous Q. douglasii was leaf-less in contrast to evergreen pines.  The C demand may have been greater in oak than in pine, resulting in increased C degrading enzyme activity. Phosphatase activity was higher in ECM-pine roots than in oak, but with little seasonal effect.  Higher P demand by pines may be a function of their year-round activity, in contrast to oaks.  Ectomycorrhizal morphotypes differed in their enzymatic profiles. Phosphatase activity was uniform with depth whereas glucoronidase and leucine-aminopeptidase activities were greater in the top 15 cm of soil.   Together, these results indicate that pine and oak ECM produce extracellular enzymes targeting a broad range of resources in soils, including C, N and P resources. However, controls on these enzymes are not clear, particularly as they relate to changes in resource supplies, demands, and organism C:N:P ratios. Our research indicates that ECM have the potential to act as saprotrophs and degrade soil organic C, thus behaving facultatively as mutualists under certain conditions.  ECM enzymatic activities can enhance our understanding of coupled C, N and P interactions and the nature and occurrence of nutrient limitation in ecosystems.