OOS 35-6
Changes in litter quality and fungal physiology enhance negative effect of simulated nitrogen addition on litter decay

Thursday, August 14, 2014: 9:50 AM
204, Sacramento Convention Center
Linda T.A. Van Diepen, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH
Eric W. Morrison, Natural Resources and the Environment, University of New Hampshire, Durham, NH
Christopher M. Sthultz, Math, Science and Technology Department, University of Minnesota, Crookston, Crookston, MN
Anne Pringle, Organismic and Evolutionary Biology, Harvard University, Cambridge, MA
Serita D. Frey, Department of Natural Resources and the Environment, University of New Hampshire, Durham, NH

Rates of nitrogen (N) deposition are increasing in industrialized and rapidly developing nations. In N-limited forest ecosystems, simulated N deposition has resulted in increased litter N concentration, coinciding with decreased carbon:N ratio, and thus changing litter quality. Litter quality is one of the main factors in determining the decay rate of litter. In addition to changes in litter quality, simulated N deposition has been found to change fungal community structure associated with decaying litter. To understand the relationship between litter quality, fungal community dynamics and litter decay in response to N deposition, we made use of an existing long-term study in which a northern hardwood forest has been exposed to simulated N deposition for 25 years (Chronic Nitrogen Addition plots, Harvard Forest, Petersham, MA). We implemented a reciprocal transplant litterbag study and measured litter mass loss, extracellular enzyme activities and fungal community composition after one and two years of decomposition. In addition, fungi were cultured from the decomposing litter. We measured in vitro decomposition capacity for a subset of cultured fungal species in conditions representative of field N availability, to understand effects of long-term simulated N deposition on fungal physiology.


After one year of decomposition, decay rate of litter with a higher N concentration was negatively affected by N treatment, while after two years all litter types showed decreased decay rates with simulated N deposition. Ligninolytic enzyme activities were negatively affected by increased litter N concentration in the first year, and by both litter N concentration and N treatment in year 2. The suppressive effect of N on ligninolytic enzymes and decay rates in the later stage of decomposition were as expected, but were unanticipated for the early stage of decomposition. Both litter N concentration and N treatment caused a shift in fungal community composition after one year of decomposition. In addition to a changed fungal community, long term N addition affected the physiology of fungal species persisting in a higher N environment. Several fungal species isolated from the N treatments showed reduced decomposition capacity compared to the same fungal species isolated from the control treatment. This cascade of effects of simulated N deposition on litter quality, fungal community and physiology, and litter decay rates indicate that complex interactions of biochemical and biological factors are at play in determining the effects of N addition on biogeochemical cycling within forest ecosystems.