The effect of tree mortality on biogeochemical response after mountain pine beetle forest infestation

Pine species in the American West are experiencing large-scale, bark beetleinduced mortality in association with a changing climate. As resulting terrestrial biogeochemical responses to this ecosystem disturbance are unclear, we hypothesized that a threshold of localized tree mortality must be exceeded before disruption of carbon and nitrogen biogeochemical cycling is observed. In order to isolate the compensatory effects of proximal healthy trees and intertwined hydrologic, energetic and rhizospheric processes, microbiological and geochemical parameters within three near-surface soil horizons were contrasted between healthy and deceased lodgepole pines (Pinus contorta) surrounded by varying extents of tree mortality. Bark beetle impact altered N cycling, as an increased proportion of the total N pool was inorganic ammonium, as opposed to more stable and less mobile organic N, which dominates in healthy systems. This change in N cycling was dependent on the extent of surrounding tree mortality in the upper soil horizons, but not in the mineral soil. The different response in the upper and lower soil horizons is likely a reflection of how altered inputs after bark beetle infestation are different in each horizon. A threshold response in the upper horizons was found for ammonium, which accumulated only under trees surrounded by at least 40% tree mortality. The elevated ammonium response was associated with increased C recalcitrance and increased relative abundance of N. Concurrently, surrounding tree mortality also affected the soil bacterial community structure with associated increases in alpha diversity and overall community structure. However, functional processes tended to correlate better with changes in C:N ratio. Collectively, these biogeochemical and microbial indicators suggest that high degrees of beetle-induced mortality shift the terrestrial environment of Colorado Rocky Mountain lodgepole pine forests from an N-limited ecosystem to one where N is in excess.