Charcoal and shrubs modify soil processes in ponderosa pine forests of western Montana

Low-elevation ponderosa pine (Pinus ponderosa Doug. ex. laws) forests of western Montana are naturally fire maintained ecosystems. However, 80–140 years of fire-exclusion has led to the formation of dense, mixed stands of ponderosa pine and Douglas-fir (Pseudotsuga menzesii (Mirbel) Franco), an understory co-dominated by graminoids and ericaceous shrubs, and low N availability. Ericaceous shrubs in particular have been found to influence soil processes in boreal ecosystems and potentially exacerbate N limiting conditions. In this set of studies, we investigated the influence of graminoid and ericoid litter chemistry on soil processes and evaluated the influence of charcoal as a sorbant of C compounds and depositional product of fire. A series of experiments were performed with two common understory plants of this ecosystem, elk sedge (Carex geyeri Boott) and kinnikinnick (Arctostaphylos uva-ursi (L.) Spreng.), an ericaceous shrub. Charcoal (100 g m−2) and glycine (5 g m−2) were applied in factorial combination to intact litter microcosms of these species. Non-ionic resin capsules were used to monitor mobile C compounds and ionic resins were used to monitor net N mineralization and nitrification in-situ. Greenhouse studies revealed that the addition of glycine and charcoal leads to a significant increase in net nitrification in shrub litter microcosms, but not sedge litter microcosms, as measured by NO3− sorption to ionic resin capsules. Charcoal and glycine also resulted in a significant increase of anthrone reactive C (soluble hexose sugars, an index of bioavailable C) in shrub litter microcosms. Analysis of leaf litter leachate from these two plant communities indicated similar nutrient concentrations, but almost 20 times more phenolic compounds in shrub leaf leachates. Charcoal was shown to be extremely effective at sorbing phenols, removing over 80% of phenolic compounds from solution. These results suggest that charcoal deposition after fire may modify a nitrification interference mechanism by sorbing plant secondary metabolites. After time, charcoal loses its ability to sorb C compounds and ericaceous litter decomposition, and subsequent release of phenolics, may interfere with nitrification once again.