Relationship between ammonia oxidizing bacteria and bioavailable nitrogen in harvested forest soils of central Alberta

Forest soils are commonly limited in nitrogen (N), and the removal of aboveground biomass in harvesting operations can exacerbate the problem. Thus, the soil organisms that facilitate the rate-limiting step in the N cycle, the oxidation of ammonium (NH4+), are of special interest in harvested environments. The objective of this study was to investigate the changes in ammonia oxidizing bacteria (AOB) communities that occurred in the years following clear cutting, and link those community shifts to availability of inorganic N forms NH4+ and nitrate (NO3−). Genetic fingerprinting targeting the amoA gene coupled with denaturing gel gradient electrophoresis was carried out over two summers on forest floor (LFH) and mineral (Ae) soils of three similar cutblocks harvested during different years. In-situ NH4+ and NO3− availability was measured over the growing seasons of 2009 and 2010, as well as a suite of physical soil characteristics. Results indicated that the AOB community composition differed in younger vs. older cutblocks, but not by soil horizon. The changes seen in the AOB paralleled the change in N bioavailability across sites, soil horizons, and sampling years, thus indicating that N bioavailability may be directly linked to AOB community composition. This link may provide the basis for the use of AOB as indicators of nutrient availability in the future.

► Assessed ammonia oxidizing bacterial (AOB) community composition and N bioavailability in a chronosequence of clear cut boreal forest soils. ► A field study of clear cut Boreal forest soils with a chronosequence approach. ► AOB community composition differed along the chronosequence, but not between LFH and mineral Ae soil horizons. ► N response to harvesting paralleled that of AOB, suggesting a relationship between the two. ► This is the first study of AOB response to harvesting in North American forest soils.