Post-harvest soil nitrate dynamics in aspen- and spruce-dominated boreal forests

It has been suggested that clear-cutting boreal forests shifts the soil N environment from ammonium-domination to nitrate-domination, which encourages aspen regeneration but inhibits spruce regeneration. We assessed nitrate production and availability in clear-cut and uncut aspen- and spruce-dominated forests in a boreal mixedwood forest in northern Alberta, Canada, 4 years after harvesting. We hypothesized that nitrate production and availability would be higher in aspen than spruce-dominated forests, and higher in clear-cuts than in uncut forests. Nitrate production occurred in all forest types, but ammonium was the dominant inorganic N form and at all sites. Clear-cutting did not increase nitrate production and neither was it higher in aspen-dominated forests. Nitrate consumption was also unaffected by harvesting. Thus clear-cutting does not appear to convert boreal forest soils into “nitrate environments”. Nitrate was produced by both autotrophic and heterotrophic soil organisms, but heterotrophic activity dominated. Nitrification was responsive to manipulation of available C levels in laboratory, but changes in soil C availability in the field treatments were not associated with significant changes in nitrate production and consumption.