Gross N transformations were little affected by 4 years of simulated N and S depositions in an aspen-white spruce dominated boreal forest in Alberta, Canada

The effects of 4 years of simulated nitrogen (N) and sulfur (S) depositions on gross N transformations in a boreal forest soil in the Athabasca oil sands region (AOSR) in Alberta, Canada, were investigated using the 15N pool dilution method. Gross NH4+ transformation rates in the organic layer tended to decline (P < 0.10, marginal statistical significance, same below) in the order of control (CK, i.e., no N or S addition), +N (30 kg N ha−1 yr−1), +S (30 kg S ha−1 yr−1), and +NS treatments, with an opposite trend in the mineral soil. Gross NH4+ immobilization rates were generally higher than gross N mineralization rates across the treatments, suggesting that the studied soil still had potential for microbial immobilization of NH4+, even after 4 years of elevated levels of simulated N and S depositions. For both soil layers, N addition tended to increase (P < 0.10) the gross nitrification and NO3− immobilization rates. In contrast, S addition reduced (P < 0.001) and increased (P < 0.001) gross nitrification as well as tended (P < 0.10) to reduce and increase gross NO3− immobilization rates in the organic and mineral soils, respectively. Gross nitrification and gross NO3− immobilization rates were tightly coupled in both soil layers. The combination of rapid NH4+ cycling, negligible net nitrification rates and the small NO3− pool size after 4 years of elevated N and S depositions observed here suggest that the risk of NO3− leaching would be low in the studied boreal forest soil, consistent with N leaching measurements in other concurrent studies at the site that are reported elsewhere.

► Four years of elevated N and S depositions had little effect on gross N transformation rates. ► Gross immobilization rates were always greater than gross mineralization or nitrification rates. ► There was little risk of nitrate leaching or N saturation.