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

Abstract: The effects of 4years 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 (30kgNha⁻¹ yr⁻¹), +S (30kgSha⁻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 4years 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 4years 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. [Copyright &y& Elsevier]