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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

Cheng, Yi, Cai, Zu-cong, Zhang, Jin-bo, Chang, Scott X.
Forest ecology and management 2011 v.262 no.3 pp. 571-578
Picea, boreal forests, forest soils, leaching, mineral soils, mineralization, nitrification, nitrogen, oil sands, organic soils, risk, sulfur, Alberta
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⁻¹yr⁻¹), +S (30kgSha⁻¹yr⁻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.