Nitrogen transformation rates are affected by cover soil type but not coarse woody debris application in reclaimed oil sands soils

Forest floor mineral soil mix (FMM) and peat mineral soil mix (PMM) are cover soils commonly used for reclamation of open‐pit oil sands mining disturbed land in northern Alberta, Canada; coarse woody debris (CWD) is another source of organic matter for land reclamation. We investigated net nitrogen (N) transformation rates in FMM and PMM cover soils near and away from CWD 4–6 years after oil sands reclamation. Monthly net nitrification and N mineralization rates varied over time; however, mean rates across the incubation periods and microbial biomass were greater (p < 0.05) in FMM than in PMM. Net N mineralization rates were positively related to soil temperature (p < 0.001) and microbial biomass carbon (p = 0.045). Net N transformation rates and inorganic N concentrations were not affected by CWD; however, the greater ¹⁵N isotope ratio of ammonium near CWD than away from CWD indicates that CWD application increased both gross N mineralization/nitrification (causing N isotope fractionation) and gross N immobilization (no isotopic fractionation). Microbial biomass was greater near CWD than away from CWD, indicating the greater potential for N immobilization near CWD. We conclude that (1) CWD application affected soil microbial properties and would create spatial variability and diverse microsites and (2) cover soil type and CWD application had differential effects on net N transformation rates. Applying FMM with CWD for oil sands reclamation is recommended to increase N availability and microsites.