Contrasting responses of gross and net nitrogen transformations to salinity in a reclaimed boreal forest soil

Plant growth is often affected by low soil nitrogen (N) availability and soil salinity in the Athabasca oil sands region (AOSR) of Alberta, Canada, as the reclaimed cover soil of peat-mineral soil mix (PMM) releases N slowly, and may be affected by salts moving upward from the underlying saline tailings sand (TS) or overburden (OB) materials used for land reclamation. Whether soil salinity affects N transformation and thus N availability is not clear. We studied the effect of soil salinity on net and gross N transformation rates in a reclaimed boreal forest soil in an incubation experiment using the N-15 isotopic pool dilution technique. To simulate the salinity in reclaimed soils, sodium chloride was added to a PMM soil to form five salinity levels represented by electrical conductivity (EC) of 2, 4, 6, and 8 dS m(-1) (based on 1:2 w/v soil to water ratio) and the control. Gross N mineralization, nitrification, and immobilization rates were significantly different among salinity levels, with the rates suppressed by high soil salinity, but they were not completely inhibited even when soil EC reached 8 dS m(-1). As the soil EC increased from 2 to 8 dS m(-1), gross N immobilization rates decreased faster than gross N mineralization rates, resulting in less negative net N mineralization rates at high salinity levels. The immobilization of NH4 (+)-N was a greater process for NH4 (+)-N consumption than nitrification and possible NH3 volatilization, regardless of the salinity level. In contrast to gross nitrification rates, net nitrification rates were not affected by soil salinity in the studied soil, indicating that the influence of soil salinity on NO3 (-)-N production and consumption was similar. We conclude that soil salinity had different effects on gross versus net rates of soil N transformation and reduced N mineralization and nitrification rates under high soil salinity would lower N availability and thus increase the need for N fertilization in reclaimed soils.