Gross nitrogen transformation rates differ in reconstructed oil-sand soils from natural boreal-forest soils as revealed using a 15N tracing method

• A tracing method is proposed to study N-cycle in reconstructed and natural soils.
• Ammonium was cycled differently in reconstructed and in natural soils.
• Positive net nitrification rates were measured only in reconstructed soils.
• N transformation rates did not differ among vegetation types.

The Athabasca Oil Sands deposit is one of the largest single oil deposits in the world. Following surface mining, companies are required to restore soil-like profiles that can support the previous land capabilities. Re-establishment of nutrient cycles in reconstructed soils is one of the most critical factors in ensuring long-term sustainability of these reconstructed boreal landscapes. We compared soils from nine reconstructed oil-sand sites with those from three natural boreal-forest sites of similar age since wildfire disturbance. We (1) measured soil total N, NH4+, and NO3− content; (2) quantified gross N transformation rates in reconstructed and natural soils using a 15N tracing method; (3) compared gross rates of N transformations in reconstructed soils under different vegetation types to compared N-cycling processes in these soils. The 15N tracing approach highlighted key distinctions in N-cycling processes in reconstructed and natural soils. In reconstructed soils, NH4+ was mainly cycled through the recalcitrant organic-N pool, whereas in natural soils, NH4+ was produced from the recalcitrant organic-N pool but predominantly consumed in the labile organic-N pool. The mineralization of NH4+ from the labile organic-N pool was also higher in natural soils compared to reconstructed soils, suggesting greater prominence of microbial N-cycling activity in the natural soils compared to the reconstructed soils. Gross nitrification rates were similar in natural and reconstructed soils, but net nitrification rates were higher and apparently of heterotrophic origin in reconstructed soils. The higher net nitrification rates in reconstructed soils indicate a surplus of N relative to microbial requirements in reconstructed soils. N-transformation rates were similar in reconstructed soils under the three types of vegetation.