Soil gross nitrogen transformations are related to land-uses in two agroforestry systems

•Gross mineralization and immobilization rates were greater in forest than in agricultural soils.•Gross nitrification rates were lower in forest than in agricultural soils.•Gross mineralization and immobilization rates were tightly coupled in forest soils.•Nitrate production and leaching risk were higher in agricultural than in forest soils. A better understanding of gross nitrogen (N) transformation rates in agroforestry systems is crucial for testing the land use management effects on soil N retention. In this study, gross N transformation rates in forest and adjacent agricultural soils in two agroforestry systems, hedgerow and shelterbelt, located near Edmonton, Alberta, Canada were investigated using the 15N labeled technique and a numerical model, FLUAZ. Gross N mineralization rates in the forested land use were higher than that in the agricultural land use for hedgerow and shelterbelt agroforestry system, and the pattern was the same for gross N immobilization rates. However, gross nitrification rates in the forested land use were lower than that in the agricultural land use for both agroforestry systems. The ratios of gross nitrification to ammonium immobilization rates and the ratios of gross N mineralization to immobilization rates in the agricultural land use were higher than that in the forested land use irrespective of the type of agroforestry system studied. Our study suggested that the agricultural land use had a lower potential to conserve N, posing a higher risk of N loss, whereas mineralization and immobilization processes were tightly coupled in the forested land use, leaving less N available for nitrification and subsequent N loss to the environment.