Depth distribution of mineralizable nitrogen pools in contrasting soils in a semi-arid climate

A better understanding of the depth distribution of soil mineralizable nitrogen (N) pools is important to improve prediction of net soil N mineralization. However, our understanding of the depth distribution of these N pools under the semi-arid conditions of western Canada is limited. This study examined the depth distribution of soil mineralizable N pools (kS, the rate constant of a nondepleting zero-order stable N pool, and NL, the size of a depleting first-order labile N pool) of six sites in western Canada chosen to vary with respect to soil zone, soil texture, and cropping system. The depth distribution of mineralizable N pools varied substantially among sites, indicating that this distribution needs to be considered in making predictions of net soil N mineralization. A single regression equation including soil total nitrogen (STN), Pool I (a labile mineralizable N pool determined through a 14-day aerobic incubation), and soil pH explained 67% of the variation in kS across sites and soil depths. In addition, 95% of the variation in NL was explained by a regression model with Pool I. Thus, although the depth distribution of soil mineralizable N pools can vary substantially among sites, the mineralizable N parameters can be adequately predicted across sites and soil depths from simple soil properties. Comparison with a study using surface soils under humid conditions in New Brunswick suggests that the relationship between NL and Pool I is applicable across a wide range of soils, climatic zones, and cropping systems, whereas the regression model to predict kS varied with climatic zone, perhaps reflecting different pedogenic processes stabilizing the organic matter in these climatic zones.