INCREASE IN MINERAL N IN SOILS DURING WINTER AND LOSS OF MINERAL N DURING EARLY SPRING IN NORTH-CENTRAL ALBERTA

Abstract

Ten field experiments were conducted on cultivated soils in north-central Alberta to determine any change in mineral N content of soils during winter, and during early spring after the soils had thawed. Soil samples were taken periodically from fall to spring to a depth of 120 (or 90) cm and were analyzed for NH₄-N and for NO₃-N. Mineral N changes occurred primarily in the top 60 cm. Between fall and late winter, there was an increase of 48 kg N ha⁻¹ of mineral N (range of 27–83) in the 60-cm depth of eight experiments set on stubble and the value increased only to 55 kg N ha⁻¹ when the sampling depth was extended to 120 (or 90) cm. Considering only the values from soil samples taken when soils were frozen, the increase in mineral N was 31 kg N ha⁻¹ (range of 14–54) in the 120-cm depth, and the average net mineral N accumulation was 0.35 kg N ha⁻¹ d⁻¹ (range of 0.26–0.43). There was a loss of mineral N during early spring of 44 kg N ha⁻¹ (range of 18–71). The two experiments on summerfallow had more over-winter accumulation of mineral N and more loss in early spring compared to the stubble experiments. This study showed large increases in the mineral N content when the soil was frozen and large decreases in the early spring. The mechanism of increase in mineral N in frozen soil was not determined. The cause of the decrease in early spring was most likely denitrification, and was not leaching of nitrate. The results of the investigation may have implications for the time of soil test sampling and for the loss of native N from cultivated soils. Key words: Ammonium N, frozen soil, mineral N, nitrate N, early spring loss