LOSS OF ORGANIC MATTER AND POTENTIALLY MINERALIZABLE NITROGEN FROM SASKATCHEWAN SOILS DUE TO CROPPING

Abstract

Although it is known that considerable loss in total organic matter has occurred due to cultivation of prairie soils, there is little information on changes in the fertility-related fraction of the organic matter. Twelve prairie surface soils representing paired virgin and cultivated coarse-, medium-, and fine-textured soils from the Brown, Dark Brown, and thin Black Chernozem and Gray Luvisol soil zones were analyzed and used to assess changes in total C, N and potentially mineralizable N. Cropping caused large losses of organic C (41–53%) and N (31–56%), but losses of potentially mineralizable N (N₀) were even greater for the Chernozems (55–67%) though not for the Gray Luvisols. N₀ in virgin soils ranged between 133 and 401 μg N/g soil and increased from Gray Luvisol to Brown to Dark Brown to thin Black Chernozem. In cultivated soils, N₀ was generally similar among soil zones and values ranged between 66 and 141 μg N/g soil. Medium-textured soils generally had the greatest organic matter content and N₀. The active N fraction (N₀/N) ranged between 4 and 20.5% and was greatest in Brown soils and in the coarser-textured soils. Losses of organic matter, N₀, and N₀/N were greatest from the coarser-textured soils. The findings of this study help to explain why a significant portion of prairie soils now require N fertilizer even after being summer-fallowed.