Present and potential future nitrogen gains from legumes in major Soil Zones of the prairies
van Kessel, C.
Two developments have enabled us to realistically estimate N, fixation and N credits from legume crops across the prairies: one is computerized processing of Statistics Canada’s Census of Agriculture crop data by Soil Climatic Zones (SCZ) rather than provincial borders; the other is availability of reliable field 15N data on N fixation (% Ndfa) for various legumes and on partitioning of N (PTN) into harvested and residual parts. Total N fixation for each legume and SCZ was computed on the basis of crop area and SCZ average yields, adjusted for abnormal precipitation, and then multiplied with crop-specific Ndfa and PTN factors. Future legume cropping and N gains were 'guesstimated' according to the most likely crop sequences for various SCZs to be in use by the year 2005. Bctwcen 1991 and 1994 legume hcctaragc increased 69%, due mainly to more pulse cropping, to a total of 2.4 million and is expected to reach 5.5 million by 2005. Dcspitc the increased pulse cropping, 85% of total N fixation in 1994 occurred still in the northern parklands and was overwhelmingly due to forage legumes. The average fixation rate was 104 kg/ha and the total 250 million kg N fixed is equal to 24% of all fertilizer N used in 1994 on 25 million ha of Prairie cropland. In comparison with recent fertilizer N costs the present legume N fixation can be valued at $220 million. Total N credit for 1994 was calculated to be 62 million kg N which is equal to 1/4 of total N fixation and suggests an average net gain of 26 kg N/ha of legume crop, although these amounts vary widely with legume type. Our N credit estimates ranged from a low of 0 kg N/ha for colored beans, to 9.4 for lentils, 18.1 for peas, 36.4 for forage hay legumes and a high of 65.4 kg N/ha for forage seed legumes. Through combination of the N credit effect with crop substitution and ‘rotation’ effects, the inclusion of forage, grain or green manure legumes in rotations can effect substantial energy savings for Prairie cropping systems. With continued expansion of legume crops and partial adoption of fallow replacement green manuring, N fixation is projected to total around 550 million kg N by 2005 with a residual N credit of about 190 million kg, indicating an average net gain of 35 kg N/ha. The N gains from legume cropping are forecast to provide most of the N required for reasonable yields by a following cereal or oilseed crop.
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