Estimating biological N2 fixation in Canadian agricultural land using legume yields

Annual biological N2 fixation by legumes and by free-living non-symbiotic organisms was estimated for all Canadian farmland from 1981 to 2006 using the Canadian Agricultural Nitrogen Budget (CANB v3.0) model. Published average N2 fixation rates were adjusted using variations in crop yield, and the total amount of N2 fixed was estimated at the Soil Landscape of Canada polygon level (1:1,000,000) and then scaled up to provincial and national levels. The adjusted N2 fixation rates ranged from 54 to 150 kg N ha−1 (average 95 kg N ha−1) for pulse crops, 57 to 201 kg N ha−1 (average 118 kg N ha−1) for soybean (Glycine max (L.) Merr.), 27 to 141 kg N ha−1 (average 79 kg N ha−1) for hay [i.e., a mixture of alfalfa (Medicago sativa L.), clover (Trifolium), timothy (Phleum pratense) and brome (Bromus) grasses, etc.] and 141 to 300 kg N ha−1 (average 218 kg N ha−1) for alfalfa. Total estimated N2 fixation for all Canadian farmland increased from 0.996 Tg in 1981 to 1.767 Tg in 2006, primarily due to an increased legume hectare over time (from 9 to 15 Mha). These four legume crops accounted for 82–89% of total N2 fixation in Canada. Provincial averages of N2 fixation by total farmland area varied from 6 to 19 kg N ha−1 in Saskatchewan, 16 to 27 kg N ha−1 in Alberta and Manitoba, 44 to 79 kg N ha−1 in Ontario and Quebec and 27 to 47 kg N ha−1 in Atlantic provinces. Because the Prairies provinces contain 75% of total farmland, the national averages of N2 fixation by total farmland was heavily weighted by the estimates for the Prairies and ranged from 16 to 29 kg N ha−1.