Estimating biological N2 fixation in Canadian agricultural land using legume yields

Abstract Annual biological N 2 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 N 2 fixation rates were adjusted using variations in crop yield, and the total amount of N 2 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 N 2 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 N 2 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 N 2 fixation in Canada. Provincial averages of N 2 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 N 2 fixation by total farmland was heavily weighted by the estimates for the Prairies and ranged from 16 to 29 kg N ha −1 .