Developing a Reactive Nitrogen Model for Canadian Agricultural Land

Global reactive N has increased over the last century to sustain global demand for food, fiber and energy crops by the production of synthetic fertilizers as well as planting of legume crops which biologically fix atmospheric nitrogen. Increases in reactive N, however, have resulted in detrimental effects on ecosystems and human health. For example, N gas emissions cause smog (NH3) and global warming (N2O), while nitrate N losses by leaching and runoff cause eutrophication of fresh waters and contamination of drinking water. A reactive N model is being developed to estimate the annual reactive N cycle for Canadian agricultural land using an mass balance approach at a soil landscape polygon scale. The N inputs include fertilizer N, biological N fixation, N deposition and N mineralization from previous organic residues. The N output contains crop N removal as human food, animal feed, N losses from gaseous forms including N2O and NH3, and leaching and runoff of NO3-. Several national databases were used as inputs of the model. The land use database is derived from the agricultural census (crop area, livestock, tillage); soil data were obtained from Canadian Soil Information System; weather data were from Canadian weather framework, and yield data were from Statistics Canada. Fertilizer N data were obtained from both provincial agriculture and Canadian fertilizer industry. The preliminary result showed that fertilizer N, biological N fixation and N deposition to Canadian farmland increased by 2.1, 1.7 and 1.2 times, respectively, while N removal by crop production only increased by 1.4 times from 1981 to 2011. Regional differences were significant. Fertilizer N losses from ammonia (NH3) emissions increased by 2.4 times and NO3- leaching losses increased by 3.0 times during this period. Management practices have to be developed and/or implemented to reduce the economic and environmental losses associated with increases in reactive N in Canadian farmland.