Nutrient Dynamics in Canadian Agricultural Systems

Derek Lynch, Nova Scotia Agricultural College, Truro, NS B2N 5E3

Abstract
Agriculture contributes significantly to greenhouse gases (GHG) in the atmosphere, particularly the potent GHG methane (CH₄) and nitrous oxide (N₂O). N₂O is produced primarily from excess reactive nitrogen (Nr) accumulating in agricultural soils, before cascading through terrestrial and aquatic systems.

During just the last few decades, the production of new Nr by humans, primarily as industrially fixed fertilizer N, has been greater than the Nr derived from all natural terrestrial ecosystems. To satisfy human needs and dietary preferences, we have had a much greater impact on the global N cycle than the global C cycle, and the negative consequences of this saturation of the earth and the atmosphere with Nr, for the health and welfare of people and of ecosystems, is only now being fully realized.

Organic farms are largely dependent on N from cultivated legumes, and recent research in Canada suggests that legumes that are actively fixing N₂ by biological nitrogen fixation (BNF) have far lower N₂O emissions than was previously estimated. Additional work has demonstrated that composting reduces N₂O emissions compared to stockpiled manure, and that under long term organic soil management, the ratio of emission of non-reactive, benign N₂ gas to N₂O improves.

Research across Canada is documenting another related benefit of organic farming; that organic farms are far less intensive in terms of overall soil nutrient loading, and livestock stocking rate, with greatly reduced risks of nutrients losses to aquatic systems. At the same time, evidence is mounting that maintaining adequate P levels is a particular challenge for all organic cropping and some livestock systems. Additional research has shown that these P deficiencies cannot be readily addressed through phytoextraction of un-acidulated rock phosphate P alone. Maintaining the P status and productivity of organic farms will require a willingness by the organic sector to both reassess permitted P inputs, and to improve the efficiency of how we use and recycle P in regional agroecosystems.

Full Presentation (PDF)

Français

Posted February 2007