The Canadian Agricultural Nitrogen Budget model (CANB v3.0) was developed to estimate two Agri-environmental Indicators; Residual Soil Nitrogen (RSN) and the Indicator of Risk of Water Contamination by Nitrogen (IROWC-N) at the 1:1 million spatial scales. The RSN (kg N ha^-1) is an estimate of the amount of N remaining in the soil after crop harvest. The IROWC-N provides an estimate of the concentration and amount of nitrate-N leaching from the rooting depth of the soil.

In the CANB v3.0 model, fertilizer N application was reconciled with provincial N sales. Storage and land losses of manure N and manure N mineralization were estimated. Biological N₂ fixation rates were adjusted by crop yields and crop N removals were estimated by crop yield and N concentration in the yield and biomass. N leaching losses in both growing and non-growing seasons were estimated by integrating the growing and non-growing drainage volumes from the daily IROWC-N model with the CANB v3.0.

Regional differences in the RSN were large. The average RSN values were less than 30 kg N ha^-1 in western Canada but greater than 66 kg N ha^-1 in eastern Canada. There was an increase in the amount of land in the high and very high RSN risk classes in Canada from 1981 (10%) to 2001 (30%) followed by a decrease to 17% by 2006.

Annual (growing and non-growing seasons) N leaching losses varied from 2.6 kg N ha^-1 in 1981 to 3.7 kg N ha^-1 in 2006, and average nitrate concentrations increased from 2.0 mg N L^-1 in 1981 to 7.0 mg N L^-1 in 2006. These results demonstrate that there is an increasing risk of RSN and IROWC-N over time in Canadian farmland and yearly climatic variability can accentuate these differences.

Keywords: Fertilizer N, manure N, biological N₂ fixation, residual soil N, nitrate leaching, Canadian Agricultural Nitrogen Budget (CANB) model