Residual Soil Nitrogen
Drury, C.F., Yang, J.Y., De Jong, R., Huffman, T., Reid, K., Yang, X.M., Bittman, S. and Desjardins, R. 2016. Residual Soil Nitrogen. Pages 114-120 in Clearwater, R. L., Martin, T. and Hoppe, T. (eds.) 2016. Environmental sustainability of Canadian agriculture: Agri-environmental indicator report series – Report #4. Ottawa, ON: Agriculture and Agri-Food Canada.
The residual soil nitrogen (RSN) Indicator provides an estimated of the amount of nitrogen which remains in the soil at the end of the growing season. It is calculated as the difference between total N inputs to agricultural soils (fertilizer and manure, N fixation by leguminous plants, wet and dry atmospheric deposition) and total N outputs (harvested crops and gaseous losses including ammonia, nitrous oxide and nitrogen gas). The Canadian Agricultural Nitrogen Budget (CANB) v4.0 model was derived to estimate the RSN Indicator in agricultural regions across Canada. Agricultural practices and climatic growing conditions vary across the agricultural regions of Canada, and this variability is reflected in the average RSN values estimated for these regions. The majority of farmland in Canada was in the moderate (28%) and low risk (24%) classes. Whereas 28% of farmland was in the high and very high risk categories, with most of this land located in southwestern Manitoba, southern Ontario, the St. Lawrence Lowlands (Quebec) and Atlantic Canada. The only regions with a majority of agricultural land in the low or very low risk categories were Saskatchewan, southern Alberta and British Columbia; however, these regions also contained pockets of higher risk. There was a national trend towards increasing risk associated with elevated residual N levels in farmland soils across Canada from 1981 to 2011. The amount of land in the very low risk class decreased from 69% to 20% in Canada between 1981 and 2011. This decrease occurred as a result of an 8% increase in land in the low risk class, a 22% increase in land in the moderate risk level, an 11% increase in land in the high risk class and a 7% increase in land in the very high risk class. On a national basis, average N inputs have almost doubled over the past 30 years, from 44.4 kg to 80.8 nitrogen/hectare, whereas average N outputs increased by 63% from 35 kg nitrogen/hectare in 1981 to 57.2 kg nitrogen/hectare in 2011. The greater increase in N inputs compared to N outputs over time has resulted in an increase of RSN values from 9.4 kg nitrogen/hectare in 1981 to 23.6 kg nitrogen/hectare in 2011. The high RSN levels are a major contributing factor in risk of water contamination by nitrogen. Methods to reduce the RSN should be applied to reduce total N input and increase nitrogen output whenever possible. These include (i) growing cover crops to capture the unused inorganic N in the soil after harvest; (2) increasing manure N efficiency and possibly reduce the amount of fertilizer required; (3) enhancing fertilizer N use efficiency by 4R techniques (right source, rate, time and place) and (4) improving crop N uptake by irrigation during drought conditions.
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