Residual effects of preceding crops and nitrogen fertilizer on yield and crop and soil N dynamics of spring wheat and canola in varying environments on the Canadian prairies

Abstract

Past agronomic practices, particularly cropping sequence and the balance between N applications and crop N removal, will influence soil N supply and crop yield potential. Nitrogen (N) fertilizer applications required to optimize crop yield and quality will be a function of the difference between crop N demand and the amount of N supplied by the soil. Therefore, assessment of the effect of past management practices on crop yield potential and soil N dynamics is important to accurately predict crop fertilizer N requirements. Field studies were conducted at seven locations representative of the varying soil types and ecozones across the Canadian prairies to determine the persistence of the effects of legume and non-legume crops and repeated N fertilizer applications on soil N supply and on the yield and N uptake of spring wheat (Triticum aestivum L.) and canola (Brassica napus L.) for two seasons after fertilization had ceased, corresponding to the fourth and fifth season after legume crop production. Field pea (Pisum sativum L.), lentil (Lens culinaris Medik.), faba bean (Vicia faba L.), canola and wheat harvested for grain, and faba bean grown as a green manure were direct-seeded in 2009. Canola was seeded in 2010, barley (Hordeum vulgare L.) in 2011, and canola again in 2012 with fertilizer N applied at 0, 30, 60, 90 and 120kgha−1 in each crop. Fertilizer N applications were then halted and spring wheat was grown in 2013 and canola in 2014, without fertilizer N applications. Very few effects of crops grown in 2009 were apparent on soil NO3-N supply, crop yield or crop N accumulation by 2013, which was the fourth year after crop growth, and no effects were observed in 2014. In contrast, fertilizer N applied from 2010 to 2012 had persistent effects, increasing soil NO3-N concentration, crop yield and crop N accumulation for two growing seasons after fertilization had ceased, although the magnitude of response varied considerably with location. By fall of 2014, effects of preceding fertilizer applications on soil NO3-N were no longer significant. Accumulation of N in wheat grain and straw was closely related (R2=0.86) to soil NO3-N across locations while the relationship between soil NO3-N and N accumulation in canola seed was not as strong (R2=0.60). Therefore, measurement of soil nitrate is a valuable tool for producers in this region to use when selecting fertilizer N application rates, but mineralization of N over the growing season will also contribute to crop N uptake. Residual effects of N fertilization can be substantial under prairie conditions, increasing N supply to crops for up to two years after the final application, and should be taken into account when assessing fertilizer use efficiency and N requirements for crop production.