Preceding crops and nitrogen fertilization influence soil nitrogen cycling in no-till canola and wheat cropping systems

Abstract

Crop rotation and nitrogen (N) fertilization can influence soil N cycling, however, less is known about their interactive effects under varying soil and climatic conditions. We examined the interactive effects of preceding crops and N fertilizer rates on soil N cycling in canola (Brassica napus L.) and spring wheat (Triticum aestivum L.) cropping systems in no-till soils in the Canadian prairies. Field pea (Pisum sativum L.), lentil (Lens culinaris Medik), faba bean (Vicia faba L.; faba bean-seed), canola and wheat grown for grain, and faba bean green manure (faba bean-GRM) were the preceding crops and were direct-seeded at 7 and 6 locations in 2009 and 2010, respectively. Canola and wheat were seeded in 2010 and 2011 (2012 for wheat at one site), respectively with N fertilizer applied at 0, 30, 60, 90 and 120kgNha−1. Above-ground residue N returned was greatest for faba bean-GRM and lowest for canola and wheat. On average across sites, apparent in-crop N mineralization (ANM) under canola was greater following faba bean-GRM than all other preceding crops, except field pea, while under wheat, ANM was greater following all the legumes than following canola and wheat. Crop N uptake increased with N fertilizer rate, but the response was generally lower following faba bean-GRM and lentil. The N budget showed that 40–65% of crop N uptake (35–100% at highest ANM site) was possibly derived from ANM, more so following legumes, as compared to 35–60% from N fertilizer. The surplus and unaccounted N were particularly pronounced when canola and wheat were preceded by legumes rather than by canola or wheat. Our findings indicate that legumes can enhance soil N supply in no-till soils, and also highlight the importance of adjusting N fertilizer rates based on preceding crops to minimize the potential for N losses.