Particulate organic matter and soil mineral nitrogen concentrations are good predictors of the soil nitrogen supply to canola following legume and non-legume crops in western Canada

Accurate estimation of potential nitrogen (N) availability from preceding crops is essential to improve N fertilizer management in agricultural soils. Labile organic N fractions such as microbial biomass N (MBN), water-extractable organic N (WEON), particulate and light fraction organic matter N (POMN, LFOMN) are sensitive to management-induced changes and have the potential to predict N availability. This study assessed the impact of preceding legume [field pea (Pisum sativum L.), faba bean (Vicia faba L.), faba bean green manure] and non-legume crops [canola (Brassica napus L.) and wheat (Triticum aestivum L.)] on labile organic N fractions, mineral N (NH4-N NO3-N), potentially mineralizable N (N0) and soil N supply (canola grain yield and N uptake), and whether these soil parameters for the top 15 cm of soil could be used as indicators of soil N supply across no-till sites in western Canada. Labile organic N fractions and N0 were similar regardless of preceding crop. Soil N supply was greatest following faba bean green manure at four of five sites. POMN was the best single predictor of soil N supply (R2=0.56 and R2-0.69 for yield and N uptake, respectively). Soil N supply was primarily related to the combined effects of POMN, mineral N and sand content, which explained 68 and 71% of the variation in grain yield and N uptake, respectively. This study demonstrated that POMN and mineral N are relatively good predictors of soil N supply to canola in western Canada. Accounting for these parameters as well as soil texture may help improve N fertilizer recommendations for canola.