Short‐term effects of Brassicaceae crop residue on soil nitrification and nodulation in the subsequent field pea

Glucosinolates (GLS) in Brassicaceae crops are known to suppress soil nitrification. Despite widespread inclusion of Brassicaceae species in pulse crop rotations, their effects on soil nitrification and root nodulation of subsequent pulse crops are poorly understood. This controlled greenhouse study examined the effect of Brassicaceae crop residues on soil nitrification and effective root nodulation in field pea ( Pisum sativum L.). Field pea was grown after spring wheat ( Triticum aestivum L.; zero GLS), Argentine canola ( Brassica napus L.; low GLS), and yellow mustard ( Sinapis alba L.; high GLS). Nine treatments were established with field pea grown on below‐ground residues (BGRs) of each preceding crop; combined BGR and above‐ground residues (AGR) from each crop; two chemical amendments, 4‐hydroxy benzyl isothiocyanate (GLS‐hydrolyzed product) and urea; and an unamended control. Soil nitrification was quantified by total soil‐available nitrogen (nitrate‐nitrogen and ammonium‐nitrogen) and ammonia‐oxidizing microbial abundance (ammonia monooxygenase subunit A [ amo A] gene copy concentration) at pre‐seeding, 100% of flowering, and 90% of physiological maturity (harvest) of field pea. Effective nodulation was quantified at flowering and harvest. At flowering, soils with Brassicaceae residues reduced soil nitrate‐nitrogen concentrations by 16.2%, amo A gene copy concentration by 80.2%, and effective root nodulation by 14.6% compared to wheat residues. Yellow mustard residues suppressed nitrification by 18.8%–35.3% and effective root nodulation in field pea by 5.0%–16.3% compared to Argentine canola residues. Overall, Brassicaceae residues transiently suppress nitrification and nodulation; however, suppressive effects diminished as residues decomposed. This study highlights the importance of residue management while underscoring the need for field‐based validation.