Nitrogen Recovery from Brassicaceae Oilseed Crops: A Function of Residue Quality

Brassicaceae oilseed crop residues contain diverse chemical compounds that have the potential to influence nitrogen (N) mineralization and hence recovery from crop residues. This study investigated how glucosinolate (GLS), carbon (C), N, lignin contents, and associated ratios (C:N and lignin:N) in selected Brassicaceae [Argentine canola (Brassica napus L.), industrial mustard (B. carinata L.), Oriental mustard (B. juncea L.), camelina (Camelina sativa L. Crantz), and yellow mustard (Sinapis alba L.] and non-Brassicaceae (spring wheat, Triticum aestivum L.) crop residues affect N recovery potential and soil N availability. A 120-d laboratory incubation was performed using 15N-labeled residues of these crops. Yellow mustard contained the highest GLS concentration (6.49 µmol g-1 of tissue) and Argentine canola contained the least (0.02 µmol g-1 of tissue). Argentine canola (17.8:1) and Oriental mustard (19.5:1) residues had the lowest lignin:N due to their high N concentrations. After 120-d, 47.1-53.7% of residue N was mineralized and recovered as ammonium-N and nitrate-N. Nitrogen mineralization from the Brassicaceae residues was initially lower than wheat, likely due to inhibitory effects of GLS degradation products on soil microbes. Strong negative correlations were found between N recovery and both GLS concentration (r = –0.737 to –0.846, days 3-28, P=0.04) and lignin:N ratio (r = –0.631 to –0.552, days 3–56, P=0.02), indicating that these biochemical properties likely delayed N availability from crop residues at the early-stage. These findings suggest that the biochemical composition of Brassicaceae residues, particularly their GLS content, can delay short-term N cycling and affect nutrient availability for subsequent crops.