Brassica cover crop effects on nitrogen availability and oat and corn yield

Cover crops are used to scavenge residual soil nitrate, with the goal of reducing N losses from agricultural fields and increasing subsequent N availability. Two experiments were conducted to determine fall-planted brassica cover crops’ effect on N availability for rotational crops. The first evaluated five species— brown mustard [Brassica juncea (L.) Czern], hybrid turnip (Brassica rapa L. × B. napus L.), radish (Raphanus sativus L.), rapeseed (Brassica napus L.), and white mustard (Sinapis alba L.)—in rotation with oat (Avena sativa L.). Th e second evaluated radish in rotation with corn (Zea mays L.). End of season cover crop biomass averaged 1160 to 6170 kg ha^–1 across experiments, locations, and years. Biomass did not differ by species. Nitrogen accumulation was generally greater than 70 kg ha^–1 (range 31–136 kg ha^–1). In the subsequent spring and summer, brassica cover crops reduced soil nitrate N levels relative to a no-cover control by 0 to 132 kg ha^–1. In Exp. 1, all brassicas reduced oat N accumulation by ≥77 kg ha^–1 and oat biomass by ≥1255 kg ha^–1 in June 2012, relative to the control. Mustards reduced oat grain yield by 505 kg ha^–1 in 2011, while radish increased oat yield by ≥578 kg ha^–1 in each year. In Exp. 2, radish did not affect corn V8 biomass N concentration, grain yield, or response to N fertilizer. Nitrogen taken up by brassica cover crops oft en is not available when the subsequent crop needs it.