Nitrate accumulation in forage brassicas

Abstract

Previous research has shown that the application of fertilizer nitrogen (N) can lead to an accumulation of nitrate nitrogen (NO3-N) in forage brassicas, particularly when N application rates exceed that required to obtain maximum crop yield. However, there are no data examining the effect of timing of fertilizer application on NO3-N accumulation. An experiment was established in 2009 with four forage brassica crops (turnips, rape, kale and swedes) and three N fertilizer treatments (early N, late N and double N) at Lincoln, Canterbury, New Zealand. The N treatments, which differed among crops depending on crop duration, consisted of rates and timings of N fertilizer. For the turnips and rape, the early N treatment consisted of 200 kg N/ha applied 16 days after sowing; the late N treatment consisted of 200 kg N/ha applied 40 days after sowing; and the double N treatment consisted of 200 kg N/ha applied 16 days after sowing and a further 200 kg N/ha applied 40 days after sowing. For the kale and swedes, the early N treatment consisted of 400 kg N/ha applied 16 days after sowing; the late N treatment consisted of 400 kg N/ha applied 72 days after sowing; and the double N treatment consisted of 400 kg N/ha applied 16 days after sowing and a further 400 kg N/ha applied 72 days after sowing. There were differences in yield between species that reflected their growth duration but no yield differences between N treatments. Averaged across all species, plant NO3-N content was greater in crops that received the double rate of N than either the early or late N treatments. Importantly, the late N treatment (3.7 mg/g) accumulated almost double (P < 0.001) the plant NO3-N content of the early N treatment (2.0 mg/g). Averaged across all N treatments, the leafy crops (rape and kale) (5.5 mg/g) had nearly double (P < 0.001) the NO3-N content of the bulb crops (turnip and swedes) (2.9 mg/g). In general, the stems had the highest NO3-N content across all crops, followed by the bulbs. Leaves had the lowest NO3-N content. Overall, these results indicate that early application of fertilizer N could be used to minimize NO3-N accumulation in forage brassicas.