Developing a critical nitrogen dilution curve for forage brassicas

Abstract

Defining the critical nitrogen concentration (N c ; g N kg )1 ) for maximum growth of forage brassicas will aid in the fertilizer management of these crops. Typically, the N c value decreases with increasing crop biomass. In this paper, we used a nitrogen (N) response experiment with kale (Brassica oleracea) to define a critical N dilution (N c = 55AE3 · biomass )0AE47 ). However, at biomass <3AE4 t ha )1 , a constant N C of 31AE2 g N kg )1 was found. This N dilution curve compared favourably with published data sets for a range of forage brassicas but was substantially different from the established N dilution curve for oilseed rape (Brassica napus). This study also found a strong relationship (R 2 = 0AE81) between the nitrogen nutrition index (NNI) and the NO 3 content of forage brassicas from a range of data sets. The NNI is the actual N concentration of the shoot as a ratio of the N c from the established curve. The relationship between NNI and NO 3 contents was significantly different between leafy forage brassica crops and root forage brassicas. For each 0AE1 increase in NNI, the proportion of total N that was in the form of NO 3 increased by 2AE7% for leaf ⁄ stem brassicas and 0AE60% for root crop brassicas. The critical dilution curve defined in this study can be used to manage fertilizer N in forage brassica crops, so that growth can be maximized but the risk of high NO 3 concentrations in the forage can be minimized.

Nitrate accumulation in forage brassicas

“…Target crop yields were 10 t/ha for the summer crops and 20 t/ha for the winter crops. The optimum N uptake for these yields would have been 190 and 270 kg N/ ha, respectively (Fletcher & Chakwizira 2012). At sowing there was 123 kg N/ha of mineral N in the top 0.9 m of soil.…”

“…A separate sample of 10 leaves, stems and bulbs was collected and freeze-dried for NO 3 -N determination. The NO 3 -N content was determined using suppressed ion exchange chromatography (Dionex Corporation, Sunnyvale, CA) on 1g of ground sample extracted in 50 ml of deionized water (Fletcher & Chakwizira 2012). Data were analysed using analysis of variance (Genstat 12.2) with two orthogonal comparisons.…”

“…Fertilizer nitrogen (N) is applied to increase forage brassica yields with the response depending on background soil N fertility and yield potential (Gramshaw & Crofts 1969;Pelletier et al 1976;McDonald et al 1981;Pearson & Thomson 1996;Wilson et al 2006;Fletcher et al 2007;Stevens & Carruthers 2008;Chakwizira et al 2009;Fletcher et al 2010;Chakwizira et al 2011;Keogh et al 2011). However, the application of fertilizer N that exceeds that required to obtain maximum crop yield can also result in the accumulation of elevated nitrate-nitrogen (NO 3 -N) in the plant (Pelletier et al 1976;McDonald et al 1981;Guillard & Allinson 1988;Jacobs & Ward 2008;Fletcher et al 2010;Fletcher & Chakwizira 2012). *Corresponding author.…”

“…These N treatments consisted of a range of timings and rates of fertilizer N that were designed to either match or exceed the N required to grow each brassica crop and achieve the potential yield. Yield responses, plant N and NO 3 -N content were presented as part of a larger analysis by Fletcher & Chakwizira (2012). In this paper we outline the more detailed tissue NO 3 -N responses observed in this trial.…”

Evaluation of effect of autumn nitrogen dose and nitrogen nutrition status on oilseed rape yield

“…are also useful for quantifying the risk of high N-NO 3 -in plants. When excess of nitrogen is applied, low-yielding brassica crops are at a greater risk of accumulating high NO 3 -contents than high-yielding crops (Fletcher and Chakwizira, 2011). …”