TECHNICAL
NITROGEN BALANCE – UNDERSTANDING MANAGEMENT AND ENVIRONMENTAL IMPLICATIONS OF NITROGEN USE IN CROP PRODUCTION Bruce Searle, Trish Fraser, Jo Sharp
Nitrogen (N) is a key nutrient for the successful production of crops, and each year growers invest significantly in applying N fertilisers. However, not all the applied N is used by the crop – and some can potentially be lost to the environment. This environmental N loss occurs via different pathways that include leaching (nutrient loss in drainage), run-off, or gaseous losses (via processes known as volatilisation and denitrification). Measurement and monitoring of each individual pathway is complex, and involves having the right equipment and expertise. In the Sustainable Vegetable System (SVS) project we have developed a method to calculate an N balance to
help understand how different production systems and N fertiliser inputs change the amount of N that is potentially at risk for environmental loss (Figure 1). The N balance is based on the idea that every crop exports a certain amount of N in the sold product. Usually more N needs to be supplied than is exported, which leaves additional N in the system. This N that is not exported becomes part of different pools of N in the crop system where it can be susceptible to loss. Calculating the N balance helps to identify in what pools this ’non-exported N’ ends up, and helps us better understand N use in the system and the risk of N losses.
An example We used this approach to calculate the N balance (see Figure 2) for data from broccoli grown in a field experiment as part of the SVS project at the Plant & Food Research farm near Lincoln in Canterbury, with fertiliser application rates of 60 kg N/ha (Medium N) and 120 kg N/ha (High N). Fertiliser N was applied as calcium ammonium nitrate (CAN) in three equal side-dressings. All other management was the same. The broccoli (cultivar ’Nobel’) was transplanted on 3 March 2021 and harvested in a single cut on 1 July 2021. Marketable yields were 6.1 t/ha with an application of 60 kg N/ha and 5.5 t/ha with an application of 120 kg N/ha, with the low yields likely reflecting a singlecut harvest. To calculate the balance, first we need to determine the inputs and outputs from the system.
Figure 1: Framework for nitrogen (N) balance. This balance identifies the ’non-exported N’ (i.e. the N in the system that is over and above that exported in marketable yield) and where the ’non-exported N’ is allocated within the system. The ’all other’ pool reflects within season environmental loss, and can be further split into leached and other (gaseous) losses if values can be quantified
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The N inputs are: • Mineralisable N – this value was obtained from the Potential Mineral N test (PMN) at transplanting. The results indicate that 86 kg N/ha will be made available to the crops from mineralisation during growth. • Mineral N – the amount of nitrate and ammonium N in the soil that is immediately available to the crop, measured at transplanting. This was 37 kg N/ha for the Medium N treatment and 54 kg N/ha for the High N treatment.
