Between October 2018 and October 2021 Genetic Technologies carried out a three-year study to investigate and quantify nitrogen (N) leaching in maize cropping systems.

Each year researchers planted maize in spring. Immediately after the subsequent harvest, some plots were either drilled into a range of winter catch crops, including annual ryegrass, or left fallow.

Throughout the year, N leaching was measured in each plot to a depth of 120 cm using ceramic suction cups and barrel lysimeters.

While management of the maize plots was done using standard good practice, an additional 200 kg N/ha above the recommended rates was applied to the maize crop to ensure excess soil N after harvest and create the potential for leaching.

This was done to have a true measure of N leaching losses. No additional N was applied to the winter option as the left-over N was considered sufficient.

During the trials, more than 90 % of N leaching losses were observed during winter (after maize harvest), and the balance occurred during spring. These results were different in a wetter than average spring.

In a wetter spring season, by December plots that measured 200 kg mineral N/ha around planting or immediately after had lost 30kg N/ha due to leaching. This compares with 0 kg N/ha of leaching if starting mineral soil N levels were only 75 kg/ha. This result highlights the leaching risks associated with front loading N at or before planting.

In the catch crop trials, nitrogen leaching losses were much higher in the maize-fallow (>60 kg N/ha) than the maize–annual ryegrass rotation plots.

Catch crops were found to reduce leaching losses by up to 90 %. This can be largely attributed to their ability to take up soil N, which reduces the concentration of soil N and therefore leaching risk.

Catch crop efficacy can be is shown in Figure 1. It illustrates the comparatively low N concentration of the leachate samples collected from the maize-catch crop plots relative to maize-fallow plots.

Figure 1: Nitrate-N concentrations for leachate samples collected from plots planted to a winter catch crop or fallowed at a Te Awamute site.

Where needed, a winter catch crop can hence be used as an effective mitigation option against N leaching after maize.

The leaching losses observed in this study were much lower than results from previous research. One of the key reasons for the difference in the results could be attributed to the depths at which the leaching losses were measured.

Most previous work has measured leaching losses at 60 cm, which we considered too shallow given the deeper rooting system for maize.

Part of our study also included a separate, but related experiment that evaluated comparative leaching losses between measurements conducted at 70 cm and 120 cm. Over a three-year period, leaching losses were on average 350 % higher when measurements were taken at 70 cm than at 120 cm.

Nitrogen 15 Experiments

To prove that maize was capable of extracting nutrients as far deep as 120 cm, a separate experiment was carried out using nitrogen 15 (15N). In nature N exists as two stable isotopes, nitrogen 14, which makes up 99.6 % of the total, and nitrogen 15 which makes up 0.4 %. Due to the negligible levels of 15N in soils, synthetic 15N can be used to trace the ability of maize roots to take up N from the soil.

Experiments with 15N were conducted on a Waikato site in the 2020/21 and 2021/22 seasons. The study involved a range of treatments including control and enriched synthetic 15N inserted in maize plots to 60 cm, 90 cm and 120 cm below the soil surface.

At harvest, maize plants’ components (leaves, stalks, grain) from each of the three treatments were sampled to trace 15N uptake.

Samples were finely ground to pass through a 100-mesh screen and sent to the Stable Isotope Facility at the University of California Davis for total N and isotopic 15N composition analysis using an automated combustion elemental spectrometer.

Significantly greater levels of 15N uptake were observed when 15N was drilled into the maize plots at all three soil depths compared to control (See Table 1).

This provides clear evidence that maize roots are capable of growing beyond 120 cm and also capable of picking up N to that depth.

For deep rooted crops such as maize, 120 cm can be considered a more accurate depth to measure soil leaching and values obtained at 60 cm could significantly over-estimate leaching losses.

Table 1: Percentage of 15N concentrations in maize plant components (leaf, stalk and grain) at grain harvest maturity on a Waikato ash soil over two maize growing seasons.

Means followed by the same letter within a column are not statistically significant

The results of these trials highlight four key findings:

1. Leaching losses in maize cropping systems are much lower than previously thought. Provided the right amount of N is applied at the right time, N leaching losses during the maize growing cycle are likely to be marginal because leaching mostly occurs during winter, post harvest.
2. Front loading N before or at the time of maize planting could potentially increase N leaching risk, especially after a wet spring season. The best approach to maximise yield and minimise potential N leaching is to apply the right amount of N when the maize plants are at the V6 growth stage or “redband gumboot high.” .
3. In the event of excess soil N after maize harvest, a winter catch crop will likely reduce the risk of N leaching losses in a maize cropping system.
4. Maize roots are not only deep but are also capable of extracting soil N at depth. Maize can therefore be used to mop up excess soil N from deeper profiles, and reduce N leaching into water ways.
5. In deep rooted crops such as maize, N leaching loss measurements conducted at shallower depths, e.g., 60 cm will significantly overestimate leaching losses.

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