SYSTEMS NITROGEN RESEARCH
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n increasing focus on the environmental sustainability of farming led a maize seed company to embark on a comprehensive research programme. “We started off with four key questions and then we set out trials and demonstrations to help answer them,” says Dr Rowland Tsimba, National Research and Agronomy Manager for Pioneerbrand seeds. The questions were how deep maize roots grow, can they extract nitrogen from depth, what are the nitrogen leaching losses under maize and what is the impact of a winter catch crop (vs a winter fallow period) on reducing nitrogen leaching losses? To quantify how far maize could root in unimpeded soils, the research team built a rhizotron - a soil-filled box with doors which could be opened to reveal a transparent perspex window. Maize seeds were planted at the top and their roots could be viewed as they grew down through the soil profile. The first rhizotron was around 2.5 metres deep but within 10 weeks of planting the roots had reached the bottom so the next season a larger, 3.8m deep rhizotron was constructed. “We used a fork-lift to plant the maize seeds in the top and we monitored the root depth on a weekly basis,” Rowland says. “At 70 days post-planting the maize plants were about 95cm high, but the roots were almost double that and within a week of the maize plant tasseling, the roots had reached the bottom of the rhizotron.” “We all knew maize roots went deep, but we were all surprised just how fast they grew and what the potential rooting depth was in an unimpeded soil. The next question was whether those deep roots were actually capable of taking up nitrogen.”
N-15 TRIAL WORK
Rhizotron showing the rooting depth of maize.
Low nitrogen loss under maize Researchers have gone to great depths to see how deep maize roots will grow to reduce nitrogen loss. Raewyn Densley reports. 44
Naturally occurring nitrogen is a mix of two stable isotopes: the vast majority (99.6%) is N-14 with the remainder being N-15. In the 1930s researchers developed a method of concentrating N-15 and today N15-enriched nitrogen can be purchased, at high cost, for research purposes. N-15 has been used extensively in nitrogen uptake studies across a range of agricultural and horticultural crops. Maize was planted using a conventional planter and a starter fertiliser of DAP was applied at 40kg N/ha. When the maize was about shin height (V6) urea was broadcast at 75kg N/ha across the trial area. “Within a week of applying the urea, we injected 30kgN/ha of N-15-enriched nitrogen to a depth of 60, 90 or 120cm under each plant within the centre row of each treatment plant,” Rowland says. “An equivalent amount of standard urea was broadcast on the centre row of the control plants at the same time.”
Dairy Exporter | www.nzfarmlife.co.nz | October 2021
