Student – cow trampling increases nitrate leaching

By Lynley Hargreaves 18/02/2015 1


Kyle Roberston
Kyle Roberston

It’s 2015, the International Year of Soils, and eighteen-year-old Kyle Robertson has been doing his bit for New Zealand dirt. By squashing soil with a press and pouring fake cow pee on top, the former Palmerston North Boys’ High School student has modelled the effect of cow trampling on nitrate leaching, with some surprising results. He explains why his Gold CREST research tells farmers to beware of overstocking on sandy loams.

Is compaction well known for increasing nitrate leaching?

There was no research that I could find that looked at the effect of compaction on nitrate leaching. I had looked into whether cow urine has a greater risk of leaching nitrate than typical fertiliser application for my Silver CREST project, and I wanted to look further into nitrate leaching mitigation. My mentor was Associate Professor David Horne from Massey University and I also had advice from Dr Brent Clothier from Plant & Food Research and Dr Alec Mackay from AgResearch.

How did you simulate nitrate leaching?

I used a bromide solution to mimic nitrate from cow urine on two types of soil: Manawatu fine sandy loam (MFSL), which is a coarse textured soil, and Tokomaru silt loam (TSL), which is a fine textured soil. I would then simulate a rain event and analyse the leachate for bromide that drained through the soil. The results showed that MFSL leached significantly more bromide than TSL in both compacted and non-compacted soil cores.

Surprisingly, compaction significantly increased bromide leaching from both soils types.

What do you think was going on?

It was previously thought that a compacted soil’s reduced flow rate would give bromide more time to diffuse throughout the soil. The reason compacted soils leached more bromide could be because compaction destroyed many macropores in the soil, causing bromide to flow through only a few of the remaining macropores. This would have limited the opportunity for bromide to diffuse through the soil and thus the compacted soils leached more bromide than non-compacted soils.

What advice would you give farmers?

Based on the results of this experiment, they should minimise the risk of soil compaction by maintaining low and suitable stocking rates; graze cows on predominantly silt soils rather than sandy soils; and farm on land not situated directly next to waterways. This could significantly reduce the risk of nitrate leaching from dairy farms into groundwater and waterways. By managing dairy cow stocking numbers, I hope this work can help dairy farmers optimise production while minimising its impact on the environment.

How did you find time to do research while at high school?

By communicating with my supervisors and planning my project timeline, I was able to conduct the experiment in a timely manner. I had to make sure I didn’t take too many extracurricular activities, as this would have made it difficult to meet deadlines for my project.

CREST is an international awards scheme designed to encourage years 0-13 students to be innovative, creative, and to problem solve in science, technology and environmental studies.

These interviews are supported by the Royal Society of New Zealand, which promotes, invests in and celebrates excellence in people and ideas, for the benefit of all New Zealanders.


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