2015 | Kyle Robertson
Palmerston North Boys’ High School
Kyle Robertson was awarded Gold CREST for his project:
“Pressure’s On: Ur-IN 2 DEEP!”
Mentors: Drs David Horne (Massey University), Brent Clothier (Plant & Food Research), Alec Mackay (AgResearch) and Andrew McLachlan (Biometrician, Plant & Food Research)
Supervising teacher: Mrs Anna Fletcher, Palmerston North Boys’ High School
Kyle’s Executive Summary states:
Dairy farming in New Zealand has expanded and intensified significantly over the past decade. Public attention has been drawn to the quality of waterways due to the resultant increased nitrogen inputs to soil. For example, concentrated nitrogen from cow urine is converted to a mobile form (nitrate) in soil. Nitrate’s mobility makes it susceptible to leaching into groundwater and waterways. If high concentrations of nitrate enter waterways, this can encourage algal growth and even cause eutrophication. Increased cattle stocking rates can also increase soil trampling, which compacts soil and affects the flow of water and solutes, such as nitrate, potentially affecting how much nitrate is leached from soil.
The aim of this project was to evaluate the effect of compaction, caused by cow trampling, on nitrate leaching. Two soil types were used for the experiment: Manawatu fine sandy loam (MFSL), which is a coarse textured soil, and Tokomaru silt loam (TSL), which is a fine textured soil. Soil cores (10 cm deep and 10.4 cm in diameter) were collected in PVC pipes. These soil testing systems were used to test the collected soil cores and cores were compacted using a hydraulic pump, mimicking an average dairy cow’s hoof pressure. The experiments were carried out in two stages:
- Preliminary feasibility trials to optimise the methodology for characterising soil cores, applying the compaction treatment and refining the overall design.
- The main trial involved characterising compacted and non-compacted cores and applying a mimic urine solution (potassium bromide*) to the cores, followed by a ‘rainfall’ event, after which bromide leachate samples were collected and analysed using ion chromatography.
*As nitrate is naturally present in soil, background nitrate could interfere with experimental results. Bromide was therefore used to mimic nitrate as it is not typically found in soil and has similar properties to nitrate.
Before compaction, MFSL had a significantly lower bulk density[1] and a significantly greater total porosity[2], macroporosity[3] and saturated hydraulic conductivity[4] (P ≤ 0.05). Although not significantly different, MFSL leached more bromide on average than TSL, prior to compaction (P > 0.05). After compaction, MFSL had a significantly greater macroporosity and saturated hydraulic conductivity than TSL (P ≤ 0.05). MFSL leached significantly more bromide than TSL (P ≤ 0.05). The greater leaching from MFSL was likely to be due to its greater hydraulic conductivity, meaning that bromide would have less time to diffuse into immobile water[5] within the soil matrix. Sandy soils, like MFSL, also typically have fewer micropores than silt soils. This would mean there would be less immobile water in the MFSL soils for bromide to diffuse into. Therefore it is not surprising that MFSL leached more on average than TSL.
Compaction significantly increased bulk density, significantly decreased total porosity, macroporosity and conductivity, but significantly increased bromide leaching from both soils types (P ≤ 0.05). A possible explanation for the increase in bromide leaching could be that the destruction of the macropores reduced connections with the micropores. Consequently, there would be less opportunity for bromide to disperse into the soil matrix and so compacted cores would have leached more bromide than non-compacted cores.
These results suggest that sandy soils are particularly vulnerable to leaching bromide (nitrate) and that soil compaction increases the risk of nitrate leaching from both coarse-textured sand and fine-textured silt soils. Based on the results of this experiment, (i) minimising the risk of soil compaction by reducing stocking rates, (ii) grazing cows predominantly on silt soils rather than sandy soils and (iii) removing cows from soils that are wet could significantly reduce the risk of nitrate leaching from dairy farms into groundwater and waterways.
Future work should include:
Investigating the effect of soil moisture on soil compaction and nitrate leaching.
Evaluating other soil types used/planned to be used for dairy farming to determine which soils are more suitable for dairy farming.
[1] Bulk density: the ratio of dry mass to the total soil volume (McLaren & Cameron 2012).
[2] Total porosity: the ratio of the total volume of pores to the total soil volume (McLaren & Cameron, 2012).
[3] Macroporosity: the ratio of the macropore (pore diameter > 30 μm) volume to the total soil volume (McLaren & Cameron, 2012).
[4] Saturated hydraulic conductivity: a soil’s ability to transmit free-water (McLaren & Cameron, 2012).
[5] Immobile water: water that does not readily move – held within micropores (pore diameter < 30 μm) – Summer, 2000.