Lysimeter Collection

Thirty six large, intact soil monolith lysimeters (500 mm diameter by 700 mm depth) were collected on 14 December 2010 from the AgResearch No. 1 Dairy Farm, at the Ruakura Research Centre, Hamilton, New Zealand (latitude 37.779 ˚S, longitude 175.315 ˚E). The soil at the collection site was a Horotiu silt loam (Typic Orthic Allophanic Soil) (Hewitt, 1998). This soil is characterised by a dark brown, moderately weak, moderately developed medium nut structure silt loam A horizon (0 – 200 mm); a yellowish brown, moderately weak, moderately developed silt loam (200-400 mm), progressing to a sandy loam B horizon (200-600 mm); underlain by a yellowish brown loose sand or gravelly sand C horizon (Singleton, 1991). Horotiu soils are moderately permeable and very well drained. They have a potential rooting depth of up to 80 cm, the topsoil is structurally stable and available water in the root zone is generally high (Singleton, 1991). The soil was under ryegrass (Lolium perenne L.) and white clover (Trifolium repens L.) pasture and had a history of regular grazing and fertiliser application. A total of 15 soil cores (7.5 cm depth) were taken randomly from the lysimeter collection site, bulked and analysed for basic soil chemical properties by NZ Labs (Table 3.1).

Table 3.1 Soil properties from lysimeter collection site Soil Properties pH 5.9 Total C (g kg-1) 55 Total N (g kg-1) 6.7 Olsen P (mg kg-1_{) 30} Sulphate sulphur (mg kg-1) 6 Potassium (cmolc kg-1) 0.65 Calcium (cmolc kg-1_{) 8.1} Magnesium (cmolc kg-1) 1.57 Sodium (cmolc kg-1) 0.14

Cation exchange capacity (cmolc kg-1) 23

Total base saturation (%) 45

The lysimeter collection procedure followed that described by (Cameron et al., 1992). The lysimeter casings were originally manufactured from steel plates (5 mm thick) that had been rolled and welded to make cylindrical casings 500 mm wide by 700 mm high. A steel internal cutting ring was welded to the inside of one end of the cylinder and extended 10 mm beyond the lower edge of the lysimeter (Figure 3.1a and b). Lysimeter casings were arranged within a marked area 1.5 m wide by 30 m long. A 1.0 m wide by 1.5 m deep trench was dug outside the marked area by a hydraulically operated digger to aid manual digging. The turf around the internal edge of the lysimeter casings was cut with a sharp knife and the turf and soil around the outer edge of the casing was dug away. The casing was then carefully pushed over the exposed monolith directly below the cutting edge, with the procedure repeated until the lysimeter casing was completely filled with the soil monolith. As a result of the internal cutting ring, there was an annular gap between the soil monolith and the casing. To prevent damage to the soil monolith, these gaps were packed with 5 mm thick lengths of wood.

The lysimeters were gently lowered onto their side to attach the base plates. The soil at the base of the lysimeter was levelled with the base of the casing. The base plates were attached by four steel rods with circular lugs (Figure 3.1d) evenly positioned around the lysimeter through lifting flanges (Figure 3.1c). The lysimeters were then removed from the trench by attaching a chain to the lugs and lifted out with the digger.

Figure 3.1 Lysimeter specifications (a) entire lysimeter; (b) the internal cutting ring; (c) lifting flanges; (d) lifting rods

The lysimeters were then brushed down to remove loose soil, and the base plate was sealed to the lysimeter casing by applying silicone RTV sealant (Dow Corning Silastic ® 1080), which, upon drying, was covered with strong polyurethane adhesive (Holdfast Gorilla Grip Express). Meanwhile vasoline (petrolatom) was liquefied in large 20 L tin drums by heating on gas-fired hotplates. Once the entire drum was liquefied, it was left to cool for approximately thirty minutes. This slightly thickened the consistency of the melted vasoline so that upon contact with the soil, it set immediately, rather than being absorbed into the soil pore spaces. The 5 mm packing lengths between the soil monolith and lysimeter casing were removed and the liquified vasoline was administered through a

40mm

(a) (c)

(b)

funnel into the annular gap between the soil monolith and the lysimeter casing. The vasoline was observed to flow freely from the delivery point, around the entire annular gap, to completely surround the outside of the soil monolith. The vasoline cooled on contact with the soil and the lysimeter casing, which later solidified to form a water-tight seal between the monolith and the casing, preventing preferential flow between the soil and the casing.

The lysimeters were transferred from the collection site using a small crane (Hiab) on the back of a truck, to an empty lysimeter trench approximately 1 km from the collection site. Drainage outlets were screwed onto the bottom of the base plates, and the lysimeters were positioned on both sides of the trench (18 on each side). Each side of the lysimeter trench had a series of 0.8 m lengths of 150 mm diameter pipes, underlain by gravel. The

exposed surface of each piece of pipe had a circular 20 mm diameter hole, which the drainage outlet on the base of the lysimeters protruded through (Figure 3.3). The area between each length of pipe was filled in with sand, to create a relatively level surface for the lysimeter installation. Once the lysimeters were in place, they were accurately

levelled, and the trench was back-filled with soil. The pasture at the collection site was suffering from drought so each lysimeter received 25 kg N ha-1 _{urea fertiliser with 10 mm} of water. The lysimeter collection site was back-filled, levelled and re-sown with pasture.