Movement of E. coli through stony soils
Irrigation of dairy shed effluent (DSE) onto land is an integral part of New Zealand’s farming practice. The use of inappropriate soils can result in contamination of ground waters with microbes and nutrients.
Animal wastes such as DSE contain microbes that cause disease in humans, including Cryptosporidium, Campylobacter, toxigenic Escherichia coli, and rotavirus. A gap in our knowledge is the ability of stony soils to treat DSE safely. The problem with stony soils is they have a limited water storage capacity compared with many non-stony soils, and pastures can rapidly dry out, which leads to low productivity. The reason for the limited water storage is that stones in the soil occupy space but are not available to store water. To compensate for their inability to store water, stony pastural soils tend to require irrigation for intensive use. However, as their soil water storage is limited, irrigation volumes need to match. Too large an irrigation volume, or rainfall after irrigation, can exceed the limited capacity of soil water storage, resulting in water draining from the soil. To maintain optimal soil water content for pasture growth, stony soils are irrigated frequently, which can result in topsoils frequently having high water content.
At high water content topsoils can be pugged by intensive stocking. In New Zealand stony soils are an extensive landscape in eastern regions of the North and South Islands, with 1.68 million hectares mapped that occur on slopes <15° and therefore have potential for intensive land use. Irrigated dairy farming has become a major land use on these soils, with 232 000 hectares under this land use in 2012. Over the last decade the Canterbury region has seen a doubling in dairy farm area, with 71% of dairy farms now located on stony soils with <400 mm of fines over stony alluvium.
Replicates of four different stony soils were collected from Canterbury as intact soil lysimeters 460 mm in diameter and up to 750 mm deep. The soils had either stones to the surface or 300–600 mm fines over stones (Figure 1). To determine leaching characteristics of the soils a pulse of DSE (25 mm depth) was applied to the soil cores followed by continuous artificial rainfall, for one pore volume, at 5 mm h–1 (Figure 2). Leachates collected from the bottom of the soil cores were analysed for Escherichia coli. The lysimeters were then treated with hoof pugging using a mechanical hoof, and the E. coli leaching characteristics of the soil determined again. E. coli breakthrough curves revealed that the potential for E. coli to leach through the soils was high for Selwyn very stony soil and low for most other soils analysed that had silty material of varying depths over stones. After pugging, which disrupts the physical structure of the topsoil, E. coli leaching increased in Mackenzie soil, which has stones to the surface (Figure 3). Increased E. coli leaching likely occurs as effluent ponds in hoof pugs and may be transmitted via a worm hole or soil structural void. For most other soil cores E. coli concentrations in soil leachates were low.
This means that in stony soils with stones close to the surface (i.e. soils with small soil water storage), shallow groundwater is potentially vulnerable to microbial contamination as irrigation or rainfall can quickly induce water movement through the soil. Furthermore, the frequent irrigation practised on stony soils maintains a higher topsoil water content than in non-irrigated soils. At high water content, topsoils are more liable to pugging by stock, which increases microbial leaching. Therefore stony soils are best managed with shallow depth irrigations that retain the irrigation water within the soil profile. Furthermore, stocking following irrigation or rainfall should be avoided when there is increased potential for the topsoil to become pugged.
Malcolm Mcleod — Landcare Research