Effect of slaughterhouse effluent and water irrigation upon aggregation in seasonally dry New Zealand soil under pasture

Abstract

Studies of the long-term (>25 years) treatment of a silt loam soil under pasture with slaughterhouse effluent showed that effluent did not affect macroaggregate stabilities as measured by wet-sieving, but increased the stability of clay aggregation as measured after selective disaggregation treatments. Highly humified organic matter was associated with the soil clays as a result of slaughterhouse effluent treatment. Studies of the similarly long-term irrigation of the same soil type with water showed that water decreased the stabilities of macroaggregates as long as they were kept wet, but increased their stabilities after air-drying. Scanning electron microscopy showed a web that became firmly bound to soils only after drying. Stabilities of aggregated clays were largely unchanged by water irrigation, though their sensitivity to the removal of polysaccharides and organically bound cations was increased by this treatment. Changes in stabilities could not be related to earthworm numbers or to the extent of mycorrhizal infection. Macroaggregate stabilities did not reflect obvious visual changes that occurred to the soil structure upon a major change in composition of the effluent. The effects of seasonal drying processes were very likely obscured by the stabilising effects of air-drying prior to wet sieving analyses for macroaggregate stabilities.