Changes in soil properties after application of dairy factory effluent to New Zealand volcanic ash and pumice soils

In field studies, we assessed the effects of irrigation with dairy factory effluent on the surface (0–10 cm) properties of 3 rhyolytic tephra soils. Horotiu and Te Kowhai soils had been irrigated for 22 years and Taupo soil for 2 years. Soil properties measured were: total organic C, total N, pH, bulk density, unsaturated hydraulic conductivity, moisture release characteristics, readily and totally available water, particle density, microbial C, soil respiration, mineralisable N, denitrification enzyme activity, and nitrification potential. Matched, non-irrigated areas were sampled for comparison. Average annual loadings (kg/ha) of C, N, and P since 1990 were 1120 kg N, 560 kg P, and 36 300 kg C onto Horotiu soil, and 760 kg N, 380 kg P, and 25 200 kg C onto the Te Kowhai soil. These post-1990 loadings were, on average, 23% less than pre-1990 values. Irrigation for 22 years onto the Horotiu and Te Kowhai soils caused no change, or a slight decrease, in total C and N in the topsoil, but microbial C and mineralisable N contents were more than doubled, and N cycling activity much increased. Soil pH was increased by up to 1.8 units. Unsaturated hydraulic conductivity was increased from 8.5 to 49.8 mm/h on the Horotiu soil, and from 6.5 to 29.0 mm/h on the Te Kowhai soil. Irrigation increased the volumetric water content between 10 and 100 kPa. Most of the changes in soil properties suggest current application rates and pasture production can be maintained or increased. The high loading and mineralisation of N in the irrigated soils raises concerns about potential leaching of nitrate; current management practices are targeted towards minimising N loadings from effluent.