The efficacy of good practice to prevent long-term leaching losses of phosphorus from an irrigated dairy farm

Recent work has highlighted the potential for phosphorus to move from P-enriched, poorly P-sorbing soils to groundwaters at the regional scale. Some work has elucidated mechanisms of loss to the vadose zone, but most are short term and therefore cannot establish their impact with time. We measured P fractions in leachate at 70-cm depth from two shallow low-P sorbing soils (moderate or fast flowing) in an intensively grazed dairy farm that received ‘best practice’ P fertiliser rates with or without dairy shed effluent. A 4 to 7% yearly increase in filterable and particulate P fractions, along with P-enriched loads annualised over the 14-yr period of measurement (2001-2015), confirmed our hypothesis that the freely draining soil with effluent applied lost significantly more P (total P [TP] = 1.46 kg ha-1 yr-1) than the same soil without effluent (TP = 0.25 kg ha-1 yr-1) or a moderately draining soil receiving P fertiliser with or without effluent applied (TP = 0.12 kg ha-1 yr-1, for both treatments). Losses were attributed to a combination of increasing soil P saturation of the soil matrix and flow that was enhanced by macropores and high hydraulic conductivity. Enrichment of FRP was also detected in a well intercepting groundwater at 10-m depth, the sources of the enrichment was unclear. These data suggest that under current best practice, where effluent is not allowed to pond and P is applied to match pasture demand, significant P losses can occur. To remediate this, effluent should be applied at a shallower depth and lower rate, or not at all, to dairy-farmed soils that exhibit similar freely draining, low P-sorption shallow soils under irrigation.