Phosphorus in humped and hollowed soils of the Inchbonnie catchment, West Coast, New Zealand: II. Accounting for losses by different pathways

Abstract

Phosphorus (P) is the limiting nutrient for the eutrophication of Lake Brunner, West Coast, New Zealand. In a previous study, the top 7.5 cm of soil in grazed dairy pastures in the Inchbonnie catch‐ment which drains into Lake Brunner was estimated to lose about 20 kg P ha‐1 year‐1. Only some of this goes into waterways and the lake, but how much and in what chemical form is unclear. Hence, my aim was to account for P losses within the farm and estimate potential losses to waterways according to a number of source treatments typical for a dairy pasture. Soil turves and lysimeters were collected, treatments imposed (control, dung equivalent to that deposited during a 24‐h grazing period, superphosphate at 30 kg P ha‐1, and dung plus superphosphate) and subjected to 330 mm of simulated rainfall over a period of 30 days, equivalent to 1 month's rainfall. Total runoff (overland now and subsurface now—shallow leachate in this study—combined) was collected and analysed for dissolved reactive P (DRP), dissolved organic P (DOP), particulate P and total P. More P was lost from the dung + fertiliser treatment than the sum of dung and fertiliser treatments. Dung and fertiliser treatments lost significantly more P than the control. More P was lost via overland flow than leaching. The loss of DOP was more important in leachate than overland flow due to the poorer sorp‐tion of DOP compared to DRP (largely comprised of orthophosphate). When aggregated to a year, P losses were estimated to be 6.6–9.7 kg ha‐1 year‐1 of which 1.6–2.2 kg ha‐1 year‐1 was via leaching. In accounting for P lost from the 0–7.5 cm soil layer of a grazed diary pasture, it was estimated that 7 kg P ha‐1 year‐1 was transferred to the effluent block, while 0.6 kg Pha‐1 year‐1 was estimated as lost via lanes and stream crossings, leaving 2.7–5.8 kg P ha‐1 year‐1 unaccounted for. This was thought to be associated with Plost from, or transferred to, deeper soil layers, effluent ponds or stand‐off or feedpads, but needs to be further investigated. Management practices to mitigate P losses in overland flow and leachate from humped and hollowed paddocks include: maintaining soil Olsen P concentrations at as low a concentration as economically optimal, using low water solubility P fertilisers and keeping cows off wet paddocks as often as practically possible.