Phosphorus in humped and hollowed soils of the Inchbonnie catchment, West Coast, New Zealand: I. Variation with age and distribution

Phosphorus (P) is the limiting nutrient for eutrophication of Lake Brunner in the South Island of New Zealand. Dairy farms within the Inchbonnie catchment are thought to be a major source of P that enters the lake. A study was conducted using farm surveys and sequential P fractionation to investigate and account for the fate, form and concentration of P in the 0–7.5 cm layer of hump and hollow land of different ages (1–13 years since development). Data indicated that, apart from two labile organic P fractions, there was no significant difference in P concentrations of hump, mid‐slope and hollow areas. However, there was a significant increase in water soluble P with time since development. This pool is often used as a predictor of potential P loss to runoff (the equivalent of 0.5% of total P year ‐1 ). By accounting for P inputs and outputs with time, the loss of P from the 0–7.5 cm layer was estimated to be about 20 kg P ha ‐1 year ‐1 . It is suspected that the majority of this would be lost to deeper soil layers or redistributed to other parts of the farm such as an effluent block. However, additional work is required to confirm this and also to decide what to do about that part which is lost to surface waters.