NUTRIENT CYCLING AND LOSSES BASED ON A MASS-BALANCE MODEL IN GRAZED PASTURES RECEIVING LONG-TERM SUPERPHOSPHATE APPLICATIONS IN NEW-ZEALAND .1. PHOSPHORUS

Phosphorus (P) cycling and losses in irrigated, sheep-grazed pastures receiving superphosphate (SP) applications for 35 years at annual rates of 0, 188 and 376 kg/ha were studied using a mass-balance approach which accounted both for P inputs to and outputs from the soil-plant-animal system. Total recoveries of applied P in the soil-plant-animal systems in the 188 and 376 kg SP/ha treatments were 94 and 83 % respectively. Approximately 52-53 % of the applied P was recovered in the soil within the major plant rooting zone (0-300 mm soil depth). These data suggest that P leaching losses from SP fertilizer, plant litter, root residue and sheep faeces were unlikely to occur beyond the major plant rooting zone. However, the transfer of excretal P to stock camps and the transport of P from SP fertilizer, plant litter and sheep faeces via the irrigation water along the border from the top to the bottom of the irrigated border strip accounted for less than 6% of the applied P. Superphosphate applications resulted in the accumulation of both soil inorganic and organic P fractions to a depth of 225 mm. The accumulation of soil inorganic P was most pronounced when SP was applied annually at the rate of 376 kg/ha, which was in excess of pasture P requirements.