NUTRIENT CYCLING AND LOSSES BASED ON A MASS-BALANCE MODEL IN GRAZED PASTURES RECEIVING LONG-TERM SUPERPHOSPHATE APPLICATIONS IN NEW-ZEALAND .2. SULFUR

Sulphur (S) cycling and losses in irrigated, sheep-grazed pastures receiving annual superphosphate (SP) applications for 35 years at rates of 0, 188 and 376 kg/ha were studied using a mass-balance approach which accounted both for S inputs to and outputs from the soil-plant-animal system. Total recoveries of applied S in the 188 and 376 kg SP/ha treatments were estimated to be 35.2 and 21.2 % respectively. Over 65.0 % of the applied S was not recovered, and this was attributed to leaching losses beyond the major rooting zone (0-300 mm), lateral S transfer by irrigation water and excretal S transfer to stock camps. The lower S recovery in the 376 compared with the 188 kg/ha per year treatment was probably due to a higher accumulation of soil phosphorus (P), which might have competed with sulphate (SO42-) for adsorption sites on soil colloids. Superphosphate applications resulted in the accumulation of soil organic S (S(o)) fractions (hydriodic acid-reducible and carbon-bonded S) to a soil depth of 300 mm in both camp and non-camp areas. However, an increase in SP applications from 188 to 376 kg/ha per year did not lead to a further accumulation of soil S. fractions. Approximately 50 % of this additional S input was estimated to be leached beyond the 300 mm depth. The presence of soil S. fractions at 300 mm soil depth, especially in camp areas, suggests that leaching occurred, involving not only SO42- but also S(o) fractions from soil, plant litter, root residues and animal excreta, as the soil had a low SO42- retention capacity. These leaching losses may limit the ability of soil S reserves to sustain adequate S for pasture production when S fertilizer application is withheld.