ESTIMATING NET ANNUAL SOIL SULFUR MINERALIZATION IN NEW-ZEALAND GRAZED PASTURES USING MASS-BALANCE MODELS

Estimates of net soil sulfur (S) mineralisation. of grazed pastures in New Zealand were made using the New Zealand Ministry of Agriculture ( MAF) S model and a recently developed simple mass-balance S model. Qua ntitative estimates of S inputs and outputs in the MAF S model were al so compared with those obtained in recent field trials. Results obtain ed showed that the MAF S model was reasonably accurate in estimating S inputs and outputs and long-term maintenance pasture S requirements b ut not short-term pasture S requirements, due to the parameters used n ot accounting for the effects of major factors affecting soil organic S mineralisation. Net annual soil S mineralisation under high-producin g pastures estimated by the simple mass-balance model was substantial, ranging from 17.7 to 29.1 kg S/ha . year, thus showing that the miner alisation of soil organic S can be a significant source of S for pastu re plant growth. The inclusion of the MAF soil organic S test into the MAF S model may improve short-term S requirements, but the soil organ ic S test needs to be calibrated with net soil S mineralisation potent ial, soil types, management practices, and pasture production. Althoug h some of the excreta S that has been transferred to stock camps may b e returned to the main grazing area, this excretal S and sulfate leach ing losses were found to be major S outputs of the main grazing area. The results suggested that the mineralisation of soil organic S may no t be sufficient to replace S losses indefinitely when S fertiliser app lications are withheld.