Estimating net annual soil sulfur mineralisation in New Zealand grazed pastures using mass balance models

Abstract

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. Quantitative estimates of S inputs and outputs in the MAF S model were also compared with those obtained in recent field trials. Results obtained showed that the MAF S model was reasonably accurate in estimating S inputs and outputs and long-term maintenance pasture S requirements but not short-term pasture S requirements, due to the parameters used not accounting for the effects of major factors affecting soil organic S mineralisation. Net annual soil S mineralisation under high-producing 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 mineralisation of soil organic S can be a significant source of S for pasture 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 organic S test needs to be calibrated with net soil S mineralisation potential, soil types, management practices, and pasture production. Although some of the excreta S that has been transferred to stock camps may be returned to the main grazing area, this excretal S and sulfate leaching losses were found to be major S outputs of the main grazing area. The results suggested that the mineralisation of soil organic S may not be sufficient to replace S losses indefinitely when S fertiliser applications are withheld.