Effects of afforestation on organic carbon, nitrogen and sulfur concentrations in New Zealand hill country soils

This paper describes results from a study of the effects of first rotation radiata pine (Pinus radiata) on major organic constituents (carbon, nitrogen and sulfur) in hill country pasture soils in New Zealand. Soil properties were compared under 17–19-year-old radiata pine and adjacent pasture at four sites on the boundary of a 9000-ha forest. No fertiliser had been applied to either forest or pasture areas since tree establishment, and the pasture sites were either ungrazed or lightly grazed. Differences between sites included soil type (influenced mainly by drainage) and soil fertility (influenced mainly by fertiliser history prior to afforestation). Concentrations of organic carbon and nitrogen were 15–43% lower in soil to a 12–18-cm depth under trees compared with pasture at three sites. On the other hand, total (organic) sulfur was significantly lower (19–44%) in topsoil under trees compared with pasture at all sites, while concentrations of plant available sulfate–sulfur were consistently and significantly higher in soil to 30-cm depth under trees compared with pasture. These findings confirmed that afforestation of hill country pasture soils resulted in net mineralization of soil organic sulfur in addition to organic carbon and nitrogen.