The potential for sown tropical perennial grass pastures to improve soil organic carbon in the North-West Slopes and Plains of New South Wales

Sown tropical perennial grass pastures may be a means to restore soil organic carbon (C) lost by cropping with conventional tillage to the levels originally present in native grass pastures. To assess this, total organic carbon and related soil properties were measured under sown tropical pastures, conventionally cultivated cropping, and native pastures on 75 Chromosols and 70 Vertosols to 0.3 m depth in the New South Wales North-West Slopes and Plains region of Australia. The impact of several perennial pasture species on soil organic carbon was also assessed in a 6-year-old, sown pasture experiment on a previously cropped Chromosol. Soil cores in 0.1-m segments to 0.3 m were analysed for total organic carbon, total nitrogen (N), pH, and phosphorus (Colwell-P). Mid-infrared scans were used to predict the particulate, humus, and resistant fractions of the total organic carbon. Bulk density was used to calculate stocks of C, N, and C fractions. In Chromosols, total organic carbon in the surface 0–0.1 m was greater under sown tropical pastures (23.1 Mg ha–1) than conventional tillage cropping (17.7 Mg ha–1), but still less than under native pastures (26.3 Mg ha–1). Similar land-use differences were seen for particulate and resistant organic C, and total N. The proportional differences between land uses were much greater for particulate organic C than other measures, and were also significant at 0.1–0.2 and 0.2–0.3 m. Subsurface bulk density (0.1–0.2 m) was lower under sown tropical pastures (1.42 Mg m–3) than conventionally tilled cropping (1.52 Mg m–3). For Vertosols, total organic carbon in the surface 0–0.1 m was greater under sown tropical pastures (19.0 Mg ha–1) and native pastures (20.5 Mg ha–1) than conventional tillage cropping (14.0 Mg ha–1). Similar land-use effects were seen for the particulate and humus organic C fractions, and total N. In the sown pasture species experiment, there was no significant difference in total N, total organic carbon, or any C fraction between soils under a native-grass species mixture, two improved tropical grass species, or a perennial pasture legume. Regular monitoring is required to better discern whether gradual changes are being masked by spatial and temporal variation. The survey results support previous research on Vertosols within the New South Wales North-West Slopes and Plains that show sown tropical grass pastures can improve total organic carbon. Improvements in total organic carbon on Chromosols have not previously been documented, so further targeted soil monitoring and experimentation is warranted for the region.