Removing Grazing Pressure from a Native Pasture Decreases Soil Organic Carbon in Southern New South Wales, Australia

Grazing management is a known influence of carbon accumulation in agricultural soil, but there is conflicting evidence on the extent. This study compared organic carbon and nitrogen stocks at the conclusion of a 5-year grazing trial on a fertilised native pasture in south-eastern Australia. The study included three grazing treatments: ungrazed, tactically grazed (set stocking with biannual rest periods) and cell grazed (intense stocking with frequent long rest periods). There was no influence of grazing treatment detected on pasture sward composition when averaged over seasons or on total nitrogen or bulk density. The cell grazing treatment had total carbon stock of 32·9 Mg C ha⁻¹ (SE = 1·8) in the 0–0·30 m soil layer, which was a significant increase (p < 0·05) relative to the ungrazed treatment at 25·6 Mg C ha⁻¹ but not statistically greater than the tactical treatment at 29·5 Mg C ha⁻¹. There was no difference detected in labile carbon stocks to 0·30 m, which indicates that differences in soil carbon due to grazing was accumulated over the 5-year trial rather than reflecting short-term seasonal impacts. We propose that a combination of factors contributed to a greater stock of soil carbon under grazed pastures including differences in plant shoot/root allocation, root growth and root turnover with defoliation under grazing as well as lower plant productivity where grazing is excluded because of shading and nutrient tie-up. This study demonstrates removing grazing pressure may lead to lower soil carbon stocks in native pastures over time and provides evidence of the potential for grazing management to increase soil carbon in the short-term.