Changes in organic carbon and selected soil fertility parameters in agricultural soils in Tasmania, Australia

Twenty-four sites in northern Tasmania on Ferrosols formed on Tertiary basalt were sampled in 1997, 2005, and 2010. The farming systems at the sites during this period were either (a) continuous pasture, (b) continuous cropping of vegetable, cereal, and other cash crops in rotation, or (c) a mix of pasture and cropping (intermittent cropping). Average organic carbon (OC) concentrations across all sites and both sample depths (0–150 mm and 150–300 mm) decreased from 4.0% in 1997 to 3.5% in 2005 and 3.3% in 2010 (P < 0.001). Average OC was greater at 0–150 mm (4.1%) than at 150–300 mm (3.1%) (P < 0.001). Average total soil nitrogen (N) in 2010 (0.42%) at 0–150 mm was not significantly different (P = 0.07) from the average in 1997 (0.38%). Soil pH in water (pH_w) showed the opposite trend to OC, increasing with time from 6.0 in 1997 to 6.2 in 2010. This reflects the consistent use of calcite and dolomite in Tasmanian farming because the natural pH_w of these soils is less than 5.5. Exchangeable calcium and magnesium were also greater in 2010 compared with 1997. Bicarbonate-extractable phosphorus (P) averaged across both depths increased (P < 0.001) steadily from 77 mg kg⁻¹ in 1997 to 95 mg kg⁻¹ in 2005 to 126 mg kg⁻¹ in 2010. At the five sites that remained under continuous cropping throughout the study, average 2010 OC in the topsoil was 3.2%, a decrease of 0.5% since 1997; pH_w was unchanged at 6.4; and bicarbonate-extractable P increased by an average of 73 mg kg⁻¹ to 211 mg kg⁻¹. For the three sites that remained in pasture the corresponding topsoil changes were 0.3% less OC to an average of 4.8%; pH_w 0.4 units greater at 6.0; and 41 mg kg⁻¹ more bicarbonate-extractable P to an average of 99 mg kg⁻¹. At the six sites converted from continuous pasture to intermittent cropping the average 2010 topsoil OC was 4.5%, a loss of 1.3%; pH_w increased by 0.5 units to 6.1; and bicarbonate-extractable P increased by 35 mg kg⁻¹ to 97 mg kg⁻¹. There was a highly significant (R² = 0.82) exponential relationship between topsoil OC in 2010 and the number of years each site had been cropped during the 38 years 1972–2010, for which reliable records were available (OC = 4.933^{e−0.018years}), which suggested an equilibrium topsoil OC concentration of about 2.5% under continuous cropping on these soils. These results indicate that Tasmanian farmers on Ferrosols continue to increase the pH_w and P fertility of their soils and that increasing cropping intensity on these soils comes at the expense of soil carbon.