Gs Oliver

Land use and management influences on surface soil organic carbon in Tasmania

The effects of environmental parameters, land-use history, and management practices on soil organic carbon (SOC) concentrations, nitrogen, and bulk density were determined in agricultural soils of four soil types in Tasmania. The sites sampled were Dermosols, Vertosols, Ferrosols, and a group of texture-contrast soils (Chromosol and Sodosol) each with a 10-year management history ranging from permanent perennial pasture to continuous cropping. Rainfall, Soil Order, and land use were all strong explanatory variables for differences in SOC, soil carbon stock, total nitrogen, and bulk density. Cropping sites had 29–35% less SOC in surface soils (0–0.1 m) than pasture sites as well as greater bulk densities. Clay-rich soils contained the greatest carbon stocks to 0.3 m depth under pasture, with Ferrosols containing a mean of 158 Mg C ha–1, Vertosols 112 Mg C ha–1, and Dermosols 107 Mg C ha–1. Texture-contrast soils with sandier textured topsoils under pasture had a mean of 69 Mg C ha–1. The range of values in soil carbon stocks indicates considerable uncertainty in baseline values for use in soil carbon accounting. Farmers can influence SOC more by their choice of land use than their day-to-day soil management. Although the influence of management is not as great as other inherent site variables, farmers can still select practices for their ability to retain more SOC.

Effect of biochar on nutrient leaching in a young apple orchard

Nutrient leaching from agricultural soils is a worldwide problem that has been implicated in deleterious impacts on the environment. Application of biochar to soil has been proposed as a means to reduce nutrient leaching and improve fertilizer use efficiency. The potential for biochar to reduce nutrient leaching and increase fertilizer use efficiency was tested by applying 47 Mg ha hardwood biochar before replanting a commercial apple () orchard, in the Huon Valley, Tasmania. Passive wick flux meters were installed at the base of the A1 horizon at a depth of 25 cm to monitor leachate volume and the concentration of nutrients leached below the A1 soil horizon over a 38 mo period. Biochar application significantly increased the concentration of phosphorous in the leachate, while having no significant effect on nitrate or potassium concentration. The volume of leachate collected in the flux meters was significantly higher in the biochar treatment, which resulted in significantly higher amounts of potassium and phosphorous being leaching from the biochar treatment than the control. Biochar application had no significant effect on either the concentration or the flux of nitrate leached from the A1 horizon. Nonetheless, nutrient application was well in excess of tree requirements, such that between 53 to 78% of the applied nitrogen, 5 to 11% of the applied phosphate, and 69 to 112% of the applied potassium were leached below the A1 horizon.

Benchmarking nitrous oxide emissions in deciduous tree cropping systems

Nitrous oxide (N2O) emissions contribute 6% of the global warming effect and are derived from the activity of soil-based microorganisms involved in nitrification and denitrification processes. There is a paucity of greenhouse gas emissions data for Australia’s horticulture industry. In this study we investigated N2O flux from two deciduous fruit tree crops, apples and cherries, in two predominant growing regions in eastern Australia, the Huon Valley in southern Tasmania (Lucaston – apples and Lower Longley – cherries), and high altitude northern New South Wales (Orange – apples and Young – cherries). Estimated from manual chamber measurements over a 12-month period, average daily emissions were very low ranging from 0.78gN2O-Nha–1day–1 in the apple orchard at Lucaston to 1.86gN2O-Nha–1day–1 in the cherry orchard in Lower Longley. Daily emissions were up to 50% higher in summer (maximum 5.27gN2O-Nha–1day–1 at Lower Longley) than winter (maximum 2.47gN2O-Nha–1day–1 at Young) across the four trial orchards. N2O emissions were ~40% greater in the inter-row than the tree line for each orchard. Daily flux rates were used as a loss estimate for annual emissions, which ranged from 298gN2O-Nha–1year–1 at Lucaston to 736gN2O-Nha–1year–1 at Lower Longley. Emissions were poorly correlated with soil temperature, volumetric water content, water filled porosity, gravimetric water content and matric potential – with inconsistent patterns between sites, within the tree line and inter-row and between seasons. Stepwise linear regression models for the Lucaston site accounted for less than 10% of the variance in N2O emissions, for which soil temperature was the strongest predictor. N2O emissions in deciduous tree crops were among the lowest recorded for Australian agriculture, most likely due to low rates of N fertiliser, cool temperate growing conditions and highly efficient drip irrigation systems. We recommend that optimising nutrient use efficiency with improved drainage and a reduction in soil compaction in the inter-row will facilitate further mitigation of N2O emissions.

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 (pHw) 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 pHw 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; pHw 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%; pHw 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%; pHw 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 = 4.933e−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 pHw and P fertility of their soils and that increasing cropping intensity on these soils comes at the expense of soil carbon.