The Soils Team of the Productivity, Irrigation, Pests and Soils project (PIPS) has collected baseline data on soil carbon across Australian orchards. This is the first step toward understanding how orchard management could be used to change soil carbon and soil health knowledge that growers could use to optimise nutrient availability and orchard performance. There is growing interest in determining whether agricultural and horticultural soils are a source, or sink, for carbon. The first step is to establish baseline values for soil carbon. We have completed a survey of 10 Australian apple orchard sites to establish baseline ranges of soil carbon stocks, soil health, and soil microorganism community composition under orchard production in Australias major apple-growing regions. Because carbon is a driver of many soil processes and a determinant of soil health, we investigated the relationships between soil carbon, soil health indicators, and soil microorganism community composition. This information can be used to measure the impacts of orchard management practices on soil carbon and soil health attributes in the future. We observed a wide variation in carbon stocks and soil health attributes among orchards. Labile carbon measurements served as strong indicators for microorganism activity and potential soil nutrient supply services. These observations warrant further analysis with microorganism community composition so we can link orchard microorganism biodiversity with soil health, soil functioning and the ecosystem services provided by apple orchard soils in Australia.
About the survey
The survey incorporated orchard sites located in the main pome fruit orchard regions in Australia. These sampling sites included orchard blocks randomly selected in the following locations for a total of 10 sites:
Donnybrook and Manjimup, WA Adelaide Hills, SA Gippsland and Shepparton, VIC Batlow and Orange, NSW Stanthorpe, QLD Tamar and Huonville, TAS We established a sampling protocol for the orchard survey to account for the spatially complex system of an orchard, both vertically and laterally. Soil samples were taken at multiple depth increments to estimate soil carbon stocks to 1 m depth. The soil samples collected were analysed for soil health parameters of total carbon and labile carbon; total and plant available nitrogen; potential nitrogen mineralisation; microorganism activity; and microorganism diversity.
Orchard carbon stocks
The apple orchards exhibited a wide variation in soil carbon (C) stocks to 1 m depth. Carbon stocks ranged from 7 kg C per square metre in Shepparton to 26 kg C per square metre in Stanthorpe (Figure 1). The orchards at Stanthorpe, Gippsland, Manjimup, Donnybrook, and Tamar contained the highest carbon stocks. By observation, these sites also showed indications of a high presence of charcoal in the soil samples, which may have contributed to their high carbon stocks. This charcoal could be historical from the original land clearing, or as a result of more recent orchard redevelopment. The carbon stock measurements from this survey provide a database of the magnitude and variability of carbon stocks in Australian apple orchards and they will be able to serve as a baseline from which the effects of orchard management on soil carbon can be monitored at these sites in the future.
Orchard soil health characteristics
A suite of soil chemical and biological properties were measured on the orchard soils. These soil health metrics were selected as potential indicators for soil carbon regulation and nutrient supply services. These soil properties were found to decline rapidly with soil profile depth at all the orchard sites. As a result of these analyses, we have established a database of soil health properties by profile depth by which to monitor and benchmark any future change in these orchards. All the soil health properties in the topsoil varied widely among the orchard sites (Table 1). Labile carbon is the part of the total organic carbon that is readily available to soil microorganisms for decomposition. Total organic carbon concentrations ranged from 20.0 g C / kg of soil in the Orange orchard to 45.0 g C / kg of soil in the Gippsland orchard. Nitrogen mineralisation rates varied from 0.32 mg N / kg of soil / day in Batlow to 4.21 mg N / kg soil / day in Adelaide Hills. In general, the orchards in Adelaide Hills, Gippsland and Tamar tended to have the highest indicators of microorganism activity, labile carbon and nitrogen mineralisation. Conversely, the Donnybrook, Manjimup, Batlow and Stanthorpe orchards tended to have the lowest indicators of microorganism activity, as would befit their coarser textures. Strong relationships were observed between various soil health attributes (Table 2). Labile carbon, microorganism activity, total nitrogen and nitrogen mineralisation were strongly related with one another. These variables all serve as indicators of microorganism activity and soil nutrient supply ecosystem services. Greater concentrations of labile or active carbon, which is carbon readily available to soil microorganisms, lead to greater microorganism activity and this highlights the health value of maintaining or enhancing labile soil carbon concentrations. The total organic carbon concentrations were strongly correlated only with total nitrogen. This emphasizes the utility of the labile carbon measure to quantify the pool of carbon that is available to soil microorganisms and is active in soil nutrient cycling processes. For example, while the Stanthorpe orchard soil had one of the highest organic carbon concentrations, probably from the presence of charcoal, it had low labile carbon concentrations and low microorganism activity. These observations imply that the carbon in the Stanthorpe soil is structurally complex and is not readily decomposed by most soil microorganisms. Our results support previous findings that labile carbon may be used as a sensitive indicator of nutrient pools and microorganism activity for determining impacts of management activities. Furthermore, the change in the labile carbon content can be used an early indicator of any change in the total carbon stock. We recommend that labile carbon be regularly monitored in standard soil testing. Table 1. Soil health characteristics at 0-10 cm depth in Australian apple orchards. Table 2. Correlation coefficients for soil health characteristics in Australian apple orchards. Orchard microorganism community composition
A healthy soil microorganism community is crucial for good soil functioning and health. There is clear evidence that healthy microorganism communities help plants to weather drought and the microorganisms can beneficially change the physical properties of soil. However, the relationships between soil microorganism diversity and soil functioning are not yet fully understood. Furthermore, it is increasingly being shown that changing management of farms and orchards can change both the soil microorganism population and the underlying soil processes. Once clarified, this understanding will then assist development of protocols for maintaining a healthy soil. The first results from our orchard survey contain new information that increases our knowledge of the make-up and functioning of the microorganism communities in Australian orchards. Furthermore, these data provide baseline information on microorganism biodiversity in the soil against which future changes can be referenced. Microorganism community composition was assessed for 8 of the survey orchard sites by measuring the relative abundance of a number of common soil microorganism groups and nitrogen cycling genes. The relative abundances of both the microorganism groups and nitrogen cycling genes varied markedly among orchard sites. Our analysis reveals the wide range of microorganism community compositions that exists across Australian orchards. These observations warrant further examination to determine how measured biodiversity relates to actual soil functioning. Acknowledgements We thank Terry Martella, Harvey Gilbert, Michael Stafford, Brad Fankhauser, Maurice Silverstein, Barry McLean (orchard manager), Michael Cunial, Daniel Nicoletti, Brad Ashland, and Howard Hansen for their cooperation with this research. The PIPS project is funded by Horticulture Australia Ltd using the apple and pear industry levy and voluntary contributions from the Institute for New Zealand Plant & Food Research and matched funds from the Australian Government. About the authors This article was written by Roberta Gentile, Robert Simpson, Brent Clothier, Carlo van den Dijssel and Karen Mason (Plant & Food Research Limited, New Zealand); Dave Cornwall (Department of Environment and Primary Industries, Victoria); and Marcus Hardie (Tasmanian Institute of Agriculture). For more information contact Michele Buntain, Tasmanian Institute of Agriculture, Michele.firstname.lastname@example.org, 03 6233 6814.