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6INTRODUCTIONMANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEIncreasing soil organic matter is widely regardedas beneficial to soil function and fertility and inagricultural production systems is integral tosustainable farming. Storing the carbon componentof organic matter in agricultural, rangeland andforest soils is also seen as one way to decreaseatmospheric carbon dioxide levels and mitigate theimpact of climate change. Consequently, there isgreat interest in quantifying the capacity of varioussoil types and land management practices to supportincreases in soil organic matter and understandinghow these changes impact soil health, ecosystemservices and carbon sequestration in the mediumand long-term.ORGANIC MATTER STATUS OFAUSTRALIAN SOILSAustralian soils are ancient. They have inherentlypoor structure, fertility and low levels of organicmatter in their surface layers – a condition madeworse by historical land clearing and subsequentland management practices. Physical and chemicalsoil constraints such as salinity, acidity, disease,compaction and sodicity impact large areas ofAustralian soils. These factors limit their productivityand act as major constraints to increasing soilorganic matter.Australian soils are low in soil organic mattercontent by global standards, with the exceptionof soils that support high net primary productivitysuch as well-managed pastures and irrigatedsystems unconstrained by water availability. Recentestimates from the 2011 State of the Environmentreport suggest climate variability and the historicclearing of native vegetation for agriculture hasresulted in a 30-70 per cent decline in soil organicmatter content. However, while soil organic matterhas declined in many systems over the past 100-
200 years, recent assessments suggest there is ahigh to moderate potential to increase the carboncontent of soils across extensive areas of Australia(see Figure 1.1).Recent measures of soil organic matter (0-30 cm)in Australian soils suggest they can contain betweenthree tonnes of carbon per hectare (0.3 per centcarbon for desert loams) and 231 tonnes of carbonper hectare (14 per cent carbon for intensive dairysoils; see http://www.daff.gov.au/climatechange/australias-farming-future/climate-change-andproductivity-research/soil_carbonfor the report).For Australian dryland agricultural soils the organiccarbon content is more typically between 20-150tonnes carbon per hectare (or about 0.7-4.0 percent depending on soil bulk density).Quantifying existing soil organic carbon stocks inkey soil types under important farm managementregimes within some of Australia’s agriculturalregions was a key output of the Soil CarbonResearch Program (SCaRP), and has providedvaluable information to baseline soil organicmatter stocks across different environments(see Figure 1.2).Management and site factors interact to influencethe actual amount of soil organic matter in relationAustralian soils are low in soilorganic matter content by globalstandards, with the exception ofsoils that support high net primaryproductivity such as well-managedpastures and irrigated systemsunconstrained by water availability.to the attainable amount of soil organic matter (asdefined by climate and soil type). While there is astrong relationship between rainfall and the amountof soil organic carbon, regional data indicates thateven within a rainfall zone soil organic carbon canvary widely and the highest variability occurs in areasof high rainfall, which support increasing biomassproduction (Griffin et al. 2013). Such variabilityis likely due to a range of factors influencing netprimary productivity, including agronomic and soilmanagement as well as variations in soil type andmoisture – the influences of which are discussed inthis publication.7MANAGING SOIL ORGANIC MATTER: A PRACTICAL GUIDEFigure 1.1 Potential for soil organic matter gainresulting from a combination of effective rain(considering amount and availability) and residuepressure, which reflects plant and livestock removalsof organic matter (Baldock et al. 2009). Figure 1.2 Soil organic carbon (tonnes per hectare)stocks in surface layers (0-10 cm) for south westWestern Australia. Areas of low confidence (e.g.subsystems that only have one site) are maskedout in white. Lines constitute agricultural soil zones:grey is remnant vegetation (Griffin et al. 2013).
Page 1 and 2: MANAGING SOILORGANIC MATTERA PRACTI
Page 3 and 4: FOREWORD1The one constant in agricu
Page 6 and 7: TABLES, FIGURES AND PLATES4MANAGING
Page 10 and 11: 018SOIL ORGANIC MATTERMANAGING SOIL
Page 12 and 13: Table 1.1 Size, composition, turnov
Page 14 and 15: Table 1.2 Functional role of soil o
Page 16 and 17: Figure 1.7 The influence of soil ty
Page 18 and 19: 16HOW MUCH ORGANIC CARBONREMAINS IN
Page 20 and 21: 0218BIOLOGICAL TURNOVEROF ORGANIC M
Page 22 and 23: 20MANAGING SOIL ORGANIC MATTER: A P
Page 26 and 27: 2403MANAGING SOIL ORGANIC MATTER: A
Page 29: less than 15 per cent of carbon inp
Page 36 and 37: Figure 4.1 Nitrogen cycling in soil
Page 39: mycorrhizal associations have been
Page 42 and 43: Table 5.1 Influence of soil charact
Page 44 and 45: Figure 5.2 Adjustment of organic ca
Page 48 and 49: 0646ORGANIC MATTER LOSSESFROM SOILM

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