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Fractionation of soil organic carbon (SOC) is crucial for mechanistic understanding and modeling of soil organic matter decomposition and stabilization processes. It is often aimed at separating the bulk SOC into fractions with varying turnover rates, but a comprehensive comparison of methods to achieve this is lacking. In this study, a total of 20 different SOC fractionation methods were tested by participating laboratories for their suitability to isolate fractions with varying turnover rates, using agricultural soils from three experimental sites with vegetation change from C3 to C4 22–36 years ago. Enrichment of C4-derived carbon was traced and used as a proxy for turnover rates in the fractions. Methods that apply a combination of physical (density, size) and chemical (oxidation, extraction) fractionation were identified as most effective in separating SOC into fractions with distinct turnover rates. Coarse light SOC separated by density fractionation was the most C4-carbon enriched fraction, while oxidation-resistant SOC left after extraction with NaOCl was the least C4-carbon enriched fraction. Surprisingly, even after 36 years of C4 crop cultivation in a temperate climate, no method was able to isolate a fraction with more than 76% turnover, which challenges the link to the most active plant-derived carbon pools in models. Particles with density >2.8 g cm−3 showed similar C4-carbon enrichment as oxidation-resistant SOC, highlighting the importance of sesquioxides for SOC stabilization. The importance of clay and silt-sized particles (

Coastal wetlands are carbon and nutrient sinks that capture large amounts of atmospheric C[O.sub.2] and runoff of nutrients. 'Blue carbon' refers to carbon stored within resident vegetation (e.g. mangroves, tidal marshes and seagrasses) and soil of coastal wetlands. This study aimed to quantify the impact of vegetation type on soil carbon stocks (organic and inorganic) and nitrogen in the surface soils (0-10 cm) of mangroves and tidal marsh habitats within nine temperate coastal blue carbon wetlands in South Australia. Results showed differences in surface soil organic carbon stocks (18.4 Mg OC [ha.sup.-1] for mangroves; 17.6 Mg OC [ha.sup.-1] for tidal marshes), inorganic carbon (31.9 Mg IC [ha.sup.-1] for mangroves; 35.1 Mg IC [ha.sup.-1] for tidal marshes), and total nitrogen (1.8 Mg TN [ha.sup.-1] for both) were not consistently driven by vegetation type. However, mangrove soils at two sites (Clinton and Port Augusta) and tidal marsh soils at one site (Torrens Island) had larger soil organic carbon (SOC) stocks. These results highlighted site-specific differences in blue carbon stocks between the vegetation types and spatial variability within sites. Further, differences in spatial distribution of SOC within sites corresponded with variations in soil bulk density (BD). Results highlighted a link between SOC and BD in blue carbon soils. Understanding the drivers of carbon and nitrogen storage across different blue carbon environments and capturing its spatial variability will help improve predictions of the contribution these ecosystems to climate change mitigation. Keywords: blue carbon, coastal wetlands, mangroves, soil carbon, soil nitrogen, temperate wetlands, tidal marshes., Introduction Occurring at the interface between land and sea, vegetated coastal wetlands are dynamic ecosystems that play an important role in global carbon and nutrient cycles (Duarte et al. 2013). [...]

Sandy soils make up a substantial fraction of cropping land in low rainfall ( Additional keywords: crop nutrition, deep ripping, soil compaction, soil fertility, water repellency. Received 14 June 2019, accepted 19 January 2020, published online 26 February 2020, Introduction The low-medium rainfall zone (3 :1), making it one of the driest rain-fed grain cropping environments in the world. In addition, rainfall is erratic with a coefficient of variation [...]

It is important to understand the stability of soil organic matter (SOM) sequestered through land management changes. In this study we assessed differences in carbon (C) stability of pasture soils that had high and low C content (2.35% vs 1.73% whole soil C in the 0-10 cm layer) resulting from long-term phosphorus fertilisation. We used soil size fractionation (fine fraction, coarse fraction and winnowing) to assess the amount of stable C and indicators of microbial decomposition capacity (catabolic profiles, metabolic quotient) to assess C stability. As a main effect throughout the 60-cm profile, C concentrations were higher in the fine fraction soil in the high (excess P fertiliser; P2) than low (no P fertiliser; PO) treatments, demonstrating a larger stable C fraction. For both P2 and PO, there was a strong correlation between C measured in the fine fraction and winnowed fraction in the 0-30 cm layer (R = 0.985, P < 0.001), but no correlation was observed for the 30-60 cm layer (R = 0.121, P > 0.05). In addition, we conducted two incubation experiments to assess C stability in the treatments with depth and to assess C stability in the physical soil fractions. For the surface soils (0-10 cm), the highest respiration occurred in fractions containing plant material, including roots (coarse fraction, 0.65 g C[O.sub.2]-C [kg.sup.-1] soil; whole soil, 1.48 g C[O.sub.2]-C [kg.sup.-1] soil), which shows that the plant material was less stable than the fine and winnowed soil fractions (0.43 and 0.40 g C[O.sub.2]-C [kg.sup.-1] soil respectively). Soil respiration, microbial metabolic quotient and substrate utilisation were similar in P0 and P2. Collectively, the data show that the increased C in P2 was associated with increased C concentrations in the more stable fine soil fraction, but with no change in the stability of the C within the fractions. Additional keywords: carbon fractions, fine fraction soil carbon, mid-infrared spectroscopy, nutrient stoichiometry, soil carbon stocks, soil organic matter. Received 20 March 2019, accepted 2 September 2019, published online 10 October 2019, Introduction Agricultural productivity is intrinsically linked to soil organic matter (SOM) dynamics, which has led to growing interest in management changes to sequester SOM and in fractionation methods to assess [...]

The abiotic protection of low molecular weight organic compounds (LMWOC) in soils may be an important regulator of C cycling. The study of the protection of LMWOC through sorption typically employs soils shaken in solution, which may be compromised by biological activity. We used [sup.14]C-labelled glucose as a LMWOC in batch assays of four different soils with contrasting physico-chemical properties (Arenosol, Luvisol, Ferralsol and Andisol). The commonly used biocides Na[N.sub.3] and Hg[Cl.sub.2] were employed alone or in concert across a range of concentrations to assess their efficacy in inhibiting microbial degradation. For short ( Additional keywords: abiotic process, LMWOCs, mercuric chloride, radiolabelled compounds, sodium azide, soil organic matter., Introduction The presence of low molecular weight organic compounds (LMWOCs), such as amino acids and carbohydrates, in soil is attributed to root exudation and turnover (Farrar et al. 2003; Boddy [...]

Organic phosphorus (P) plays an important role in the soil P cycle. It is present in various chemical forms, the relative amounts of which vary among soils, due to factors including climate, land use, and soil type. Few studies have investigated co-variation between P types or stoichiometric correlation with the key elemental components of organic matter-carbon (C) and nitrogen (N), both of which may influence P pool structure and dynamics in agricultural soils. In this study we determined the organic P speciation of twenty Australian Red Chromosols soils, a soil type widely used for cropping in Australia. Eight different chemical forms of P were quantified by 31P NMR spectroscopy, with a large majority (>90%) in all soils identified as orthophosphate and humic P. The strongest correlations ([r.sup.2] = 0.77-0.85, P Additional keywords: carbon, diester phosphate, monoester phosphate, nitrogen, organic P, solution [sup.31]P NMR spectroscopy, stoichiometry., Introduction Organic phosphorus (P) comprises a substantial proportion of total P in most soils (Williams and Steinbergs 1958; Harrison 1987). Although it is not directly plant available, it plays a [...]

Better understanding the spatial distribution of soil organic carbon (SOC) stocks is important for the management and enhancement of soils for production and environmental outcomes. We have applied digital soil mapping (DSM) techniques to combine soil-site datasets from legacy and recent sources, environmental covariates and expert pedological knowledge to predict and map SOC stocks in the top 0.3 m, and their uncertainty, across South Australia's agricultural zone. In achieving this, we aimed to maximise the use of locally sourced datasets not previously considered in national soil C assessments. Practical considerations for operationalising DSM are also discussed in the context of working with problematic legacy datasets, handling large numbers of potentially correlated covariates, and meeting end-user needs for readily interpretable results and accurate maps. Spatial modelling was undertaken using open-source R statistical software over a study area of ~ 160 000 [km.sup.2]. Legacy-site SOC stock estimates were derived with inputs from an expert-derived bulk-density pedotransfer function to overcome critical gaps in the data. Site estimates of SOC were evaluated over a consistent depth range and then used in spatial predictions through an environmental- correlation regression-kriging DSM approach. This used the contemporary Least Absolute Shrinkage and Selection Operator penalised-regression method, which catered for a large number (63 numeric, four categorical, four legacy- soil mapping themes) of potentially correlated covariates. For efficient use of the available data, this was performed within a k-fold cross-validation (k = 10) modelling framework. Through this, we generated multiple predictions and variance information at every node of our prediction grid, which was used to evaluate and map the expected value (mean) of SOC stocks and their uncertainty. For the South Australian agricultural zone, expected value SOC stocks in the top 0.3 m summed to 0.589 Gt with a 90% prediction interval of 0.266-1.086 Gt. Additional keywords: digital soil mapping, geostatistics, LASSO, soil organic carbon, pedotransfer function, spatial analysis. Received 13 August 2014, accepted 21 August 2015, published online 12 October 2015, Introduction Soil organic carbon (SOC) is an important measure of soil condition, with links to soil characteristics that underpin plant water- and nutrient-use efficiencies and agricultural productivity (Baldock and Skjemstad [...]

The application of biochar technology for soil amendment is largely based on evidence about soil fertility and crop productivity gains made in the Amazonian Black Earth (term preta). However, the uncertainty of production gains at realistic application rates of biochars and lack of knowledge about other benefits and other concerns may have resulted in poor uptake of biochar technology in Australia so far. In this review, we identify important opportunities as well as challenges in the adoption of biochar technology for broadacre farming and other sectors in Australia. The paper highlights that for biochar technology to be cost-effective and successful, we need to look beyond carbon sequestration and explore other opportunities to value-add to biochar. Therefore, some emerging and novel applications of biochar are identified. We also suggest some priority research areas that need immediate attention in order to realise the full potential of biochar technology in agriculture and other sectors in Australia. Additional keywords: biochar characterisation, contaminants, herbicide efficacy, heavy metals, nitrous oxide, plant growth media, regulations., Introduction Biochar is a carbon (C)-rich, solid material produced by thermal decomposition of organic material or biomass in the absence, or under limited supply, of oxygen (Lehmann and Joseph 2009). [...]

Identifying drivers of variation in soil organic carbon (OC) at a regional scale is often hampered by a lack of historical management information. Focusing on red-brown-earth soils (Chromosol) under dryland agriculture in the Mid-North and Eyre Peninsula of South Australia, our aims were 2-fold: (i) to provide a baseline of soil OC stocks (0.3 m) and OC fractions (mid-infrared predictions of particulate, humus, and resistant OC in 0.1 m samples) in cropping and crop-pasture systems; and (ii) to evaluate whether the inclusion of management-based indices could assist in explaining regional-level variation in OC stocks and fractions. Soil OC stocks in both regions varied ~20 Mg [ha.sup.-1], with higher OC stocks in the Mid-North (38 Mg [ha.sup.-1]) than the Eyre Peninsula (29.1 Mg [ha.sup.-1]). The humus OC fraction was the dominant fraction, while the particulate OC was the most variable. Environmental variables only partially explained soil OC variability, with vapour pressure deficit (VPD) offering the greatest potential and likely acting as an integrator of temperature and moisture on plant growth and decomposition processes. Differences between broad-scale cropping and crop-pasture systems were limited. In the Mid-North, variability in soil OC stocks and fractions was high, and could not be explained by environmental or management variables. Higher soil OC concentrations (0.1 m) in the Eyre Peninsula cropping than crop-pasture soils were largely accounted for in the particulate OC fraction and are therefore unlikely to represent a long-term stable OC pool. Use of the management data in index format added some explanatory power to the variability in OC stocks over the main environmental variables (VPD, slope) within the Eyre Peninsula cropping soils only. In the wider context, the management data were useful in interpreting differences between regional findings and highlighted difficulties in using uninformed, broad-scale management categories. Additional keywords: Chromosol, legume, pasture, soil organic matter, vapour pressure deficit., Introduction Agricultural management causes disruption to the natural balance in soil carbon and nutrient cycling, resulting in a decline in the organic carbon (OC) content of agricultural soils (Lal 2004b). [...]

Significant interest exists within Australia and internationally to quantify the impact of land use and applied management practices on stocks of soil organic carbon. This has resulted from the recognition [...]