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Eucalyptus pellita and Acacia mangium are widely planted in the tropics and managed over five-to-fifteen-year rotations for wood and wood products. This study compares the growth of routine plantings of A. mangium with trial plantings of E. pellita and nearby native forests across six broadly distributed locations on Melville Island, northern Australia. In addition to tree growth measurements, litter mass and chemical properties, and topsoil (0–10 cm) physical and chemical parameters were assessed. Melville Island has a pronounced seasonal rainfall and soils that range from loamy sand to sandy loam textures; across the sites sampled soils comprised 65–85% sand and 8–16% clay. Across our sites, the maximum mean annual increment of E. pellita was 31.12 m3/ha/year compared to 14.64 m3/ha/year for A. mangium , with stand volume strongly positively correlated with clay content. On a site basis tree volume growth correlated strongly with soil clay content, with lower survival on sites with less clay in the predominantly sandy soils. The higher E. pellita growth compared with A. mangium reduced available phosphorous and calcium in topsoil, indicating that addition of P and Ca in subsequent rotations will be important to maintain and increase E. pellita productivity. The N 2 -fixing species, A. mangium , significantly increased nitrogen in litter (1.88 ± 0.04%) and in top soil (0.15 ± 0.01%) compared to E. pellita (litter 0.93 ± 0.05%; topsoil 0.11 ± 0.01%) so that carbon to nitrogen (C/N) ratios in Acacia litter (23.90 ± 0.81) and soil (23.06 ± 0.91) were significantly lower than in Eucalyptus (litter 46.30 ± 2.66; top soil 27.66 ± 1.65). Topsoil C/N also decreased with increasing soil clay content in all land uses (A. mangium, E. pellita, native forests) indicating better conditions for decomposer activity and turnover in sites with higher clay content soils. The Eucalyptus plantation drawdown of N and PO 4 3- in topsoil suggests that future plantings of E. pellita may benefit from additional N fixed into biomass and soil organic matter by the preceding A. mangium plantations. Tree growth correlations with rainfall and soil characteristics showed growth is best explained by annual rainfall, soil texture (as a percentage of clay or of sand), soil electrical conductivity, and soil bulk density. Overall, in tropical long dry season regions, for plantation development on sandy soils, E. pellita is a better choice than A. mangium. • Eucalyptus pellita had higher productivity than Acacia mangium. • While A. mangium increased soil N, E. pellita tended to decline soil N status. • E. pellita was more efficient at taking up soil available P and Ca. • E. pellita growth is influenced by rainfall, bulk density, and sand content. • Stocking rate should be higher in clayey sites with higher water availability. [ABSTRACT FROM AUTHOR]

Tillage and nitrogen (N) fertilization have been shown to affect soil abiotic and biotic properties, but it is unclear how these impact long-term tillage systems in subtropical climates. We studied a 48-y wheat cropping trial in semi-arid, subtropical Australia that applied different tillage practices (no-till [NT] and conventional till [CT]) and N fertilization rates (0 and 90 kg N ha-1, 0 N and 90 N, respectively) in a Vertisol from 0 to 10 cm soil depth. Total organic carbon (TOC) significantly increased only when N fertilizer was applied under NT. Total nitrogen (TN) concentrations increased when N fertilizer was applied but were unaffected by tillage. N-acetyl-β- D -glucosaminidase (NAG) activity was highest when N was applied and α-glucosidase (α-Glu) activity was lowest in the NT and 0 N treatments. Activities of β-glucosidase, phenol oxidase and phosphatase were unaffected by the management practices. While the bacterial community composition appeared most distinct in NT and 0 N conditions, fungal community composition, and bacterial and fungal alpha diversity remained unaffected by the treatments. Although wheat yield was only significantly influenced by N fertilization rates, both NT and N fertilization may be preferable for management as these soils had the most favorable TOC, TN, and enzymatic activity. These differences seen between treatments here are less pronounced than reported elsewhere, with this likely a result of the high clay content of the soil which can have a buffering capacity, or that these treatments have less of an impact over longer time scales. Nevertheless, we conclude that no-tillage and N fertilization remain important in influencing soil properties and functioning over decades in this Vertisol in a semi-arid subtropical region. • Total organic carbon significantly increased when N was applied under no-till (NT). • Total N increased when N fertilizer was applied but there was no effect of tillage. • N-acetyl-β- D -glucosaminidase activity was highest when N was added. • Bacterial community composition was most distinct in NT and no N addition treatment. • Fungal community composition and bacterial alpha diversity were not affected. [ABSTRACT FROM AUTHOR]

The expansion of irrigation infrastructure in Tasmania, Australia, has led to intensified irrigated crop production, including for vegetables. This has resulted in increased irrigation of texture contrast soils, and other soils with high clay content in the subsoil. Widely used for rain-fed grain production in other parts of Australia, these soils have been subject to a range of soil amelioration approaches in an effort to improve their soil physical characteristics and capacity to produce crops. Subsoil manuring – the process of placing organic soil ameliorants at a target depth using a specially modified deep ripper – has proven to be successful in increasing yields of rain-fed grain crops. Locally available organic materials (chicken manure and poppy seed meal) were placed in the upper zone of a clay subsoil at three sites used for irrigated vegetable production. The crops grown were green peas for processing and carrots for seed production. Normal grower management practices were the reference against which treatments were compared. Subsequent measurements showed no consistent or significant benefit from subsoil manuring one year after treatment under the soil and crop management conditions used in this research. • Subsoil manuring has shown productivity benefits in rain-fed grain production. • Irrigated vegetables were grown on treated texture contrast and high clay soils. • No significant differences were found as a result of subsoil manuring treatments. [ABSTRACT FROM AUTHOR]

[Display omitted] • FTIR spectroscopy of clay is sensitive to temperature during bushfires. • Boron leached from combusted bark decreases d11B of clays. • No direct influence of combustion temperature on d11B of clays. • Clays have higher d11B after experiencing a high severity fire. • Both FTIR spectroscopy and B isotope could be viable fire proxies. Bushfire occurrences will likely be exacerbated by climate change, thus requiring a model to forecast and manage their impacts. However, a robust bushfire model requires new proxies that can infer fire severity responses to past climate variability. A key test to the viability of new fire proxies is whether they record fire severity in the affected soil. We address this by testing Attenuated Total Reflectance- Fourier Transform Infrared (ATR-FTIR) spectroscopy and boron (B) isotopes in soil clay fractions from Yengo National Park, southeastern Australia, as proxies for bushfire severity. The isotopic results were also compared to that of clays that reacted with experimentally combusted bark. ATR-FTIR spectroscopy constrains the soil temperature to between 225 and 500 °C during high severity fires, based on the lack of dehydroxylation peak characteristics and the increased aromatic to aliphatic organic peak ratios in clays, compared to that of low severity sites. The isotope composition of the non-exchangeable B fraction in clays is lighter after reacting with leaching solutions of bark combusted at higher temperatures. Combustion temperature does not affect the B isotope fractionation during B adsorption onto clays. Changes to the B isotope composition of clays could instead be justified by the varying B concentration and B isotope compositions of the leaching solutions. In Yengo soil clay fractions, sites that experienced a high severity fire show higher δ 11B values by about 1.5 ‰, compared to low severity sites- at odds with observations from our experiment using combusted bark. The combustion of leaves from tree crowns in high severity fires could account for the increase in δ 11B of clays post-fire. In summary, FTIR spectroscopy of clays could be useful for constraining soil temperature during bushfires, while the B isotope composition of clays appears as a promising proxy for fire severity. [ABSTRACT FROM AUTHOR]

Droughts are increasing in frequency and severity in many regions of the world and there are uncertainties how recurrent drought will impact acid sulfate soils, which can undergo extreme and persistent acidification (pH < 4) following oxidation. Using column experiments, we induced a 9 week drought/drying and 9 week rewetting phase in two acid sulfate soil profiles with sulfuric (pH < 4) and hypersulfidic materials from the Lower River Murray region of Australia's largest river system. These soils had not yet recovered from the extreme period of the 'Millennium Drought' between 2007 and 2010. pH, redox potential, dissolved metals and greenhouse gases (CO 2 , CH 4 and N 2 O) were measured at multiple depths in each column every 3 weeks during the drying and rewetting phases. The solid phase of the Sulfuric and Hypersulfidic clay soil profiles in both column experiments were analysed for complete acid-base accounting (actual and retained acidity, potential acidity in the form of pyrite, and acid neutralising capacity) and reactive metals at the beginning and end of the experiment. Residual pyrite, present in high concentrations in the Sulfuric clay soil column, oxidised during the drying period, further lowering the pH (<4) and mobilising dissolved metals (Al, Cd, Cu, Ni, Zn). More reactive Fe and Al phases were formed during the drying-rewetting cycle while reactive Mn decreased. The soil pH did not recover (i.e. increase) during the rewetting phase in both the Sulfuric and Hypersulfidic clay soils, likely due to barriers to microbial reduction reactions, although Fe-oxidising bacteria were likely still active as CO 2 was released. Acid sulfate soils may not recover in inter-drought periods and more severe impacts can be expected following recurrent droughts. • Droughts expose acid sulfate soils causing severe and prolonged impacts. • Column experiments simulated repeated drought in acid sulfate soils. • Soil pH decreased and metals increased during the drying phase. • Soils did not recover upon rewetting. • Recurrent drought in acid sulfate soils creates significant additional risks. [ABSTRACT FROM AUTHOR]

Savanna regions are increasingly developed for agriculture to support population growth, food demand and export economies. This is driving interest in the conversion of natural savanna to cattle grazed pastures or horticultural crops in Northern Australia. Savanna clearing leads to aboveground carbon (C) loss but impacts below-ground are less certain as studies have focused on high rainfall regions, low reactive clay soils and shallow depths of 0.3 m or less. To examine impact of land use change (LUC) in Northern Australia, we sampled the upper 1.0 m soil profile of: 1) savanna woodlands, 2) old and young C4 perennial cattle grazed pastures and 3) old and young, melon-sorghum rotation fields. Soil C concentrations and soil bulk density in the upper 0.3 m were significantly greater in cattle grazed pastures than savanna woodland, such that savanna to pasture LUC significantly increased soil C density from 30.2 ± 1.5–43.4 ± 3.0 Mg C ha-1 over 28 years. Soil bulk density in the upper 0.7 m of savanna woodland was significantly less than that in the melon-sorghum rotations. In contrast, soil C concentrations below 0.5 m were significantly greater in the savanna woodland than in the 3 y.o. melon-sorghum rotation fields. There was no significant difference in soil C density between savanna woodland and 12 y.o. melon-sorghum rotations. This may relate to the beneficial effects of ploughing in sorghum as a green manure, or simply that the decrease in soil C concentration was offset by increased soil C density in the upper layers of soil where most C is located. In contrast, conversion of savanna to cattle grazed pasture sites is equivalent to a soil sequestration rate of 0.34 Mg C ha-1 y-1, approximately 50% of the observed net ecosystem productivity estimated using eddy covariance methods. Changing the depth to which soil C density was estimated changed significant differences amongst land-use systems. Whereas, estimating soil C density using a 'fixed depth' or 'equivalent mass' method did not change the statistical significance. There is an increasing focus on the potential of soil carbon sequestration especially in north Australia and understanding potential change and impact of accounting methods is essential for meaningful policy development. Policy settings to maintain, or improve, soil C storage and soil health may contradict agricultural development goals. • Soil C was measured to 1.0 m under savanna woodland, pasture and cropping systems. • Converting savanna to pasture significantly increased soil C density in the upper 0.3 m. • Melon-sorghum rotation cropped soil had significantly greater bulk density than savanna in the upper 0.5 m. • Converting savanna to melon-sorghum rotation cropping did not significantly change soil C density. • Estimating soil C density to 0.3, 0.6 or 1.0 m changed statistical significant differences. [ABSTRACT FROM AUTHOR]