Soil organic phosphorus and microbial community composition as affected by 26years of different management strategies

Agricultural management can affect soil organic matter chemistry and microbial community structure, but the relationship between the two is not well understood. We investigated the effect of crop rotation, tillage and stubble management on forms of soil phosphorus (P) as determined by solution 31P nuclear magnetic resonance spectroscopy and microbial community composition using fatty acid methyl ester analysis in a long-term field experiment (26years) on a Chromic Luvisol in New South Wales, Australia. An increase in soil organic carbon, nitrogen and phosphorus compared to the beginning of the experiment was found in a rotation of wheat and subterranean clover with direct drill and mulching, while stubble burning in wheat-lupin and wheat-wheat rotations led to soil organic matter losses. Microbial biomass was highest in the treatment with maximum organic matter contents. The same soil P forms were detected in all samples, but in different amounts. Changes in organic P occurred mainly in the monoester region, with an increase or decrease in peaks that were present also in the sample taken before the beginning of the experiment in 1979. The microbial community composition differed between the five treatments and was affected primarily by crop rotations and to a lesser degree by tillage. A linkage between soil P forms and signature fatty acids was tentatively established, but needs to be verified in further studie

Organic amendments as phosphorus fertilisers: chemical analyses, biological processes and plant P uptake

As phosphorus (P) fertilisers become increasingly expensive there is a need to find innovative ways to supply crops with P. Organic amendments (OA) can contain high concentrations of total P, although the P is present in various forms. We aimed to determine the forms of P and carbon (C) in a range of OA and the effect of these OA on soil microbial biomass, P release, arbuscular mycorrhizal (AM) colonisation, and plant P uptake. Four OA were investigated: two chicken litters (CHK-STR and CHK-SD, one with straw bedding and one with sawdust bedding), a pig litter (PIG-STR) and a municipal waste compost (COMP). An incubation experiment and a plant growth experiment were conducted in which OA and INORG-P were supplied at 15 mg P kg−1 soil and a zero P control was included. All OA had high P concentrations and did not result in an increase in the soil microbial biomass C. There were few temporal changes in available P throughout the incubation experiment suggesting that solubilisation and/or mineralisation of P occurred at a similar rate as conversion of P to unusable forms. Of the OA, PIG-STR had the largest proportion of orthophosphate P and bicarbonate extractable P, and it provided the most P to plants. While CHK-STR had a higher proportion of orthophosphate P and bicarbonate extractable P than CHK-SD, both CHK-STR and CHK-SD provided plants with similar amounts of P. This could be because CHK-SD had a higher proportion of phytate, which can be rapidly mineralised to orthophosphate, and/or because plants in the CHK-SD had higher rates of arbuscular mycorrhizal (AM) colonisation compared with CHK-STR. This study provides new insights into plant and microbial responses to OA which could help in the development of sustainable food production systems.J.E. Mackay, L.M. Macdonald, R.J. Smernik, T.R. Cavagnar

Organic chemistry insights for the exceptional soil carbon storage of the seagrass Posidonia australis

The high organic carbon (OC) stores in seagrass meadows have led to their recognition as significant Blue Carbon sinks, though the diagenetic conditions that enable OC retention in seagrass soils remain poorly understood. In this study, seagrass soils were sampled from a Posidonia australis meadow in Oyster Harbour (Albany; south-western Australia) to investigate the preservation of sedimentary OC. We analysed soil characteristics (colour, grain size and redox potential), radiocarbon age, and characterised the soil organic matter (OM) using solid state CP/MAS 13C NMR spectroscopy to examine the preservation of OM down the soil profile. There was minimal change in organic composition over 1,700 years of accumulation, indicating long-term OM preservation. Primarily, this preservation appears to be driven by the recalcitrance of seagrass detritus buried in anoxic soils. The majority (70–83%) of total sedimentary OM comprised components directly attributable to seagrass origins (lignin, carbohydrate and black carbon-like matter), while the remainder consisted mostly of protein, some of which may have been present in seagrass biomass, along with likely contributions from algae and microbes. Although black carbon originates from organic matter combustion, here we provide evidence that the 13C NMR signal identified as black carbon-like matter in our soils is possibly associated with seagrass-derived organic matter consisting of degraded lignin products or other non-pyrogenic aromatics. The increase in the relative abundance of this black carbon-like matter with aging suggests its selective preservation. The relative abundances of carbohydrates significantly decreased with age down core (i.e. they appeared to be selectively decomposed), while lignin and protein did not show any quantitative changes in relative abundance (non-selective preservation). These findings demonstrate the exceptional preservation of P. australis derived OC, which contributes to our understanding of the higher OC storage capacity of Posidonia compared to other seagrass species.Oscar Serrano, Mohammad Rozaimi, Paul S. Lavery, Ronald J. Smerni

Do organic inputs alter resistance and resilience of soil microbial community to drying?

Abstract not availableE.-L. Ng, A.F. Patti, M.T. Rose, C.R. Schefe, R.J. Smernik, T.R. Cavagnar

Measurement of soil carbon oxidation state and oxidative ratio by (13)C nuclear magnetic resonance

Extent: 14p.The oxidative ratio (OR) of the net ecosystem carbon balance is the ratio of net O₂and CO₂ fluxes resulting from photosynthesis, respiration, decomposition, and other lateral and vertical carbon flows. The OR of the terrestrial biosphere must be well characterized to accurately estimate the terrestrial CO₂sink using atmospheric measurements of changing O₂ and CO₂levels. To estimate the OR of the terrestrial biosphere, measurements are needed of changes in the OR of aboveground and belowground carbon pools associated with decadal timescale disturbances (e.g., land use change and fire). The OR of aboveground pools can be measured using conventional approaches including elemental analysis. However, measuring the OR of soil carbon pools is technically challenging, and few soil OR data are available. In this paper we test three solid-state nuclear magnetic resonance (NMR) techniques for measuring soil OR, all based on measurements of the closely related parameter, organic carbon oxidation state (Cox). Two of the three techniques make use of a molecular mixing model which converts NMR spectra into concentrations of a standard suite of biological molecules of known Cox. The third technique assigns Cox values to each peak in the NMR spectrum. We assess error associated with each technique using pure chemical compounds and plant biomass standards whose Cox and OR values can be directly measured by elemental analyses. The most accurate technique, direct polarization solid-state ¹³C NMR with the molecular mixing model, agrees with elemental analyses to ±0.036 Cox units (±0.009 OR units). Using this technique, we show a large natural variability in soil Cox and OR values. Soil Cox values have a mean of −0.26 and a range from −0.45 to 0.30, corresponding to OR values of 1.08 ± 0.06 and a range from 0.96 to 1.22. We also estimate the OR of the carbon flux from a boreal forest fire. Analysis of soils from nearby intact soil profiles imply that soil carbon losses associated with the fire had an OR of 1.091 (±0.003). Fire appears to be a major factor driving the soil C pool to higher oxidation states and lower OR values. Episodic fluxes caused by disturbances like fire may have substantially different ORs from ecosystem respiration fluxes and therefore should be better quantified to reduce uncertainties associated with our understanding of the global atmospheric carbon budget.W. C. Hockaday, C. A. Masiello, J. T. Randerson, R. J. Smernik, J. A. Baldock, O. A. Chadwick and J. W. Harde

Does the chemical nature of soil carbon drive the structure and functioning of soil microbial communities?

Abstract not availableE.-L. Ng, A.F. Patti, M.T. Rose, C.R. Schefe, K. Wilkinson, R.J. Smernik, T.R.Cavagnar

Fire-derived organic matter retains ammonia through covalent bond formation

Fire-derived organic matter, often referred to as pyrogenic organic matter (PyOM), is present in the Earth's soil, sediment, atmosphere, and water. We investigated interactions of PyOM with ammonia (NH₃) gas, which makes up much of the Earth's reactive nitrogen (N) pool. Here we show that PyOM's NH₃ retention capacity under ambient conditions can exceed 180 mg N g⁻¹ PyOM-carbon, resulting in a material with a higher N content than any unprocessed plant material and most animal manures. As PyOM is weathered, NH₃ retention increases sixfold, with more than half of the N retained through chemisorption rather than physisorption. Near-edge X-ray absorption fine structure and nuclear magnetic resonance spectroscopy reveal that a variety of covalent bonds form between NH₃-N and PyOM, more than 10% of which contained heterocyclic structures. We estimate that through these mechanisms soil PyOM stocks could retain more than 600-fold annual NH₃ emissions from agriculture, exerting an important control on global N cycling.Rachel Hestrin, Dorisel Torres-Rojas, James J. Dynes, James M. Hook, Tom Z. Regier, Adam W. Gillespie, Ronald J. Smernik, Johannes Lehman

A benchmark quantum yield for water photoreduction on amorphous carbon nitride

Published online: August 14, 2017Amorphous carbon nitride (a-CN) is a less-explored but promising photocatalyst for hydrogen production. Despite an extended visible light absorption (EVLA) its low quantum efficiency (QE) for water photoreduction is a long standing problem. This implies that EVLA is not proportionally translated into collection of large amounts of photogenerated electrons. Minimizing the mismatch between light-absorption and charge-collection remains a scientific challenge. Here a sponge-like hierarchical structure of a-CN that addresses this apparent mismatch is reported. Combined experimental and finite difference time domain simulations demonstrate the ability of the a-CN sponge to induce scattering for total internal light reflection that promotes localized charge carrier generation. Diffused reflectance and transient fluorescence decay studies show good agreement with simulations with a 40% enhanced light-trapping and an ≈23 times longer electron lifetime in spongy a-CN compared with that of the bulk material. The result is a new high benchmark for hydrogen production of 203.5 µmol h⁻¹ with a QE of 6.1% at 420 nm in a reaction system of 10 vol% triethanolamine and 1 wt% Pt cocatalyst. The enhanced water photoreduction is a result of amenable photophysical and electrochemical attributes existing within the a-CN sponge.Mohammad Z. Rahman, Patrick C. Tapping, Tak W. Kee, Ronald Smernik, Nigel Spooner, Jillian Moffatt, Youhong Tang, Kenneth Davey, and Shi-Zhang Qia

Frequency Versus Quantity: Phenotypic Response of Two Wheat Varieties to Water and Nitrogen Variability

Due to climate change, water availability will become increasingly variable, affecting nitrogen (N) availability. Therefore, we hypothesised watering frequency would have a greater impact on plant growth than quantity, affecting N availability, uptake and carbon allocation. We used a gravimetric platform, which measures the unit of volume per unit of time, to control soil moisture and precisely compare the impact of quantity and frequency of water under variable N levels. Two wheat genotypes (Kukri and Gladius) were used in a factorial glasshouse pot experiment, each with three N application rates (25, 75 and 150mgNkg−1 soil) and five soil moisture regimes (changing water frequency or quantity). Previously documented drought tolerance, but high N use efficiency, of Gladius as compared to Kukri provides for potentially different responses to N and soil moisture content. Water use, biomass and soil N were measured. Both cultivars showed potential to adapt to variable watering, producing higher specific root lengths under low N coupled with reduced water and reduced watering frequency (48h watering intervals), or wet/dry cycling. This affected mineral N uptake, with less soil N remaining under constant watering × high moisture, or 48h watering intervals × high moisture. Soil N availability affected carbon allocation, demonstrated by both cultivars producing longer, deeper roots under low N. Reduced watering frequency decreased biomass more than reduced quantity for both cultivars. Less frequent watering had a more negative effect on plant growth compared to decreasing the quantity of water. Water variability resulted in differences in C allocation, with changes to root thickness even when root biomass remained the same across N treatments. The preferences identified in wheat for water consistency highlights an undeveloped opportunity for identifying root and shoot traits that may improve plant adaptability to moderate to extreme resource limitation, whilst potentially encouraging less water and nitrogen use

Determination of neo- and d-chiro-Inositol Hexakisphosphate in Soils by Solution 31P NMR Spectroscopy

The inositol phosphates are an abundant but poorly understood group of organic phosphorus compounds found widely in the environment. Four stereoisomers of inositol hexakisphosphate (IP6) occur, although for three of these (scyllo, flea, and D-chiro) the origins, dynamics, and biological function remain unknown, due in large part to analytical limitations in their measurement in environmental samples. We synthesized authentic neo- and n-chiro-IP6 and used them to identify signals from these compounds in three soils from the Falkland Islands. Both compounds resisted hypobromite oxidation and gave quantifiable P-31 NMR signals at delta = 6.67 ppm (equatorial phosphate groups of the 4-equatorial/2-axial conformer of neo-IP6) and delta = 6.48 ppm (equatorial phosphate groups of the 2-equatorial/4-axial conformer of D-chiro-IP6) in soil extracts. Inositol hexakisphosphate accounted for 46-54% of the soil organic phosphorus, of which the four stereoisomers constituted, on average, 55.9% (myo), 32.8% (scyllo), 6.1% (neo), and 5.2% (n-chiro). Reappraisal of the literature based on the new signal assignments revealed that neo- and D-chiro-IP6 occur widely in both terrestrial and aquatic ecosystems. These results confirm that the inositol phosphates can constitute a considerable fraction of the organic phosphorus in soils and reveal the prevalence of neo- and D-chiro-IP6 in the environment. The hypobromite oxidation and solution P-31 NMR spectroscopy procedure allows the simultaneous quantification of all four IP6 stereoisomers in environmental samples and provides a platform for research into the origins and ecological significance of these enigmatic compounds