Search

Ameliorating biological attributes of agricultural soils is desirable, and one avenue is introducing beneficial microbes via commercial biostimulant products. Although gaining popularity with farmers, scientific evaluation of such products in field-grown crops is often lacking. We tested two microbial products, Soil-Life™ and Nutri-Life Platform®, in a commercial sugarcane crop by profiling bacterial and fungal communities in soil and roots using high throughput phylogenetic marker gene sequencing. The products, one predominantly consisting of Lactobacillus and the other of Trichoderma, were applied as a mixture as per manufacturers’ instructions. Additives included in the formulations were not listed, and plots that did not receive the product mixture were the controls. The compositions of bacterial communities of soil and sugarcane roots, sampled 2, 5 and 25 weeks after application, were unaffected by the products. Soil fungal communities were also unaffected, but sugarcane roots had a greater relative abundance of three unidentified taxa in genera Marasmius, Fusarium and Talaromyces in the treated plots. Sugarcane yield was similar across all treatments that included a 25% lower nitrogen fertiliser rate. Further research must examine if the altered root fungal community is a consistent feature of the tested products, and if it conveys benefits. We conclude that putative biostimulants can be evaluated by analysing the composition of microbial communities. DNA profiling should be complemented by cost-benefit analysis to build a public information base documenting the effects of microbial biostimulants. Such knowledge will assist manufacturers in product development and farmers in making judicious decisions on product selection, to ensure that the anticipated benefits of microbial biostimulants are realised for broad acre cropping.

Soil extracellular enzyme activity (EEA) represents a critical bottleneck in the release of bioavailable nutrients from organic materials. However, quantifying spatial and temporal dynamics of EEA remains challenging. Techniques which measure the activity of, or directly sample free enzymes in situ may assist in understanding the short-term exoproteomic responses of microbes and roots to substrates, but few tools exist to explore EEA with minimal disturbance. We explore the potential of in situ microdialysis to directly sample soil enzymes, measuring their activity using a modified enzyme assay. We hypothesise that the technique's bias towards free solutes will also allow differentiation of free and stabilised enzyme pools. As little is known about the efficiency of microdialysis to sample enzymes from soil, recovery of a protease standard was quantified from solution and soil, finding that enzyme recovery is hindered at lower soil moisture contents. We further measured the response of native protease activity after the addition of soybean litter to clay and sandy soils, finding microdialysis observed greater EEA in litter-amended treatments than controls in both soil types. In comparison, EEA as measured by conventional extraction-incubation methods was only greater in amended clay soils. In a final experiment, hydrolytic enzyme activity of free and stabilised clay soil fractions were estimated using microdialysis. Free enzymes contributed 9% of total hydrolytic activity in soil without litter, increasing to 46% in litter-amended soil, suggesting fresh litter promoted a transient increase in the production of free exoenzymes by soil microbes. In contrast, the addition of litter had no significant effect on stabilised EEA. In spite of the obvious challenges involved in applying microdialysis as a method for soil protein sampling, this method offers new possibilities for investigating challenging spatial and temporal aspects of enzyme dynamics and protein availability in soils.

Soil incubation studies commonly use archived, dry stored soil samples. Numerous studies have detected a profound impact of dry storage on soil microbial community composition and activity. However, comparatively few studies have explored the impact of storage on the short-term mineralisation of soil organic matter (SOM) after moistening dry soils to revive and stabilise microbial activity (“pre-incubation”). We examined whether pre-incubation of dry stored soil of contrasting texture (Alfisol and Vertisol) can recover native soil microbial functions in fresh soil or soil stored for 3 weeks, 3 or 36 years. The oldest samples had a delayed CO2 response after moistening, but matched CO2 production rates of younger samples within hours. The two soil types displayed contrasting CO2 emission responses after moistening; the Alfisols had a rapid but short-lived CO2 emission peak (the so-called “Birch Effect”) while the Vertisols demonstrated a long, sustained release of CO2 reflecting its higher abundance of microsite abundance and microaggregates which may protect SOM and microorganisms during adverse conditions like drying. After 10 days of pre-incubation, older Alfisol samples had higher protease activity and response to amino acid addition demonstrating altered microbial physiological responses relating to nitrogen (N) compared to fresher counterparts. However, such responses in Vertisols were unchanged suggesting high clay soils with high potential microsite abundance may improve preservation of soil N functions through dry storage or even drought. This work highlights both limitations and possibilities of incubating dry archived soils for SOM mineralisation studies.

Delivering nutrients from mineral or organic fertilizers out of synchrony with crop uptake causes inefficiencies and pollution. We explore methodologies for evaluating sorbents as additives to organic agricultural wastes to retain nitrogen in an exchangeable form and deliver at rates that approximate the uptake capacity of roots. Focussing on ammonium (NH4+) as the main inorganic nitrogen form in the studied wastes (sugarcane mill mud, poultry litter), we tested geo-sorbents and biochar for their ability to retain NH4+. Sorption capacity was ranked palagonite

Soil organic matter (SOM) is crucial to soil health, supporting most of the soil properties that benefit plant growth and ecosystem services including carbon sequestration, nutrient recycling and water infiltration. Recent study is exposing the soil microbial community as not only decomposing SOM, but also providing the primary source of chemicals for its formation. All else equal, SOM formation is theoretically greatest when microbes maximise enzyme efficiency (ratio of enzyme activity to carbon loss from respiration) and biomass efficiency (biomass gain per unit substrate added). Our study examines the relationship between microbial composition and metrics of these two efficiencies. We hypothesised that both will increase with higher ratios of soil fungi to bacteria. We manipulated microbial composition through sustained use of selective microbial inhibitors in microcosms with tropical soil, alongside variation of litter quality and diversity and the presence or absence of a simulated root exudate. Both litter and inhibitor treatments significantly changed soil microbial composition and function, and enzyme efficiency and biomass efficiency were both higher in microbial communities with more fungi, supporting our hypothesis. Structural equation modelling suggested that the observed efficiency changes did indeed occur in part via changes in microbial composition after accounting for direct effects of treatments. Taken together the results provide some support for the hypothesis that soil fungi benefit SOM formation.

Microdialysis is a technique that can be used to sample fluxes of nitrogen (N) in soils with minimal disturbance. To advance our understanding of the technique and improve N recovery, we compared a common membrane type (10 x 0:5 mm probe length and width, 20 kDa molecular weight cut-off; MWCO) with alternative length and MWCO configurations (30 mm; and 100 kDa MWCO). We hypothesised that the alternative membranes would improve recovery of low molecular weight N via increased surface area and membrane pore size. The test environments, sampled at fixed pump flow rates, were: (i) stirred 100 mu M N standard solution containing organic (amino acid) and inorganic (ammonium, nitrate) N; (ii) soil spiked with 100 mu M standard N solution; and (iii) in situ boreal forest soil. In general, long membranes recovered more N, but the magnitude of improved recovery varied with test environment. Long membranes recovered more inorganic N regardless of flow rate, except ammonium in stirred solution, where length had no effect at slow flow rates. Long membranes also recovered more organic N from stirred solution regardless of flow rate, and recovered most N at slow flow rates in spiked soil. Longer membranes recovered more amino acids in situ in forest soil, with improved resolution of individual amino acids, but were biased towards soluble, mobile forms. MWCO did not affect N recoveries, indicating that in the test conditions, membrane length had greater control than pore size. We discuss the bottlenecks of microdialysis application in soil research and conclude that optimised membrane configurations will advance its use as a tool for quantifying nutrient fluxes in soils. (C) 2017 Elsevier Ltd. All rights reserved.

This study sought to identify whether piggery effluent-derived nitrogen sources can be formulated with urea and nitrification inhibitors to better synchronize nitrogen (N) supply with crop demand than conventional urea fertilizer alone. A 288 pot pasture growth and leaching growth accelerator trial (5 pasture cuts) was completed with a factorial treatment structure of three N sources (2.63 g N [kg soil]−1 applied as 100% urea-N, 8% struvite-N + 92% urea-N, and 8% piggery pond sludge-N + 92% urea-N), five rates of three nitrification inhibitors (including 3,4-Dimethylpyrazole phosphate, DMPP; limonene+ethanol; and dicyandiamide, DCD), and matrix encapsulated forms of these inhibitors. Applying a combination of piggery sludge with urea increased N uptake during the first 4 weeks of plant growth (by 65%), though total N uptake throughout the trial (22 weeks) did not differ across the N-sources. The microbial community of the soil to which the sludge was added was significantly different from the un-amended soil at the conclusion of the trial. All inhibitor formulations significantly decreased leaching losses of mineral-N relative to the control (by 14 to 61%). The use of DMPP decreased initial nutrient uptake, deferring uptake until later in the experiment. Inhibitor addition resulted in microbial community effects that persisted throughout the trial. The study demonstrated that a piggery-derived N-source and a nitrification inhibitor can be used to manipulate plant N uptake to occur later or earlier in a growing period with equal cumulative uptake, achieving an 11% increase in residual N store, and decreased N leaching losses.

Microdialysis is emerging as a sensitive tool in environmental sciences, allowing for in situ sampling of solutes with minimal disturbance to soil environments. This perspective presents the theoretical foundations and practical applications of the technique with a focus on its use in soil, microbial and plant sciences. Using small probes (usually 0.5 mm diameter) fitted with permeable membranes, soil solutes are sampled, so that solute flux (and potentially concentration) can be calculated at root-relevant scales. However, physical and biological characteristics of soil systems impose challenges to sampling efficiencies with microdialysis, particularly where solute concentrations are low. Experimental parameters such as perfusate flow rates, probe design and sampling times can also influence the findings, and reduce the comparability between studies. We explore how equipment setup and experimental conditions can be optimised for soil environments, and how standardisation and calibration techniques may improve cross-study comparability. We show that the technique's strength lies in the ability to integrate many soil factors into a biologically relevant measure of solute availability. Microdialysis has so far provided new insight regarding the bioavailability of soil nitrogen (N) and phosphorus (P), where fluxes have been related to root uptake rates and gaseous fluxes from soil. The technique has also been used as a root simulator, mimicking exudation of organic acids to mobilise soil P, and to measure the potential contribution of transpiration-induced mass flow on N availability at the root surface. With further development, and paired with sensitive analytical methods and equipment, microdialysis has much potential to explore fragile and dynamic soil processes in biologically-active zones such as the rhizosphere, and could contribute to solving challenges relating to under- and oversupply of nutrients to plants. We conclude that, with further advancements and critical evaluation, microdialysis could become an important instrument in the soil analysis toolkit.

Improvement in fertilizer use efficiency is a key aspect for achieving sustainable agriculture in order to minimize costs, greenhouse gas emissions, and pollution from nutrient run-off. To optimize root architecture for nutrient uptake and efficiency, we need to understand what the roots encounter in their environment. Traditional methods of nutrient sampling, such as salt extractions can only be done at the end of an experiment, are impractical for sampling locations precisely and give total nutrient values that can overestimate the nutrients available to the roots. In contrast, microdialysis provides a non-invasive, continuous method for sampling available nutrients in the soil. Here, for the first time, we have used microCT imaging to position microdialysis probes at known distances from the roots and then measured the available nitrate and ammonium. We found that nitrate accumulated close to roots whereas ammonium was depleted demonstrating that this combination of complementary techniques provides a unique ability to measure root-available nutrients non-destructively and in almost real time.

Cotton cropping systems in Australia have poor nitrogen (N) use efficiency, largely due to over-application of N fertiliser. The N mineralisation from soil organic N reserves is often overlooked, or underestimated despite recent studies indicating that it may contribute the majority of N exported with the crop. Predicting N mineralisation is a major challenge for agricultural industries worldwide, as direct measurements are time-consuming and expensive, but there is considerable debate as to the most reliable methods for indirect estimation. Additionally, laboratory incubations assess potential (rather than actual) mineralisation, and may not be representative of N cycling rates in the field. We collected 177 samples from most major Australian cotton growing regions, and assessed their mineralisation potential using ex situ laboratory incubations, along with an assessment of potential indicators routinely measured in soil nutrient tests. Additionally, at three unfertilised sites we conducted in situ assessment of mineralisation by quantifying soil N at the beginning of the growing season, and soil and crop N at the end of the season. We found that Australian cotton cropping soils had substantial mineralisation potential, and that soil total N and total carbon were correlated with mineralisation, and have potential to be used for prediction. Other potential indicators such as carbon dioxide production and ammonium and nitrate concentrations were not correlated with mineralisation. In parallel studies of ex situ and in situ mineralisation, we found ex situ laboratory incubations overestimated mineralisation by 1.7 times on average. We discuss findings in terms of management implications for Australian cotton farming systems.