Ann McNeill

Acquisition of phosphorus and nitrogen in the rhizosphere and plant growth promotion by microorganisms

The rhizosphere is a complex environment where roots interact with physical, chemical and biological properties of soil. Structural and functional characteristics of roots contribute to rhizosphere processes and both have significant influence on the capacity of roots to acquire nutrients. Roots also interact extensively with soil microorganisms which further impact on plant nutrition either directly, by influencing nutrient availability and uptake, or indirectly through plant (root) growth promotion. In this paper, features of the rhizosphere that are important for nutrient acquisition from soil are reviewed, with specific emphasis on the characteristics of roots that influence the availability and uptake of phosphorus and nitrogen. The interaction of roots with soil microorganisms, in particular with mycorrhizal fungi and non-symbiotic plant growth promoting rhizobacteria, is also considered in relation to nutrient availability and through the mechanisms that are associated with plant growth promotion.

The X-factor: visualizing undisturbed root architecture in soils using X-ray computed tomography

Although roots play a crucial role in plant growth and development through their acquisition and delivery of water and nutrients to the above-ground organs, our understanding of how they interact with their immediate soil environment largely remains a mystery as the opaque nature of soil has prevented undisturbed in situ root visualization (Perret et al., 2007). However, new developments in non-invasive techniques such as X-ray computed tomography (CT) provide, for the first time, an exciting opportunity to examine detailed root architecture in three dimensions (3-D) in undisturbed soil cores (Fig. 1). Although other non-invasive 3-D visualization procedures exist, X-ray CT is viewed as the most appropriate technique for studies of soil:root interactions as the presence of iron and manganese ions may provide interference when alternative techniques such as Nuclear Magnetic Resonance (NMR) are used (Heeraman et al., 1997). Detailed understanding of interactions between roots and their immediate soil environment is vital when considering issues such as land degradation as soil structure is a primary factor determining the availability of edaphic resources such as water and nutrients (Lynch, 1995), and is extrinsically linked to plant productivity (Moran et al., 2000). In view of the rapidly increasing human global population and the threat posed by climate change, maximizing crop yields and developing sustainable soil management strategies are vital for food security. X-ray CT overcomes some of the limitations associated with previous methodologies for studying roots by providing Fig. 1. X-ray CT image of roots of a 3-week-old Zea mays (L.) plant grown in a soil column (loamy sand, Newport series). 1 pixel1⁄444 lm.

Net nitrogen balances for cool-season grain legume crops and contributions to wheat nitrogen uptake: a review

The removal of nitrogen (N) in grain cereal and canola crops in Australia exceeds 0.3 million t N/year and is increasing with improvements in average crop yields. Although N fertiliser applications to cereals are also rising, N2-fixing legumes still play a pivotal role through inputs of biologically fixed N in crop and pasture systems. This review collates Australian data on the effects of grain legume N2 fixation, the net N balance of legume cropping, summarises trends in the soil N balance in grain legume–cereal rotations, and evaluates the direct contribution of grain legume stubble and root N to wheat production in southern Australia. The net effect of grain legume N2 fixation on the soil N balance, i.e. the difference between fixed N and N harvested in legume grain (Nadd) ranges widely, viz. lupin –29–247 kg N/ha (mean 80), pea –46–181 kg N/ha (mean 40), chickpea –67–102 kg N/ha (mean 6), and faba bean 8–271 kg N/ha (mean 113). Nadd is found to be related to the amount (Nfix) and proportion (Pfix) of crop N derived from N2 fixation, but not to legume grain yield (GY). When Nfix exceeded 30 (lupin), 39 (pea) and 49 (chickpea) kg N/ha the N balance was frequently positive, averaging 0.60 kg N/kg of N fixed. Since Nfix increased with shoot dry matter (SDM) (21 kg N fixed/t SDM; pea and lupin) and Pfix (pea, lupin and chickpea), increases in SDM and Pfix usually increased the legume’s effect on soil N balance. Additive effects of SDM, Pfix and GY explained most (R2 = 0.87) of the variation in Nadd. Using crop-specific models based on these parameters the average effects of grain legumes on soil N balance across Australia were estimated to be 88 (lupin), 44 (pea) and 18 (chickpea) kg N/ha. Values of Nadd for the combined legumes were 47 kg N/ha in south-eastern Australia and 90 kg N/ha in south-western Australia. The average net N input from lupin crops was estimated to increase from 61 to 79 kg N/ha as annual rainfall rose from 445 to 627 mm across 3 shires in the south-east. The comparative average input from pea was 37 to 47 kg N/ha with least input in the higher rainfall shires. When the effects of legumes on soil N balance in south-eastern Australia were compared with average amounts of N removed in wheat grain, pea–wheat (1:1) sequences were considered less sustainable for N than lupin–wheat (1:1) sequences, while in south-western Australia the latter were considered sustainable. Nitrogen mineralised from lupin residues was estimated to contribute 40% of the N in the average grain yield of a following wheat crop, and that from pea residues, 15–30%; respectively, about 25 and 15 kg N/ha. Therefore, it was concluded that the majority of wheat N must be obtained from pre-existing soil sources. As the amounts above represented only 25–35% of the total N added to soil by grain legumes, the residual amount of N in legume residues is likely to be important in sustaining those pre-existing soil sources of N.

Stable isotope techniques in the study of biological processes and functioning of ecosystems.

1. Fundamentals of Stable Isotope Chemistry and Measurement T.E. Dawson, P.D. Brooks. 2. Carbon Isotope Discrimination and Plant Water-Use Efficiency: Case Scenarios for C3 Plants J.S. Pate. 3. Extraction and Analysis of Plant Water for Deuterium Isotope Measurement and Application to Field Experiments J.V. Turner, et al. 4. The Use of Stable Isotopes of Water for Determining Sources of Water for Plant Transpiration G. Walker, et al. 5. What do delta15N Signatures tell Us about Nitrogen Relations in Natural Ecosystems? G. Stewart. 6. Assessing N2 Fixation in Annual Legumes using 15N Natural Abundance M. Unkovich, J.S. Pate. 7. The Use of 15N to study Biological Nitrogen Fixation by Perennial Legumes M.B. Peoples, et al. 8. Source/Sink Interactions in Crop Plants: Application of 13CO2 and Urea-15N Techniques in Quantitative Analysis J.A. Palta. 9. Use of Enriched 15N Sources to study Soil N Transformations I.R.P. Fillery, S. Recous. 10. Stable Isotope Techniques using Enriched 15N and 13C for Studies of Soil Organic Matter Accumulation and Decomposition in Agricultural Systems A. McNeill. 11. Source Identification in Marine Ecosystems: Food Web Studies using delta13C and delta15N A.J. Smit. 12. delta13C as an Indicator of Palaeoenvironments: A Molecular Approach K. Grice. Index.

Non-destructive quantification of cereal roots in soil using high-resolution X-ray tomography

One key constraint to further understanding plant root development is the inability to observe root growth in situ due to the opaque nature of soil. Of the present non-destructive techniques, computed tomography (CT) is best able to capture the complexities of the edaphic environment. This study compared the accuracy and impact of X-ray CT measurement of in situ root systems with standard technology (soil core washing and WinRhizo analysis) in the context of treatments that differed in the vertical placement of phosphorus fertilizers within the soil profile. Although root lengths quantified using WinRhizo were 8% higher than that observed in the same plants using CT, measurements of root length by the two methodologies were highly correlated. Comparison of scanned and unscanned plants revealed no effect of repeated scanning on plant growth and CT was not able to detect any changes in roots between phosphorus treatments that was observed using WinRhizo. Overall, the CT technique was found to be fast, safe, and able to detect roots at high spatial resolutions. The potential drawbacks of CT relate to the software to digitally segment roots from soil and air, which will improve significantly as automated segmentation algorithms are developed. The combination of very fast scans and automated segmentation will allow CT methodology to realize its potential as a high-throughput technique for the quantification of roots in soils.

Subsoil constraints to crop production on neutral and alkaline soils in south-eastern Australia: a review of current knowledge and management strategies

Crop yield variability and productivity below potential yield on neutral and alkaline soils in the semiarid Mediterranean-type environments of south-eastern Australia have been attributed, in part, to variable rooting depth and incomplete soil water extraction caused by physical and chemical characteristics of soil horizons below the surface. In this review these characteristics are referred to as subsoil constraints. This document reviews current information concerning subsoil constraints typical of neutral and alkaline soils in south-eastern Australia, principally salinity, sodicity, dense soils with high penetration resistance, waterlogging, nutrient deficiencies and ion toxicities. The review focuses on information from Australia (published and unpublished), using overseas data only where no suitable Australian data is available. An assessment of the effectiveness of current management options to address subsoil constraints is provided. These options are broadly grouped into three categories: (i) amelioration strategies, such as deep ripping, gypsum application or the use of polyacrylamides to reduce sodicity and/or bulk density, deep placement of nutrients or organic matter to overcome subsoil nutrient deficiencies or the growing of ‘primer’ crops to naturally ameliorate the soil; (ii) breeding initiatives for increased crop tolerance to toxicities such as salt and boron; and (iii) avoidance through appropriate agronomic or agro-engineering solutions. The review highlights difficulties associated with identifying the impact of any single subsoil constraint to crop production on neutral and alkaline soils in south-eastern Australia, given that multiple constraints may be present. Difficulty in clearly ranking the relative effect of particular subsoil constraints on crop production (either between constraints or in relation to other edaphic and biological factors) limits current ability to develop targeted solutions designed to overcome these constraints. Furthermore, it is recognised that the task is complicated by spatial and temporal variability of soil physicochemical properties and nutrient availability, as well as other factors such as disease and drought stress. Nevertheless, knowledge of the relative importance of particular subsoil constraints to crop production, and an assessment of impact on crop productivity, are deemed critical to the development of potential management solutions for these neutral to alkaline soils.

Long-term effects of crop rotation, stubble management and tillage on soil phosphorus dynamics

The effects of various management practices on soil phosphorus (P) dynamics were investigated in a field experiment in New South Wales, Australia, during 24 years of different crop rotation, stubble management, and tillage treatments. Topsoil samples collected at the beginning of the trial and after 6, 12, 18, and 24 years were analysed for resin-extractable P, inorganic and organic P, and total P. According to the calculated P input–output budget, 9–14 of the 20 kg P/ha added as superphosphate annually remained in the system, depending on the treatment. The measured increase in total P in 0–0.20 m did not differ between treatments, showing an accumulation rate of only 9 ± 2 kg P/ha.year. These results suggest a loss of 4 ± 2 kg P/ha.year, presumably into lower soil layers. Resin-extractable P at 0–0.10 m increased by 1.7 kg P/ha.year, irrespective of the treatment. The increase in total P after 24 years was almost completely accounted for by the increase in total extractable inorganic P. Changes in organic P paralleled changes in organic carbon, with a significant loss in treatments with stubble burning (wheat–lupin rotation and continuous wheat), and a significant accumulation in a wheat–subterranean clover rotation with stubble retention and direct drilling. We conclude that on the time scale of this experiment, the dynamics of carbon and organic P are closely linked.

The Nitrogen Cycle in Terrestrial Ecosystems

The terrestrial nitrogen (N) cycle comprises soil, plant and animal pools that contain relatively small quantities of biologically active N, in comparison to the large pools of relatively inert N in the lithosphere and atmosphere, but that nevertheless exert a substantial influence on the dynamics of the global biogeochemical N cycle. After carbon (ca. 400 g kg−1) and oxygen (ca. 450 g kg−1), N is the next most abundant element in plant dry matter, typically 10–30 g kg−1. It is a key component of plant amino and nucleic acids, and chlorophyll, and is usually acquired by plants in greater quantity from the soil than any other element. Plant N provides the basis for the dietary N (protein) of all animals, including humans.

Whole plant response of crop and weed species to high subsoil boron

Within the semi-arid region of south-eastern Australia, high levels of subsoil boron (B) in alkaline soil can limit production of dryland crops. The aim of this research was to investigate the whole plant response to a range of subsoil-extractable B concentrations for a number of crop and weed species common to agricultural areas of South Australia. Specifically, the objectives were to determine (a) the morphological response of the entire root system to high subsoil B and (b) the available B concentrations in subsoil critical for expression of shoot traits commonly used in selection of B tolerance. Barley grass (Hordeum glaucum L.), crop barley (Hordeum vulgare) variety Clipper and breeders’ line VB9953, fababean (Vicia faba var. Fjiord), Lincoln weed (Diplotaxis tenuifolia L.), prickly lettuce (Lactuca serriola), and evening primrose (Oenothera stricta L.) were grown in sealed PVC cylinders (500 mm deep by 150 mm diam.) containing a sandy soil. The concentration of extractable B in the topsoil (0–0.20 m), considered non-toxic, was 0.5 mg/kg for all cylinders but a range of B treatments (0.5, 2.4, 4.3, 6.8, or 12.2 mg/kg) was applied directly to the subsoil (0.30–0.50 m). Increasing the concentration of extractable B in the subsoil decreased root dry weight in this region, but did not reduce water use from subsoil by barley grass or evening primrose. The response of the roots in the topsoil and subsequent responses in the shoot also differed among species. Symptoms of B toxicity in shoots of all the species were observed at subsoil-extractable B concentrations of 12.2 mg/kg and at lower concentrations in some of the crop and weed species. Shoot growth, total water use, and root growth in topsoil of Clipper and Lincoln weed were severely impaired by high subsoil-extractable B, as was topsoil root growth in evening primrose, with the reduction in the weed species being mostly associated with a decrease in taproot dry weight. Barley grass, VB9953, evening primrose, and to a lesser extent fababean and prickly lettuce, maintained shoot growth at all subsoil-extractable B concentrations, despite a reduction in subsoil water use by VB9953. Prickly lettuce and VB9953 also sustained root growth in the topsoil whilst fababean and barley grass increased root growth in the topsoil in response to high subsoil extractable B. There was no direct relationship between the quantity of B accumulated in shoots and detrimental effects on growth. Furthermore, there appeared to be no direct relationship between water uptake and B uptake since irrespective of the effect of subsoil B on either subsoil or total water use, shoot B concentration increased in all the species/genotypes as subsoil B increased. The degree to which plants were deemed to exhibit tolerance was, therefore, highly dependent upon the trait used for assessment. One suggestion in the current study is that shoot dry matter in B toxic soil can be a consistent parameter for considering varieties for tolerance to B toxicity.

Waterlogging in Australian agricultural landscapes: a review of plant responses and crop models

Abstract. This review summarises reported observations of the effects of waterlogging on agricultural production in Australia and briefly discusses potential remediation strategies. Inconsistencies are demonstrated in the current indicators used for assessment of waterlogging potential across agricultural landscapes as well as in parameters measured in waterlogging studies. It is suggested that predictions of waterlogging potential for landscapes should be based on a minimum dataset that includes pedological, topographical, and climate data for the defined area, as well as observations of plant morphological appearance and visible surface water. The review also summarises the effects of low oxygen concentration in soil on rhizosphere processes, and discusses evidence for direct effects on plant physiology of reductions in soil oxygen caused by waterlogging. Finally, the review describes current crop growth, water use, and yield simulation models used in Australia (SWAGMAN, DRAINMOD, and APSIM) that incorporate waterlogging stress. It is suggested that there is scope for modifications to these models based on recent improved understanding of plant physiological responses to waterlogging and on further research. The review concludes that improvements in modelling waterlogging outcomes to assist growth and yield predictions should ultimately enhance management capacity for growers.