Mycorrhizal Symbioses of Cotton Grown on Sodic Soils: A Review from an Australian Perspective
The majority of terrestrial plants form some type of mycorrhizal symbiosis. This established symbiosis therefore exists in most commercially important crops, which includes cotton. Arbuscular mycorrhizal fungus (AMF) can colonise 50%–90% of cotton root length under field and controlled conditions. Mycorrhization improves growth and nutrient uptake (especially phosphorus) of cotton, particularly at the early growth stages. Mycorrhizal symbioses help plants to counter the stresses imposed by physical and chemical soil constraints; however, adverse environmental conditions may restrict the mycorrhizal associations and consequently may reduce nutrient uptake and impair plant growth. In Australia, cotton is mainly grown on sodic soils that contain more than 6% of the total cations as exchangeable sodium. High levels of sodium in the soil create adverse physical and chemical soil conditions that may negatively affect mycorrhizal symbioses of cotton. This review discusses the cotton mycorrhizal colonisation, plant growth, and disease protection effects, potential negative effects of physical and chemical properties of sodic soils, and influences of some agronomic management practices. In addition, the research gaps were identified and some practical applications of the research outcomes were suggested.
Spatial variability of soil phosphorus in grazing systems
Phosphorus (P) use efficiency has been identified as a key issue for Australian grazing systems. This project examined the spatial variability in soil P concentration from two separate surveys of grazed pasture fields. A field on the central tablelands of NSW had a range in Bray P of 1.2 to 140 mg/kg and a COV of 107%. The other field on the northern tablelands of NSW reported a range in Colwell P from 13.0 to 121.1 mg/kg and a COV of 59%. Maps of the spatial variability of soil P demonstrated that there is a relationship with field elevation. Application of critical P values to both fields enabled an estimation of the value of site specific fertiliser management. For one field, fertiliser inputs could potentially be isolated to 37% and the other 56% if nutrient additions were targeted at responsive areas. The opportunity for increased fertiliser use efficiency through site specific management (SSM) warrants further investigation. Research is required into both the value of SSM and the techniques that might enable the development of this strategy.
Application of X-ray computed tomography to quantify fresh root decomposition in situ
Background and aims: Much of our understanding of plant root decomposition and related carbon cycling come from mass loss rates calculated from roots buried in litter bags. However, this may not reflect what actually happens in the soil, where the interactions between root and soil structure presents a more complex physico-chemical environment compared to organic matter isolated in a porous bag buried in disturbed soil. This work investigates the potential of using X-ray micro-computed tomography (CT) to measure root decomposition in situ. Methods: Roots of 'Vicia faba' L. were excised from freshly germinated seeds, buried in re-packed soil cores and cores incubated for 60 days. Changes in root volume and surface area were measured using repeated scans. Additional samples were destructively harvested and roots weighed to correlate root mass with root volume. The method was further applied to an experiment to investigate the effects of soil bulk density and soil moisture on root decomposition. Results: Root volume (X-ray CT) and root mass (destructive harvest) decreased by 90 % over the 60 day incubation period, by which stage, root volume and mass had stabilised. There was a strong correlation (R² = 0.97) between root volume and root mass. Conclusions: X-ray CT visualization and analysis provides a unique toolbox to understand root decomposition in situ.
Ecological Succession, Hydrology and Carbon Acquisition of Biological Soil Crusts Measured at the Micro-Scale
The hydrological characteristics of biological soil crusts (BSCs) are not well understood. In particular the relationship between runoff and BSC surfaces at relatively large (>1 m²) scales is ambiguous. Further, there is a dearth of information on small scale (mm to cm) hydrological characterization of crust types which severely limits any interpretation of trends at larger scales. Site differences and broad classifications of BSCs as one soil surface type rather than into functional form exacerbate the problem. This study examines, for the first time, some hydrological characteristics and related surface variables of a range of crust types at one site and at a small scale (sub mm to mm). X-ray tomography and fine scale hydrological measurements were made on intact BSCs, followed by C and C isotopic analyses. A 'hump' shaped relationship was found between the successional stage/sensitivity to physical disturbance classification of BSCs and their hydrophobicity, and a similar but 'inverse hump' relationship exists with hydraulic conductivity. Several bivariate relationships were found between hydrological variables. Hydraulic conductivity and hydrophobicity of BSCs were closely related but this association was confounded by crust type. The surface coverage of crust and the microporosity 0.5 mm below the crust surface were closely associated irrespective of crust type. The δ ¹³C signatures of the BSCs were also related to hydraulic conductivity, suggesting that the hydrological characteristics of BSCs alter the chemical processes of their immediate surroundings via the physiological response (C acquisition) of the crust itself. These small scale results illustrate the wide range of hydrological properties associated with BSCs, and suggest associations between the ecological successional stage/functional form of BSCs and their ecohydrological role that needs further examination.
Field application of a DNA-based assay to the measurement of roots of perennial grasses
Background and aims: DNA-based methods present new opportunities for overcoming the difficulties of accurately identifying and quantifying roots of different plant species in field soils. In order to quantify species-specific root biomass from measurements of DNA, consideration needs to be given to replication and ability to recover roots for calibration purposes in order to account for spatial, temporal and inter- and intra-species variation in DNA content of roots and distribution of roots within the soil profile. Methods: This paper develops the field application of a DNA-based technique for direct quantification of roots in soils. The method was applied to a field experiment to investigate differences in root growth of acid-soil resistant and sensitive genotypes of perennial pasture grasses in an acid soil. DNA was extracted directly from soil and species-specific DNA was quantified using quantitative real-time PCR prior to estimation of root biomass. Results: Root growth of the perennial grasses was quantified using the DNA-based technique, although separate calibration procedures were needed to convert DNA content to root mass for each species, soil layer and sampling date. Compared to acid-soil resistant genotypes, lesser root growth in acid soil layers and reduced above-ground dry matter production was observed for acid-soil sensitive genotypes. Conclusions: The DNA-based method allowed genotypic differences in root growth to be assessed directly in soil and was advantageous for rapid processing of a large number of samples. However, high replication was still required to overcome spatial variability and separate calibrations were required for different species and soil depths across sampling times. The technique demonstrated greater root growth of acid-soil resistant perennial grasses which was beneficial for their establishment and persistence.
Using common PA tools and GPS livestock tracking to examine the variability in soil nutrients across grazing landscapes
Precision Agriculture (PA) is changing how producers manage their land. PA involves the use of sensors and management strategies that target the spatial and temporal variability that occurs across a landscape. The introduction of PA has increased profitability and resource use efficiency across many agricultural systems and is now widely applied in cropping and horticultural enterprises. However, development of PA strategies for grazing systems has largely been ignored, possibly due to the complex relationships that exist when considering soil, plant and animal interactions across variable pastoral landscapes. There is a growing interest in the potential of PA management strategies, for example Site Specific Nutrient Management (SSNM) to assist in increasing the fertiliser use efficiency in grazing systems (Simpson et al., 2011). Technologies such as soil EM38 mapping and plant vigour sensors (Crop Circle - Active Optical Sensor) have been extensively used in PA cropping operations and these tools offer some ability to monitor the soil and plant systems in a pasture. The more recent development of GPS livestock tracking has now unlocked the ability to monitor the spatial and temporal variability of the animal component of a grazing system. The integration of these technologies holds significant potential in providing an understanding of how grazing systems vary and how this variability can be managed, particularly through SSNM. This study aims to investigate how common PA tools such as soil EM38 and plant vigour sensors along with GPS tracking information from livestock can be used to understand the spatial distribution of soil nutrients in grazing systems. It is anticipated that this information will lead to an understanding of how producers can zone pasture paddocks to apply SSNM strategies in a similar way to what is currently applied in cropping systems.
Root hair length and rhizosheath mass depend on soil porosity, strength and water content in barley genotypes
Selecting plants with improved root hair growth is a key strategy for improving phosphorus-uptake efficiency in agriculture. While significant inter- and intraspecific variation is reported for root hair length, it is not known whether these phenotypic differences are exhibited under conditions that are known to affect root hair elongation. This work investigates the effect of soil strength, soil water content (SWC) and soil particle size (SPS) on the root hair length of different root hair genotypes of barley. The root hair and rhizosheath development of five root hair genotypes of barley ('Hordeum vulgare' L.) was compared in soils with penetrometer resistances ranging from 0.03 to 4.45 MPa (dry bulk densities 1.2-1.7 g cm⁻³). A "short" (SRH) and "long" root hair (LRH) genotype was selected to further investigate whether differentiation of these genotypes was related to SWC or SPS when grown in washed graded sand. In low-strength soil (<1.43 MPa), root hairs of the LRH genotype were on average 25 % longer than that of the SRH genotype. In high-strength soil, root hair length of the LRH genotype was shorter than that in low-strength soil and did not differ from that of the SRH genotype. Root hairs were shorter in wetter soils or soils with smaller particles, and again SRH and LRH did not differ in hair length. Longer root hairs were generally, but not always, associated with larger rhizosheaths, suggesting that mucilage adhesion was also important. The root hair growth of barley was found to be highly responsive to soil properties and this impacted on the expression of phenotypic differences in root hair length. While root hairs are an important trait for phosphorus acquisition in dense soils, the results highlight the importance of selecting multiple and potentially robust root traits to improve resource acquisition in agricultural systems.
Effect of plant density on yield and root traits of two Trifolium subterraneum cultivars
Trifolium subterraneum is the most widely sown annual pasture legume in the P-deficient soils of southern Australia. Controlled-environment studies have demonstrated that variation exists between genotypes of this legume to acquire P and yield in low-P soils, and there appears to be a plant density effect on these traits. However, the magnitude of this effect is largely unknown. Two cultivars of T. subterraneum, that differ significantly for the aforementioned traits when using the same sowing rate, were grown to determine differences in shoot growth, P uptake and root traits with changing plant density. Microswards of both cultivars were grown at five plant densities and five P levels. Yield and P content of shoots and roots were determined after 5 weeks growth. Root samples were assessed for diameter, length and root hair length. Shoot dry mass of both cultivars increased in response to increasing P supply and increasing plant density. Differences between the cultivars for shoot yield were most pronounced at low plant densities and diminished as plant density increased. This response was particularly evident at lower soil-P levels, whereas maximum yield was relatively independent of plant density in the high-P soil. In contrast, differences between cultivars for root morphological traits such as specific root length were maintained regardless of plant density. The results demonstrate that plant density effects sward P-acquisition and hence shoot yield achieved in the P-deficient soil. Accurate screening for P-acquisition and shoot yield across the T. subterraneum genome therefore requires a uniform plant density comparable to densities observed in the field. The identification of T. subterraneum cultivars capable of improved growth in low-P soils would improve P-use efficiency in Australian soils which are often P-deficient and require annual applications of P fertiliser for high yields. This would consequently lead to greater resilience of the agricultural sector.
Root proliferation in response to P stress and space: implications for the study of root acclimation to low P supply and P acquisition efficiency
Aims The experiment was conducted to understand how root morphological traits contributed to the contrasting P-acquisition efficiencies of two Trifolium subterraneum cultivars.
Methods Phosphorus acquisition, root length proliferation and root acclimation to P deficiency were investigated by varying the P supply (20-300 mg P kg−1) and planting density (1-12 plants pot−1) of clover micro-swards.
Results The shoot yield of both cultivars increased in response to increased P supply and increased planting density. Higher planting densities increased shoot yield by reducing the time to leaf canopy closure. In P-deficient soil, the ‘less P-efficient’ cultivar acquired less P and yielded relatively poorly compared with the ‘more P-efficient’ cultivar when planting density was low (<6 plants pot−1). However, the P acquisition and yields of the cultivars were equivalent when planting density was high (~6-12 plants pot−1). Both cultivars proliferated nutrient-foraging roots in response to P limitation and space (i.e. lower planting densities). However, the proliferation responses of the cultivars differed, particularly in response to space.
Conclusions Differential proliferation responses, combined with differences in specific root length, explained how the cultivars differed in P-acquisition efficiency. The results indicate that inappropriate cultural conditions (e.g. planting densities) may confound attempts to define P-efficient root traits and to identify genotypes with improved P-acquisition efficiency.
Foraging for better root traits: phosphorus acquisition efficiency in a critical pasture species
Pastures grown on P-deficient soils in temperate southern Australia use mixtures of grasses and legumes. The main legumes (Trifolium and Medicago spp.) are highly productive across a wide range of environments but have high 'critical' P requirements (i.e. the P supply needed for near-maximum yield) relative to the grasses with which they grow. Improving the P-efficiency of the most important legume (T. subterraneum), or developing the agronomic merit of alternatives that are already P-efficient (e.g. Ornithopus spp.) would deliver reductions in P fertiliser inputs, improve farm incomes, and achieve better use of scarce nutrient resources. Here we describe research to improve the P efficiency of T. subterraneum. Field and controlled-environment experiments, with various pasture legume species, have demonstrated that substantial differences in the nutrient foraging potential of their roots determines their requirement for P fertiliser. Three key root morphology traits ensure efficient P acquisition from low P soil: development of high root length, high specific root length and long root hairs. Ornithopus spp. deploy an "optimal" combination of these root traits, efficiently maximising soil exploration to capture more P and to yield well in low P soils. In contrast, Trifolium subterraneum develops long roots in response to low P but has low specific root length and short root hairs which limit its ability to explore soil for P. Within T. subterraneum, variation exists in specific root length and root proliferation. These key factors determine intra-specific variation in P acquisition with the best genotypes achieving twice the yield of the worst in low P soil. The short root hairs on T. subterraneum (0.2-0.4 mm) are a major factor limiting P acquisition efficiency. Wider studies of nutrient foraging root traits among genetically-allied Trifolium species from the Section Trichocephalum revealed substantial differences in propensity for root foraging (11-35 cm root/cm3 soil) and root hair length (0.2-0.5 mm) but, like T. subterraneum, no genotypes tested to date have root foraging traits in the optimal combinations achieved by Ornithopus spp. To drive further substantive change in the P efficiency in the key pasture legume, T. subterraneum, it will be necessary to break through apparent intra-specific 'boundaries' for specific root length and root hair length by identifying radical ecotypic outliers, inter-specific introgression or directed mutagenesis.