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

Effect of competition from a C₄ grass on the phosphorus response of a subtropical legume

Tropical pasture systems are typically dominated by C₄ grasses growing on nitrogen (N) deficient soils. Under these conditions, N₂-fixing legumes should have a competitive advantage, yet low legume contents are often reported in these systems. This work investigates whether below-ground competition for phosphorus (P) is limiting the ability of legumes to compete in swards of C₄ grasses when grown in a sand matrix. The external P requirement of a subtropical legume (butterfly pea, 'Clitoria ternatea' L.) and a C₄ grass (buffel grass, 'Cenchrus ciliaris' L.) were initially determined in a P-response experiment. Four rates of P (4.6-78.2 mg P kg-¹ of Colwell P) were subsequently selected to investigate the growth response of the butterfly pea when grown with and without competition from a sward of N-deficient buffel grass. Shoot dry matter was determined over successive cuts and P uptake determined at the final harvest at 72 days. Buffel grass dominated the mixed swards and reduced the shoot dry matter production of the butterfly pea by >50% relative to the pure swards. A significant difference in the soil P response curve and shoot P uptake of butterfly pea was not detected between pure swards and those with competition from buffel grass. The ability of C₄ grasses to acquire and convert resources (i.e. light, water and nutrients) more efficiently into shoot dry matter is likely to be a major factor resulting in grass-dominated pastures in tropical systems

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

Mycorrhizal Symbiosis and Nutrient Acquisition of Cotton ('Gossypium Hirsutum L.) in Sodic Vertosols

Mycorrhizal symbioses are considered the most important mutualism on Earth. This symbiosis occurs between plant roots and mycorrhizal fungi in the rhizosphere. Mycorrhizal hyphae facilitates the exploration of a greater volume of soil by plant roots and increased water and nutrient uptake, especially phosphorus (P) which is immobile in the soil. This consequently improves plant growth, particularly at early growth stages. Cotton (Gossypium hirsutum L.) is a mycorrhizal dependant crop and mycorrhization clearly improves early growth and nutrient uptake. Although mycorrhizal fungi also alleviate abiotic stresses, adverse abiotic soil condition might restrict mycorrhizal colonisation and associated nutrient uptake. One possible adverse environmental condition is elevated soil sodicity. In Australia the majority of cotton growing regions are sodic. Sodicity creates adverse physical and chemical conditions, including waterlogging, hard-setting, high bulk density, high pH, and high soil solution sodium (Na), which may affect mycorrhizal colonisation of cotton plants. This thesis aimed to investigate the percentage of root length colonised by mycorrhizae and nutrient uptake of cotton in a range of naturally sodic soils. This thesis also attempted to estimate the relative hyphal contribution to early P uptake of cotton in sodic soil conditions. A series of glasshouse experiments was conducted in order to assess mycorrhizal colonisation and a number of different colonisation techniques in moderately (ESP 10– 15) and highly (ESP>15) sodic soils. Standard techniques to inoculate cotton plants in sodic soils were unsuccessful until live hyphal cultures were introduced. 15) sodic soils. Standard techniques to inoculate cotton plants in sodic soils were unsuccessful until live hyphal cultures were introduced. In a separate glasshouse experiment, changes in mycorrhizal colonisation and nutrient uptake of cotton in a range of naturally non-sodic and low-sodic soils from cotton production areas in southern Queensland and northern New South Wales, with different exchangeable Na percentages (ESP) (ranged between 1.4 and 9.8) was investigated. Linear mixed model analysis showed minimal effects of sodicity, when ESP was less than 10, on mycorrhizal colonisation, associated plant growth and nutrient uptake. Principle component and regression analysis showed that other sources of variation including soil pH and soil extractable P, rather than sodicity, might drive cotton colonisation in Vertosols with low to moderate ESP. The colonisation percentage was positively linearly correlated with P, Mg, and Zn uptake of cotton plants. An isotope experiment was established to assess the mycorrhizal colonisation and nutrient uptake of cotton plants under highly (ESP 21) and less (ESP 7) sodic soil conditions with two rates of applied P. The relative hyphal contribution to P uptake was quantified using dual isotope labelling techniques (32P and 33P). Root colonisation and P uptake of mycorrhizal cotton plants reduced by 16% and 20%, respectively, in highly sodic soil as compared to plants in low sodic soil, however, the relative proportion of P delivered via hyphal pathways (32P from root-free hyphal compartment) was similar. Under high P conditions, the relative increase in the proportion of 33P (root + hyphae compartment) taken up by inoculated plants was greater in the low sodic soil relative to the high sodic soil. Mycorrhization improved early seedling vigour, and nutrient uptake. Overall, these results confirmed that early growth and nutrient uptake of cotton, especially P and Zn, benefits from mycorrhizal association in both non-sodic and sodic soils. However, mycorrhizal colonisation establishment in sodic soils is not as straight forward as in non-sodic soils. In the absence of fresh hyphal material, reliance on spore germination to colonise cotton roots might be unsuccessful in moderately- and highlysodic soils. Therefore, the presence of a fresh mycorrhizal hyphal network within the inoculum source may play an important role in initiating colonisation of cotton roots in sodic soils with ESP greater than 15. These results indicate that higher levels of sodicity restrict mycorrhizal colonisation of cotton. Reduced colonisation and hyphal exploration of the soil, possibly due to the physical and chemical constraints imposed by highly-sodic soil, rather than poorer mycorrhizal function, might be one of the responsible factors for limited early P uptake of cotton in highly-sodic soil. However, mycorrhizal colonisation and associated nutrient uptake (P, Zn, Ca, Mg, K, Mn) of cotton was not dominated by sodicity and associated physical/chemical conditions (waterlogging, high bulk density, high pH, high soil solution Na) occurring in soils when ESP is less than 10. Further investigation into mycorrhizal spore density, species diversity and mycorrhizal proliferation under sodic soil conditions is warranted

Eggshell uniformity and the relationship between shell structures and shell translucency, examined by Computated Tomography (CT) and Scanning Electron Microscopy (SEM)

While it has been determined that eggshell translucency is caused by the accumulation of moisture within the structures of the shell, it is still unclear precisely which shell structures are related to translucency. As part of a larger study, initial work aimed to determine how many shell samples from a single egg were required to adequately identify any features or abnormalities of that shell. SEM and CT analysis of shell samples is a time consuming process so avoiding unnecessary replication would allow for more eggs to be examined

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

Variation in P-acquisition efficiency among Trifolium subterraneum genotypes and the role of root morphology traits

Trifolium subterraneum is widely grown in the P-deficient soils of southern Australia. However, this pasture legume has a high critical external P requirement and requires annual applications of P-fertiliser for high productivity. Twenty six cultivars or lines of T. subterraneum were grown to determine: (i) the difference between cultivars in shoot growth and P uptake under low P supply, and (ii) the root morphology traits important for P acquisition. Micro-swards of each cultivar were grown with a topsoil layer that was either deficient in P for plant growth (40 mg P applied kg-1 ) or had P supplied in excess of the critical requirement for maximum yield (250 mg P applied kg-1; “luxury P”). The subsoil was P-deficient (0 mg P applied kg-1 ). Yield and P content of shoots, topsoil and subsoil roots were determined after 5 weeks growth. Root samples were assessed for diameter, length and root hair length. When luxury P was supplied, all cultivars were equally highly productive. However, in P-deficient soil shoot yield ranged from 38% to 71% of maximum yield. Root morphology traits such as total root length of the cultivars ranged from 63 to 129 m, and correlated well with cultivar plant P acquisition (R~0.86). Topsoil root length density (14-26 cm-3 ) and topsoil specific root length (99 to 172 m g-1 ) varied between cultivars. Variation was also observed for traits such as root hair length (0.19-0.33 mm) and root diameter (0.30-0.35 mm). These traits were used to calculate the total surface area of the root hair cylinder for each cultivar, which correlated well with cultivar plant P acquisition (R~0.83). The results demonstrated that there is potential to identify cultivars of T. subterraneum for improved P acquisition and higher yields in low P soil

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

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 m(2)) 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 (13)C 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

Mycorrhizal colonisation of cotton in soils differing in sodicity

Despite the reported importance of mycorrhizal symbioses for early growth and nutrient acquisition of cotton, little is known about how sodicity affects this relationship. Changes in mycorrhizal colonisation and nutrient uptake of cotton in a range of naturally non-sodic (exchangeable sodium percentages (ESP