Robert M. Norton

Factors regulating the contributions of fixed nitrogen by pasture and crop legumes to different farming systems of eastern Australia

On-farm and experimental measures of the proportion (%Ndfa) and amounts of N2 fixed were undertaken for 158 pastures either based on annual legume species (annual medics, clovers or vetch), or lucerne (alfalfa), and 170 winter pulse crops (chickpea, faba bean, field pea, lentil, lupin) over a 1200 km north-south transect of eastern Australia. The average annual amounts of N2 fixed ranged from 30 to 160 kg shoot N fixed ha−1 yr−1 for annual pasture species, 37–128 kg N ha−1 yr−1 for lucerne, and 14 to 160 kg N ha−1 yr−1 by pulses. These data have provided new insights into differences in factors controlling N2 fixation in the main agricultural systems. Mean levels of %Ndfa were uniformly high (65–94%) for legumes growing at different locations under dryland (rainfed) conditions in the winter-dominant rainfall areas of the cereal-livestock belt of Victoria and southern New South Wales, and under irrigation in the main cotton-growing areas of northern New South Wales. Consequently N2 fixation was primarily regulated by biomass production in these areas and both pasture and crop legumes fixed between 20 and 25 kg shoot N for every tonne of shoot dry matter (DM) produced. Nitrogen fixation by legumes in the dryland systems of the summer-dominant rainfall regions of central and northern New South Wales on the other hand was greatly influenced by large variations in %Ndfa (0–81%) caused by yearly fluctuations in growing season (April–October) rainfall and common farmer practice which resulted in a build up of soil mineral-N prior to sowing. The net result was a lower average reliance of legumes upon N2 fixation for growth (19–74%) and more variable relationships between N2 fixation and DM accumulation (9–16 kg shoot N fixed/t legume DM). Although pulses often fixed more N than pastures, legume-dominant pastures provided greater net inputs of fixed N, since a much larger fraction of the total plant N was removed when pulses were harvested for grain than was estimated to be removed or lost from grazed pastures. Conclusions about the relative size of the contributions of fixed N to the N-economies of the different farming systems depended upon the inclusion or omission of an estimate of fixed N associated with the nodulated roots. The net amounts of fixed N remaining after each year of either legume-based pasture or pulse crop were calculated to be sufficient to balance the N removed by at least one subsequent non-legume crop only when below-ground N components were included. This has important implications for the interpretation of the results of previous N2 fixation studies undertaken in Australia and elsewhere in the world, which have either ignored or underestimated the N present in the nodulated root when evaluating the contributions of fixed N to rotations.

Next generation of elevated [CO2] experiments with crops: A critical investment for feeding the future world

A rising global population and demand for protein-rich diets are increasing pressure to maximize agricultural productivity. Rising atmospheric [CO(2)] is altering global temperature and precipitation patterns, which challenges agricultural productivity. While rising [CO(2)] provides a unique opportunity to increase the productivity of C(3) crops, average yield stimulation observed to date is well below potential gains. Thus, there is room for improving productivity. However, only a fraction of available germplasm of crops has been tested for CO(2) responsiveness. Yield is a complex phenotypic trait determined by the interactions of a genotype with the environment. Selection of promising genotypes and characterization of response mechanisms will only be effective if crop improvement and systems biology approaches are closely linked to production environments, that is, on the farm within major growing regions. Free air CO(2) enrichment (FACE) experiments can provide the platform upon which to conduct genetic screening and elucidate the inheritance and mechanisms that underlie genotypic differences in productivity under elevated [CO(2)]. We propose a new generation of large-scale, low-cost per unit area FACE experiments to identify the most CO(2)-responsive genotypes and provide starting lines for future breeding programmes. This is necessary if we are to realize the potential for yield gains in the future.

Environmental effects on seed composition of Victorian canola

Data from advanced breeding experiments between 1985 and 1994 were used to determine the effects of region, year and environment on the quality of canola grown across Victoria. Estimates from these unbalanced data were made using residual maximum likelihood. Environmental effects were large relative to cultivar effects for oil and protein content, while the reverse occurred for glucosinolate content. High oil contents (and low seed protein contents) were correlated with cooler spring temperatures and higher spring rainfall. Oil contents were lowest, on average, in canola grown in dry years, or from the hotter regions, such as the Mallee, and were highest in canola from the cooler, wetter regions, such as south-western and north-eastern Victoria. Fatty acid composition varied with year and region. Means for saturated fatty acid content averaged 6.4 ± 0.1%. The oleic acid content averaged 60.3 ± 0.4% and was higher in canola grown in central Victoria and the Wimmera, and in most years, in north-eastern Victoria compared with other regions. Low temperatures and low rainfall reduced oleic acid content. Linoleic acid content averaged 19.7 ± 0.3% and linolenic acid averaged 10.4 ± 0.3%, with the content of these fatty acids negatively correlated with the content of oleic acid. Erucic acid levels were below 0.6% in all regions.

Impacts of global change on diseases of agricultural crops and forest trees

The fourth assessment report of the Intergovernmental Panel on Climate Change projects rising levels of greenhouse gas and global temperature. The well-known dependence of plant diseases on weather has long been exploited for predicting epidemics and to time applications of control measures for tactical disease management. Fingerprints of inter-annual climatic variation on pathogens have recently been shown in literature linking pathogen abundance to atmospheric composition. Past reviews have dealt with impacts of changing atmospheric composition and climate on diseases, regional or country-wide assessments of climate change impacts and impacts on specific disease/pathogen or pathogen groups. All agree on paucity of knowledge prompting a need to generate new empirical data on host‐pathogen biology under a changing climate. Focused on experimental research, the purpose of this review is to summarize published and unpublished studies on plant pathogens and diseases in free-air CO2 enrichment (FACE) facilities and open top chambers and other current non-FACE research to offer a summary of future research needs and opportunities. Critical review of recent literature on the influence of elevated CO2 and O3 on agriculture and forestry species forms a major part of the treatise. Summaries of unpublished or ongoing experimental research on plant pathogens from FACE studies are included as a catalogue of work in this neglected area. The catalogue and knowledge gaps are intended as a resource for workers initiating research in this area as well as the general scientific community grappling with the design and scope of next generation of FACE facilities.

Australian grains free-air carbon dioxide enrichment (AGFACE) facility: design and performance

The AGFACE project commenced in June 2007 at Horsham (36°45′07″S, 142°06′52″E; 127 m elevation), Victoria, Australia. Its aim is to quantify the interactive effects of elevated atmospheric carbon dioxide concentration (e[CO2]), nitrogen, temperature (accomplished by early and late sowing times), and soil moisture on the growth, yield, and water use of wheat (Triticum aestivum L.) under Australian conditions. The main engineering goal of the project was to maintain an even temporal and spatial distribution of carbon dioxide (CO2) at 550 μmol/mol within AGFACE rings containing the experimental treatments. Monitoring showed that e[CO2] at the ring-centres was maintained at or above 90% of the target (495 μmol/mol) between 93 and 98% of the operating time across the 8 rings and within ±10% of the target (495–605 μmol/mol) between 86 and 94% of the time. The carbon dioxide concentration ([CO2]) measured inside the rings declined non-linearly with increasing distance downwind of the CO2 source and differed by 3–13% in concentration between the two canopy heights in each ring, but was not affected by wind speed or small variations in [CO2] at the ring-centres. The median values for model-predicted concentrations within the inner 11-m-diameter portion of the rings (>80% of the ring area) varied between 524 and 871 μmol/mol but remained close to target near the centres. The design criteria adopted from existing pure CO2 fumigating FACE systems and new ideas incorporated in the AGFACE system provided a performance similar to its equivalent systems. This provides confidence in the results that will be generated from experiments using the AGFACE system.

Increasing mycorrhizal colonisation does not improve growth and nutrition of wheat on Vertosols in south-eastern Australia

Most crops host arbuscular mycorrhizal fungi (AMF). Canola and other brassicas are some of the few exceptions. This study examined AM fungal colonisation, uptake of phosphorus (P) and zinc (Zn), growth, and yield of wheat following brassicas and crops that host AMF in 5 crop-sequence experiments in southern New South Wales and Victoria. All experiments were on alkaline Vertosols, similar to soils in the northern wheatbelt on which low AM fungal colonisation of wheat following canola, or long-fallow, has been reported to induce poor crop growth. Soils with a broad range of extractable P concentrations were chosen. AM fungal colonisation of wheat was generally lower following brassicas than hosts of AMF, although this varied with year and location. The effect on wheat AM fungal colonisation levels did not vary between brassicas with differing levels and types of root glucosinolates. Low AM fungal colonisation did not affect early wheat growth, pre-anthesis P and Zn uptake, or yield. A positive relationship between AM fungal colonisation and grain Zn and P concentrations occurred in one experiment. High levels of colonisation by AMF did not protect crop roots from damage by root pathogens and high levels of pathogen damage made interpretation of results difficult in some instances. As these findings are consistent with results from an experiment on an acidic Kandosol in southern New South Wales, it appears farmers do not need to consider the degree to which wheat will be colonised by AMF when planning crop sequences in south-eastern Australia.

Will intra-specific differences in transpiration efficiency in wheat be maintained in a high CO2 world? A FACE study

This study evaluates whether the target breeding trait of superior leaf level transpiration efficiency is still appropriate under increasing carbon dioxide levels of a future climate using a semi-arid cropping system as a model. Specifically, we investigated whether physiological traits governing leaf level transpiration efficiency, such as net assimilation rates (A(net)), stomatal conductance (g(s)) or stomatal sensitivity were affected differently between two Triticum aestivum L. cultivars differing in transpiration efficiency (cv. Drysdale, superior; cv. Hartog, low). Plants were grown under Free Air Carbon dioxide Enrichment (FACE, approximately 550 µmol mol⁻¹ or ambient CO₂ concentrations (approximately 390 µmol mol⁻¹). Mean A(net) (approximately 15% increase) and gs (approximately 25% decrease) were less affected by elevated [CO₂] than previously found in FACE-grown wheat (approximately 25% increase and approximately 32% decrease, respectively), potentially reflecting growth in a dry-land cropping system. In contrast to previous FACE studies, analyses of the Ball et al. model revealed an elevated [CO₂] effect on the slope of the linear regression by 12% indicating a decrease in stomatal sensitivity to the combination of [CO₂], photosynthesis rate and humidity. Differences between cultivars indicated greater transpiration efficiency for Drysdale with growth under elevated [CO₂] potentially increasing the response of this trait. This knowledge adds valuable information for crop germplasm improvement for future climates.

Does phosphorus stimulate the effect of elevated [CO2] on growth and symbiotic nitrogen fixation of grain and pasture legumes?

Abstract. The effect of elevated [CO2] (700 μmol/mol) and phosphorus (P) supply on the growth and symbiotic N2 fixation of chickpea (Cicer arietinum L.), field pea (Pisum sativum L.) and barrel medic (Medicago truncatula Gaertn.) were investigated in the glasshouse. The effect of elevated [CO2] on the growth and N2 fixation at various growth stages of the chickpea and field pea plants (grown on a Vertosol) were also examined. Elevated [CO2] generally increased the aboveground biomass of chickpea (by 18–64%), field pea (by 24–57%) and barrel medic (by 49–82%), but the effect was greater when P was non-limiting. Elevated [CO2] only stimulated grain yield of chickpea (by 70%) and field pea (by 21%) if P supply was adequate. Elevated [CO2] reduced the grain protein concentration of chickpea (by 17–18%) regardless of P input, but increased that of field pea (by 11%) when soil P was limiting but had no effect at adequate P. The percentage of shoot N derived from the atmosphere (%Ndfa) of the three legumes was unaffected by elevated [CO2] regardless of soil P supply. Elevated [CO2] increased the amount of N fixed by chickpea (by 20–86%), field pea (by 44–51%) and barrel medic (by 114–250%) under P fertilisation, but had no significant effect when soil P was deficient. These results suggest that the predictions of future climates on the potential contribution of legumes to maintaining soil N fertility will depend on the particular response of a species to soil P status.

Intraspecific variation in growth and yield response to elevated CO2 in wheat depends on the differences of leaf mass per unit area

In order to investigate the underlying physiological mechanism of intraspecific variation in plant growth and yield response to elevated CO2 concentration [CO2], seven cultivars of spring wheat (Triticum aestivum L.) were grown at either ambient [CO2] (~384μmolmol-1) or elevated [CO2] (700μmolmol-1) in temperature controlled glasshouses. Grain yield increased under elevated [CO2] by an average of 38% across all seven cultivars, and this was correlated with increases in both spike number (productive tillers) (r=0.868) and aboveground biomass (r=0.942). Across all the cultivars, flag leaf photosynthesis rate (A) increased by an average of 57% at elevated [CO2]. The response of A to elevated [CO2] ranged from 31% (in cv. H45) to 75% (in cv. Silverstar). Only H45 showed A acclimation to elevated [CO2], which was characterised by lower maximum Rubisco carboxylation efficiency, maximum electron transport rate and leaf N concentration. Leaf level traits responsible for plant growth, such as leaf mass per unit area (LMA), carbon (C), N content on an area basis ([N]LA) and the C:N increased at elevated [CO2]. LMA stimulation ranged from 0% to 85% and was clearly associated with increased [N]LA. Both of these traits were positively correlated with grain yield, suggesting that differences in LMA play an important role in determining the grain yield response to elevated [CO2]. Thus increased LMA can be used as a new trait to select cultivars for a future [CO2]-rich atmosphere.

Nitrogen use and crop type affect the water use of annual crops in south-eastern Australia

The effect of management and crop selection on water use and profile drying was investigated using 2 series of experiments conducted in the Victorian Wimmera. The effect of applied nitrogen on growth and water use of canola was assessed from 3 field experiments on a Vertosol soil. Across these sites, 140 kg N/ha increased crop water use by a mean of 30 mm, and the biological response averaged 3.68 t/ha of shoot dry matter and seed yield increased by 73% from 1.46 to 2.52 t/ha. The additional nitrogen enabled roots to go deeper into the soil and also to extract water to higher tensions, but the increases in water use were far less than the growth and yield responses. Estimated average soil evaporation was 120 mm across these experiments, but ranged from 26 to 57% of total water use. It was concluded that increased crop vigour in response to applied nitrogen does increase total water use, but the main way that water-use efficiency increases is through reduced soil evaporation. In a second series of experiments, the growth, yield, and water use of wheat, canola, linseed, mustard, and safflower were compared across 4 sites with differing soil moisture contents. Wheat was the highest yielding crop at all sites. Mustard and canola produced similar amounts of biomass and seed yields, whereas linseed produced seed yields that were generally less than the brassica oilseeds. Safflower grew well and produced large amounts of biomass at all sites, but this increased growth did not necessarily translate into increased seed yields. Safflower yielded less seed than all other crops at the 2 dry sites, but yields were similar to canola at the wetter sites. On 2 drier sites, soil water extraction occurred to approximately 1 m for all crops, and all available water was used within that zone by all crops. Where the soil was wet beyond 1 m, safflower was able to extract water from deeper in the profile than the other crops and generated a soil water deficit of about 100 mm more than the other crops at maturity. This deficit persisted into the subsequent autumn–winter period. The potential of using safflower as a management option to extract water from deep in the profile, and so create a soil buffer, is discussed.