J. F. Angus

Biofumigation: Isothiocyanates released from Brassica roots inhibit growth of the take-all fungus

The presence of root tissue of the brassicas canola and Indian mustard inhibited growth of pure cultures of the fungal pathogen which causes take-all of wheat [Gaeumannomyces graminis (Sacc.) Arx and Oliver var.tritici, abbreviated as Ggt]. Ggt growth was generally inhibited more in the presence of Indian mustard roots than canola roots. Dried irradiated roots were consistently effective in reducing Ggt growth, but growth inhibition by young live roots and macerated roots was not consistent. The inhibitory compound(s) were shown to be volatile because the symmetry of Ggt growth was not affected by the proximity of theBrassica tissue. Volatile breakdown products from maceratedBrassica roots were identified using a gas chromatograph-mass spectrometer. The major compounds found were isothiocyanates (ITCs). Canola roots released mostly methyl ITC and Indian mustard roots released mostly phenylethyl ITC. Low concentrations of these and related compounds inhibited growth of Ggt in pure culture when supplied as the vapour of pure chemicals in concentrations within the range expected during breakdown ofBrassica roots in soil.

Benchmarking water-use efficiency of rainfed wheat in dry environments

Attainable water-use efficiency relates attainable yield, i.e. the best yield achieved through skilful use of available technology, and seasonal evapotranspiration (ET). For wheat crops in south-eastern Australia, there is a common, often large gap between actual and attainable water-use efficiency. To evaluate whether this gap is only an Australian problem or a general feature of dry environments, we compared water-use efficiency of rainfed wheat in south-eastern Australia, the North American Great Plains, China Loess Plateau, and the Mediterranean Basin. A dataset of published data was compiled (n = 691); water-use efficiency (WUEY/ET) was calculated as the ratio between actual grain yield and seasonal ET. Maximum WUEY/ET was 22 kg grain/ha.mm. Average WUEY/ET (kg grain/ha.mm) was 9.9 for south-eastern Australia, 9.8 for the China Loess Plateau, 8.9 for the northern Great Plains of North America, 7.6 for the Mediterranean Basin, and 5.3 for the southern-central Great Plains; the variation in average WUEY/ET was largely accounted for by reference evapotranspiration around flowering. Despite substantial differences in important factors including soils, precipitation patterns, and management practices, crops in all these environments had similarly low average WUEY/ET, between 32 and 44% of attainable efficiency. We conclude that low water-use efficiency of Australian crops is not a local problem, but a widespread feature of dry environments. Yield gap analysis for crops in the Mallee region of Australia revealed low availability of phosphorus, late sowing, and subsoil chemical constraints as key factors reducing water-use efficiency, largely through their effects on soil evaporation.

Arbuscular mycorrhizae in wheat and field pea crops on a low P soil: increased Zn-uptake but no increase in P-uptake or yield

Few field studies have investigated the contribution of arbuscular mycorrhizal fungi (AMF) to agricultural systems. In this study, the role of AMF in nutrition and yield of dryland autumn-sown wheat and field pea was examined through a 2-year crop sequence experiment on a red loam (Kandosol) in SE Australia. The soil was P-deficient and had low levels of root pathogens. In Year 1, levels of AMF were increased by growing subterranean clover or LinolaTM and decreased by growing canola or through maintenance of bare fallow with herbicides or tillage. In Year 2, hosts of AMF (wheat and field pea) and non-mycorrhizal canola were grown with 0 P or 20 kg ha−1 of P as superphosphate. Yields of all Year 2 crops were increased by P-fertiliser. Year 1 treatment led to 2–3 fold variation in colonisation by AMF at each P-level for Year 2 wheat and field pea. High colonisation did not correspond with greater crop growth, yield, or uptake of P, K, Ca, Cu or S in wheat or field pea. However, total crop Zn-uptake and grain Zn concentration were positively correlated with colonisation by AMF, due to enhanced Zn-uptake after anthesis. For wheat, high colonisation also corresponded with reduced Mn-uptake and lower grain Mn concentrations. In a glasshouse experiment using a second P-deficient Kandosol, inoculation of wheat with Glomus intraradices and Scutellospora calospora enhanced uptake of Zn and P when no P-fertiliser was applied. We conclude that high colonisation by AMF is unimportant for productivity of the major field crops grown on the Kandosol soils that occupy large areas of cropland in temperate SE Australia, even under P-limiting conditions. Investigation of the factors that control functioning of arbuscular mycorrhizae under field conditions, especially temperature, is required.

'Haying-off', the negative grain yield response of dryland wheat to nitrogen fertiliser II.Carbohydrate and protein dynamics

Changes in carbohydrate and protein in stems, leaves, spikes, and grain between anthesis and maturity were measured in 3 dryland wheat crops whose responses to applied nitrogen (N) ranged from increases in grain yield through to decreases in grain yield. This decrease in grain yield, known as haying-off, was described in Paper I in this series. Measurements reported there showed that apparent retranslocation, defined as the decrease in weight of vegetative organs during grain filling, was generally greater for crops of high-N status than for those of low-N status. Retranslocation in this context is the process of moving compounds assimilated before anthesis to the grain. The largest source of assimilates available for retranslocation in all crops at anthesis was water-soluble carbohydrates (WSC) contained in the stems and spikes, and represented a potential contribution of 34-50% to yield for the most severely hayed-off crops. The absolute amount of WSC present in high-N crops was less than that in low-N crops, despite a greater biomass. The lack of this form of assimilate available for retranslocation was the greatest single contributor to the yield reduction of the crops of high-N status. The quantity of protein retranslocated increased with crop N status, but the amounts involved were smaller than the quantity of WSC. Virtually all of the WSC reserves were utilised in all crops, in contrast to the protein reserves which were poorly retranslocated in the hayed-off crops. Most of the WSC was contained in the stems and most of the protein in the leaves. The potential contribution of retranslocated WSC and protein from leaves was more difficult to estimate because of an apparent loss of 40-50% of leaf tissue after anthesis. The nature of the loss was estimated from the amounts of acid detergent fibre (ADF; fibre not solubilised by hot acid detergent) present at anthesis and maturity. Since ADF comprises cellulose and lignin which decompose slowly, the loss of 30-37% of ADF was applied as a correction factor in calculating potential retranslocation from leaves. There was no loss of stem ADF. Using the correction, the potential retranslocation of leaf protein and leaf WSC was equivalent to 6-15% of yield. The export of all WSC and protein failed to account for the total decrease in leaf biomass, even after correction of leaf losses. We identified hemicellulose as an additional and previously unsuspected source of carbohydrate for retranslocation. Unlike WSC, the amount of leaf and stem hemicellulose at anthesis increased with crop N status, and the increase in hemicellulose between anthesis and maturity was equal to 10-17% of yield.

Effects of grazing on wheat growth, yield, development, water use, and nitrogen use

The effect of grazing by sheep during the late vegetative and early reproductive phases was measured on long-duration wheat crops in 2 experiments on farms in southern NSW. In both experiments, grazed and non-grazed crops were compared with different N-fertiliser strategies. In the first experiment, grazing 40 dry-sheep equivalents (DSE)/ha for 19 days increased grain yield from 2.30 to 2.88 t/ha in a season with a dry early spring. The second experiment, in a more favourable season, compared 6 durations of grazing by an average of 32 DSE/ha. The effects of grazing varied from no yield reduction with 15 days of grazing to a reduction from 5.97 to 3.98 t/ha with 51 days of grazing. In both experiments grazing caused slower crop development, with about 1 day’s delay in anthesis and maturity for every 4–5 days of grazing. Different patterns of water use by grazed and non-grazed crops, combined with delayed development, explained much of the effects of grazing on yield. The soil accumulated more water during grazing, which was used during grain filling when water-use efficiency for grain production was high. Delayed development also allowed grazed crops to respond to later rain. In the second experiment, grazing resulted in a net loss of 38 kg N/ha from the crop. Despite reduced N levels, the grazed crops showed no greater ability than grain-only crops to recover fertiliser N. The effect of the low recovery was that N removed during grazing was not efficiently replaced by fertiliser.

Soil water extraction by dryland crops, annual pastures, and lucerne in south-eastern Australia

The extraction of soil water by dryland crops and pastures in south-eastern Australia was examined in 3 studies. The first was a review of 13 published measurements of soil water-use under wheat at several locations in southern New South Wales. Of these, 8 showed significantly more water extracted by crops managed with increased nitrogen supply or growing after a break crop. The mean additional soil water extraction in response to break crops was 31 mm and to additional N was 11 mm. The second study used the SIMTAG model to simulate growth and water-use by wheat in relation to crop management at Wagga Wagga. The model was set up to simulate crops that produced either average district yields or the potential yields achievable with good management. When simulated over 50 years of weather data, the combined water loss as drainage and runoff was predicted to be 67 mm/year for poorly managed crops and 37 mm for well-managed crops. Water outflow was concentrated in 70% of years for the poorly managed crops and 56% for the well-managed crops. In those years the mean losses were estimated to be 95 mm and 66 mm, respectively. The third study reports soil water measured twice each year during a phased pasture-crop sequence over 6.5 years at Junee. Mean water content of the top 2.0 m of soil under a lucerne pasture averaged 211 mm less than under a subterranean clover-based annual pasture and 101 mm less than under well-managed crops. Collectively, these results suggest that lucerne pastures and improved crop management can result in greater use of rainfall than the previous farming systems based on annual pastures, fallows, and poorly managed crops. The tactical use of lucerne-based pastures in sequence with well-managed crops can help the dewatering of the soil and reduce or eliminate the risk of groundwater recharge. Additional keywords: water-use, groundwater recharge, simulation, dryland salinity, soil dewatering. J.s, R.lt, les, per,n Herwa AR3 Watersd p J.us et a

Comparison of canola, Indian mustard and Linola in two contrasting environments. II. Break-crop and nitrogen effects on subsequent wheat crops

Abstract The main canola-growing region in Australia is southern New South Wales where previous studies showed higher yield and grain protein of wheat growing after brassicas compared with wheat grown after wheat. This advantage, called the break-crop effect, was studied using winter oilseeds in two field experiments, one in this region and the other in central western New South Wales which is generally drier during the growing season, warmer throughout the year and is currently considered marginal for oilseeds. The effect of nitrogen (N) fertilizer on the response of wheat to previous crops was also investigated by considering both the soil N remaining from fertilizer applied to the break crops, and N applied to the subsequent wheat crops. The experiments were conducted over three years, with two phases of an oilseed-wheat sequence and a wheat-wheat control sequence at each site. Both sites had low baseline levels of soil mineral-N and average levels of root-disease inoculum. At the drier site the inoculum of wheat leaf and root pathogens remained during both phases but there were no break-crop effects. The effect of previous crops on the yield and protein of a subsequent wheat crop could be explained by the amount of residual soil mineral N. At the wetter site, wheat responses to previous crops could be explained by the amounts of residual soil mineral N in one phase of the experiment when there was no root disease of wheat. Under these conditions, yield and grain protein generally increased in response to increasing levels of soil mineral N. The exception was the yield of wheat after Linola which decreased when an excessive amount of residual N resulted in greater vegetative biomass, rapid depletion of soil water and decreased yield. In the other phase, when root disease was present, break crops increased yield of a subsequent wheat crop by 30% and grain protein by 1.3% compared to wheat growing after wheat, and among the oilseeds the brassicas gave a greater break-crop benefit than Linola. Application of fertilizer N to wheat growing after wheat failed to compensate for the disadvantage, indicating that residual N was not responsible for the differences. The break-crop benefit of the oilseeds extended to the second successive wheat crop for the phase in which root diseases were present, with increases of 13% in grain yield. The break-crop effect at the wetter site confirms previous observations of this benefit of oilseeds in general, and brassicas in particular, in southern New South Wales. The absence of any break-crop effect at the drier site suggests that root disease of wheat was less severe, possibly because the inoculum was less infective during the dry springs.

Reduced growth of autumn-sown wheat in a low-P soil is associated with high colonisation by arbuscular mycorrhizal fungi

Autumn-sown wheat (Triticum aestivum) was studied over two seasons in south-eastern Australia, on a low-P soil where indigenous arbuscular mycorrhizal fungi (AMF) were known to provide little nutritional benefit to crops. It was hypothesised that AMF would be parasitic under these circumstances. Shoot dry mass and water soluble carbohydrate (WSC) reserves in roots and shoots were measured for wheat grown with or without P-fertiliser, in plots where crop sequences had produced either high or low colonisation by AMF. Application of P-fertiliser greatly increased crop growth and decreased colonisation by AMF. At tillering, colonisation by AMF ranged from 24 to 66% of root length when no P was applied and from 11 to 32% when P was applied. At each P-level, high colonisation correlated with reductions of around 20% in stem and root WSC concentrations (first season) or shoot WSC content and shoot dry mass (much drier second season). Impacts on yield were not significant (first season) or largely masked by water-stress and frost (second season). While the major fungal root diseases of the region were absent, interactions between crop sequence and other unknown biotic constraints could not be discounted. The results are consistent with the parasitic impacts of colonisation by AMF being induced primarily through the winter conditions experienced by the crops until anthesis. It is concluded that wheat in south-eastern Australia may benefit from reduced colonisation by AMF, which could achieved through selected crop sequences or, perhaps, targeted wheat breeding programs.

Rate of soil acidification under wheat in a semi-arid environment

The rate of acidification under wheat in south-eastern Australia was examined by measuring the fluxes of protons entering and leaving the soil, using the theoretical framework of Helyar and Porter (1989). Monthly proton budgets were estimated for the root zone (0–90 cm layer) and for the 0–25 and 25–90 cm layers. After an annual cycle, the root zone was alkalinized by 0.5 to 3.1 kmol OH- ha-1. The alkalinity originated from the mineralization of the organic anions contained in the organic matter. The budget was near neutrality in the 0–25 cm layer (range: −1.0 to 1.4 kmol H+ ha-1), whereas there was net alkalinization in the 25–90 cm layer (1.7 to 2.3 kmol OH- ha-1). In the 0–25 cm layer, the acidity produced in autumn by mineralization of organic nitrogen was counterbalanced by the alkalinity released from crop residues. The main acidifying factor in this layer was leaching of NO3- during early winter (2.4 kmol H+ ha-1). Nitrate added through leaching was the main alkalinizing factor in the 25–90 cm layer, as added NO3- was taken up by the roots or denitrified in this layer. Urea fertilization had almost no effect on the rate of acidification, as little NO3- was leached out of the root zone. The factors acidifying the soil under wheat were limited in this environment because of the small amout of NO3- leached and the retention of the crop residues.

Modelling nutrient responses in the field

Models of the yield responses of crops to applied nutrients are a recent addition to the methods available for making fertilizer recommendations. They have a place in integrating nutrient information with information on other factors which affect yield and its response to added nutrients. This review deals with nitrogen models classified into three groups: those which predict yield-response curves based on empirical factors; those which simulate the yield response from complex simulation models of many processes regulating crop growth and the soil environment; and those which aim to simulate yield and selected processes based on simplified functional relationships which apply to a target region or industry. Three case studies representing the three classes of model are drawn from research on dryland wheat in different parts of Australia. They show examples in which models provide information which is unobtainable from experimental procedures and which provide information useful to farmers in making decisions about fertilizers.