Projections of climate change impacts on crop yields are inherently uncertain1. Uncertainty is often quantified when projecting future greenhouse gas emissions and their influence on climate2. ...However, multi-model uncertainty analysis of crop responses to climate change is rare because systematic and objective comparisons among process-based crop simulation models1, 3 are difficult4. Here we present the largest standardized model intercomparison for climate change impacts so far. We found that individual crop models are able to simulate measured wheat grain yields accurately under a range of environments, particularly if the input information is sufficient. However, simulated climate change impacts vary across models owing to differences in model structures and parameter values. A greater proportion of the uncertainty in climate change impact projections was due to variations among crop models than to variations among downscaled general circulation models. Uncertainties in simulated impacts increased with CO2 concentrations and associated warming. These impact uncertainties can be reduced by improving temperature and CO2 relationships in models and better quantified through use of multi-model ensembles. Less uncertainty in describing how climate change may affect agricultural productivity will aid adaptation strategy development and policymaking.
► Conceptual frameworks link agricultural practices to Great Barrier Reef pollution. ► Dissolved inorganic N exports from catchments were well related to N Surpluses in cropped lands. ► Catchment ...fine sediment exports were well related to field-scale erosion in grazing lands. ► Government targets for reducing pollutant exports to the Reef will not be met with current BMPs. ► The frameworks are applicable to other pollutants with particulate and/or soluble phases.
Chemical and sediment losses from agricultural lands are threatening coastal marine and aquatic ecosystems in many parts of the world. This is an acute problem in Australia, where the condition of Great Barrier Reef (GBR) ecosystems is threatened by increased pollutant loads from agricultural lands, and Governments have enacted policies to reduce pollutant exports. These policies raise the question of how to identify changes in land management that will effectively reduce exports. The scale of the GBR catchments (> 400,000km2) precludes detailed modelling investigations, especially within the time scale of policy implementation. Therefore, we developed conceptual frameworks linking agricultural land management to river pollutant exports for two contrasting agricultural pollutants posing threats to the health of GBR ecosystems; dissolved inorganic nitrogen (DIN) and fine sediment (silt and clay), based on a synthesis of past studies. We argue that nitrogen (N) Surpluses (N inputs relative to crop N off-take), are the primary driver of DIN losses from agricultural land to rivers. Similarly, previous studies in GBR grazing lands and elsewhere have quantitatively defined how sediment losses from hill slopes, gullies and stream banks are related to grazing land condition, ground cover and riparian management, which are products of recent climate and grazing practices. From these frameworks we derive relationships between firstly, estimated N Surplus and DIN exports, and secondly ground cover and river fine sediment exports. Using these relationships we examine how DIN and fine sediment exports to the GBR may respond to a range of management scenarios for reducing N inputs, and increasing ground cover and improving riparian management. We predict that widespread adoption of the most extreme scenarios would approximately meet water quality improvement targets set/implied by governments for these two pollutants. However, it is unlikely that these extreme scenarios will be adopted to the extent needed and in the time frames set by current policy. In particular, the agri-environmental management practices defined in this study for N are generally unproven in GBR cropping systems, the required levels of pasture cover and riparian management are generally beyond current experience, and it can take decades to improve land condition, and so reduce erosion rates after cover increases. We also show that the approach taken is applicable to other pollutants, such as total N, that combine characteristics of the pollutants considered here. For the case of total N, the reductions in pollutant loads are not as great smaller relative to targets than for DIN or fine sediments.
•Processes generating agricultural pollutants were similar to those in temperate environments.•Reducing N losses will primarily be achieved by reducing N applications to high value crops.•Maintaining ...ground cover and pasture biomass, especially during the dry season, controls erosion.•There are uncertainties over breakdown and fate of pesticides in these tropical environments.•High spatial and temporal rainfall variability complicates management of agricultural pollutants.
The environmental consequences of agriculture are of growing concern. One example of these consequences is the effect of agricultural pollutants on the Great Barrier Reef (GBR), a world heritage-listed ecosystem lying off the tropical north-eastern coast of Australia. Pollutants from agricultural lands (fine sediments and attached nitrogen (N) mainly from grazing lands, and dissolved N and pesticides mainly from cropping) in catchments draining into the GBR lagoon threaten the health and resilience of this ecosystem. Government actions are prompting farmers to adopt new management practices to reduce pollutant exports from their farms. However, previous agricultural research has, with the exception of erosion, largely focussed on production rather than environmental impacts. Also, the relevance of research conducted in other regions, e.g. Europe and North America, with different climates, soils and agricultural systems may be limited. Thus, there may not be a strong knowledge base underpinning actions to improve water quality. In this paper, we review research on the relationship between management of agricultural lands and pollutant exports in GBR catchments, and compare this knowledge with experience in other regions. Despite the differences in climate and agricultural systems, there are similarities in the causes and management of N and pesticide losses from cropping lands. Substantial N fertiliser is applied to high value crops in GBR catchments, and the primary path to reducing N losses from cropped lands will be through reducing N applications. Other practices may become effective in these crops once current (high) rates of N application and reduced. Herbicides are widely used, and practices that reduce herbicide runoff have recently been developed and demonstrated in most of the main cropping systems. However, there are still uncertainties over breakdown and fate of pesticides, especially new products, in these tropical environments. The principles of reducing erosion in grazing lands are well understood, and centre on maintaining ground cover and biomass of pastures, especially during the dry season and droughts. Highly variable rainfall makes this principle challenging to achieve in practice. In addition, it has recently become clear that gully networks caused by livestock grazing are much more important sources of sediment that previously thought. Practices such as targeted vegetation management will be important strategies for reducing gully erosion. Despite these advances in practice effectiveness, it is clear that much more is needed to have agricultural systems that are compatible with a sustainable and resilient GBR. As the demand for food increases in coming decades, and agriculture expands and intensifies in tropical countries, the experience in GBR catchments will help guide the development of more sustainable agricultural systems in these countries.
► Unlike experience other cropping systems, N losses are likely to be no higher in irrigated sugarcane production systems than is rainfed systems. ► N losses can be reduced whilst yields are ...maintained through reducing N applications in Australian irrigated sugarcane production. ► Reducing N applications to give long-term N surpluses of 50
kg
ha
−1
yr
−1 are predicted to maintain yields but reduce N losses by 50–57%. ► Improved irrigation management could also help reduce N losses but generally to a much lesser extent than reduced N fertiliser applications.
There is concern about environmental impacts of cropping in catchments of Australia's Great Barrier Reef, especially losses of nitrogen (N) from cropping systems. Sugarcane production in the Burdekin region in the dry tropics stands out from other crops/regions because it is grown with the highest applications of irrigation water and N fertiliser rates of any sugarcane producing region in Australia, attributes which may enhance losses of N. Little is known about N losses from sugarcane production systems, especially irrigated systems. We measured parts of the water and N balance over three sugarcane crops at three contrasting sites in different parts of the Burdekin region, covering a range of soil types/textures and irrigation managements. The experimental data were used to parameterise the APSIM-Sugarcane cropping systems model, and the model then used to ‘infill’ missing data and develop more complete water and N balances for each of the crops at the three sites. The model was also used to simulate long-term yields and N losses through runoff and leaching below the root zone at the sites under a range of N fertiliser and irrigation management practices. Unlike the experience in other cropping systems, N losses through runoff and leaching below the root zone were not higher at our sites than measured in rainfed sugarcane production systems. The long-term simulations showed there were clear opportunities for reducing N losses while maintaining yields through reducing N fertiliser application rates. Simulations results suggested that long-term N surpluses of 50
kg
ha
−1
yr
−1, considerably less than those during the experiment or common in the study region, were sufficient to maintain yields but reduce N losses by 50–57%. So, N fertiliser management should aim to keep surpluses to that level. Improved irrigation management could also help reduce N losses but generally to a much lesser extent than reduced N fertiliser applications. Research is required to confirm these predicted benefits, and investigate potential interaction between N fertiliser and irrigation management practices, and impacts of other management practices.
Increasing the precision of nitrogen (N) fertiliser management in cropping systems is integral to increasing the environmental and economic sustainability of cropping. In a simulation study, we found ...that natural variability in year-to-year climate had a major effect on optimum N fertiliser rates for sugarcane in the Tully region of north-eastern Australia, where N discharges pose high risks to Great Barrier Reef ecosystems. There were interactions between climate and other factors affecting crop growth that made optimum N rates field-specific. The regional average optimum N fertiliser rate was substantially lower than current industry guidelines. Likewise, simulated N losses to the environment at optimum N fertiliser rates were substantially lower than the simulated losses at current industry fertiliser guidelines. Dissolved N discharged from rivers is related to fertiliser applications. If the reductions in N applications identified in the study occurred in the Tully region, the reduction in dissolved N discharges from rivers in the region would almost meet current water quality improvement targets. Whilst there were many assumptions made in this exploratory study, and there are many steps between the study and a practically implemented dynamic N fertiliser recommendation system, the potential environmental benefits justify field validation and further development of the concepts identified in the study.
•Sugarcane N response differs when rainfall is above or below average.•The effect of rainfall depends on soil-type and harvest date (i.e. growing season).•Interactions are complex so optimal N application rates are field specific.•Knowledge of seasonal climate information could reduce sugarcane N rates by 47%.•This reduction in N rates would have important water quality benefits.
► We examine seasonal herbicide runoff losses from irrigated sugarcane farms. ► Paddock herbicide losses are compared to catchment water quality monitoring. ► Seasonal contrasts exist in patterns of ...herbicide movement. ► Dry season irrigation runoff poses particular threats to aquatic ecosystem health. ► Furrow irrigation adds additional risk of off-site herbicide movement.
Irrigation is vital to most of the sugarcane produced in Australia's ecologically sensitive Great Barrier Reef catchment area, although little is known regarding pesticide losses under irrigated sugarcane production. This study determined the dynamics of off-site paddock-scale pesticide movement and subsequent concentrations in local receiving environments in fully irrigated sugarcane farming systems of the lower Burdekin floodplain region, the largest sugar producing area in Australia. Chemical movement (both mass and concentration) in paddock surface run-off followed a similar pattern across sites in the region for several of the commonly applied herbicides such as diuron, atrazine and ametryn. Highest losses (loads and event concentrations) occurred in the first irrigation run-off events following application, with subsequent irrigation losses tailing off rapidly. Significant losses could also occur during wet season rainfall run-off events from paddocks with recent pesticide applications. There was a strong seasonal signal evident in catchment monitoring results. Pesticide concentrations in nearby receiving creek systems were invariably an order of magnitude or more lower than values collected at paddock-scale, highlighting the considerable dilution that takes place over relatively short distances. While the concentrations found in receiving creek systems were considerably lower than direct paddock run-off, they regularly exceeded some ecological guidelines and results of pesticide risk modeling suggested concentrations, particularly under dry season conditions, posed considerable ecological risk to aquatic ecosystems.
► We simulate the impact of management change and climate change on N loss in sugarcane farms. ► We investigated the interactions between management, soil type, location and climate change. ► New ...management designed to meet water quality goals will not be greatly affected by climate change. ► Improved management could reduce N losses by up to 66%, compared with traditional practices. ► The frequency of years with high N losses was predicted to increase due to climate change for all management systems.
Nitrogen (N) lost from cropping is one of the major threats to the health of the Great Barrier Reef (GBR) in northern Australia, and there are government initiatives to change farming practices and reduce N losses from farms. Sugarcane is the dominant crop in most catchments draining into the GBR lagoon, especially those of the Mackay Whitsunday region (8400km2) where sugarcane represents>99% of cropping in the catchments, and is grown with large applications of N fertiliser. As farmers and farming systems adapt to a future requiring lower environmental impact, the question arises whether climate change may influence the effectiveness of these changes, an issue rarely considered in past water quality studies. To address this question we used the APSIM farming-systems model to investigate the complex interactions between a factorial of five proposed sugarcane management systems, three soil types, three sub-regional climatic locations and four climate change projections (weak, moderate and strong, with historical climate as a ‘control’). These projections, developed from general circulation models and greenhouse gas emission scenarios, estimated that median annual rainfall would be reduced by up to 19%, and maximum and minimum temperatures increased by up to 0.5°C and 0.6°C, respectively. Management practices, such as tillage, fallow management and N inputs, were grouped into five systems according to the perceived benefits to water quality. For example; management System A grouped together zero tillage, soybean rotation crops, reduced N inputs and controlled traffic practices. While at the other end of the scale, System E included many severe tillage operations, bare fallows, high N inputs and conventional row spacing; practices that are still used in some areas. Importantly, this study parameterised controlled traffic systems, which is considered an important component of ‘best’ management in the GBR catchment, but for which water quality benefits have yet to be widely quantified. The study predicted that the improvement in farm management needed to meet water quality improvement goals will not be greatly affected by climate change. However, without any interventions, the frequency of years with very high N losses, and hence extreme ecological risk, was predicted to increase by up to 10–15%. Compared with traditional practices, improved management systems were predicted to reduce N losses by up to 66% during these years. The results support continued adoption of improved management systems to achieve proposed water quality targets in both the current and a range of potential future climates. However, there are important uncertainties about the effects of elevated atmospheric CO2 concentration on plant assimilation rates and the characterisation of extreme climate events that deserve further study.
Experimental field data are used at different levels of complexity to calibrate, validate and improve agro-ecosystem models to enhance their reliability for regional impact assessment. A ...methodological framework and software are presented to evaluate and classify data sets into four classes regarding their suitability for different modelling purposes. Weighting of inputs and variables for testing was set from the aspect of crop modelling. The software allows users to adjust weights according to their specific requirements. Background information is given for the variables with respect to their relevance for modelling and possible uncertainties. Examples are given for data sets of the different classes. The framework helps to assemble high quality data bases, to select data from data bases according to modellers requirements and gives guidelines to experimentalists for experimental design and decide on the most effective measurements to improve the usefulness of their data for modelling, statistical analysis and data assimilation.
► Sugarcane residue retention has highly site-specific affects on soil C sequestration and soil fertility. ► Effects were not related to the time (1–17 years) of residue retention, or the environment ...in which sites were located. ► Composition of soil C was a more responsive to residue management than amount of soil C. ► As well, different methods of soil C fractionation gave different information about soil C. ► We conclude that modelling will be useful to understand residue management impacts on C sequestration in sugarcane crops.
Sugarcane crop residues contain substantial quantities of C and plant nutrients, but there have been relatively few studies of how sugarcane residues enrich the soil and contribute to C sequestration, and most studies have been undertaken at only one or a few sites. The purpose of this study was to address these knowledge gaps by determining the magnitude and time scale of changes in soil concentrations of total C, C fractions and plant nutrients following retention of sugarcane residues. C fractions were determined by two different methods. We sampled soils from five experiments, in contrasting environments, where sugarcane residues had been either retained or removed for between 1 and 17 years. Changes in the concentration of both soil C and plant nutrients were highly site-specific and not in proportion to the period that residues were retained: for example, soil C (0–250mm) decreased by 0.9gkg−1 and 0.5gkg−1 at sites where residues had been retained for 1 and 17 years, respectively, but increased by 2.0gkg−1 at a site with residues retained for 6 years. Soil C composition, defined by the KMnO4 oxidation and particulate organic C-ultraviolet photo-oxidation fractionation (POC-UV) schemes, appeared to be a more sensitive indicator of changes in residue management, indicating that increases in readily-oxidisable C and particulate organic C, respectively, after 1 year of retaining instead of burning residues. The two methods provided different information that was complementary in understanding changes in soil C. The KMnO4 method identified downward movement of C fractions in the profile to 250mm, while the labile fractions measured by the POC-UV scheme appeared to be more sensitive to early changes in residue management (after 1 year). While recent studies have found that several concentrations of KMnO4 reduced all C fractions by a similar magnitude and thus concentrated on the fraction oxidised by the 333mM concentration of KMnO4, we found that use of both this and the 33mM concentration enabled a greater understanding of changes in C pools due to residue management.
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