M. E. Probert

An overview of APSIM, a model designed for farming systems simulation

The Agricultural Production Systems Simulator (APSIM) is a modular modelling framework that has been developed by the Agricultural Production Systems Research Unit in Australia. APSIM was developed to simulate biophysical process in farming systems, in particular where there is interest in the economic and ecological outcomes of management practice in the face of climatic risk. The paper outlines APSIMs structure and provides details of the concepts behind the different plant, soil and management modules. These modules include a diverse range of crops, pastures and trees, soil processes including water balance, N and P transformations, soil pH, erosion and a full range of management controls. Reports of APSIM testing in a diverse range of systems and environments are summarised. An example of model performance in a long-term cropping systems trial is provided. APSIM has been used in a broad range of applications, including support for on-farm decision making, farming systems design for production or resource management objectives, assessment of the value of seasonal climate forecasting, analysis of supply chain issues in agribusiness activities, development of waste management guidelines, risk assessment for government policy making and as a guide to research and education activity. An extensive citation list for these model testing and application studies is provided.

APSIM's water and nitrogen modules and simulation of the dynamics of water and nitrogen in fallow systems

Abstract APSIM (Agricultural Production Systems Simulator) is a software system which provides a flexible structure for the simulation of climatic and soil management effects on growth of crops in farming systems and changes in the soil resource. The focus of this paper is the predictive performance of APSIM for simulation of soil water and nitrate nitrogen in contrasting soils (vertisols and alfisols) and environments. The three APSIM modules that determine the dynamics of water, carbon, and nitrogen in the soil system (viz. SOILWAT, SOILN and RESIDUE v.1) are described in terms of the processes represented, with particular emphasis on aspects of their coding that differ from their precursors in CERES and PERFECT. The most fundamental change is in SOILN, which now provides a formal balance of both carbon and nitrogen in the soil and includes a labile soil organic matter pool that decomposes more rapidly than the bulk of the soil organic matter. Model performance, in terms of prediction of soil water and nitrate, is evaluated during fallows, thereby avoiding complications arising from water use and nitrogen uptake by a crop. One data set is from a long-term experiment on a vertisol in southeast Queensland which studied two tillage treatments (conventional and zero tillage) in combination with fertiliser nitrogen inputs for the growth of wheat; soil water and nitrate were measured twice each year (pre-planting and post-harvest). The second comes from experiments at Katherine, Northern Territory, where legume leys growing on alfisols were chemically killed and ensuing changes in soil water and nitrate were measured during a single season. For both datasets, the predictive ability of the model was satisfactory for water and nitrate, in terms of both the total amounts in the whole profile and their distribution with depth. Since neither of these datasets included measurements of the runoff component of the water balance, this aspect of model performance was evaluated, and shown to be generally good, using data from a third source where runoff had been measured from contour bay catchments.

Simulation of grain protein content with APSIM-Nwheat

Abstract apsim -Nwheat is a wheat crop system simulation model within the apsim framework which consists of modules that incorporate aspects of soil water, nitrogen, residues, wheat ( Triticum aestivum L.) crop development and growth, including grain protein content. apsim -Nwheat has been validated for soil water, soil N, crop phenology, biomass production and yield. However, previous analyses have indicated that model performance was poor in terms of grain protein simulations particularly under very high or very low N input conditions together with terminal drought. The original routine for grain protein content simulated grain protein as a function of independent dry matter and N accumulation into the grain. To constrain grain protein content simulations under very high and very low N input conditions, without effecting other simulations, a simple but physiologically sound link was incorporated between dry matter and N translocation to the grain. An upper boundary of daily protein transfer to the grain was set to 4% N (22.8% grain protein), and a lower boundary was set at 1.23% N (7% grain protein). In addition, grain N was initialised with up to 3% N (17.1% grain protein) at the beginning of the grain filling phase, depending on the availability of tissue N. The new modified grain protein routine was tested across field data sets from a wide range of growing conditions and showed an improved performance with a RMSD, consistent across environments and soil types, at or below 0.35% N (2% grain protein). Independent tests, including data from different climates and a sensitivity analysis confirmed the improved simulation of grain protein under a wide range of conditions. The improved model was found to be reliable and robust enough to be used for specific simulation experiments to study grain protein interactions with management, soil types and environments in different climatic regions.

Use of modelling to explore the water balance of dryland farming systems in the Murray-Darling Basin, Australia

Abstract The Agricultural Production Systems Simulator (APSIM) modelling framework was used to explore components of the water balance for a range of farming systems in the Murray-Darling Basin (MDB) of Australia. Water leaking below the root zone of annual crops and pastures in this region is leading to development of dryland salinity and delivery of salt to waterways. Simulation modelling was used to identify the relative magnitude of transpiration, soil evaporation, runoff and drainage and to explore temporal variability in these terms for selected locations over the 1957–1998 climate record. Two transects were used to explore the impact of climate on water balance, with all other factors held constant, including the soil. An east–west transect at approximately latitude 33°S demonstrates the primary effect of annual average rainfall ranging from 300 to 850 mm. A north–south transect along approximately the 600 mm rainfall isohyet demonstrates a secondary effect of rainfall distribution, with the fraction of annual rainfall received in winter months rising from 40% in the north to 70% in the south. Water excess (i.e. runoff plus drainage) is strongly episodic, with 60% simulated to occur in 25% of years. Longer term cycles are also evident in the time series simulations, with strong below average periods from 1959–1968 and 1979–1988 interspersed with extended periods of above average water excess from 1969–1978 and 1989–1993. Water excess was highest for the annual wheat farming system and lowest for perennial lucerne pasture. Other systems that mix summer and winter annuals (opportunity cropping) or include wheat and lucerne pasture in different temporal combinations (phase farming and companion cropping) were intermediate in their simulated water excess. These differences in water balance of the farming systems simulated were associated with differences in grain and forage yields that will affect their economic viability. The predictions of annual water excess derived from the dynamic, daily time-step modelling using APSIM for a wheat based farming system were of similar magnitude as those predicted by the Zhang et al. (2001) static model for shallow rooted pasture catchments, whilst continuous lucerne was similar to predictions for deep rooted forest catchments. To capture the effect of rainfall distribution between winter and summer an additional term was added to the Zhang model. This modified function captured 88% of the variation in the APSIM predictions of annual average water excess from annual wheat systems for 78 locations in the MDB.

Role of modelling in improving nutrient efficiency in cropping systems

The applicability of models in addressing resource management issues in agriculture has been widely promoted by the research community, yet examples of real impacts of such modelling efforts on current farming practices are rare. Nevertheless, simulation models can compliment traditional field experimentation in researching alternative management options. The first objective of this paper is, therefore, to provide four case study examples of where models were used to help research issues relating to improved nutrient efficiency in low-input cropping systems. The first two cases addressed strategies of augmenting traditional farming practices with small applications of chemical fertilizer (N and P). The latter two cases explicitly addressed the question of what plant genetic traits can be beneficial in low-nutrient farming systems. In each of these case studies, the APSIM (Agricultural Production Systems Simulator) systems model was used to simulate the impacts of alternative crop management systems.The question of whether simulation models can assist the research community in contributing to purposeful change in farming practice is also addressed. Recent experiences in Australia are reported where simulation models have contributed to practice change by farmers. Finally, current initiatives aimed at testing whether models can also contribute to improving the nutrient efficiency of smallholder farmers in the SAT are discussed.

The development of strategies for improved agricultural systems and land-use management

There is a pressing need for better management of agricultural lands in much of the developing world. Understanding what changes are needed and how they might be stimulated is the central task of agricultural systems research. But past experience in systems research in non-agricultural fields, where systems research methods developed, shows that a scientific approach to management has had limited effect on what managers actually do. In agriculture, farming-systems research methodology has demonstrated the importance of farmer involvement if research is to change the practice of agriculture. Yet, although participation appears necessary, it has proved to be insufficient in systems where there are strong resource constraints.

Occurrence and simulation of nitrification in two contrasting sugarcane soils from the Australian wet tropics

The concentration of ammonium-nitrogen (NH4+-N) frequently exceeds that of nitrate-N (NO3--N) in Australian wet tropical sugarcane soils. The amount of mineral N in soil is the net result of complex processes in the field, so the objective of this experiment was to investigate nitrification and ammonification in these soils under laboratory conditions. Aerobic and saturated incubations were performed for 1 week on 2 wet tropical soils. Net NO3--N increased significantly in both soils during both types of incubation. A second series of aerobic incubations of these soils treated with NH4+-N and inoculated with subtropical nitrifying soils was conducted for 48 days. Nitrification in the wet tropical soils was not significantly affected by inoculation, and virtually all added N was nitrified during the incubation period. Mineral N behaviour of the 48-day incubations was captured with the APSIM-SoilN model. As nitrification proceeded under laboratory conditions and was able to be captured by the model, it was concluded that nitrification processes in the wet tropical soils studied were not different from those in the subtropical soils. Processes that remove NO3- from the soil, such as leaching and denitrification, may therefore be important factors affecting the proportions of NH4+-N and NO3--N measured under field conditions.

Phosphorus requirements of tropical grazing systems: the northern Australian experience

Background and aimsExtensive grazing is common on low phosphorus (P) soils in tropical areas. In this study we aimed to use experience and research results from northern Australia to investigate methods to manage low P status to efficiently raise animals without exploiting soil P resources.ScopeSimple “farm-gate” P balances were calculated for four cattle breeding and growing operations. Estimated P balances were slightly negative (outputs>inputs) for extensive breeding operations (0.02 to 0.04 kg/ha/year depending on P supplementation), slightly positive for a system growing young animals with small annual P fertilizer applications, and negative for a mixed grazing-cropping enterprise. In northern tablelands pastoral environments, responses to P application have remained unchanged over the last 50 years, with >80% of paddocks remaining P limited. Liveweight gain responses to P fertilizer in most experiments have been <4 kg LWG/kg P applied as fertilizer.ConclusionsLittle or no P fertilizer is used in most grazing systems. Supplying P directly to animals as a feed supplement can economically improve animal performance and contribute to an improvement in the P balances of grazing systems. Guidelines for supplement use have been devised. Further research is required to determine the best application strategies to overcome P deficits, and the best farming systems to minimise P requirements.

Using systems modelling to explore the potential for root exudates to increase phosphorus use efficiency in cereal crops

Enhanced citrate release from crop roots has been one of the recent breeding targets for increased phosphorus (P) use efficiency (PUE), due to the potential of root citrate to solubilise soil P. However, it is unclear about the level of citrate efflux required to significantly impact on crop PUE in different soils. This paper presents a modelling approach to assess the field level impact of root exudates on crop PUE. The farming systems model, APSIM, was modified to include the effect of root citrate efflux on P availability in soil, crop P uptake and growth. With parameters derived from literature, the model was used to simulate the long-term impact of root citrate across soil and climatic conditions. Preliminary results showed contrasting long-term and short-term impacts due to either the accumulated effect of solubilisation or the depletion of soil P reserve. The major impact of enhanced citrate efflux is to increase the efficiency of applied P. The enhanced model enables simulations of a wide range of combinations of Genotype by Environment by Management (GxExM) scenarios, to address knowledge gaps, and to assist in design of field testing for validating the performance of new wheat varieties across environments. Highlights? Citrate release from plant roots can solubilize phosphorus (P) bonded in soil. ? It is unclear how much citrate is required to significantly impact on crop P use efficiency (PUE). ? A modelling approach is developed to assess impact of citrate efflux on crop PUE. ? It enables to explore short and long-term impact of citrate efflux on crop PUE. ? Results show major impact of enhanced citrate efflux is to increase the PUE of applied P.