4. The OVERSEER Nutrient Budget Model and Estimates of N Leaching
The OVERSEER® nutrient budget model is an empirical, annual time-step model (Ledgard et al. 1999). It provides average estimates of the fate of the nutrients N, P, K and S in kg/ha/year, ignoring year-to-year variability due to climate. The model contains a number of internal databases with nutrient concentrations of fertilisers, animals, products, crop framework, and crop residues. These are used for estimating the nutrient inputs or outputs on a per-hectare basis.
From an environmental perspective, N is often the nutrient of most interest because of its potential effects on water quality (e.g. Boothroyd et al. 2000). The OVERSEER® nutrient budget model provides average estimates of the fate of N for a range of pastoral, arable and horticultural systems, including N leaching. It gives long-term average annual estimates (ignoring year-to-year variability) of N flows and for the pastoral and horticultural systems it is assumed that the soil organic N is at an equilibrium level. An N balance model concept is used whereby S N inputs = S N outputs.
In pastoral systems, N leaching is determined by the amount of N in fertiliser, farm dairy effluent and that excreted in urine and dung by grazing animals. The latter is calculated from the difference between N intake by grazing animals and N output in animal products, based on user inputs of stocking rate or production and an internal database with information on the N content of pasture and animal products. The loss factor for urine or dung is dependent on soil and rainfall, based on a summary of New Zealand and overseas research.
Figure 2 summarises data from field studies where N leaching has been estimated under dairy farm grazing systems. The study at Dexcel No.2 dairy with three different rates of N fertiliser is the most detailed and long-term dairy farmlet experiment in New Zealand (Ledgard et al. 1999) and highlights the relatively large increases in N leaching with N fertiliser input. A farmlet study at the Westpac Trust Taranaki Research Station compared an intensive nil-N fertiliser farmlet under standard winter grazing management with a farmlet which simulated cows going onto a feed-pad from April to July, and highlights the significance of N excreted in autumn and winter to total leaching losses (Ledgard et al. unpublished). The other two farm sites in Otago and Southland in Figure 2 refer to measurements made over two years on commercial dairy farms (Monaghan et al. 2002). Over all sites and treatments, there was good agreement between measured and modelled estimates of N leaching with values differing by <20% in all cases.
Figure 2: Comparison of average annual N leaching measured in dairy farm systems with estimates using the OVERSEER® nutrient budget model.
In cropping systems, N leaching is a product of the monthly N surplus (S N inputs – N in products) and estimated monthly drainage (based primarily on rainfall). The N inputs from fertiliser, supplements or irrigation are calculated from user-defined values for the amount of fertiliser, supplements or irrigation used in their farm system and values for the N concentration of these inputs from the database within the model. An important N input occurs from mineralisation due to cultivation and is calculated to decline exponentially with increasing time out of pasture, with some adjustment for soil group, rainfall and region (temperature effect). The N output in product is a function of user-defined yield and an average N concentration of the product from the model database.
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