Abstract
Most dryland grain growers in Australia retain all or most of their crop residues to protect the soil from erosion and to improve water conservation but retaining stubbles with a high carbon-to-nitrogen (C:N) ratio can affect N availability to crops. A simulation experiment was conducted to investigate the effects of N fertilizer application rate and residue retention on soil N dynamics. The simulation used seven N fertilizer application rates (0, 25, 50, 75, 100, 150 and 200 kg N ha−1) to wheat (Triticum aestivum) over 27 years (1990–2016) at four locations across a gradient in annual rainfall in Victoria, Australia. Nitrogen immobilization, denitrification and N leaching loss were predicted and collectively defined as sources of N inefficiency. When residues were retained, immobilization was predicted to be the biggest source of inefficiency at all simulated sites at N application rates currently used by growers. Leaching became a bigger source of inefficiency at one site with low soil water-holding capacity, but only at N rates much higher than would currently be commercially applied, resulting in high levels of nitrate (NO3−) accumulating in the soil. Denitrification was an appreciable source of inefficiency at higher rainfall sites. Further research is necessary to evaluate strategies to minimize immobilization of N in semi-arid cropping systems.
Highlights
World population and food demand will both continue to increase in the three decades, at least
We applied the Agricultural Production Systems sIMulator (APSIM), which is based on a set of biophysical modules that simulate biological and physical processes in farming systems [29], to quantify the relative importance of immobilization, denitrification and leaching as sources of N inefficiency in wheat-based farming systems in southeast Australia
In the case of N at sowing, there was a threshold level of fertilizer application at each site above which increases were rapid and large. This coincided with N balance becoming positive (Figure 4)
Summary
World population and food demand will both continue to increase in the three decades, at least. Fischer and Connor [1] conclude that staple grain crop yields need to increase by 1.1% p.a. relative to 2010 levels to meet increasing demand, but future yield gains must be supported with environmentally appropriate inputs of plant nutrients [2]. Nitrogen (N) is an essential macro-nutrient for plant growth and a key agricultural input that has been increasingly used in food production since the invention of the HaberBosch process to reduce atmospheric N2 gas to reactive forms of N [3]. Nitrogen is the most limiting nutrient for non-leguminous crop production in most agricultural areas in the world. N can be lost from the soil-plant system
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