Soil nitrogen: can pasture yields be increased by capitalising on seasonal trends in mineralisation and immobilisation?

Matthew T. Harrison1, Karen M. Christie1, Andrew P. Smith2, Richard P. Rawnsley1, Rachelle Meyer3 and Richard J. Eckard2

1 Tasmanian Institute of Agriculture, University of Tasmania, Cradle Coast Campus, Burnie, Tasmania 7320,

2 Faculty of Veterinary and Agricultural Sciences, The University of Melbourne, Victoria 3010

3Australian-German College of Climate and Energy Transitions, University of Melbourne, Parkville, Victoria 3010, Australia


Decomposition of organic nitrogen (N) into inorganic N is known as mineralisation. This process and its converse, immobilisation, may occur simultaneously and continuously under the controls of soil temperature, moisture, texture and organic N content. As plants utilise inorganic nitrogen as the primary source of N, the rate of net mineralisation (i.e., the difference between gross mineralisation and gross immobilisation) may be a determinant of subsequent pasture growth. Here we examine how the timing of inorganic N fertilisation influences net N mineralisation and pasture growth. We simulated the long-term effects of adding urea fertiliser in August, when pasture growth was near its peak, or in December, when pasture growth was often lowest; the latter treatment being designed to test whether N would be immobilised and released from soil organic matter in the following growing season(s). Pastures were cut on a monthly basis with litter returned to the soil. Application of 100 kg N/ha in December resulted in 7-14 kg N/ha.year greater net mineralisation within and across all years in the simulation relative to N fertiliser applied in August. These trends were consistent in both the vertosol and chromosol soils, suggesting that time of year of fertilisation had a significant and sustained influence on subsequent net N mineralisation. N fertilisation in August partially relieved N stress and stimulated growth (growth rates in August were on average 4 kg DM/ha higher than in December), but also resulted in lower net N mineralisation relative to fertiliser applied in December. We found that greater N utilisation of pastures in the August-fertilisation treatment in concert with litter of higher N concentration resulted in lower potential for immobilisation compared with the December-fertilisation treatment.


The Australian Society of Agronomy is the professional body for agronomists in Australia. It has approximately 500 active members drawn from government, universities, research organisations and the private sector.

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David Marland Photography Graham Centre for Agricultural Innovation, Charles Sturt University

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