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, email@example.com
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.