Fate and recovery of nitrogen applied as slow release brown coal-urea in field microcosms: ¹⁵N tracer study

The over-use of synthetic nitrogen (N) fertilisers for crop production can cause environmental pollution through leaching and gaseous losses, resulting in low N use efficiency (NUE). Previous work has shown that brown coal (BC) combined with urea can slow down the fertiliser-N release to better synchronise soil N supply with crop N demand. The study aimed to evaluate the impact of granulated BC-urea (BCU) applied to sweet corn on NUE, fate and recovery of fertiliser-N using an ¹⁵N tracer technique. In this in-field microcosm study, 10 atom percent enriched ¹⁵N-labelled urea (46% N) and BCU (20% N) were applied as N fertilisers at rates of 90 or 180 kg N ha^-1. On average, BCU fertiliser reduced the urea-derived ¹⁵N losses as nitrous oxide (N₂O) by 64%, ammonia (NH₃) by 73% and downward movement of total N by 59% compared to urea. Reduced losses of applied BCU fertiliser-¹⁵N were associated with significantly increased microbial immobilisation, soil retention and availability of fertiliser-¹⁵N to plants for longer periods of time, compared with urea. As a result, BCU enhanced cob yield by an average of 23%, ¹⁵N uptake by 21% and fertiliser NUE by 21% over urea. The plant recovery of fertiliser-¹⁵N was significantly higher from BCU (59%) than the recovery from urea (38%). Moreover, mining of native soil-N was lower when the N-fertiliser source was BCU cf. urea, suggesting that BCU could be used as a more N-efficient alternative to urea in cropping systems.