Unused fertiliser nitrogen in arable soils—its contribution to nitrate leaching

✍ Scribed by Andrew J Macdonald; David S Powlson; Paul R Poulton; David S Jenkinson

• Publisher: John Wiley and Sons
• Year: 1989
• Tongue: English
• Weight: 623 KB
• Volume: 46
• Category: Article
• ISSN: 0022-5142
• DOI: 10.1002/jsfa.2740460404

✦ Synopsis

Nitrate present in arable soils in autumn is at risk to leaching during the following winter. To see whether unused nitrogen fertiliser was a major source of this nitrate, "N-labelled fertiliser was applied to I 1 winter wheat crops at rates of between 47 and 234 kg N ha-' in spring. The experiments were on three contrasting soil types in south-east England.

On average, 17% of the N ffom spring-applied labelled fertiliser remained in the 0-23 ern soil layer at harvest (range, 7-36%) but only a small proportion was in inorganic forms (ammonium + nitrate). This was never more than 5 kg N ha-' and averaged only 1.3 % of the fertiliser N applied (range, 0-4-36 %). Between 79 and 98 % of the inorganic N in soils at harvest was unlabelled, being derived f f o m the mineralisation of organic N rather than ffom unused fertiliser. The amount of unlabelled N was much greater where wheat was grown afer ploughing up grass or grasslclover leys than where it was grown in all-arable rotations. When wheat was grown without N fertiliser, soil inorganic N content at harvest was no lower than in plots given fertiliser at rates up to 234 kg N ha-'.

This work indicates that, for soil growing winter wheat, almost all of the nitrate at risk to leaching over the winter period comes from mineralisation of organic N , not f f o m unused fertiliser applied in spring. Consequently, even a drastic reduction in N fertiliser use would have little efect on nitrate leaching.