Role of a Nitrification Inhibitor to Improve Nitrogen Use Efficiency of Fertiliser and Manure in Vegetable Systems.

Use of manures and fertilisers in vegetable production systems can result in very high nitrogen (N) loads (eg. 600 kg N ha^-1 for a 15 week crop). High N combined with high water inputs creates conditions conducive for high leaching and denitrification losses, reducing nitrogen use efficiency (NUE) and creating environmental concerns. These losses can be exaggerated when sandy soils with high leaching potential are used. Nitrification inhibitors, such as DMPP, provide one way of reducing losses, including from both manure and fertilisers. A small plot experiment conducted on a vegetable crop (celery) showed that for ammonium fertiliser treatments only, DMPP increased the apparent NUE by an average of 10% relative to non-inhibitor fertiliser treatments (from 28 to 38%). The apparent NUE with the DMPP application to manure only treatments was increased by 4.6% (from 5.7 to 10.3%). The lower NUE with manure is due to the high organic N content of the manures which is not immediately available for plant use. Use of the DMPP with fertiliser improved N recovered in the plant and soil from a single application of enriched fertiliser (¹⁵NH₄ and ¹⁵NO₃, 10 atom% enrichment) as measured in a ¹⁵N microplot. DMPP use increased the recovery of fertiliser N in the plant and soil by 13% (increase from 66 to 79%) and 19% (increase from 52 to 71%), with the variation dependent upon the time of application of the ¹⁵N-enriched fertiliser in the growing season. Where manure was incorporated into the treatments, and spiked with ¹⁵N, addition of DMPP led to an additional 13% of the applied N being recovered in the plant and soil (increase from 60 to 73%),   relative to non-amended manure. DMPP can improve NUE in vegetable production, most noticeably when background levels of N are not elevated by organic N inputs.