Soil N process inhibitors alter nitrogen leaching dynamics in a pumice soil

Abstract

A field lysimeter experiment, using a free-draining pumice soil, was carried out to investigate the effect of different soil nitrogen (N) process inhibitors on the fate of 15N-labelled cow urine. The treatments were a urease inhibitor (Agrotain; N-(n-butyl) thiophosphoric triamide), 2 nitrification inhibitors (dicyandiamide, DCD; 4-methylpyrazole, 4MP), a combination (DCD+Agrotain), a urine control, and a nil urine. The inhibitors were mixed with cow urine, which was then applied in a single application (equivalent to 775 kg N/ha) to lysimeters in autumn and monitored over the following 196 days. DCD and 4MP similarly reduced nitrate leaching by 59%, from 114 to 47 kg N/ha compared with the urine control. Of the DCD applied, 58% of it (8.7 kg/ha) was recovered in leachate, and represented an N loss of 5.8 kg/ha. The presence of Agrotain reduced ammonia (NH3-N) emissions by 64% (equivalent to 70 kg N/ha) over the short term (first 20 days), but led to large leaching losses of urea-N (25 kg N/ha) over the medium term (76 days). The Agrotain-DCD combination resulted in even larger losses of urea-N (45 kg N/ha). The major N component measured in leachate (below 450 mm) was ammonium-N, which constituted about 60% of the average total N (205 kg N/ha) leached. Ammonium-N leaching was rapid and almost entirely driven by macropore flow processes. Further research is required in deeper soil profiles, and in relation to climatic risk of high early rainfall on fresh urine patches, to determine the importance of macropore processes on N loss under typical on-farm soil conditions.