The impact of urease inhibitor on the bioavailability of nitrogen in urea and in comparison with other nitrogen sources in ryegrass (Lolium perenne L.)

Abstract

Improving nitrogen (N)-use efficiency of applied urea is critical to maximise its uptake and decrease environmental impact. Two glasshouse-based studies were conducted to investigate the potential of incorporating urea fertiliser with urease inhibitor (N-(n-butyl) thiophosphoric triamide (NBPT) or ‘Agrotain’) to enhance fertiliser N uptake efficiency. Topsoil (0–0.075 m, Typic Haplustepts silt loam) from a pasture site near Lincoln, Canterbury, New Zealand, was collected and ryegrass (Lolium perenne L.) was grown from seed in standard plant trays maintained at soil moisture contents of 75–80% field capacity. Urea, Agrotain-treated urea, ammonium nitrate, ammonium sulfate, or sodium nitrate, were applied in granular form at rates equivalent to 25 or 50 kg N/ha with 4 replicates. Herbage was harvested 21 and 42 days after application of treatments to assess dry matter (DM) production, N uptake, leaf amino acid, ammonium (NH4+) and nitrate (NO3–) concentrations, and nitrate reductase activity (NRA). In a separate pot experiment, granular 15N urea (10 atom%) with or without Agrotain was applied to ryegrass at 25 kg N/ha. At 0.5, 1, 2, 3, 5, 10, and 21 days after treatment application, 3 pots per treatment were destructively sampled to determine urea hydrolysis, herbage DM, and 15N uptake. In both experiments, Agrotain-treated urea improved bio-availability of added N and resulted in significantly higher herbage DM yield and N uptake than urea alone or other forms of N fertilisers. Agrotain-treated urea applied at 25 kg N/ha increased N response by 66% compared with urea alone (and by greater proportions compared with the other fertiliser forms). Agrotain-treated urea applied at 25 kg N/ha produced significantly higher uptake efficiency (13 g DM/g of applied N) than at 50 kg N/ha (5 g DM/g of applied N). Tissue amino acids, NH4+ and NO3– contents, and NRA were not significantly influenced by any type of fertiliser. Results from the 15N experiment support the suggestion that a delay in urea hydrolysis by Agrotain provided an opportunity for direct plant uptake of an increased proportion of the applied urea-N than in the case of urea alone. Treating urea with Agrotain thus has the potential to increase N-use efficiency and herbage production.