The impacts of urease inhibitor and method of application on the bioavailability of urea fertiliser in ryegrass (Lolium perenne L.)
Degree GrantorUniversity of Canterbury
Degree NameDoctor of Philosophy
The use of urea fertiliser has been associated with relatively poor nitrogen (N) use efficiency (NUE) due to heavy N losses such as gaseous emissions of ammonia (NH₃) and nitrous oxide (N₂O) and nitrate (NO₃⁻) leaching into surface and ground waters. Improving N use-efficiency of applied urea is therefore critical to maximise its uptake and to minimise its footprint on the environment. The study was conducted under laboratory-glasshouse conditions (Chapter 2-4)and lysimiter-field plot studies (Chapter 5). In chapter 2, Two glasshouse-based experimentswere conducted to investigate the potential of incorporating urea fertiliser with ureaseinhibitor, (N-(n-butyl) thiophosphoric triamide (nBTPT) or ‘Agrotain’) to enhance fertiliser N uptake efficiency. Urea, with or without Agrotain, was applied to Ryegrass (Lolium perenne L.) grown in standard plant trays maintained at soil moisture contents of 75–80% field capacity, at rates equivalent to 25 or 50 kg Nha⁻¹. These treatments were compared with other common forms of N fertilisers (ammonium nitrate, ammonium sulphate and sodium nitrate). In a separate pot experiment, granular ¹⁵N urea (10 atom %) with or without Agrotain, was applied at 25 kg Nh⁻¹ to track N use-efficiency and the fate of ¹⁵N-labelled fertiliser. In both experiments, Agrotain-treated urea improved bioavailability (defined as the fraction of total soil N that can interact with a biological target in the plant or that can be taken up by plant) of added N and resulted in significantly higher herbage DM yield and N uptake than urea alone or other forms of N fertilisers. 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.
In chapter 3, two more glasshouse-based experiments were conducted to investigate if urea applied in fine particle application (FPA), with or without Agrotain, had any effect on fertiliser-N uptake efficiency (defined as the difference in N uptake between the fertiliser treatment and the control as a percentage of the amount of N applied) under optimum soil moisture (75-80% field capacity) and temperature (25 °C) conditions, in comparison with other common forms of N fertilisers applied, either in FPA or in granular form. In a separate pot experiment, ¹⁵N urea (10 atom %), with or without Agrotain, was applied to either shoots or leaves only or to the soil surface (avoiding the shoots and leaves) to determine urea hydrolysis, herbage DM and ¹⁵N uptake. In both experiments, herbage DM yield and N uptake were significantly greater in the FPA treatments than in those receiving granular application. Agrotain-treated urea FPA resulted in significantly higher N response efficiency (difference between the dry matter produced by the various fertiliser treatments and the control, divided by the amount of N applied) than urea FPA alone or other forms of N fertilisers. Results from the ¹⁵N experiment support the idea that Agrotain treatment improves the N response of urea applied in FPA form due to a delay in hydrolysis of urea, thus providing herbage an extended opportunity to absorb added urea directly through leaves, cuticles and roots.
A further glasshouse-based study was conducted to investigate the effect of Agrotain and irrigation on urea hydrolysis and its movement in a Typic Haplustepts silt loam soil (Chapter 4). A total of 72 repacked soil cores (140 mm inner diameter and 100 mm deep) were used - half (36) of these cores were adjusted to soil moisture contents of 80% field capacity (FC) and the remaining 36 cores to 50% FC. Granular urea, with or without Agrotain, was applied at a rate equivalent to 100 kg N ha⁻¹. Twelve pots were destructively sampled at each day after 1, 2, 3, 4, 7, and 10 days of treatment application to determine urea hydrolysis and its lateral and vertical movement in different soil layers. Agrotain-treated urea delayed urea hydrolysis compared with urea alone during the first 7 days of its application. This delay in urea hydrolysis by Agrotain enabled added urea to disperse and move away from the surface soil layer to the sub-surface soil layer both vertically and laterally. In contrast, most urea in the absence of Agrotain hydrolysed within 2 days of its application. Irrigation after 1 day resulted in further urea movement from the surface soil layer (0-10 mm) to the sub-soil layer (30-50 mm) in Agrotain-treated urea. These results suggest that Agrotain delayed urea hydrolysis and allowed more time for rainfall or irrigation to move the added urea from the surface layer to sub-soil layers where it is likely to make good contact with plant roots. This distribution of urea in the rooting zone (0-200 mm) has the potential to enhance N use efficiency and minimise N losses via ammonia (NH₃) volatilisation from surface-applied urea.
Finally, a field study using lysimeters (300 mm inner diameter and 400 mm deep), and small field plots (1 m² in area) was established using a silt loam Typic Haplustepts soil (Soil Survey Staff 1998) to investigate the effect of FPA and granular applications of urea, with or without Agrotain, on N losses and N use efficiency (Chapter 5). The five treatments were: control (no N) and ¹⁵N-labelled urea (10 atom %), with or without Agrotain, applied to lysimeters or mini plots (un-labelled urea), either in granular form to the soil surface or in FPA form (through a spray) at a rate equivalent to 100 kg N ha⁻¹. Gaseous emissions of NH₃ and N₂O, NO₃⁻ leaching, herbage production, N response efficiency, total N uptake and total recovery of applied ¹⁵N in the plant and soil were determined up to 63 days. Urea-alone and urea with Agrotain, applied in FPA form, was more effective than its granular form and reduced N2O emissions by 5-12% and NO3- leaching losses by 31-55%. Urea-alone applied in FPA form had no significant effect in reducing NH₃ losses compared with granular form. However, urea with Agrotain applied in FPA form reduced NH₃ emissions by 69% compared with the equivalent granular treatment. Urea-alone and with Agrotain applied in FPA form increased herbage dry matter production by 27% and 38%, and N response efficiency compared with the equivalent granular urea application, respectively. Urea applied in FPA form resulted in significantly higher ¹⁵N recovery in the shoots compared with granular treatments – this was improved further when urea in FPA form was applied with Agrotain. Thus, treating urea with Agrotain in FPA under field conditions has the potential to delay its hydrolysis, minimise N losses and improve N use efficiency and herbage production. The lower dry matter production and N-response efficiency to urea applied in FPA form in Chapter 3 are probably because of additional factors such as lower application rates (25 kg N ha⁻¹ ) or lack of interception of urea by the leaves. Applying urea in FPA form is a good management strategy and I conclude that combining FPA urea with Agrotain has the potential to increase N use efficiency and herbage production further.