Report for Defra Project NT2601

NT2605 Final Report WP3 Optimum use of Agrotain

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Component report for Defra Project NT2605 (CSA 6579)

WP3 Optimum use of nBTPT (Agrotain) urease inhibitor

Lead Authors

Dr Catherine J Watson and Dr Nasir A Akhonzada, Queen’s University of Belfast

November 2005

Contents

1. EXECUTIVE SUMMARY………………………………………………………………………………….4
2. INTRODUCTION…………………………………………………………………………………………...5
3. EXPERIMENTAL DESIGN, TREATMENTS AND METHODS………………………………...…..…8
4. RESULTS AND DISCUSSION………………………………………………………………………….12
5. CONCLUSIONS……………………………………………….………………………………………….32
6. FUTURE RESEARCH REQUIREMENTS………………………………………………………...…...34
7. REFERENCES……………………………………………………………………………………………35
8. APPENDICIES…………………………………………………………………………………………….37

Abbreviations

1. Executive Summary

The current study evaluated the effect of rate of application of the urease inhibitor nBTPT, trade name ‘Agrotain’, (0, 100, 250, 500, 750 or 1000 ppm w/w active ingredient nBTPT) on NH3 volatilisation in three formulations of urea (coated, added to the urea melt or in UAN solutions), at three temperatures (5, 15 and 25C) and with four contrasting soil types. Ammonia volatilisation was studied for up to 21 days after surface N application, using ventilated soil enclosures, under laboratory conditions. Asymmetrical sigmoidal curves were fitted to cumulative daily NH3 volatilisation data and the time of maximum rate of loss (Tmax) and maximum daily loss rate were estimated. In addition, the stability of nBTPT in different formulations was investigated by high pressure liquid chromatography.

Agrotain was highly effective in lowering NH3 volatilisation from urea. The average
% inhibition over all soils, temperatures and formulations was 61.2%, 69.9%, 74.2%, 79.2% and 79.8% for the 100, 250, 500, 750 and 1000 ppm nBTPT concentration, respectively. The % inhibition with nBTPT was lower at 15C compared to 5C or 25C and was lower in UAN solution than in granular products. It was suggested that this effect may be due to the speed of formation of the oxygen analogue in soil and/or its stability. There was little difference between the melted and coated granular products in reducing NH3 loss or in soil N transformations.There was little additional benefit in using concentrations of nBTPT above 500 ppm in any formulation. The inhibitor not only lowered total NH3 volatilisation and the maximum daily rate, but delayed the time of maximum rate of loss (Tmax). Under field conditions, delaying Tmax would increase the opportunity of rain falling to move urea below the soil surface and lower NH3 loss.

Ammonia loss from unamended urea varied with soil type and temperature and ranged from 8.2% to 31.9% of the N applied. It was not possible to explain the variation in NH3 emission with temperature or soil properties.

The stability of nBTPT was highly dependent on temperature in all fertiliser formulations, with the rate of degradation being greatest at 25C. nBTPT was much less stable in the coated urea fertilisers than in either the melted or UAN products. The nBTPT in the melted products maintained in bulk quantities at 4C was stable at all concentrations.

2. Introduction

2.1.The NT26 Research programme

The NT26 research programme was set up by Defra to investigate the nitrogen (N) loss pathways, the environmental and economic impacts, and the response of agricultural and horticultural crops to different forms of fertiliser-N. The NT2605 project was part of a suite of projects in this programme as shown below (Final report submission dates shown in brackets).

• Nitrogen fertilising materials (June 2003)
• Production and use of nitrogen fertilisers (August 2003)

• Evaluation of urea-based nitrogen fertilisers (October 2003)

• The behaviour of some different fertiliser-N materials (March 2004)

• Ammonia emissions from nitrogen fertilisers – wind tunnel construction (March 2004)

• Spreading accuracy of solid urea fertilisers (August 2005)

The following leading UK agri-environment research organisations participated in all NT26 projects (except NT2610), including the NT2605 project reported here.

• ADAS UK Ltd
• Edinburgh University (EU)
• Warwick HRI (HRI)
• Institute of Grassland and Environmental Research (IGER), North Wyke
• Queens University, Belfast (QuB)
• Rothamsted Research (RR)
• SAC Commercial Ltd (SAC)

The project was led by Peter Dampney, Principal Research Scientist, ADAS Boxworth Research Centre, Cambridge who was the main point of contact with the Defra NT26 Steering Group.

2.2.The NT2605 project

The NT2601, NT2602 and NT2603 projects provided the basis for the field experimental and other work carried out in NT2605 in cropping seasons 2003/04 and 2004/05. The overall aim of the project was to develop working decision support systems (DSS) to evaluate the agronomic, environmental and economic impacts that would result from changes in the use of different fertiliser-N materials in UK agriculture. More specifically, project work packages (WP) covered the following topic areas:-

Reporting of the NT2605 has been structured into a suite of 8 component reports, one for each work package plus an over-arching Executive Summary for the whole project. Each report is self contained with its own Executive Summary, but interacts with data and conclusions from other WPs where appropriate.

2.3. WP3 Optimum use of nBTPT (Agrotain) urease inhibitor

Fertiliser urea can be an inefficient N source due to rapid hydrolysis by the soil enzyme urease, leading to NH3 volatilisation. The efficiency of urea can be improved by soil incorporation or by use of urease inhibitors. As 97% of all N fertilisers used in the UK are top-dressed to crops, there is little scope for the widespread use of soil incorporation as a mitigation strategy to lower NH3 emissions from urea. Urease inhibitors delay the rate of urea hydrolysis and hence prevent localised zones of high pH, which are conducive to NH3 volatilisation. Many compounds have been evaluated as urease inhibitors (Martens and Bremner, 1984, Mulvaney and Bremner, 1978, Watson, 2000), however, few meet the requirements of being effective at low concentrations, non-toxic, stable, inexpensive and compatible with urea. N-(n-butyl) thiophosphoric triamide (nBTPT), a structural analogue of urea, is currently the most promising. Its urease inhibitory activity in soil is associated with the activity of its derivative, the oxygen analogue, N-(n-butyl) phosphoric triamide (NBPTO). The factors that affect the rate of conversion to the oxygen analogue have not been fully elucidated and will probably depend on a number of biotic and abiotic soil properties. The conversion to nBTPT to its oxygen analogue is generally rapid, occurring within minutes or hours in aerobic soils (Byrnes and Freney, 1995).

Until recently, the only European work with nBTPT had been on grassland by Watson (2000) in Northern Ireland, who showed that nBTPT had considerable potential for improving the efficiency of urea under temperate conditions. Coating nBTPT onto urea granules lowered NH3 volatilisation and increased N recovery and dry-matter yield of ryegrass compared with urea alone. The inhibitor was active at low concentrations and there appeared to be little benefit in using concentrations above 0.1% (w/w) nBTPT (Watson et al., 1994). The beneficial effects of coating urea with nBTPT have been confirmed in recent field trials with grassland and tillage land during 2003, in the Defra funded project NT2603 (Dampney et al., 2004).

In considering the potential for use of nBTPT in Europe, further work is required to evaluate alternative formulation methods and to determine the optimum rates to reduce NH3 loss. The inhibitor can be used to coat urea granules, be added to the urea melt during manufacture, or be added to urea ammonium-nitrate (UAN) solutions prior to surface application to soil. There is no published information on the efficacy of nBTPT to lower NH3 loss when added to the urea melt or used in UAN solutions. The stability of nBTPT in different formulations under different storage conditions is also an important consideration requiring further investigation.

The current study aimed to evaluate the effect of rate of nBTPT (0, 100, 250, 500, 750 or 1000 ppm w/w) on NH3 volatilisation in three formulations of urea (coated, added to the urea melt or in UAN solutions), at three temperatures (5, 15 and 25C) and with four contrasting soils. Ammonia volatilisation was studied for up to 21 days after N fertiliser application, using ventilated soil enclosures, under laboratory conditions. The stability of nBTPT in different formulations was investigated by high pressure liquid chromatography.

3. Experimental design, treatments and methods

3.1. Soils

Representative surface samples (0-10 cm) of two arable (ADAS Gleadthorpe, GL and Boxworth, BX) and two grassland soils (ADAS High Mowthorpe, HM and IGER North Wyke - Debathe site, DB) with different chemical and physical properties, were collected in February 2004. These soils were used in Work package 1b, thus providing a comparison of field and laboratory based measurements of ammonia emissions. The field moist soils were air dried at 30ºC for a minimum of 18 hours and coarsely sieved through a 6 mm sieve to remove large stones and plant debris. A 0.5 kg sub-sample of each soil was ground (2 mm) prior to other soil analyses.

3.2. Soil analyses

Soil chemical (pH, cation exchange capacity, loss on ignition, % C, extractable P, K and Mg) and physical analyses (% sand, % silt and % clay) were determined according to standard methodology (MAFF, 1986) and are shown in Table 1. Soil available extractable P, K, and Mg were expressed on a soil volume basis (i.e. mg l -1). The gravimetric moisture content of the soils at field capacity was determined using the Haines method (Rowell, 1994), as the retained water capacity under a tension of 0.05 bar. Urease activity was measured on soil rewet to field capacity and allowed to equilibrate for one week at 15 C. Urease activity was based on the determination of NH4+-N released during soil incubation with THAM (Tris (hydroxyl-methyl) aminomethane) buffer and urea solution at 37ºC for 2 hours (Tabatabai, 1982). The NH4+-N released was determined using an automated continuous flow wet chemistry analyser (Skalar, SAN++), following soil extraction with 2M KCl containing 5 μg ml-1 PMA (phenyl mercuric acetate) to stop urease activity.

3.3. Fertiliser

The granular products were produced with Agrotain added to the urea melt, by fluidized bed granulation, to give target concentrations of 0, 100, 250, 500, 750 and 1000 ppm nBTPT on a urea weight basis. Agrotain refers to the clear green liquid containing 25% nBTPT as the active ingredient. The nBTPT was in a mixed solvent consisting of 10% by weight of N-methyl-pyrrolidone (NMP) with the balance consisting of propylene glycol. No Agrotain was added to the 0 ppm nBTPT product so there was no NMP or propylene glycol present in these granules. The melted products were manufactured on 22nd January 2004 and analysed for nBTPT concentration on 28th January 2004. Approximately 2 kg of each of the granular products was delivered to Queen’s University, Belfast (QUB) on 6th February 2004 and was stored in a cold room at 4C in sealed bags, prior to use. The same batch of product was used throughout work package 3.

Urea was spray impregnated (i.e. coated) with Agrotain at ADAS Boxworth, to give target concentrations of 0, 100, 250, 500, 750 and 1000 ppm nBTPT on a urea weight basis. To achieve the different concentrations, a standard volume (24 mls) of Agrotain or Agrotain diluted with propylene glycol was used to spray a 6 kg batch of urea using a high velocity centrifugal seed dressing machine. The coating of the urea products commenced on 13 February 2004. However, as there was concern that the coated products may not be stable over the long-term, additional products were coated on 2 April 2004, 21 May 2004, 30 July 2004 and 11 January 2005. The coated products were stored in a cold room at 4C in sealed bags, prior to use.

The urea ammonium nitrate (UAN) solution was sourced supplied by ADAS (Nuram 37 product, 37 % N w/v, bulk density 1.289 kg l-1) and Agrotain was added immediately prior to use to give target concentrations of 0, 100, 250, 500, 750 and 1000 ppm nBTPT on a urea weight basis.

The concentration of nBTPT in each of the fertiliser products was validated by high pressure liquid chromatography (HPLC), with ultra violet (UV) detection, at QUB at the start of each experimental run (Appendix 1). Independent analyses of nBTPT showed good agreement with QUB. There was also good agreement between nBTPT concentrations determined by LC-UV and LC-MS (liquid chromatography-mass spectroscopy).

In February 2004 the variability of nBTPT concentration in the granular products (melted and coated) was investigated by analysing 10 x 100 mg samples (to 10 mls water) of each product (melted and coated) and target concentrations (0, 100, 250, 750 and 1000 ppm nBTPT). Results are shown in Appendix 2. The nBTPT concentration in the coated products was more variable that in the melted products, as indicated by the higher standard deviation. In addition, analytical reproducibility was investigated on selected products by analysing the same solution ten times, and was found to be excellent (Appendix 3).

3.4. Experimental design

Each NH3 volatilisation run consisted of four soils (BX, GL, HM & DB), six concentrations of the inhibitor nBTPT (0, 100, 250, 500, 750, 1000 ppm) and three formulations (Agrotain impregnated onto urea granules (coated), Agrotain added to the urea melt (melted) and Agrotain added to UAN solution). In addition, two unfertilised samples of each soil were also incubated in each run to act as controls. Each experimental run required 80 jars of soil, which were arranged in a completely randomised design over five shelves in the cabinet; with 40 jars being ventilated from the right hand side and 40 jars being ventilated from the left hand side of the cabinet. Using three temperature regimes (5ºC, 15ºC and 25ºC) and four replicates required twelve separate runs each lasting up to 21 days. An error in the volume of UAN applied resulted in 2 runs being repeated and the number of replicates for the UAN treatment being reduced from 4 to 3. Statistical analysis showed that there was no significant positional effect within the cabinet on any of the parameters studied.

3.5. Measurement of NH3 volatilisation

Ammonia volatilisation was studied for 1521 days after fertiliser application in a dark controlled environment cabinet using ventilated enclosures at sufficient flow rates to promote maximum NH3 removal (11 changes of headspace volume per minute). Air-dried soil was placed in cylindrical screw-top plastic jars (80 mm diameter, 85 mm height) and re-wet with deionised water to field capacity. The weight of each soil type per jar was different, which ensured a standard headspace volume of 176 cm3 in each jar. The moist soil was allowed to equilibrate in loosely capped jars at 15ºC for one week before the fertiliser was applied. The fertiliser granules were 2.8 – 3.35 mm in size and were uniformly applied to the soil surface to give 50 mg N per treatment (equivalent to approximately 100 kg N ha-1). The UAN solutions containing the required concentration of nBTPT were mixed immediately prior to application to the soil surface using a pipette, to give 50 mg N per jar.

The lid of each jar was fitted with an input and an output port. Air at a high relative humidity (60- 65%) was blown over the soil surface in each jar using an air compressor and flow meters, at a rate of 2 l minute-1 into traps containing 40 mls of 20 mM orthophosphoric acid, which were replaced daily. The acid was transferred to labelled screw top 60 ml plastic jars and the weight recorded. The samples were stored at 4ºC, prior to analysis of NH3-N using a TRAACS 800 continuous flow analyser (Bran and Luebbe, 1989). The total NH3 loss from the soils receiving no fertiliser was very low and averaged0.006 mg N over 21 days. The NH3-N volatilised was expressed as a percentage of that applied, after adjusting for the controls.

Each jar was weighed prior to the application of fertiliser and at the end of the incubation to determine the moisture loss (by weight difference). At the end of the incubation the total soil in the jar was extracted in 2M KCl and NH4+-N and NO3- -N concentrations in the soil extracts were determined using an automated continuous flow wet chemistry analyser (Skalar, SAN++ 2003). Any NO2--N present was included in the NO3--N determinations. Residual urea was determined colorimetrically (Mulvaney and Bremner, 1979). The KCl soil extracts were stored at 4ºC and analysed within one week. All N concentrations were expressed as a percentage of that applied, after adjusting for the mineral N concentrations in the control samples.

3.6. Stability of nBTPT during storage

The stability of 1000 ppm nBTPT was investigated during 2004 in the coated, melted and UAN products stored in a cool dry fertiliser store. The melted product was the same as that used in the experimental runs to measure NH3 volatilisation. It was manufactured on 22 January 2004. The coated product was spray impregnated with Agrotainmanufactured by ADAS on 13 February 2004. Both granular products were stored in bulk in sealed plastic bags at 4C until 24 March 2004.