Can urease and nitrification inhibitors improve nitrogen use efficiency in WA cropping?

Key messages

1. CSBP field trials have been evaluating Urea Sustain (urea coated with urease and nitrification inhibitors) to boost nitrogen use efficiency and grower returns.
2. The results show inhibitors can be effective in improving yield, nitrogen use efficiency and grower returns.
3. Across seven replicated field trials in 2022 and 2023, Urea Sustain boosted wheat yields by an average of 5.6% and increased protein by 0.3% over urea. In conditions conducive to nitrogen losses, wheat yield increases were as high as 700kg/ha (25% higher than urea).

Aims

To compare the nitrogen losses between urea and Urea Sustain (urea coated with NBPT and DMPP inhibitors) in WA cropping systems by investigating yield, plant nitrogen uptake and residual soil nitrogen.

Introduction

Nitrogen losses from ammonia (NH³) volatilisation, denitrification and leaching have become an area of concern as growers look to decrease environmental impacts and improve profitability. Increasing nitrogen use efficiency (NUE) helps growers maximise yield and economic return, while decreasing nitrogen (N) losses.

Nitrogen fertiliser losses from volatilisation have been reported at up to 30% of applied N in field trials in WA¹. Losses are dependent on fertiliser type, environmental conditions (temperature, rainfall, wind speed) and soil properties (clay content, CEC and pH)². Losses can be high two to six days after application if there is insufficient rainfall to wash the urea into the soil profile.

Leaching losses occur when N, most often as nitrate (NO³-) dissolved in water moves through the soil profile, draining below the root zone. Rates of loss are highly dependent on soil type, N application rate and rainfall, but N leaching losses on sands have been measured at up to 72kg/ha and modelled as being higher³.

Any applied fertiliser not converted to grain represents an economic loss as well as a potential environmental impact. With current NUE for N applications estimated at 30–60% it makes sense to investigate technologies and products that help crops take up more fertiliser N.

The inhibitors assessed as part of this study are a urease inhibitor (NBPT) and a nitrification inhibitor (DMPP), which are aimed at reducing volatilisation and leaching.

NBPT slows down the conversion of urea to NH³ and increases the proportion of N available for the crop. International literature suggests that NBPT can boost wheat yields by an average of 10% where NH3 volatilisation is an issue⁴.

DMPP aims to slow down the conversion of ammonium (NH⁴+) to NO³- by soil bacteria. By keeping N in the positively charged NH⁴+ form rather than the negatively charged NO³- form, the N applied should be less susceptible to leaching.

CSBP has introduced a new product, Urea Sustain, which is urea coated with both NBPT and DMPP. This is designed to decrease the potential for volatilisation and leaching and increase NUE. This paper summarises the findings of wheat field trials from the 2022 and 2023 seasons and outlines a field study using ¹⁵N recovery to investigate the benefits of the inhibitors in WA cropping systems.

Method

Field trials

In 2022 and 2023 seven replicated small-plot field trials were conducted across the wheatbelt comparing urea to Urea Sustain top-dressed post seeding in wheat at different rates of N. Locations and key soil and seasonal information for each trial are presented below (Table 1).

Rates of applied N ranged from 0kg/ha up to 126kg/ha in all 2023 trials, but only to 84kg/ha in Three Springs in 2022.

Table 1. Locations, soil and seasonal details for field trials used in this study

Additional details regarding wheat varieties, sowing dates, previous rotations and top-dressing timings are available at www.csbpresults.com.au.

Trials were fully randomised and maintained in line with farmer standards throughout the season. They were assessed for N uptake by tissue testing during the season and assessed for yield using a small plot header with grain samples taken for grain quality and nutrient status.

All soil and plant samples were analysed by the CSBP Laboratory in accordance with methodologies detailed on their website www.csbplab.com.au.

After the season, carbon intensity (a measure of kg CO₂ equivalent per tonne of grain) was estimated for all trials using the University of Melbourne G-GAF calculator5.

¹⁵N microplot study

In addition to these field trials there were also three unconfined microplot comparisons using ¹⁵N-enriched urea and ¹⁵N-enriched Urea Sustain. Two were conducted at Mingenew (16 June and 20 July topdressing) on grey sand and one at the Muntadgin field trial. Replicated plots of 1.5m x 1.5m were pegged within the farmer-sown crop and spread with ¹⁵N urea or ¹⁵N Urea Sustain at the equivalent of 42kg N/ha in Mingenew and 63kg N/ha in Muntadgin (27 June). Nil N plots were also included at each site.

At maturity, a quadrat of each plot (11% of the area) was plant sampled to allow analysis of plant material and grain for total N and ¹⁵N uptake. Soil cores were taken from within each plot down to 30cm to assess residual soil ¹⁵N supplies. At the Mingenew sites plant roots were also removed from the area within the quadrats and assessed for ¹⁵N content.

¹⁵N recovery in soil, plant and grain material was measured by Environmental Analysis Laboratory at Southern Cross University.

Results

Field trials

All sites except Gnowangerup showed a significant response to applied N and N application boosted protein in all trials. Table 1 shows the yield achieved with nil N and the yield and protein at the highest rate of N applied for each source (urea or Urea Sustain). Full trial details for each site can be found at www.csbpresults.com.au.

Table 2. Summary results from seven wheat trials in the 2022 and 2023 seasons showing mean grain yields from nil N controls and the highest N rate treatments of urea and Urea Sustain and protein content from these treatments. Significance relates to the comparison of the highest N-rate treatments.

When all trials and N rates were aggregated and analysed Urea Sustain was found to boost yields by an average of 5.6% over urea at the same rate (Figure 1). This higher yield was accompanied by slightly higher protein (average of 0.3% higher for Urea Sustain, results not shown).

The increase in yield also meant a 4.4% lower estimated carbon intensity for wheat grown with Urea Sustain relative to urea in these trials.

Figure 1. Relative yield benefits (LHS) and reduction of estimated carbon intensity (RHS) from Urea Sustain compared to urea when top dressed at Z11–13. Coloured dots are means of three field replicates comparing equal N rates. Boxplots show the interquartile range and medians (vertical lines).

As soil and meteorological conditions can play a large part in the performance of inhibitors it is not surprising that there is a wide range of performance results.

Where conditions were most conducive to N losses (Three Springs, 2022 and Cascades, 2023) we saw the largest increases in N uptake, yield and protein and these trial results are presented in more detail below.

Three Springs 2022

In 2022 the Three Springs trial showed a 700kg/ha yield and 0.95% protein response at the highest N rate of 84kg/ha (Figure 2).

Figure 2. Wheat yield (t/ha) and grain protein (%) responses to N rates supplied as Urea Sustain or urea top-dressed at Z11–13 at Three Springs in 2022. Each dot represents the mean of three replicates.

Cascade 2023

In Cascade in 2023 we saw an 800kg/ha yield response, and a 2.3% increase in protein at the highest N rate of 126kg/ha. In addition to this, soil testing after harvest showed significantly more residual mineral N in the 0-10cm layer in plots that received Urea Sustain, with NO³- N and NH⁴+ N found to be an average of 30% higher than in urea plots. The difference tended to increase with N rate, with the highest rate plots showing a 100% increase in residual mineral N with Urea Sustain (Figure 4).

Figure 3. Wheat yield (t/ha) and grain protein (%) responses to N rates supplied as Urea Sustain or urea top-dressed at Z11–13 in Cascade in 2023. Each dot represents the mean of three replicates.