N2O Loss Equals Dollars Lost

TAKE HOME MESSAGES

• Nitrous oxide losses from nitrogen fertiliser products compared in this trial were low: 1g N/ha/day or less.
• Granular urea and granular urea plus inhibitor fertiliser provided the greatest yield benefit this season.
• Potential benefits from different fertiliser products will be realised if seasonal conditions and timing of plant growth are matched.

BACKGROUND

Most grain growers try to carefully match inputs of fertiliser application to crop demand. The major driver for this is economics because inappropriate nitrogen (N) fertiliser application equates to dollars lost; something that farmers can ill afford.

Nitrous oxide (N₂O) is a greenhouse gas which has worldwide agricultural, environmental and societal implications. Part of the reason for this is that it has a global warming potential (GWP) 298 times that of carbon dioxide. GWP is a measure of how much heat a greenhouse gas can trap in the atmosphere compared with carbon dioxide. N₂O is produced by two chemical processes: nitrification and denitrification. The presence of favourable levels of nitrogen, soil carbon and moisture influences these processes (DAFF, 2011).

The process of nitrification requires oxygen and a moist, but not waterlogged, soil in which ammonium (NH₄⁺) is converted to nitrate (NO₃^–), releasing N₂O as a by-product.

The process of denitrification occurs in waterlogged soils as oxygen is not required. Nitrate (NO₃^–) is converted into nitrogen gas (N₂), with N₂O an intermediate product.

Previous work in low and medium rainfall environments has generally shown that N₂O losses tend to be small when compared with the amount of nitrogen (N) required to grow a crop. As a result, the focus of this work was to investigate options to improve nitrogen efficiency and ensure that grain productivity was maximimsed relative to N₂O emissions.

Slow-release, or treated, nitrogen fertiliser products have been promoted as an option for farmers wanting to improve nitrogen efficiency and to reduce losses. These products inhibit or slow down the nitrogen cycle at various points to reduce ammonia volatilisation, nitrate leaching and N₂O emissions. The different products are best utilised if they are matched to the appropriate environmental conditions.

This trial investigated four types of fertiliser, including:

• Urea (46%N): a granular nitrogen fertiliser that is broken down by urease enzyme and becomes available to the plant as ammonium and nitrate.
• Green urea (46%N): a granular urea product coated with urease inhibitor designed to reduce volatilisation. This product delays the conversion of urea to ammonium by suppressing urease activity for a period of time and allowing the fertiliser to move into the soil where it is less susceptible to volatilisation. It is suited to situations in which rainfall immediately following fertiliser top-dressing is insufficient to wash the fertiliser into the soil.
• ENTEC (46%N): a nitrification inhibitor that slows the activity of nitrosomonas bacteria which in turn delays the conversion of ammonium to nitrite and then nitrate. Nitrogen is retained in the ammonium form longer and is therefore less prone to leaching as nitrate.
• Agrocote Max (43%N): an external polymer coating around the urea granule core. Nutrients are released gradually and can be taken up as the plant grows, reducing the potential for leaching.

AIM

To measure nitrous oxide losses from inhibitor treated and slow release fertiliser products and their effect on wheat yield and quality.

METHOD

A complete randomised block design was used to compare grain yield and quality. Treatments focused on ‘in-season’ N applications, with top-dressing timed to coincide with GS31. Treatments included a nil fertiliser control, granular urea, urease inhibitor (Green Urea), nitrification inhibitor (ENTEC) and polymer coated slow release fertiliser (Agrocote Max).

Demonstration scale measurements of N₂O emissions following fertiliser application and rainfall were measured from sealed PVC static chambers of approximately 30cm diameter positioned between crop rows of 30.5cm (12 inch) spacing.

N₂O was drawn from airtight chambers via medical syringes into evacuated vials. N₂O flux measurements were collected between 12pm to 3pm at intervals of 0, 30 and 60 minutes; one day prior to, one day after and approximately seven days following a rain event (13, 15 and 23 August, GS31). Ambient and soil temperatures were measured and soil (0-10cm) was collected to enable testing for moisture and nitrogen at each sampling. Samples were analysed at the Queensland University of Technology.

Location:                         Horsham

Replicates:                      four

Sowing date:                   9 May

Sowing rate:                    70kg/ha

Crop type:                        Wallup wheat

Fertiliser:                          at sowing MAP (55kg/ha)

Herbicides: 2 September Triflur X^® (2L/ha) + Avadex^® Xtra (2L/ha)

15 August      Velocity^® (670mL/ha) + MCPA LVE (330mL/ha) + Lontrel™ (170mL/ha) + Hasten (1%)

Seeding equipment:         BCG Gason parallelogram cone seeder (knife points, press wheels, 30cm row spacing)

Pre-sow soil (12 April)     Plant available water (PAW) (0-100cm): 22mm

Soil nitrogen (0-100cm): 48kg N/ha

In-crop soil (13 August) PAW (0-10cm): 34mm

Soil N (0-10cm):               24kg N/ha

Rainfall:                            GSR – 341mm (Decile 7)

4.4mm fell the evening before sampling

16.4mm occurred after top-dressing, until the day of N₂O sampling (Table 1)

Table 1. Horsham rainfall 13-23 August 2013.

* Indicates sampling date.

Table 2. Fertiliser application rates.

RESULTS AND INTERPRETATION

Yield, quality and return on investment

Applying 46kg/ha of nitrogen by means of any fertiliser product provided a yield benefit. The yield of urea (4.5t/ha) and Green urea (4.2t/ha) treatments were the highest and applying ENTEC (4.1t/ha) was of greater yield benefit than Agrocote Max (3.4t/ha).

There was no significant difference between protein and grain quality. Grain quality fell within ASW classification, which implies that nitrogen was limiting.

When the return on investment was calculated, the urea treatment returned a favourable $7.5 for every dollar spent on nitrogen (Table 3). Green Urea and ENTEC returned more than $4 for every $1 spent.

During the season, the Agrocote Max treatments were observed to have grown less than other treatments with nitrogen applied. It also took a long time to break down: it was still visible on the soil surface after 21mm of rain which fell during the N₂O sampling period. Perhaps this product is suited to a higher rainfall area and/or earlier season applications at sowing.

In the case of the urease inhibitor treated fertiliser (Green Urea), the true benefit of the product was not realised because regular follow-up rainfall after application reduced the risk of volatilisation. Green Urea is most effective if nitrogen fertiliser is applied to the soil in the absence of follow up rainfall (conditions suited to volatilisation).

Rainfall following nitrogen application was unlikely to have increased soil water levels to a level sufficiently high (water-logging) for the ENTEC treatment to be most effective.
The in-crop nitrogen application at GS31 of fertiliser products that delay the nitrogen cycle may have been too late for the crop to realise a yield benefit.

Table 3. Grain yield, quality, income and treatment costs.

Note: Grain prices used were from Horsham (refer to pp. 18). Grain quality was ASW.

Soil water and nitrogen

Although not replicated, upon the first N₂O sampling, plant available soil water was 34mm and this did not appear to change greatly over the 10 day sampling period. Soil nitrogen measured immediately prior to top-dressing was approximately 24kg N/ha, but there were no measurable differences between treatments over the sampling duration. Given these values, it was hypothesised that soil moisture and available nitrogen were not sufficient to result in high emissions.

N₂O flux

N₂O losses from all fertiliser treatments were low. Ten days following the application of 46 kg N/ha and 16.6mm of rain, peak N₂O flux levels were still generally 1g N/ha/day or less for all treatments. Spatial variability most likely explains why the nil fertiliser treatment had the highest and second highest emissions on the second and third monitoring days. Replicated trials can account for such spatial variability and these are being conducted by DEPI in the Wimmera.

Figure 1. N2O flux for various inhibitor and coated fertiliser treatments and mean soil moisture (0-10cm) prior to, immediately following and one week after fertiliser and rain falling between 13 and 23 August.

COMMERCIAL PRACTICE

As growers know, when soil moisture and nitrogen are not limiting, in-season nitrogen fertiliser topdressing provides a yield benefit and a return on investment. In this season, where rainfall was favourable, urea provided a greater return on investment than other nitrogen inhibitors or coated products.

The true benefit of polymer coated or inhibitor treated fertiliser is likely to be realised if matched to appropriate seasonal conditions and timing of plant growth.

Findings of very small N₂O losses align with previous work in low rainfall environments, indicating a relatively low loss of N from the cropping system. Last year, static chamber N₂O losses from Rupanyup resulted in peak emissions of up to 4.5gN/ha/day following 10mm rainfall (83kg N/ha top-dressed and high baseline soil nitrogen).

Soil testing, paddock history, seasonal forecasts and/or Yield Prophet^® will continue to help growers improve nitrogen use efficiency per unit of N₂O emitted.

This project will continue in 2014.

REFERENCES

Trenkel ME. (2010) Slow and controlled release and stabilized fertilisers: An option for enhancing nutrient use efficiency in agriculture. International Fertiliser Industry Association (IFA), Paris, France.

DAFF ‘Reducing nitrous oxide emissions fact sheet’ 2011

ACKNOWLEDGMENTS

This trial was funded through the DAFF AOTGR1-956996-222 ‘Efficient grain production compared with N₂O emission’ project.