Joel Williams, Integrated Soils
Reductions in nitrogen fertiliser use and associated greenhouse gas (GHG)emissions is of course high on the agenda in recent times – you’d have to be living under a rock to have missed this memo! In many countries around the world, we are seeing legislation from governments striving to limit the release of GHGs in order to meet emissions and sustainability targets. The use of low efficiency nitrogen fertilisers is of course one of the major sources, contributing both ammonia (NH3+) and nitrous oxide (N2O). Production systems that are dependent on external N inputs are highly vulnerable to the impacts of this impetus and farmers who have been experimenting and trialling lower N strategies will be well poised to weather the upcoming changes.
There is no silver bullet to addressing the nitrogen dilemma, a multi-pronged approach that integrates many tools into the toolbox is required. One such strategy involves the use of stabilised nitrogen fertilisers – often called enhanced efficiency fertilisers – which make use of chemical inhibitors to slow N transformations in the soil and prevent losses. Urease inhibitors (UI) slow ammonia production while nitrification inhibitors (NI) slow nitrate formation(which can ultimately be converted to N2O during anaerobic conditions).These inhibitors are very targeted and they have been proven highly effective in reducing production of these twoGHGs1. However, there has been some concern raised about their potential side effects on soil biology – this is something I have been asked numerous times over the years. There are two recent studies that have investigated this question and I thought it was worth sharing their findings. The bottom line was that both studies found no effect of UI and only a mild and temporary effect of NI but let’s expand with a touch more detail…
Firstly, a short-term study2 in Canada applied urea-ammonium nitrate (UAN)with and without both urease and nitrification inhibitors and followed up monitoring the effects on the soil microbiome for 16 days. The addition of the inhibitors to UAN had only minor effects on the abundance or activity of target and non-target N-cycling microbial groups which was temporarily observed around day 9 after fertiliser application, but no longer evident by the end of the study period. Along with these minor and transient impacts, a significant ~68% reduction in N2Oemissions was achieved2.Secondly, a more recent and longer term paper3 from Ireland studied the effects of UI and NI on soil microbial communities and biological function in a grassland soil over a 5 year period.
After5 years of repeated applications, there was no impact of either inhibitor on non-target microbes while function and abundance of N cycling communities were for the most part, unaffected by fertilisation or the use of inhibitors; although, the NI did reduce the abundance of a bacteria which produceN2O. Although the inhibitors had a fairly negligible effect, this study found the fertiliser itself did have an impact on the fungal community structure but no impact on bacterial communitystructure3.
So it appears nitrogen inhibitors are not only effective at stabilising nitrogen inputs in the soil, they also have a minor effect on the soil microbiome, making them a potential valuable tool as part of an integrated nitrogen management approach. That said however, there are some reports of urease inhibitor shaving negative effects on plant growth by entering the plant and supressing the urease enzyme internally. From a plant nutrition perspective, the ureas enzyme catalyses the breakdown of urea, liberating the embedded N to be utilised by the plant to ultimately synthesise amino acids and proteins. Consequently, using a UI which can block the activity of this key enzyme internally can lead to a build up of urea and additionally prevent adequate protein synthesis (of course important for plant health and quality).
A study from as early as 1989 highlighted that plant uptake of UI increased both leaf-tip necrosis and urea concentrations to toxic levels in both wheat and sorghum4. Another study with maize demonstrated that UI can heavily interfere with urea nutrition, limiting uptake as well as the following assimilation pathway5. Lastly, a very recent study from earlier this year applied foliar urea with a UI onto pineapples– a reduction in urease activity was observed which corresponded in high levels of urea and diminished levels of ammonium, amino acids and protein in the pineapple leaves6. Combined, these studies all indicate that UI are taken up by plants, can influence N uptake and disrupt N metabolism and hence protein synthesis. That said, keep in mind that plants do make use of a range of N sources beyond just urea so utilisation of ammonium, nitrate, amino acids, proteins and bacterial endophytes can still function and support plant growth; however, the broader goal of optimising plant growth and production should aim to support all sources and pathways of N nutrition.
In part 2 of this article, we will explore the potential of some of the alternatives to chemical inhibitors, namely C-based inputs.
References:
1. Urease and Nitrification Inhibitors—As Mitigation Tools for Greenhouse Gas Emissions in Sustainable Dairy Systems: A Review. (2020). doi.org/10.3390/SU12156018
2. Short-term response of soil N-cycling genes and transcripts to fertilization with nitrification and urease inhibitors, and relationship with field-scale N2O emissions. (2020). doi.org/10.1016/J.SOILBIO.2019.107703
3. Assessing the long-term impact of urease and nitrification inhibitor use on microbial community composition, diversity and function in grassland soil. (2022). doi.org/10.1016/J.SOILBIO.2022.108709
4. Potential phytotoxicity associated with the use of soil urease inhibitors. (1989). doi.org/10.1073/PNAS.86.4.1110
5. The urease inhibitor NBPT negatively affects DUR3-mediated uptake and assimilation of urea in maize roots.(2015). doi.org/10.1073/pnas.8
6.4.11106. Transient application of foliar urea with N-(n-Butyl) thiophosphoric triamide on N metabolism of pineapple under controlled condition. (2022). doi.org/10.1016/J.SCIENTA.2021.110822