Skip to main content

Targeting N2O emission hot-spots in dairy pastures for mitigation action: microbes, stable isotope methods and modelling

Periodic Reporting for period 2 - Target-N2O (Targeting N2O emission hot-spots in dairy pastures for mitigation action: microbes, stable isotope methods and modelling)

Reporting period: 2020-11-01 to 2021-10-31

Areas of dairy farm pastures with high stocking densities have been identified as hot-spots of emissions of the powerful greenhouse gas, nitrous oxide (N2O). This is due to enhanced excretal deposition and pugging/poaching of the soil leading to edaphic conditions which can stimulate soil N2O emissions. As such, mitigation strategies which target such farm areas have been suggested (e.g. application of nitrification inhibitors), but there is limited information on the efficacy of such technologies directly applied to these hot spot areas. Specific objectives are to 1) establish factors influencing N2O emissions and DMPP performance in soils with a history of high livestock impact; (2) determine if the microbial N-cycling community is functionally distinct in areas of soil with a history of high livestock impact compared to standard areas of pasture; (3) determine spatially appropriate N turnover rate constants and urine patch N2O emission factors, with and without DMPP, in hot-spot feature soils, and (4) model paddock and farm-scale implications and conduct cost-benefit analysis of a targeted DMPP mitigation strategy.

The conclusions of the action include:
- emissions from areas impacted highly by livestock are generally higher than when applied to standard areas of pasture
- temperature, inhibitor application rate and degree of vegetative cover are important for determining the efficacy of nitrification inhibitors in reducing N2O emissions
- the microbial community composition changes more rapidly in response to urine application in areas highly impacted by livestock compared to when applied within a standard area of pasture
- DMPP reduced emissions by 49% from urine applied to standard pasture in a temperate dairy farm, but only by 24% in a gateway on the same farm, indicating a lower efficacy in soils highly impacted by livestock
- options to reduce N2O emissions from areas impacted by livestock must not rely on inhibitors alone - farmers should consider additional options such as pads to capture nutrients deposited to these areas and attempting to maintain vegetative cover in these areas
Through conducting spatial sampling campaigns, we characterised the soil and vegetation characteristics as a function of distance from a farm-scale feature with high cattle occupancy. We established a gradient of impact by livestock from low (standard pasture), medium (gateway soil) and high (sacrifice paddock) impact by livestock. This resulted in soils increasing in bulk density, dissolved organic C and decreasing in degree of vegetative cover. We conducted an incubation experiment with intact soil cores taken from across this transect and monitored N2O, CO2 and CH4 emissions and soil properties following cattle urine application. Cumulative N2O emissions increased alongside the gradient of impact by livestock, and the temporal dynamics of N2O emission and mineral N were affected by degree of impact by livestock. Results from sequencing analysis provided evidence that the microbial communities in the contrasting areas do respond differently to perturbation by livestock urine (Obj. 2). Specifically, the microbial diversity decreases upon urine application, likely due to the persistence of taxa that are able to withstand high salt and nutrient loads. This effect was much more pronounced in the areas highly impacted by livestock, suggesting the microbial community is potentially primed in these areas. The rapid response of the microbial community may also contribute to the observation of higher N2O emissions from these areas.

A field trial was conducted on an intensive dairy farm in sub-tropical NSW, Australia (Fig. 1), to determine whether the nitrification inhibitor, DMPP, would be effective in reducing nitrification and subsequent N2O emissions from an area of the farm receiving greater stocking densities (a gateway). Under the conditions of our study DMPP (1.5 kg ha-1) was ineffective in reducing nitrification rates or N2O emissions. We tested increasing rates of DMPP application in the laboratory but found no effect on DMPP performance when increasing from 1 to 10 % of the urine-N applied. In the return year two field trials were conducted, one on a temperate sheep farm and the other on an intensive dairy farm. On the sheep farm an area was created to simulate sheep congregation by placing large feed troughs in the field and rotating them periodically to achieve an area which was disturbed by livestock. Emissions of N2O were greater from urine applied to the area around the feeding troughs compared to the standard area of pasture. While DMPP reduced emissions slightly in the standard pasture, the inhibitor was not effective in reducing emissions of N2O from the livestock impacted area even though applied at a rate of 10 kg ha-1, which is ten times the rate that would normally be applied alongside fertilizers. On the intensive dairy farm in North Wales, N2O emissions from cattle urine applied to a gateway area or an area of standard pasture, with and without DMPP applied at 10 kg ha-1. As was expected emissions were higher from the gateway area, and here the DMPP was effective in reducing N2O emissions by 49% in the standard pasture, but only by 24% in the gateway. Taken together the results suggest that a higher application rate of DMPP is required for urine than would typically be applied with fertilizers. Additionally, nitrification inhibitors are concluded to be not as effective in livestock congregation areas. This is because there is limited vegetation in these areas to mop up any urine-N held in the ammonium form for longer via the inhibitors.
Our results have demonstrated contrasting nitrogen cycling dynamics in soils with a history of livestock impact, suggesting these areas of dairy farms should be modelled as spatially explicit zones when assessing N losses. Results from our field trial in a sub-tropical dairy farm showed N2O emission factors were not significantly greater from urine deposition to soil near a gateway, but were higher from urine applied to a gateway on a temperate dairy farm. Thus there is evidence that these areas should be disaggregated within national greenhouse gas inventories. The nitrification inhibitor, DMPP, did not reduce N2O emissions from gateway soils in the field trial in NSW, Australia, adding to a growing body of evidence of a variable effect of DMPP applied at a rate of 1.5 kg ha-1. The field trials conducted on temperate sheep and dairy farms show more promise for the inhibitor in reducing N2O emissions from urine patches when applied at a higher rate of 10 kg ha-1, which is a specific recommendation to be communicated to industry. Results from the project provide a better understanding of N cycling and N2O emissions from emission hotspots of intensive dairy farms, leading to improvements in national greenhouse gas inventories and in quantification of the contribution of livestock to climate change. Practical advice will has been generated for the use of inhibitors on farm, relevant to farmers interested in reducing their carbon footprint, the dairy industry for developing sustainability roadmaps and policy makers assessing livestock greenhouse gas mitigation strategies within agri-environment schemes.
Figure 1 Field trial & greenhouse gas chambers in an intensive dairy farm in Casino, NSW.