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

Periodic Reporting for period 1 - Target-N2O (Targeting N2O emission hot-spots in dairy pastures for mitigation action: microbes, stable isotope methods and modelling)

Reporting period: 2018-11-01 to 2020-10-31

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. The Target-N2O project aims to determine the agronomic and environmental cost-benefits of a targeted nitrification inhibitor (DMPP) mitigation strategy in both northern and southern hemisphere intensive dairy farms. 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.
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. Soils from this study are currently being analysed for N cycling gene abundance and amplicon sequencing to determine the microbial community composition in these areas, towards Obj. 2.

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). The gateway area had a greater bulk density and labile C contents than standard pasture, indicating impact through livestock via poaching and pugging and excretal deposits. 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. A similar field trial will be conducted in the return year (2021) on a temperate dairy farm to complete Obj. 3. Additional work for Obj. 1 in the return year will determine the efficacy of DMPP in reducing nitrification rates and DMPP degradation rates within a range of soil types, soil temperatures, soil moisture contents, degree of compaction and combinations of the above. This will help elucidate a mechanistic understanding of the variable efficacy of DMPP observed in the field.

Using DairyMod we modelled the effects of nitrification inhibitor application on N gains and losses in temperate and sub-tropical intensive dairy farms. We specifically simulated standard areas of pasture and emission hotspot areas (through modifying bulk density, saturated hydraulic conductivity and degree of plant cover), running the model over a 20 year period. We simulated no urine deposition, single urine deposition and overlapping urine patches applied within each season at each site. Using a spreadsheet approach we calculated a partial N budget accounting for the environmental N loses and gains due to inhibitor application. In all our simulations, the nitrification inhibitor delayed rather than reduced N2O emissions from urine patches. Nitrification inhibitor application was, however, effective in reducing NO3- leaching by up to almost 50%; therefore, it has the potential to reduce indirect N2O emissions. A blanket nitrification inhibition strategy was more effective at reducing N losses compared to when the inhibitor was applied only to emission hotspot areas. Work is currently underway to assess the economic implications of each nitrification inhibitor application strategy.
At the end of the project we expect to deliver a decision-support framework which will inform farm managers on the best practice in terms of nitrification inhibitor application and use on dairy farms. This will cover a range of soil types and temperatures representing contrasting climatic zones, nitrification inhibitor application rates and expected duration of efficacy. 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. We expect this was due to contrasting microbial community composition and functioning between the areas, with analysis underway to confirm this. 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. This does not provide evidence that these areas should be disaggregated within national greenhouse gas inventories. Whether the same phenomenon holds true in a temperate dairy farm will be explored in the return phase. The nitrification inhibitor, DMPP, did not reduce N2O emissions from gateway soils in the field trial, adding to a growing body of evidence of a variable effect of DMPP in the field. We expect this may have been linked to warm temperatures resulting in rapid DMPP degradation. From our modelling study and economic analysis we aim to assess contrasting nitrification inhibitor application methodologies to see whether they are a financially viable mitigation strategies for dairy farmers. Results from the project will 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 also be 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. The results are also expected to be of interest to the wider community interested in sustainable food production.