Dung Beetles Help Reducing Greenhouse Gas Emissions From Cattle Farming
Categories: On The Farm
Pat level emissions
At the level of the individual dung pat, dung beetles were found to reduce methane (CH4) emissions by 14.5% and nitrous oxide (N2O) emissions by 2.0%, resulting in an overall reduction of 7% in CO2equivalents over the lifecycle of a pat (59 days) (Fig. 2a; Supplementary Material 2: Additional Results, Table S.1). This overall effect resulted from dung beetles significantly reducing CH4 fluxes from pats over the first 20 days of the pat lifetime, as compared with the fluxes from pats without dung beetles. After this period, the gas fluxes (both with and without beetles) stabilized to the same level as from the dung free pasture (treatment x day, F7,217 = 15.87, P < 0.001). Dung beetles had no significant effect on the N2O fluxes (treatment x day, F7,217 = 0.65, P = 0.71) (Supplementary Material 2: Additional Results, Fig. S.1).
Reduction of GHG emissions by dung beetles at the level of (a) dung pats, (b) pastures and (c) the entire life cycle of beef and milk production. For (a), we show the reduction during the lifespan of a dung pat (59 days); for (b), we show mean daily flux from the pasture (including dung pats), and for (c), we refer to the entire life cycle of one kg of milk or one kg of meat. On the top row, we show effects on CH4 emissions, on the lower row we weight together the effects on CH4 and N2O fluxes as CO2 equivalents (see Methods for coefficients used).
Pasture level emissions
At the level of the pasture, dung beetles also had a substantial effect on total GHG fluxes. Overall, emissions from dung offset the CH4 sink of the pasture soil, turning the pasture from a modest CH4 sink into a source. However, the presence of dung beetles reduced the mean daily CH4 emissions during the grazing season by 17% as compared with the situation where no dung beetles were present. On an annual scale, taking into account the non-grazing period, the dung beetle effect on the pasture CH4 fluxes was 21% (Fig. 2b). Dung beetles slightly reduced the N2O fluxes from the pasture-by 5% during the grazing season and by 0.1% over the course of the year (Supplementary Material 2: Additional Results, Table S.2). In terms of the two gases combined into CO2 equivalents (see Methods for details), dung beetles reduced the emissions by 12% during the grazing season and by 0.5% annually (Supplementary Material 2: Additional Results, Table S.2). Clearly, as fluxes from dung pats are a major component of the total fluxes from a pasture, the pasture-level GHG balance is strongly dependent on the number of grazing days (Fig. 3). With a longer grazing period, overall CH4 and N2O emissions increase, and so does the relative mitigating effect of the dung beetles (Fig. 3).
The effect of the grazing season length on the annual fluxes of (a) CH4 and (b) CH4 and N2O combined (as carbon dioxide equivalents, 100 year time horizon) from a pasture with dung pats and dung beetles, with dung pats but no dung beetles, and no dung pats. Negative values indicate an ecosystem sink and positive values indicate a source to the atmosphere.
Lifecycle of meat and milk production emissions
At the level of the full life cycle of meat and dairy products, the reduction of GHG emissions by dung beetles accounts for only 0.08% of the total life cycle assessment based GHG emissions of milk when Land Use and Land Use Change (LULUC) related emissions are not included, and for 0.05% when LULUC emissions are included. For beef, the reductions are 0.13% when LULUC emissions are not included and 0.07% when LULUC emissions are included (Table 1, Fig. 2c). This is due to the fact that total N2O and CH4 emissions from dung pats deposited on grazing land account for only 1–4% and 0.1–0.4% respectively, of the total GHG emissions of milk and beef production (see Table 1).
Table 1: Breakdown of the life cycle assessment based greenhouse gas emissions of milk and beef produced in Finland.
If mitigation techniques are not implemented then greenhouse gas emissions from agriculture are projected to rise to 8.2 billion tonnes of CO2 equivalents by 20303. Manure-related sources are also projected to increase during this time period – N2O by 29% and CH4 by 20%3. The potential for mitigation of these emissions is huge, but at present few techniques are cost effective3,4. One way of reducing GHG emissions from cattle farming for milk and beef is to graze the livestock on open grassland, rather than using intensive grain fed systems4. However, this study and others6,7, show that dung additions to a pasture can turn it from a sink to a source of CH4. Dung beetles may help mitigate these effects through the aeration and burial of dung pats7. In this paper, we assessed the contribution of dung beetles to reduction of GHG emissions in Finland at three nested scales – the dung pat, the pasture ecosystem and the whole lifecycle of milk or beef production. At the lower two levels, dung beetles were found to play an important role in reducing GHG emissions: during the grazing season, beetles reduced GHG emissions from pats and pastures by up to 7% and 12%, respectively, mainly through large reductions in CH4 emissions. Yet, over the full lifecycle of beef and milk production, the impact of dung beetle mediated effects was dwarfed by other impacts, suggesting a limited net impact in the context of highly-intensive production systems such as that of Finland. Below, we address each finding in turn.
At the pat level, dung beetles significantly reduced fluxes of CH4, with less-consistent effects on N2O. As proposed by Penttilä et al.7, the effect on CH4 fluxes can likely be traced to an oxygenating effect on the dung pat interior: As CH4 is formed under anaerobic conditions, holes dug by the beetles may enhance the drying of dung pats and increase the availability of oxygen in the deeper parts of the pats, thus increasing aerobic decomposition, decreasing anaerobic decomposition and reducing methanogenesis16.
At the pasture level, CH4 production in dung pats makes them ‘hotspots’ for GHG fluxes as compared to pasture without dung. Thus, any changes in the fluxes from these pats may result in large relative changes for the whole pasture – although the absolute changes may be fairly small. Taking into account the non-grazing season (approximately 250 days in Finland), during which no new dung pats are added and emissions from previous dung pats approach zero, dung beetles contribute an annual reduction of 21% in CH4 emissions and 0.4% in N2O and CH4 emissions combined, per one hectare of pasture. The amount of actively grazed pasture needed to sustain the current Finnish cattle stock of 1 million heads (assuming that around 80% of them graze outside17), is approximately 960 km2 (pasture land calculator, which incorporates default local conditions,
https://portal.mtt.fi/portal/page/portal/Artturi/Artturikirjasto/Laskurit/Laidunalan_hallinta). Over such an area, dung beetles can reduce the annual country-scale emissions of CH4 and N2O by 900 t of CO2equivalents, compared to a situation where no beetles were present. This effect size equals 0.12% of total emissions from manure management systems (0.72 Mt CO2e18). However, the amount of dung deposited on pastures, and therefore the fraction of manure on which dung beetles may act, is naturally proportional to the length of the grazing season (see Fig. 3). This period is short in Finland, but much longer in many countries in Central and Southern Europe19 (in southern Italy, cattle may be kept outdoors the whole year round). Thus, mitigation of GHGs using dung beetles becomes a more efficient strategy the longer cows are grazed on pastures. Longer grazing seasons may also benefit dung beetle populations20, increasing the abundance and diversity of dung beetles in the pasture, and hence increasing dung removal over the long term. It is also important to note that the relative dung beetle effect on the total pasture flux is dependent on the GHG fluxes and the CH4 sink strength from the dung-free parts of the pastures, and may vary with several biotic and abiotic factors6 (see Supplementary Material 3: Sensitivity Analysis). However, although potential changes in fluxes, due for example to changes in abiotic conditions, such as rainfall and temperature, will change the absolute flux estimates for the total pasture (dung pats and dung free areas), we expect the dung beetle effect to remain relatively stable. Our sensitivity analysis shows that this is the case and that particularly at the LCA level the total dung beetle effect changes little with simulated changes to fluxes (see Supplementary Material 3: Sensitivity Analysis).