Dung Beetles Increase Greenhouse Gas Fluxes from Dung Pats in a North Temperate Grassland

Abstract

Soil fauna plays a critical role in various ecosystem processes, but empirical data measuring its impact on greenhouse gas (GHG) emissions from rangelands are limited. We quantified the effects of dung beetles on in situ CO2, CH4, and N2O emissions from simulated cattle dung deposits. Soil in meadows of the semiarid Nebraska Sandhills was treated with three treatments (dung pats with exposure and without exposure to dung beetles, and a no dung control). A closed‐chamber method was used to measure GHG fluxes at 0, 1, 2, 3, 7, 10, 14, 21, 28, and 56 d after dung placement in the early season (June–August) and late season (July–September) in 2014 and 2015. The greatest dung beetle abundance was 6 ± 2 beetles per quarter pat on Day 7; the abundance decreased to <2 ± 0.6 on Day 14 and 28 and zero on Day 56. Dung beetles increased fluxes of CO2 by 0.2 g C d−1 m−2, N2O by 0.4 mg N d−1 m−2 (only in late season 2015), and CH4 by 0.2 mg C d−1 m−2. These increases were due to beetle‐made macropores that facilitated gas transport in wet dung (initial moisture = 4.6 g g−1 on a dry‐weight basis) within 7 d after dung placement. Seasonal environmental differences resulted in greater CO2, N2O, and CH4 fluxes in the early season than in the late season. This study concluded that dung beetles increased GHG fluxes from early‐ and late‐season dung deposits on meadows of the semiarid Nebraska Sandhills. Core Ideas Dung beetle activities on dung pat enhanced CO2–C emission. The effect of dung beetle activities on N2O emission from dung pat was inconsistent. Soil moisture and temperature had a significant effect on GHG emission.

Carbon Neutral: The Failure of Dung Beetles (Coleoptera: Scarabaeidae) to Affect Dung-Generated Greenhouse Gases in the Pasture

“…Generally, dung-only showed highly variable CO 2 trajectories, with some studies showing increases ( Slade et al 2016b , Evans et al 2019 , Fowler et al our data) or decreases ( Penttilä et al 2013 , Iwasa et al 2015 , Piccini et al 2017 , Fowler et al 2020c ) despite using identical methodologies in some cases. Thus, CO 2 does not follow a familiar process as it does for CH 4 and N 2 O.…”

“…2 ), which changes the microbial community through the mixing of dung and soil ( Yokoyama et al 1991a , Slade et al 2016b ). Furthermore, this releases and increases dung nutrients ( Yokoyama et al 1991a , Nichols et al 2008 , Evans et al 2019 ) and moisture ( Penttilä et al 2013 , Evans et al 2019 , Fowler et al 2020c ) in surrounding soils; this sparks soil microbial consumption ( Fowler et al 2020c ), particularly for N 2 O-producing microbes within the soil ( Yokoyama et al 1991a ), because of the increased dung surface area. Therefore, we hypothesized that more constant activity (manual mixing on 1 d vs. dung beetle activity for 7 d), greater abundance (7 g vs. 15 g dung beetles), and soil-disrupting activities (dwelling vs. tunneling) would result in treatment-based GHG reductions—this was not the case.…”

“…5 and 6 ). Periodic CH 4 ( Penttilä et al 2013 , Slade et al 2016a , Evans et al 2019 ) and N 2 O spikes ( Penttilä et al 2013 , Iwasa et al 2015 , Slade et al 2016a , Fowler et al 2020c ) are common and likely related to dung/soil moisture ( Penttilä et al 2013 , Evans et al 2019 ) and/or soil disturbance ( Fowler et al 2020c )—abiotic factors heavily influenced by dung’s natural decay process. Understandably, since dung beetles affect both the dung and soil matrix simultaneously, studies often focus on these two GHGs, despite beetle related CH 4 reductions and N 2 O productions occurring only 50–63% of the time, respectively ( Table 1 ).…”

“…5 ). Explanations for this increase included: 1) dung beetle respiration ( Iwasa et al 2015 ), 2) enhanced gas transport ( Evans et al 2019 ), or 3) increased microbial populations and subsequent respiration from fresh resources ( Fowler et al 2020c ). While gas transport is unknown, our recent trace gas analysis indicates CO 2 changes were not based on dung beetle respiration ( Fowler et al 2020c , Piccini et al 2017 ).…”

“…Dung beetles improve pasture health by actively dismantling and burying dung, which: 1) increases soil organic matter (‘humification’) and refills water tables through increased soil percolation; 2) reduces pollution, pests, and pestilence through competitive exclusion and removal; and 3) encourages cooperation among earthworms, soil arthropods, and plant species to bury seeds, enrich the soil, and/or pollinate plants ( Lavelle et al 2006 , Nichols et al 2008 , Ridsdill-Smith and Edwards 2011 , Doube 2018 ). Only recently have researchers studied the relationship between soil, dung, dung beetles, and GHGs as part of a larger resource recycling strategy ( Penttilä et al 2013 , Iwasa et al 2015 , Hammer et al 2016a , Slade et al 2016a , Piccini et al 2017 , Evans et al 2019 ). Enteric fermentation and deposited, applied, and flooded manure and dung ( Mbow et al 2019 ) produce 66% CH 4 of all agricultural emissions, with deposited manure and dung also producing >50% N 2 O net agricultural emissions ( Jia et al 2019 )—this does not include the emissions generated by applied synthetic fertilizer to animal food crops.…”