Stimulatory and inhibitory nutrient effects on methane emissions from farm dams in southeast Australia: Field evidence to inform management

Small freshwater systems play a disproportionate role in global methane (CH 4 ) budgets, yet the drivers of their emissions remain poorly resolved. High nutrient concentrations are often associated with elevated CH 4 emissions, typically by stimulating primary production that increases organic matter inputs and promotes sediment anoxia. However, most studies focus on broad indicators like total nitrogen (TN) and phosphorus (TP), overlooking the roles of other solutes (e.g., iron, sulphate) and physicochemical factors (e.g., dissolved oxygen, turbidity). The role of warming in amplifying these effects also remains unclear, especially in small agricultural ponds. Here, we conducted a two-year field study across 89 temperate small agricultural ponds (commonly referred to as farm dams) spanning 1,498 km in southeastern Australia. We collected water chemistry data alongside 36,408 hourly CH 4 flux measurements (diffusive + ebullitive) to identify which water quality parameters stimulate or suppress methane emissions, and whether there are any interactive effects with temperature. These fluxes averaged 262 kg CH 4 ha −1 yr −1 , more than double the mean reported for temperate reservoirs and nearly four times higher than fluxes from lakes and small ponds. We found that TN, dissolved iron (Fe), and turbidity are strong positive drivers of CH 4 flux in farm dams while sulphate and DO have inhibitory effects. Temperature had a consistent positive main effect on microbial metabolism but showed no interactive effects with any nutrient, DO, or turbidity, challenging assumptions from laboratory studies that warming amplifies nutrient-driven emissions. By identifying turbidity and temperature, both of which can be measured using satellite remote sensing, as key predictors of CH 4 flux, our findings highlight a promising opportunity for future landscape-scale monitoring of emission hotspots and provide useful insight for water-quality management. This study provides field-based evidence that CH 4 dynamics in farm dams are shaped by the complex and multivariate effects of nutrients and water-quality parameters. • Farm dams emit 262 kg CH 4 ha −1 yr −1 , 2 to 4 times higher than lakes and reservoirs. • Turbidity, iron, nitrogen stimulate CH 4 ; sulphate, dissolved oxygen inhibits it. • Temperature boosts CH 4 emissions but shows no nutrient interaction effects. • Turbidity and temperature strongly predict CH 4 , showing potential for remote sensing. • Findings support fencing and livestock exclusion to reduce emissions.