| Summary: | Abstract Through the microbial breakdown of organic matter and production of greenhouse gases (GHGs), small agricultural dams or ponds have recently been shown to make a relatively large contribution to freshwater ecosystem carbon cycling. However, current estimates of their total carbon dioxide‐equivalent (CO2‐e) emissions lack inclusion of both seasonal and diel fluctuations. In addition, the atmospheric emissions of nitrous oxide from these often eutrophic systems have yet to be established. Here, we quantified the diffusive winter emissions of carbon dioxide (CO2), methane (CH4), and nitrous oxide (N2O) from 12 small agricultural dams within southeast Australia over a 24‐h period. The winter CO2‐e emissions of small agricultural water bodies were ~92% lower than previous summer estimates, at 1.02 g·m−2·d−1, while N2O contributed just 3.2% of this total. We also show that diel cycles do not significantly affect winter CO2, CH4, or N2O emission rates, and we discuss the likely carbon sources to these systems, through analyses of stable carbon isotopes (δ13C). The results from this study fill key gaps in our knowledge of agricultural dam GHG production and global atmospheric emissions, aiding their inclusion into future GHG budgets.
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