Extreme drought influences N2O hot moment intensity and duration

Extreme weather events such as prolonged flooding and extended drought are predicted to increase in frequency and intensity due to climate change. Drying and rewetting influence soil nutrient cycling and greenhouse gas emissions, particularly where nutrient inputs are high such as in agricultural systems. Flooding and drought events therefore directly influence climate change, nutrient fate and nutrient use efficiency. Soil wetting events can stimulate nitrous oxide (N₂O) hot moments (disproportionately high emission rates over a short temporal period). Antecedent soil moisture conditions influence these hot moments, however this relationship and the mechanisms underlying it are not yet fully understood.

Characterisation of N₂O hot moments in response to current and future climatic conditions is essential to inform land management practices and nutrient application regimes. This work explores the relationship between hydrological events and resultant hot moment dynamics, and aims to elucidate the mechanisms fundamental to these processes.

In this study, soil samples were subjected to four treatment conditions (n=5) for a 14-day dry period: 5%, 20%, 35% and 50% water filled pore space (WFPS). After this period, all soils were fertilised (100 kg N ha^-1 ammonium nitrate) and simultaneously wetted to 90% WFPS for a further 14 days, to stimulate an N₂O hot moment. Gas emissions (N₂O, CO₂, CH₄) and soil chemistry (NO₃^-, NH₄⁺, dissolved organic carbon) were analysed throughout the 28-day incubation, and untargeted metabolomics analysis was conducted on day 14 of the dry period.

Our results showed hot moments to intensify under pre-drought conditions, with 5% and 20% WFPS considered a drought, versus 35% and 50% WFPS considered moist. For the first time, we showed extreme drought (5% WFPS) to significantly influence hot moment dynamics compared with moderate drought and moist conditions, with emissions occurring more abruptly and to a greater intensity over a 3-day, versus > 14-day, timeframe. Possible explanations for this shift include microbial osmolyte accumulation during drought and secretion upon rewetting, resulting in a labile C pool (immediate C availability); microbial cell death during drought or rewetting (immediate C availability via necromass); or shifts in microbial community structure, or gene expression rate, following rewetting. Untargeted metabolomics analysis is being conducted to determine the extent of osmolyte accumulation between treatments, including the nature of said osmolytes for indication of species likely involved in accumulation, and to probe any disparities in active microbial metabolic pathways, and therefore function, between treatments.

In summary, our results indicate antecedent conditions to significantly influence N₂O hot moments following wetting, with extreme droughts appearing to shift biogeochemical process dynamics compared with dry-to-moist conditions. Microbial activity, function and substrate availability may play explanatory roles in this shift, with untargeted metabolomics promising a powerful tool to probe underlying functional mechanisms.