The impact of drought and rewetting onN(2)Oemissions from soil in temperate andMediterraneanclimates

The potential for soils to produce nitrous oxide (N2O) is impacted by past moisture conditions; however, the extent of this impact is not fully understood. We conducted the first review of this, using two literature searches and a meta-analysis. We found 36 studies out of a possible 735 that described experiments where soil moisture conditions had been controlled, such that the impact of antecedent moisture levels could be separated from contemporary moisture levels and attributed to N2O emissions. Of those studies, 14 (130 data points) used the appropriate experimental design and presented suitable data that could be standardized for the meta-analysis. We found that the degree to which the soil was rewetted and the water filled pore space (WFPS) the soil was brought to were significant explanatory variables (p= < 0.0001). The larger the difference between the dry and wet states of the soil and the higher the WFPS of the soil after rewetting, the larger the hot moment, with an exponential increase once the soil is anaerobic. Substrate availability and fertiliser quantity and type were also important controls on the amount of N2O emitted during the hot moment (p= < 0.0001). However, controls with a constant WFPS can have the same anaerobicity and substrate concentrations, yet much lower emissions, so we suggest that it is the bioavailability of and how the substrates are utilized by the microbial community, and thus how they are primed by the drought, that is the main causal mechanism. Unfortunately, there is still a large uncertainty regarding how microbial population structure, relative gene abundances and gene expression profiles change according to antecedent dry/wet cycles. We suggest several areas of improvement for future studies and the development of drought-impact curves. These would show the relationship between N2O emissions and the length of drought (we found no studies that have investigated this) and N2O emissions and the severity of drought (e.g. the difference between 20% and 40% WFPS). Highlights We investigated the main causal mechanisms of N2O hot moments after soil is rewetted. This is the first meta-analysis and synthesis of the literature regarding N2O hot moments. The more anaerobic the soil and the more C and N substrates, the larger the hot moment. We provide recommendations for future studies and outline key gaps in the research.