Effects of dry-wet cycles on nitrous oxide emissions in freshwater sediments: a synthesis

BACKGROUND: Sediments frequently exposed to dry-wet cycles are potential biogeochemical hotspots for greenhouse gas (GHG) emissions during dry, wet and transitional phases. While the effects of drying and rewetting on carbon fluxes have been studied extensively in terrestrial and aquatic systems, less is known about the effects of dry-wet cycles on N(2)O emissions from aquatic systems. As a notable part of lotic systems are temporary, and small lentic systems can substantially contribute to GHG emissions, dry-wet cycles in these ecosystems can play a major role on N(2)O emissions. METHODOLOGY: This study compiles literature focusing on the effects of drying, rewetting, flooding, and water level fluctuations on N(2)O emissions and related biogeochemical processes in sediments of lentic and lotic ecosystems. RESULTS: N(2)O pulses were observed following sediment drying and rewetting events. Moreover, exposed sediments during dry phases can be active spots for N(2)O emissions. The general mechanisms behind N(2)O emissions during dry-wet cycles are comparable to those of soils and are mainly related to physical mechanisms and enhanced microbial processing in lotic and lentic systems. Physical processes driving N(2)O emissions are mainly regulated by water fluctuations in the sediment. The period of enhanced microbial activity is driven by increased nutrient availability. Higher processing rates and N(2)O fluxes have been mainly observed when nitrification and denitrification are coupled, under conditions largely determined by O(2) availability. CONCLUSIONS: The studies evidence the driving role of dry-wet cycles leading to temporarily high N(2)O emissions in sediments from a wide array of aquatic habitats. Peak fluxes appear to be of short duration, however, their relevance for global emission estimates as well as N(2)O emissions from dry inland waters has not been quantified. Future research should address the temporal development during drying-rewetting phases in more detail, capturing rapid flux changes at early stages, and further explore the functional impacts of the frequency and intensity of dry-wet cycles.