An intense precipitation event causes a temperate forested drainage network to shift from N₂O source to sink

Abstract

Nitrous oxide (N2O) evasion from streams and rivers is a significant, yet highly uncertain, flux in nitrogen cycle models. Most global estimates of lotic N2O emission assume that evasion rates are proportional to inorganic nitrogen inputs to a stream or river. However, many field studies do not detect relationships between lotic N2O evasion and dissolved nitrogen concentration, highlighting the need for better understanding of process‐based controls on this flux. This study reports 4‐yr time series of pN2O and N2O evasion from eight nested streams and rivers and detects an abrupt change in N2O dynamics associated with an intense rainstorm. This rainstorm, and the associated hydrologic flood event, pushed forested reaches across the watershed from consistent N2O sources to prolonged N2O sinks. We attribute this shift to disturbance of incomplete denitrification in the stream network and surrounding watershed, although alternate hypotheses are also discussed. There was continued availability of nitrate (NO3−) for in‐stream processing, eliminating the possibility that NO3−‐availability limited N2O production, and post‐storm N2O‐to‐nitrate ratios were lower than pre‐storm ratios suggesting that the large storm affected in‐situ nitrogen processing rates. The sustained period of post‐storm N2O undersaturation resulted in net negative evasion for five of the eight study sites in 2018, which mitigated emissions over the 4‐yr study. This nonlinear response in N2O dynamics illustrates the potential importance of storm events to control lotic N2O production and emissions.

Anthropogenic activities significantly increase annual greenhouse gas (GHG) fluxes from temperate headwater streams in Germany

“…Moreover, little is known about the interactive effects of land use and wastewater effluent inflows on riverine GHG fluxes, and whether land use is the overarching controlling factor. Under temperate climatic conditions, pronounced seasonality regulates the availability of nutrients and to some extent the O2 in lotic ecosystems, which are both key factors driving instream GHG production and gas exchange rates (Borges et al, 2018;Rocher-Ros et al, 2019;Herreid et al, 2021;Aho et al, 2022). Cold winter periods are generally characterized by low instream carbon and nitrogen processing, which results in nutrient accumulation (e.g., Herreid et al, 2021), while high instream C and N processing are characteristic of warm summer periods (e.g., Borges et al, 2018;Aho et al, 2021Aho et al, , 2022.…”

“…Under temperate climatic conditions, pronounced seasonality regulates the availability of nutrients and to some extent the O2 in lotic ecosystems, which are both key factors driving instream GHG production and gas exchange rates (Borges et al, 2018;Rocher-Ros et al, 2019;Herreid et al, 2021;Aho et al, 2022). Cold winter periods are generally characterized by low instream carbon and nitrogen processing, which results in nutrient accumulation (e.g., Herreid et al, 2021), while high instream C and N processing are characteristic of warm summer periods (e.g., Borges et al, 2018;Aho et al, 2021Aho et al, , 2022. Seasonality in precipitation regulates discharge, whereby heavy precipitation events or snowmelt during spring result in high discharge events.…”

Advances in Catchment Science, Hydrochemistry, and Aquatic Ecology Enabled by High-Frequency Water Quality Measurements

“…A similar study conducted in Europe estimated the nocturnal deficit at 39% . This demonstrates that rivers and streams may be important sources of CO 2 and other greenhouse gases, e.g., methane (CH 4 ) and nitrous oxide (N 2 O); however, high-frequency measurements of their concentrations are to date limited …”

“… 138 This demonstrates that rivers and streams may be important sources of CO 2 and other greenhouse gases, e.g., methane (CH 4 ) and nitrous oxide (N 2 O); however, high-frequency measurements of their concentrations are to date limited. 139 . Moreover, these studies highlight a blind spot that arises when only discrete daytime measurements of CO 2 are made and show that diel cycles in autotrophic and heterotrophic processes in streams need further consideration, necessitating continuous high-frequency in situ monitoring.…”

Nonlinear dynamics, resilience, and regime shifts in aquatic communities and ecosystems: an overview

“…Expectations for increased frequency and intensity of heatwaves, floods, and storms are motivating research to investigate the consequences of these events for aquatic ecosystems. Aho et al (2022) measured nitrous oxide evasion from streams and observed large shifts associated with an intense storm and subsequent wet period. Changes in evasion were persistent, which reflects alterations to nitrogen cycling caused by the storm.…”