Coastal Wetland Soil Nitrogen Concentrations and Process Rates Respond to Management History and Fire Disturbance

Abstract:

Management regimes can alter tidal wetland nutrient cycling by directly changing soil properties and processes, and by influencing plant community dynamics. Additionally, management history may influence ecosystem response to disturbance. In Suisun Marsh, California, USA, hydrology and salinity of many former tidal wetlands are manipulated to provide seasonal habitats for migratory waterfowl, with little-known consequences for soil nutrient dynamics. To better understand how coastal wetland soil nutrients respond to management history and disturbance, we quantified soil nitrogen concentrations and process rates in a natural experiment after the Branscombe Fire burned portions of Suisun Marsh in October 2018. We measured paired burned-unburned patches in both tidally-influenced and seasonally-impounded wetlands. Baseline soil nitrogen dynamics differed by wetland management history. Impounded wetlands had lower soil nitrogen process rates (9% of tidal potential nitrification rates and 12% of tidal potential denitrification rates), along with little soil extractable nitrate (2.86 mg N-NO₃^- kgdw^-1 impounded; 30.23 mg N-NO₃^- kgdw^-1 tidal). Soil nitrogen response to fire disturbance also differed by wetland management history. In tidal wetlands, burned areas experienced a short-lived pulse of nitrate after fire (> 100 mg N-NO₃^- kgdw^-1), with little evidence for changes in nitrogen process rates. In impounded wetlands, nitrate concentrations remained low after fire, while both potential nitrification and potential denitrification rates were lower in burned areas; potential nitrification rates fell below detectable levels immediately after fire but recovered in later sampling periods to unburned levels. Low extractable soil nitrate and potential denitrification rates may indicate substrate limitation of potential denitrification in impounded wetlands, with implications for nitrogen removal capacity across managed landscapes. Further studies on soil redox conditions and pH will be necessary to understand the mechanisms driving low potential nitrification rates in impounded wetlands. The long-term implications of fire disturbance on nitrogen dynamics in these wetlands merits further study, including incorporating plant and water nitrogen fluxes.