Nitrous oxide fluxes and dissolved N gases (N2 and N2O) within riparian zones along the agriculturally impacted San Joaquin River

Riparian buffer zones play an important role in reducing nitrogen inputs to surface waters, but may contribute to the greenhouse effect by emitting N2O. This study investigated N2O fluxes within three abundantly vegetated riparian zones within the agriculturally impacted San Joaquin River, California. Fluxes were measured in the dry season of 2010 and 2011 at four positions perpendicular to the river: outside riparian zone, inside riparian zone, river bank and benthic sediments. Subsurface dissolved N-2/Ar and N2O concentrations were measured from groundwater wells in the riparian zone simultaneously with N2O fluxes. Mean N2O fluxes from the outside riparian zone (6.5 mg +/- 2.3 N2O m(-2) d(-1)), were higher than the inside riparian zone (2.7 +/- 0.7 mg N2O m(-2) d(-1)) and four times higher than bank sediments (1.6 +/- 0.2 mg N2O m(-2) d(-1)). Fluxes from benthic sediments were similar to the outside riparian zone averaging 4.4 +/- 1.0 mg N2O m(-2) d(-) (1). Dissolved excess N-2 (relative to Ar) and N2O concentrations in groundwater ranged from 1.53 to 10.2 mg L-1 and 0.0-6.0 mu g L-1, respectively. A spatial trend was evident along the river flowpath with higher N2O concentrations at upstream sites. The mean N2O-N/NO3--N ratio was 0.024 with a peak of 0.34. Variations in riparian soils were complex and N2O fluxes were primarily controlled by soil pH, %WFPS, NH4+ and NO3-, while benthic N2O fluxes were regulated by variations in dissolved oxygen and river flow. Higher fluxes in the riparian soils in 2011 were attributed to several months of flooding that significantly impacted groundwater tables and nutrient availability. Dissolved N2O from groundwater within the riparian zones was not found to be a significant factor contributing to atmospheric fluxes. These results suggest that riparian zones within the agriculturally impacted San Joaquin River were a significant source of N2O when elevated NO3- was present. Different controlling factors for fluxes within benthic sediments suggested that riparian vegetation did not play a role in NO3- concentrations or fluxes within the surface water.