Effects of riparian vegetation and morphology on nitrogen fluxes in agricultural ditches

Background/Question/Methods

Intensive cultivation in the California Central Valley has resulted in the loss of ~95% of riparian habitat. In addition, first-order streams have been replaced by agricultural ditches to convey irrigation and drainage water. These ditches may be crucial places to mitigate agricultural nitrogen export to the environment. Interest is growing among farmers and conservation partners to plant riparian hedgerows along ditches and to reform ditches to include small riparian floodplain benches. By using farm field edges, riparian habitat could be increased while minimizing loss of productive farm land. This novel riparian habitat may convey a suite of ecosystem services, including reducing nitrogen loading to the environment.

We initiated a study during the 2013 summer irrigation season to evaluate nitrogen retention/removal in agricultural ditches. Fourteen ditches were selected in Yolo County, CA; seven sites featured riparian plantings while seven did not. Of the planted sites, three also had floodplain benches. All sites were evaluated for physical and vegetation factors using the California Rapid Assessment Method for Riparian Wetlands (CRAM). Attenuation of inorganic nitrogen in ditch surface waters was evaluated along 150m-long longitudinal transects for eight weeks. Riparian soil inorganic nitrogen was also sampled for evidence of lateral nitrogen buffering.

Results/Conclusions

Ditches with riparian plantings generally had higher CRAM scores than unplanted ditches (p=0.0139); planted ditches with benches scored higher than ditches with plantings but no benches (p=0.0010).

Two planted and one unplanted ditch could not be used for longitudinal transects because they stopped flowing before water sampling began. The remaining six unplanted ditches and three of the planted ditches did not attenuate nitrate along longitudinal transects. Only two ditches significantly attenuated nitrate in ditch water; these two ditches had both plantings and benches (p=0.0012 and 0.0039). These findings suggest that nitrate attenuation in ditch waters may be more dependent on processes promoted by benches, than on the presence of a planting alone.

Riparian soil nitrate was significantly lower on planted ditches (p=0.0247). Soil nitrate was disproportionately lower that soil ammonium, suggesting that denitrification was the dominant pathway of nitrogen removal. However, soil nitrate was not significantly different between planted sites with benches and those without. This suggests that denitrification in planted riparian soils is promoted more by vegetation than by benches.

These initial findings suggest that riparian plantings affect N retention/removal in agricultural ditches and on their banks, and that riparian benches have distinct effects from plantings alone.