Human Behaviors Interact with Biophysical Characteristics to Affect Hotspots of Nitrogen Mobilization in Residential Lawns in Baltimore, MD

Abstract:

Lawns are ubiquitous in the urban environment and an important setting for examining how human-environment interactions affect ecosystem functioning, such as nitrogen (N) cycling. Lawns are often considered exporters of N due to fertilizer applications and thus are targeted for various mitigation strategies, such as fertilizer restrictions, to minimize N export from lawns onto impervious surfaces where N can be quickly transported to downstream ecosystems. Interestingly, relatively few studies have examined hydrobiogeochemical variability within lawns where landscape gradients, such as topography, interact with social norms, homeowner preferences and parcel configuration to generate hotspots of N mobilization. In this study, we examined how topography and homeowner decisions (i.e. fertilizer application, front versus backyard management) affected within parcel hydrobiogeochemical variability and hotspots of N mobilization.

We selected sixteen lawns on which to measure several hydrobiogeochemical characteristics related to N cycling along a topographic gradient. We also conducted experimental rainfalls to quantify N fluxes in runoff and leachate. Study lawns were located in an exurban neighborhood with large parcels, a suburban neighborhood with small parcels, and in an institutional setting. We found lawns were hydrobiogeochemically variable. Fertilized lawns did not have different patterns of N cycling (i.e. denitrification, nitrogen mineralization rates) than unfertilized lawns, but did have significantly higher N fluxes in runoff. Suburban front yards had higher N fluxes in runoff than backyards, regardless of fertilizer application, potentially due to N deposition from vehicles. In addition, swales in suburban front yards had significantly lower saturated infiltration rates than other locations potentially resulting from compaction from foot traffic. Together these results suggest that suburban front yards are most vulnerable to mobilizing N due to their proximity to impervious surfaces and a complex suite of characteristics such as N deposition, soil compaction, and downspout placements that increase runoff potential. These results suggest that mitigating N mobilization from lawns may require techniques beyond fertilizer restrictions to ones that retain runoff on lawns.