Spatial and Temporal Patterns of Nitrogen Mobilization in Residential Lawns

Identifying locations prone to exporting nitrogen (N), also called export control points, within residential landscapes, is key to determining N mitigation strategies. Within residential landscapes, lawns have the potential to act as either a sink of N via uptake and denitrification, or a source of N via additions such as fertilizer. Lawns draining to impervious surfaces are more likely to be sources of N loading to receiving water bodies through directly connected curb and sewer flow paths. We utilized small-scale rainfall experiments to examine whether hydrobiogeochemical measurements of potential denitrification and saturated infiltration rates were predictive of N mobilization, and how potential export control points (locations within the upper quartile of N mobilization values) varied spatially and temporally on residential lawns in Baltimore, Maryland. We found potential denitrification, but not infiltration, was predictive of N mobilization in runoff and leachate, only on fertilized lawns. Potential export control points occurred more often in the late summer and fall and 85% were on fertilized lawns. Applying fertilizer shortly before a rainfall event increased the N mobilization in runoff and leachate by an order of magnitude. Suburban front yards also had more potential export control points compared to backyards, which is notable as front yards are surrounded by impervious surfaces increasing their vulnerability to transporting N to downstream ecosystems. These findings highlight the spatial and temporal variability of N mobilization on lawns. Targeting locations such as vulnerable front yards, or behaviors, such as timing of fertilizer application, may be useful N mitigation strategies.