Elusive hotspots of nitrogen transport and retention in residential lawns in Baltimore, MD

Residential landscapes are typically considered exporters of nitrogen to downstream ecosystems. However, watershed scale studies have found that some residential areas retain more nitrogen than expected and it has been suggested that lawns could be a potential sink for nitrogen in these landscapes. Despite their ubiquity, predicting the environmental performance of residential lawns can be challenging. Variable management practices, complicated land use histories, and underlying geomorphology interplay to affect the ecology and hydrobiogeochemistry of lawns. For this study, we targeted locations in residential lawns we predicted to be sinks or sources for N transport and used simulated rainfall experiments to test these predictions.

N transport occurs when accumulated nitrogen is mobilized by a transport vector, such as water. We hypothesized a hotspot of N transport would occur in a location that generates large amounts of runoff and has low biogeochemical activity rates, while the reverse would be true for a hotspot of N retention. Using saturated infiltration rates and potential denitrification rates collected along a topographic gradient from a sampling effort in September 2017, we identified 48 locations in residential lawns at the extremes of those measures. We sorted these sites into categories we predicted to be hotspots of N transport or hotspots of N retention. At the 48 locations, we measured nitrate and ammonium in runoff and leachate generated from an experimental rainfall using a Cornell Sprinkle Infiltrometer in April 2018. We also measured denitrification rates on soil cores collected directly after the rainfall experiment.

We found that nitrate fluxes in leachate were larger at locations with high infiltration rates and low denitrification rates. These conditions occurred most in swales suggesting that these are important areas of focus for mitigating N transport in residential landscapes. We plan to repeat this experiment to examine seasonal variation in N transport at these locations.