Winter “hot” for biogeochemical processes in urban ecosystem
Winter is often seen as a time when biological activity freezes; nevertheless, we posit that it can be a hotbed for nitrogen (N) biogeochemistry in urban ecosystems. While humans hunker down in cities for the winter, so do urbanized animal species including American crows (Corvus brachyrhynchos), which gather in large nocturnal roosts in some cities. We are investigating the unique interplay between the roosting behavior of American crows, crow-mediated N inputs, the creation of biogeochemical “hotspots” and “hot moments”, and the shifts of N biogeochemistry in urban ecosystems. We hypothesize that crow populations roosting in high densities within urban forest patches create “hot spots” of pulsed N availability via fecal deposition, altering the pattern of N cycling, and affecting N losses via denitrification and leaching. In this study, we examine two sets of questions: what are the spatial and temporal patterns of “hot moments” created by crows; and what are the biogeochemical consequences of “hotspots” created from crow roosting? To answer these questions, we use visual crow tracking and radio telemetry to evaluate crow distribution patterns combined with soil sampling, lysimeter sampling, and gas chromatography to measure N availability and transformations in cities in central New York State.
In central New York, we have documented a consistent winter crow roost in Auburn, NY, with an average of over 40,000 individuals. Additionally, we have monitored shifting winter crow roosts in Ithaca and Cortland, NY, with counts of 4,000 and 8,500 crows, respectively, in the 2014-15 winter. Preliminary radiotracking of five crows in Ithaca suggests high fidelity of individuals to roosting locations. The dominant form of N input at roost sites was ammonia. Roost sites receive disproportionately higher crow-mediated N input as compared to non-roost sites in the form of fecal deposition, with roost sites in Auburn receiving an average of 88.6 mg NH4-N/m2/day (range 53.6-118.1 mg NH4-N/m2/day) and non-roost sites receiving an average of 0.182 mg NH4-N/m2/day (range 0.154-0.218 mg NH4-N/m2/day). Eight non-roost sites investigated in Ithaca, NY received similar low N inputs. Previous data suggests that high rates of nitrification exist after the winter roosting season, converting pools of NH4-N to NO3-N, with NO3-N dropping quickly during the spring and summer months. Our data suggests that, even in an otherwise cold winter, “hotspots” created by animal-mediated N input play an important role in urban ecosystems.