Characterization of soil aggregates in green bioswales in relation to carbon sequestration, aggregate stability, and microbial activity

Green Infrastructure (GI) plays a crucial role in reducing stormwater runoff and providing ecological benefits in urban areas. Aggregation is a key process in many soil functions as it influences carbon storage, greenhouse gas emissions, nutrient cycling, hydraulic properties, and biotic activity. In this study, we investigated soil aggregation processes and stability in GI. Soil samples were collected from six bioswale sites in New York City that had two different designs - streetside infiltration swales and enhanced tree pits. The soil samples were taken from the inlet, center, and outlet positions (relative to stormwater input) of each site. These samples were then tested for 1) macro and micro aggregate sizes; 2) distribution of soil organic carbon (SOC) and nitrogen; 3) aggregate stability; and 4) microbial biomass and activity relevant to carbon and nitrogen cycles in macroaggregates. Our results showed that 60% g/g of the soil aggregates at these GI sites were smaller than 2 mm and had high structural stability. Microaggregates between 1-2 mm had the highest SOC and accounted for 60% g/g of all microaggregate size classes. GI aggregates are formed from the breakdown of macroaggregates into intermediate microaggregates. The newly formed microaggregates contained more stable SOC than macroaggregates and bonds within microaggregates were stronger than bonds grouping microaggregates, which is not consistent with a classical model of aggregate formation in natural soils. Microbial biomass and activity were correlated with the carbon and nitrogen content of aggregates and with GI type, allowing for the identification of microbial hot spots. These results suggest that aggregation processes in human-engineered soils included in GI play an important role in sustaining carbon and nitrogen cycles.