Core Ideas Carbon loss from soil beneath impervious surfaces would not exceed 1.92 Pg globally. Net nitrogen mineralization rates beneath impervious surfaces were primarily related to nitrification. Temporally, nitrate accumulation was evident beneath impervious surfaces. Soil microbial biomass carbon was the key factor affecting soil carbon beneath the concrete slabs. Soil carbon beneath homes was positively related to soil volumetric water content. Soil sealing by impervious surfaces is a major disturbance caused by urbanization and has been shown to reduce soil carbon and nitrogen significantly. However, the degree to which these changes are driven by the initial disturbance (i.e., top soil removal) or post‐construction processes is not clear. A controlled field study consisting of three treatments including concrete slab (SLB), home with crawl space (CRW), and control (CNT) was conducted to monitor changes in soil properties immediately after sealing and over a 15‐mo time frame. At depth 10 to 20 cm (first layer beneath the concrete slab) soil carbon decreased by 30.4% (±3.4%) from 2.3 (±0.4) kg m–2 at Month 0 to 1.57 (±0.36) kg m–2 at the end of experiment with a rate of 0.04 (±0.01) kg m–2 per month (p = 0.001, F1,7 = 9.27). At this depth soil, carbon and nitrogen fluctuated seasonally at CRW and CNT plots. At depth 20 to 30 cm, the soil carbon beneath the CRW (1.14 ± 0.21 kg m–2) was higher than that beneath the SLB (0.93 ± 0.23 kg m–2, p = 0.01, F2,23 = 5.26), suggesting that partially permeable impervious surfaces such as CRW can support limited inputs of carbon and organic matter. In addition to carbon and nitrogen, temporal changes of microbial biomass carbon and nitrogen mineralization rates were also monitored. Nitrogen mineralization rates also decreased below impervious surfaces as evidenced by zero net ammonium production rate. However, a significant increase in mineralization rates was observed in warm periods of the year beneath SLB treatments.

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Two intense rainfalls [Hurricane Joaquin (2015) and Hurricane Matthew (2016)], one year apart, provided a unique opportunity to examine changes in dissolved organic matter (DOM) dynamics in coastal blackwater rivers under extreme flooding conditions in the southeastern United States. Two sites along Waccamaw River (a coastal blackwater river) and the outflow of 18 sub-basins of Yadkin-Pee Dee Basin were sampled during and after the flooding events. The peaks of dissolved organic carbon (DOC) and nitrogen (DON) concentrations were observed 18 and 23 days after peak discharge in 2015 and 2016, respectively. Moreover, DOM aromaticity and abundance of humic substances significantly increased during the same period. Separation of discharge hydrograph into surface runoff and subsurface flow suggested that temporal changes were mainly due to contributions from subsurface flow flushing organic matter from wetlands and organic-rich riparian zones. The spatial analysis highlighted the key role of the forested wetlands as the only land use that significantly correlated with both DOM quantity (DOC and DON load) and DOM composition (i.e., aromaticity). The Yadkin-Pee Dee River basin alone exported more than 474 million kg DOC into the ocean during high-flow conditions from the 2016 event, indicating that such extreme short-term events mobilized enormous amounts of organic carbon and nitrogen to the ocean. Considering the predicted increase in frequency and intensity of extreme rainfall events in the eastern U.S., the results of this study can shed light on changes in DOM dynamics that may occur under such conditions in the near future

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