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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Fires alter terrestrial dissolved organic carbon (DOC) exports into water, making reliable post-fire DOC monitoring a crucial aspect of safeguarding drinking water supply. We evaluated DOC optical sensors in a pair of prescribed burned and unburned first-order watersheds at the Santee Experimental Forest, in the coastal plain forests of South Carolina, and the receiving second-order watershed during four post-fire storm DOC pulses. Median DOC concentrations were 30 and 23mgL−1 in the burned and unburned watersheds following the first post-fire storm. Median DOC remained high during the second and third storms, but returned to pre-fire concentrations in the fourth storm. During the first three post-fire storms, sensor DOC load in the burned watershed was 1.22-fold higher than in the unburned watershed. Grab samples underestimated DOC loads compared with those calculated using the in-situ sensors, especially for the second-order watershed. After fitting sensor values with a locally weighted smoothing model, the adjusted sensor values were within 2mgL−1 of the grab samples over the course of the study. Overall, we showed that prescribed fire can release DOC during the first few post-fire storms and that in-situ sensors have adequate sensitivity to capture storm-related DOC pulses in high-DOC forest watersheds.

While impervious surface expands with global urbanization, understanding the quality and quantity changes of soil organic carbon (SOC) under impervious surfaces is essential to assess the impacts of urbanization on the SOC pool and cycling. By comparing soils under impervious surfaces with surface and subsurface soils from adjoining open areas, we present a systematic study on the SOC signature under impervious surfaces. SOC concentration barely changed when comparing soils under impervious surfaces with subsurface soil from the nearby open area; however, the depletion on SOC was 35−62% when it was compared with surface soils. Regardless of comparison with surface or subsurface soils, bulk-level 13 C NMR spectra and specific molecular biomarkers showed a depletion in carbohydrates and an increase in aromatics in SOC composition. Such an alteration was greater with coverage by concrete slabs than simulated home structures built on crawl spaces and was greater as the coverage duration of residential home structures increased. Long-term coverage of residential home structures suppressed microbial degradation and selectively increased the sequestration of plant suberin-and lignin-derived carbon, which would likely increase the residence time of SOC. This study highlights a possible impact of urbanization on the SOC signature and emphasizes that biogeochemical impacts on SOC vary with the type of impervious surface and coverage time.