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Landscape fragmentation has created large areas of forest edge. Understanding how soil organic carbon (SOC) stocks within forest edges respond to fragmentation is essential to assess carbon budgets; however, the causes and magnitude of edge effects on SOC stocks have been poorly characterized. Here, we quantify the edge effects on SOC stocks within and across urban to rural forest patches from three fragmented urban forests to a large patch of rural forest. The SOC stocks within 20 m of the rural forest edge (1.86 kgC m−2) is on average 80% lower than the interiors of rural forest (10.47 kgC m−2). We found that biotic factors, including annual litterfall mass (R2 > 0.94), peak leaf area index (R2 > 0.92), and fine-root mass density (R2 > 0.77), explained the spatial variation in SOC stocks within the rural forest. In urban forests, human activities at forest edges led to contrasting edge effects on SOC stocks, for instance, the SOC stocks at the east edges (4.74 kgC m−2) were 56% greater than at the west edges (3.03 kgC m−2) explained by the adjacent land uses (e.g., paved roads vs. non-paved soils) and in-situ litterfall management. We also found significant differences in summer soil temperature (ΔTS > 2.8 °C) and soil moisture (ΔVWC > 0.05 m3 m−3) between the east and west forest edges. Our results reveal that the factors responsible for the edge effects on SOC stocks in rural forests are biotic factors, while heterogeneous human activities at the local scale lead to complex edge effects on urban forest SOC stocks.

Soil respiration (Rs) determines land surface carbon balance; however, there have been few studies that measured Rs in heterogeneous urban landscapes. Here, we investigated the spatial and temporal variations in Rs in six land cover types (mixed forest, deciduous broadleaf forest, evergreen needleleaf forest, lawn, wetland, and bare land) in Seoul Forest Park, Republic of Korea, between March 2013 and September 2014, which included a wet (2013) and an extremely dry (2014) summer. Spatially, there was a three-fold difference (0.48–1.45 kgC m−2) in annual Rs among the six land cover types. The soil organic carbon stock at a depth of 0.1 m explained 72% of the spatial variation in the annual Rs across the land cover types. During the entire study period, the soil temperature explained 82–97% of the temporal variation in Rs among different land cover types. Comparing the two summers, the 2014 drought only resulted in a decrease in Rs in the lawn plots (25%), which was driven by a reduction in the leaf area index and the fine root density. The temperature sensitivity of Rs in 2014 (dry summer) compared to 2013 (wet summer) was significantly lower in mixed forest, deciduous broadleaf forest, and lawn, and did not change in evergreen needleleaf forest, wetland, or bare land. The differences in Rs in these drought responses highlight the importance of the careful selection of land cover type during park planning to better manage carbon cycles.

Rapid urbanization has created large areas of impervious surface areas globally. As there is little carbon input by plants into soils under impervious surfaces, soil organic carbon (SOC) stocks under impervious surfaces generally have been overlooked in the urban carbon budgets. Here we investigate SOC stocks under impervious concrete surfaces and vegetative surfaces across soil profiles to a depth of 5 m in urban housing complexes in Seoul, Republic of Korea. In the top 1 m of the profile, SOC stocks under vegetative surfaces were three times greater than those under impervious surfaces. However, we discovered that unexpectedly high SOC stocks appeared in deeper soil layers under both surface types, which led to comparable SOC stocks at a depth of 5 m beneath the impervious surface (16.9 ± 1.9 kgC m−2) and at the vegetative surface (22.3 ± 2.2 kgC m−2). Consequently, the ratio of SOC stocks at depths of 1 m to 5 m were 16% in impervious surfaces and 34% in vegetative surfaces, suggesting conventional soil sampling at 1 m depth could miss large SOC. Stable isotope data ( δ 13C and δ 15N) combined with historical aerial photographs revealed that cropland that existed until the 1970s formed the high SOC cultural layer in deeper soils. Our results highlight that deep soils under impervious surfaces could be overlooked carbon hotspots in urban ecosystems. We believe this finding could help city planners and policy makers to assess regional carbon budgets and to reduce carbon footprint by recycling the deep SOC excavated from various construction projects towards sustainable urban development.

Urban parks offer valuable ecosystem services to citizens and they have long been recognized for their recreational service; however, less attention has been paid to their carbon sequestration value. Here, we report on soil organic carbon (SOC) stocks in an urban park, Seoul Forest Park, which was built in 2004. We had two objectives: (1) to estimate SOC stocks (to a depth of 1 m) in different land-cover types (wetland, forest, lawn, and bare soil) and (2) quantify the change in the SOC concentration in topsoil in different land-use types over a 10 year period (2003–2013). We found a tenfold difference in SOC stocks across the different land-cover types within the park. Wetland soils had the highest stocks of SOC (13.99 ± 1.05 kg m −2 ), followed by forest, lawn, and bare soils. We found that a “cultural layer” that preserved previous land use history located deep in the soil profile substantially increased SOC stocks in the wetland. SOC concentrations in the topsoil were approximately three times higher in 2013 than in 2003 (256 ± 130%). The normalized difference vegetation index (NDVI) derived from MODIS and Landsat satellite images revealed that land-use history, expansion of plant areas and growth of plants could explain the increase in SOC concentrations in topsoil over the 10 year period. These findings imply that urban park soils could act as a carbon sink, and understanding the land-use history and the choice of land-cover types in park planning can substantially influence the carbon budget of urban parks.