Spatial Variation of Soil Organic Carbon and Total Nitrogen in the Coastal Area of Mid-Eastern China

Soils play an important role in sequestrating atmospheric CO₂. Coastal tidal flats have been intensively reclaimed for food security and living spaces worldwide. We aimed to identify the changes of soil organic carbon (SOC) and total nitrogen (TN) following coastal reclamation and their spatial variation in the coastal area of mid-Eastern China to provide information for coastal cropland management. We measured SOC and TN of 463 soil samples in the coastal plain of mid-Eastern China. The results showed that SOC and TN increased highly from the uncultivated coastal tidal flat (2.49 g·kg^-1 and 0.21 g·kg^-1, respectively) to the cropland (10.73 g·kg^-1 and 1.3 g·kg^-1, respectively). After long-term cultivation, SOC and TN in the old farmland (12.98 g·kg^-1 and 1.49 g·kg^-1, respectively) were greater than those in the young farmland (5.76 g·kg^-1 and 0.86 g·kg^-1, respectively). The density of SOC in the uncultivated coastal tidal flat, young farmland, and old farmland were 0.68 kg·C·m^-2, 1.52 kg·C·m^-2, and 3.31 kg·C·m^-2, respectively. The density of TN in the uncultivated coastal tidal flat, young farmland and old farmland were 0.05 kg·N·m^-2, 0.23 kg·N·m^-2, and 0.38 kg·N·m^-2, respectively. The C/N (11.17) in the uncultivated coastal tidal flat was highest comparing to that in the young and old farmland due to lower nitrogen. The C/N increased from 6.78 to 8.71 following cultivation. Reclaimed coastal tidal flats had high carbon and nitrogen sequestration potential that not only mitigated the threat of global warming, but also improved soil fertility for crop production. Coastal management of cropland should consider the spatial distribution of SOC and TN to improve ecosystem services of coastal soils.