Estimation of changes in soil organic carbon in farmlands in Fuzhou City using Landsat vegetation data and 1:50 000 soil database

Carbon pools in farmlands are critical drivers of change in global carbon stock and a small change in these pools could have a huge influence on CO₂ concentration in the atmosphere, causing global greenhouse condition. Though medium and small scale soil databases have often been used in estimating soil organic carbon (SOC) in farmlands, remote sensing image and large-scale soil database have rarely been used. In this study, we combined remote sensing and carbon cycle model to estimate SOC in farmland of Fuzhou City (in subtropical China) using Landsat TM and Landsat 8 OLI remote sensing images plus 1:50 000 high-resolution soil database. In the study area, SOC in farmlands was calculated for 1987 and 2016. The purpose of the study was to determine whether the study area was source or sink of SOC and the dynamics of SOC pool in the region. The results showed that:1) there was a strong linear relationship between soil basal respiration and soil organic carbon density (SOCD) in farmlands in Fuzhou City. The determinant coefficient R² was 0.637 for 1987 and 0.752 for 2016. 2) For the study period, SOCD in farmlands increased from eastern coastal area to western inland area. In general, SOCD and soil organic carbon storage (SOCS) increased by 0.20 kg·m^-2 in the eastern coastal area and 2.946×10⁵ kg·m^-2 in western inland area, indicating that farmlands in Fuzhou City served as a weak "carbon sink" in the past 30 years. 3) In terms of soil type, yellow earth, red earth and paddy soil contributed the largest to "carbon sink" across the soil groups, with respective SOCD increases of 0.70 kg·m^-2, 0.40 kg·m^-2 and 0.19 kg·m^-2. Fluvo-aquic soil, latosolic red soil, aeolian soil, coastal solonchak and purplish soil were the biggest contributors to "carbon source" across the soil group. Comparison of SOCS of different soil types in 1987 with those in 2016 showed that SOCS was highest in paddy soil, accounting for over 90% of the total carbon storage in Fuzhou City. 4) In terms of administrative region, Cangshan District, Changle District, Mawei District and Lianjiang County were "carbon source" areas. Then Minhou County, Minqing County, Fuqing City, Jin'an District, Yongtai County and Luoyuan County were "carbon sink" areas. The lowest SOCS was in Cangshan District in Fuzhou City, with less than 0.5% of total SOCS. Fuqing City always ranked the highest in Fuzhou City, accounting for over 20.0% of the total SOCS. In general, the spatial dynamics of SOCD in farmlands in Fuzhou City during the 30-year period were significant. Additionally, there were differences in different soil types and administrative regions in terms of SOCD. In future, it was beneficial to conduct farmland management based on soil type in different farmlands and SOC in the administrative regions.