The soil of urban green spaces is severely degraded due to human activities during urbanization, and it is crucial to investigate effective measures that can restore the ecological functions of the soil. This study investigated the effects of plant growth promoting bacteria (Bacillus clausii) and Fe-modified biochar on soil fertility increases and mechanisms of carbon sequestration. Additionally, the effects on C-cycling-related enzyme activity and the bacterial community were also explored. Six treatments included no biochar or Bacillus clausii suspension added (CK), only Bacillus clausii suspension (BC), only biochar (B), only Fe-modified biochar (FeB), biochar combined with Bacillus clausii (BBC), and Fe-modified biochar combined with Bacillus clausii (FeBBC). Compared with other treatments, the FeBBC treatment significantly decreased soil pH, alleviated soil alkalization, and increased the alkali-hydro nitrogen content in the soil. Compared to the individual application of FeB and BC, the FeBBC treatment significantly improved aggregates’ stability and positively improved soil fertility and ecological function. Additionally, compared to the individual application of FeB and BC, the soil organic carbon (SOC), particulate organic carbon (POC), and soil inorganic carbon (SIC) contents for the FeBBC-treated soil increased by 28.46~113.52%, 66.99~434.72%, and 7.34~10.04%, respectively. In the FeBBC treatment, FeB can improve soil physicochemical properties and provide bacterial attachment sites, increase the abundance and diversity of bacterial communities, and promote the uniform distribution of carbon-related bacteria in the soil. Compared to a single ecological restoration method, FeBBC treatment can improve soil fertility and carbon sequestration, providing important reference values for urban green space soil ecological restoration.

Since the late 1990s, the Yangtze River Delta (YRD) has experienced profound growth in economic scales and urban size. However, it is still unclear how much energy is consumed from both fossil fuel and electricity usage for transportation sectors (TCO₂). We take 10 sampled cities in the YRD as examples and examine their city-level sustainable levels from 1990 to 2018. Then, we observed that SHSN (Shanghai, Suzhou, Nanjing) are in leading positions, followed by WCN (Wuxi, Changzhou, Ningbo) and NXH (Nantong, Xuzhou, Hefei). We found that the cumulative TCO₂ in SHSN from 1990 to 2018 is the highest among groups, which is mainly due to the earlier industrialization in history. In 2018, SHSN had the highest TCO₂ (623.9 × 10⁴ t), WCN was 311.9 × 10⁴ t, and NXH was 166.4 × 10⁴ t. TCO₂ per capita in SHSN reached its minimal (≈0.12 t) in 2018 among 29 years, while WCN and NXH shared the same levels (≈0.07 t). This could be attributed to the dense population and a series of low carbon policies announced in SHSN and WCN. NXH is still in the stage of high demands on economic-centered development. The primary source for TCO₂ in the YRD is fossil fuels. The TCO₂ contributed by transportation electricity usage is continually increasing, especially after 2010. This phenomenon represents that electricity can be a significant part of the YRD’s transportation sectors’ energy consumption shortly. A complex estimation model uncovers the complexity between the economy, environment, and carbon emissions in the YRD, which indicated that the decrease of TCO₂ in YRD could not be regulated solely by economic or environmental interventions. This study highlighted the urgency for socio-economic adjustments from carbonized to decarbonized structures in the YRD.