Soil organic carbon of an intensively reclaimed region in China: Current status and carbon sequestration potential

Whether the carbon emission from green roofs can be effectively mitigated by recycling waste building material as green roof substrate during five-year operation?

Green roof (GF) as an important role of urban ecosystem services is more and more focused on carbon sequestration for the mitigation of climate change, which there is still a gap of longer period of investigation on carbon sequestration on GF. This work aims to quantify the carbon sequestration on green roofs from 2012 to 2017 by measuring and calculating parameter on substrate organic carbon and plant organic carbon, when using waste building material substrate (WBMS) as GF substrate for the recycling of waste solid. Green roof group 2 (waste building material substrate (WBMS) as substrate) and green roof group 1 (local natural soil (LNS) as substrate), planting same three native plants (N. auriculata, L. spicata, and L. vicaryi), were both three substrate depth of 20 cm, 25 cm, and 30 cm, respectively. Results show that both innovative WBMS and LNS were a great capability of carbon sequestration and carbon storage on green roofs. Carbon storage of green roof group 1 and green roof group 2 was 65.6 kg C m^-2 and 72.6 kg C m^-2, respectively. Annual mean carbon sequestration of the WBMS was 1.8 times higher than LNS. The overall average carbon sequestration (12.8 kg C m^-2 year^-1) in green roof group 2 using WBMS was 1.1 times than corresponding in green roof group 1 (11.4 kg C m^-2 year^-1 using LNS). WBMS substrate and L. vicaryi could be considered as the most adaptable green roof configuration, which can be a recommendation to promote the carbon sequestration and the function of green roof for the better urban ecosystem services. Future work may focus on the GF carbon model, water interface, long-term monitoring, environmental impact, water quality and quantity, synthesized effect on GF ecosystem, low impact development (LID), management and simulation, and combination on intelligent urban system, based on LCA.

Shift in soil organic carbon and nitrogen pools in different reclaimed lands following intensive coastal reclamation on the coasts of eastern China

The impacts of coastal reclamation on carbon (C) and nitrogen (N) sinks of coastal wetlands remain unclearly understood. This study was conducted to investigate the alterations of soil organic C and N (SOC and SON) pools following conversion of Phragmites australis salt marsh into fishpond, wheat and rapeseed fields and town construction land through reclamation along Jiangsu coast in eastern China. Coastal reclamation significantly increased stocks of soil total, labile and recalcitrant organic C and N (SLOC, SLON, SROC, and SRON), and concentrations of water-soluble organic C (WSOC), microbial biomass C and N (SMBC and SMBN), cumulative CO₂-C mineralization (MINC) following conversion of P. australis salt marsh into fishpond, wheat and rapeseed fields. However, coastal reclamation reduced SOC, SLOC, SROC, SRON, WSOC, SMBC, SMBN, and MINC following conversion of P. australis salt marsh into town construction land. Our results suggest that coastal reclamation affects C and N sinks of coastal wetlands by changing SOC and SON pools size, stability and dynamics changes following conversion of P. australis salt marsh into other land use types. This finding were primarily attributed to alterations in quantity and quality of exogenous materials returning the soil, and soil physiochemical properties as affected by coastal reclamation.

Variance in bacterial communities, potential bacterial carbon sequestration and nitrogen fixation between light and dark conditions under elevated CO2 in mine tailings

This study is the first to show the response of bacterial communities with primary carbon and nitrogen fixers to elevated CO₂ (eCO₂) in light and dark conditions based on 6 months of culture growth. Carbon sequestration and nitrogen fixation were analyzed by ¹³C and ¹⁵N isotope labeling using ¹³C-labeled CO₂ and ¹⁵N-labeled N₂, followed by pyrosequencing and DNA-based stable isotope probing (SIP) to identify carbon fixers and nitrogen fixers. The results indicated that eCO₂ decreased the Chao 1 richness, and the eCO₂-light treatment exhibited the highest Shannon diversity. In addition, eCO₂ (in either light or dark conditions) greatly increased the relative abundances of bacteria belonging to the classes Betaproteobacteria and Alphaproteobacteria. The ¹³C atom % in the mine tailings increased from 1.108 to 1.84 ± 0.11 under light conditions and 1.52 ± 0.17 under dark conditions after 6 months of culture growth. Ribulose-1,5-bisphosphate carboxylase/oxygenase (RubisCO) form I-coding gene (cbbL) copy numbers were 164.30-fold and 40.36-fold higher than RubisCO form II-coding gene (cbbM) copy numbers in the heavy fractions with a buoyant density of 1.7388 g·mL^-1 relative to the buoyant density gradients of DNA fractions obtained under eCO₂-light and eCO₂-dark treatment, respectively. The Proteobacteria-like cbbL genes were dominant in the carbon fixers. In addition, the ¹⁵N atom % in the mine tailings increased from 0.366 to 0.454 ± 0.021 in light conditions and 0.437 ± 0.018 in dark conditions. Furthermore, uncultured nitrogen-fixing bacteria were the dominant nitrogen fixers in light conditions, and bacteria harboring the Bradyrhizobium-like nifH and Leptospirillum-like nifH genes were the dominant nitrogen fixers in dark conditions. These first data for a mine tailing ecosystem are inconsistent with those obtained for a range of other ecosystems, in which the effects of CO₂ were limited to several nonphotoautotrophic communities and different nitrogen fixers.

Prediction of soil organic carbon in an intensively managed reclamation zone of eastern China: A comparison of multiple linear regressions and the random forest model

Organic carbon is a key component of soils and plays a fundamental role in soil fertility and climate change. Determining the importance of potential drivers of soil organic carbon (SOC) and thus predicting the distribution of SOC are important for measuring carbon sequestration or emissions. Coastal wetlands are precious land resources that are currently undergoing rapid reclamation in China. The alternations in soil physicochemical conditions caused by reclamation can strongly impact the cycle of organic carbon. However, identification of the important drivers of SOC dynamics and prediction of SOC using the potential drivers remain largely unclear. In this study, we used classification and regression tree (CART) to identify the importance of the potential drivers of SOC at 241 sites from an intensively managed reclamation zone of eastern China. Multiple linear regressions (MLR) and random forest (RF) models were applied to predict the distribution of SOC using continuous variables, such as the contents of Cl, CaO, Fe₂O₃, Al₂O₃, SiO₂, clay, silt, and sand as well as the soil pH, along with categorical variables, such as land use and reclamation duration. The results indicate that the soil/sediment pH was the most important variable impacting SOC, followed by the Cl and silt contents. The RF and MLR involving all predictor variables produced much higher R² and lower error indices than the RF and MLR models involving independent variables (pH and CaO). RF performed much better than MLR as it revealed much lower error indices (ME, MSE, and RMSE) and a higher R² than MLR. The superiority of RF in predicting SOC is related to its capability to deal with non-linear and hierarchical relationships between SOC and predictors. Analyses of land use effects on SOC dynamics indicated that paddy soils were superior in sequestering SOC than other land use types, which is likely ascribed to the rapid desalination and dealkalization of paddy field management. Therefore, paddy field management is recommended as an environment-friendly approach for managing newly reclaimed lands.

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.