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Background and aims: The accumulation of soil organic carbon (SOC) is a crucial process in mitigating climate change and ensuring soil quality. Subtropical plantations in China have shown high potential for enhancing SOC sequestration due to their high carbon sink capacity. However, the dynamics and compositions of SOC after long term afforestation are still poorly understood. Methods: We examined the dynamics of SOC and its fractions of different soil layers (0–5 cm, 5–10 cm, 10–20 cm, 20–40 cm, 40–80 cm, 80–100 cm) in a mixed broadleaf-conifer plantation (BCP), a Masson pine plantation (MP), and a Slash pine plantation (SP), and an enclosed forest (EF) in subtropical China. Both physical fractions (particulate organic carbon (POC), soil clay organic carbon (SOCclay), soil silt organic carbon (SOCsilt)) and chemical fractions (iron-aluminum-bound organic carbon (Fe/Al-OC), and calcium-bound organic carbon (Ca-OC)) were determined. Additionally, soil physicochemical properties were further analyzed. Results: Our results showed the SOC storage in the EF, BCP, MP, and SP in 2019 increased by 82%, 238%, 126%, and 103% respectively compared to 1984. The proportions of MAOC to SOC were respectively as follows: SP (65.9%) > BCP (64.9%) > EF (61.6%) > MP (53.9%). The mean proportions of SOCclay to MAOC were as follows: BCP (71.1%) > EF (66.9%) > MP (58.5%) > SP (51.0%). The mean proportions of Fe/Al-OC to MAOC were as follows: EF (53.3%) > SP (48.3%) > MP (47.9%) > BCP (39.8%). All these proportion increased with soil depth. RDA and PLS-SEM analysis revealed that soil nitrogen content and electrochemical properties (pH and EC) were the main factors that controlled SOC content and fractions. Conclusions: Subtropical plantations have significant soil carbon sequestration capabilities, but they are currently nearing saturation. Soil nitrogen content may be key factor limiting further enhancement of carbon sequestration. Due to the high proportion of MAOC and relative consistent SOC storage, BCP and SP are recommended as priorities for plantation regeneration in subtropical China. [ABSTRACT FROM AUTHOR]

Aims: Soil organic carbon (SOC) storage is mainly governed by inputs from plant and microbial sources. How the relative contributions of these two sources in forest soils vary along broad-scale environmental gradients is poorly understood. The contributions of these two carbon sources in different soil layers are also elusive. Methods: We used amino sugar and lignin phenol as biomarkers to indicate microbial- and plant-derived soil carbon. The concentrations of amino sugar and lignin phenol in topsoil (0–10 cm) and subsoil (30–60 cm) of major forest types along a latitude gradient in China were investigated. Results: The concentration of soil amino sugar decreased along the latitude in topsoil, which is mainly controlled by the variations in mean annual temperature and soil nitrogen. The concentration of lignin phenols decreased along latitude in topsoil, mainly controlled by soil carbon/nitrogen ratio. The microbial-derived carbon was mainly composed of fungi-derived in topsoil, while bacteria-derived dominated in subsoil. Climate and soil properties are primary factors controlling the persistence of microbial residues and lignin phenols in topsoil, and soil clay is the crucial factor in the subsoil. Conclusions: Both microbial residuals and lignin phenols in topsoil showed a decreasing trend with increasing latitude, but controlled by different sets of environmental factors. Subsoil microbial residues and lignin phenols showed no trend along the latitude and are mainly influenced by soil clay content.These findings provide new insights on how climate and vegetation affect soil carbon persistence, revealing significant distribution patterns of forest soil microbial residuals and lignin phenols along a latitude gradient. [ABSTRACT FROM AUTHOR]

Soil organic carbon (SOC) is a crucial medium of the global carbon cycle and is profoundly affected by multiple factors, such as climate and management practices. However, interactions between different SOC fractions and land-use change have remained largely unexplored in karst ecosystems with widespread rock outcrops. Owing to the inherent heterogeneity and divergent response of SOC to land-use change, soil samples with close depth were collected from four typical land-use types (cropland, grassland, shrubland, and forestland) in the karst rocky desertification area of China. The aim of this study was to explore the responses of SOC dynamics to land-use types and underlying mechanism. The results showed that land-use type significantly affected SOC contents and its fractions. Compared with cropland, the other three land uses increased the total organic carbon (TOC), microbial biomass carbon (MBC), and non-labile organic carbon (NLOC) contents by 6.11–129.44%, 32.58–173.73%, and 90.98–347.00%, respectively; this demonstrated that a decrease in both labile and recalcitrant carbon resulted in SOC depletion under agricultural land use. Readily oxidized organic carbon (ROC) ranged from 42 to 69%, accounting for almost half of the TOC in the 0–40-cm soil layer. Cropland soil showed significantly higher ROC:TOC ratios than other land-use types. These results indicated that long-term vegetation restoration decreased SOC activity and improved SOC stability. Greater levels of soil exchangeable calcium (ECa) and clay contents were likely responsible for higher stabilization and then accumulation of SOC after vegetation restoration. The carbon pool index (CPI) rather than the carbon pool management index (CPMI) exhibited consistent variation trend with soil TOC contents among land-use types. Thus, further study is needed to validate the CPMI in evaluating land use effects on soil quality in karst ecosystems. Our findings suggest that land-use patterns characterized by grass or forest could be an effective approach for SOC-sequestration potential and ensure the sustainable use of soil resources in the karst area. [ABSTRACT FROM AUTHOR]

Chloramphenicol, a broad-spectrum antibiotic employed for controlling bacterial infections, presents an intriguing aspect in terms of its environmental fate in soils. 14C-labeled chloramphenicol was used to explore its mineralization and residue characteristics in three distinct agricultural soils in China. The findings revealed a nuanced pattern in the fate of 14C-chloramphenicol, with notable variations among the different soils under investigation. The chloramphenicol extract residue exhibited a reduction of 18.04% in sandy clay soil, 23.04% in clay loam soil, and 21.73% in loamy clay soil. Notably, the mineralization rate in sandy clay soil was 25.22% surpassed that in the other two soils, particularly during the initial stages of incubation. Over time, the diminishing extract residue underwent conversion into minerals and bound residue. The formation rate of bound residue was increased from 44.59 to 53.65% after adding 10% manure, suggesting that chloramphenicol easily binds with soils rich in organic matter. The bound residue is predominantly localized in the humin fraction across all soils. Additionally, the sterilized soil experiments indicated the pivotal role of microorganisms in influencing the fate of chloramphenicol under the specified experimental conditions. In conclusion, this study offers valuable insights into the environmental dynamics of chloramphenicol in soils, emphasizing the importance of soil composition, organic matter content, and microbial activity. The findings contribute to a scientific understanding of the environmental safety implications associated with chloramphenicol usage. [ABSTRACT FROM AUTHOR]

The Mg isotopic compositions of siliciclastic components (δ26Mgsili) of sediments and sedimentary rocks have been commonly used to constrain the intensity of continental weathering, based on observations of, (1) an upward enrichment of 26Mg in modern weathering profiles, (2) preferential uptake of 26Mg in soil clays, and (3) general positive correlations between weathering intensities and δ26Mgsili values. However, not all weathering profiles display an upward increase of δ26Mgsili, and not all soil clays enrich in 26Mg, complicating the interpretations of δ26Mgsili data of sediments and sedimentary rocks. To further explore the controls of δ26Mgsili of sediments and sedimentary rocks, here we measured δ26Mgsili, mineralogical compositions and major element compositions of deep sea sediments from South China Sea (SCS) and carbonate rock samples from two late Paleozoic sections in South China. Carbonate samples show decreasing trends of both normalized Mg concentration (Mgnorm) and δ26Mgsili values with the increase of weathering intensity, while SCS sediments samples do not show any correlation between Mgnorm or δ26Mgsili values and weathering intensity. A three-stages weathering model is developed to explain the observed Mgnorm and δ26Mgsili data. In the new model, weathering can be divided into three stages, (1) the preliminary weathering stage characterized by dissolution of primary minerals and formation of secondary clay minerals in saprolite, (2) the transitional stage with massive vermiculite and chlorite formation on the bottom of soil, and (3) the advanced stage showing dissolution of vermiculite and chlorite in the top of soil. The new model explains an δ26Mgsili increase and Mgnorm decrease with an increase weathering intensity in preliminary weathering stage and decrease of Mgnorm and δ26Mgsiliwith increase weathering intensity in advanced weathering stage. The revised weathering model warrants the application of Mg isotope in the study of continental weathering in deep time. [ABSTRACT FROM AUTHOR]