Natural and anthropogenic activities affect soil heavy metal pollution at different spatial scales. Quantifying the spatial variability of soil pollution and its driving forces at different scales is essential for pollution mitigation opportunities. This study applied a multivariate factorial kriging technique to investigate the spatial variability of soil heavy metal pollution and its relationship with environmental factors at multiple scales in a highly urbanized area of Guangzhou, South China. We collected 318 topsoil samples and used five types of environmental factors for the attribution analysis. By factorial kriging, we decomposed the total variance of soil pollution into a nugget effect, a short-range (3 km) variance and a long-range (12 km) variance. The distribution of patches with a high soil pollution level was scattered in the eastern and northwestern parts of the study domain at a short-range scale, while they were more clustered at a long-range scale. The correlations between the soil pollution and environmental factors were either enhanced or counteracted across the three distinct scales. The predictors of soil heavy metal pollution changed from the soil physiochemical properties to anthropogenic dominated factors with the studied scale increase. Our study results suggest that the soil physiochemical properties were a good proxy to soil pollution across the scales. Improving the soil physiochemical properties such as increasing the soil organic matter is essentially effective across scales while restoring vegetation around pollutant sources as a nature-based solution at a large scale would be beneficial for alleviating local soil pollution.

Microalgae are known for their efficient removal of hexavalent chromium (Cr(VI)) through biosorption and bioaccumulation, yet the subsequent release of Cr(VI) upon cell death remains a challenge. The reduction of Cr(VI) to the less toxic trivalent chromium [Cr(III)] is another critical remediation strategy that mitigates the risk of Cr(VI) re-release, but research on microalgal reduction of Cr(VI) is scarce. In this study, a microalgal strain designated as MQ-1 was isolated from chromium-contaminated mine effluent, demonstrating the capability to tolerate and remove Cr(VI). Phylogenetic analysis revealed that MQ-1 is closely related to the genus Chlorella; hence, it is classified as Chlorella sp. MQ-1. This strain exhibited robust growth at Cr(VI) concentrations below 2 mg/L, achieving a removal rate higher than 82% for initial Cr(VI) concentrations between 0.5 and 1 mg/L after a 5-day incubation period. Mechanistic studies revealed that MQ-1 promoted the removal of Cr(VI) mainly through intracellular bioreduction and bioaccumulation processes, in which more than 60% of Cr(VI) was reduced to the less toxic Cr(III) and stocked in the cells. A two-stage cultivation strategy, involving initial biomass accumulation followed by Cr(VI) treatment, significantly enhanced the removal efficiency, which was further accelerated under illuminated conditions. Notably, MQ-1 cultures with initial OD₆₈₀ values of 4 and 6 accomplished 84.28% and 91.31% Cr(VI) removal from 2 mg/L solutions, respectively, within 30 hours under light exposure. These findings highlight the potential of MQ-1 to utilize renewable solar energy to reduce Cr(VI) and to mitigate the risk of its re-release into the environment. This characteristic positions MQ-1 as a potentially sustainable and cost-effective solution for Cr(VI) remediation and suggests its significant potential for large-scale implementation in bioremediation strategies aimed at Cr(VI)-contaminated waters.

Microalgal amendments can improve soil structure by regulating extracellular polymeric substances (EPSs). However, the mechanisms underlying this process in red soils (characterized by high clay content and susceptibility to acidification) under different farming practices remain unclear. This study examined how Chlorella vulgaris (C. vulgaris) amendment influences EPS composition to enhance soil aggregate stability under arable land and rice paddy farming. A five-month pot experiment using a completely randomized design was conducted to investigate the effects of Chlorella vulgaris amendment on soils cultivated with Pennisetum × sinese and rice, two economically important crops commonly grown in South China. At the end of the experiment, Chlorella vulgaris amendment substantially increased both the mean weight diameter (MWD) and geometric mean diameter (GMD) of soil aggregates under both farming systems. Excitation–emission matrix (EEM) fluorescence spectroscopy revealed distinct changes in soil EPS components between the two farming types. Under arable land farming, humic-like and protein-like EPSs were dominant in Chlorella vulgaris-amended treatments, with fluorescence intensities more than doubling compared to the control. Conversely, under rice paddy farming, soil fulvic acid was the main component and showed a moderate increase. Partial least squares path modeling (PLS-PM) demonstrated that protein-like and humic-like EPSs had the strongest direct effects on aggregate stability in arable land red soil, while fulvic acid was the key factor in rice paddy red soil. The present study demonstrates that Chlorella vulgaris amendment improves aggregate stability in red soils through farming-specific, EPS-mediated pathways, providing a quantitative framework for researchers and land managers seeking to apply microalgal amendments for red soil enhancement and sustainable land management.

Soil cyanobacteria in tropical forests is understudied despite its important role in soil biochemical process and plant growth. Under a nitrogen (N) deposition background in tropical forests, it is important to learn how soil cyanobacterial communities respond to N deposition and whether phosphorus (P) mediated this response. A fully two-factor (N and P additions) factorial design with four blocks (replicates), each including a 12 × 12 m plot per treatment (Control, +N, +P, and +NP) were established in a tropical secondary forest in 2009. In July of 2022, soil cyanobacteria at 0–10 cm and 10–20 cm depths in the experimental site were collected and analyzed using a metagenomic method. The impact of N and P additions on soil cyanobacteria remained consistent across the different soil depths, even though there was a significant contrast between the two layers. The effect of N addition on soil cyanobacteria did not significantly interact with P addition. N addition increased soil N availability and decreased soil pH but did not significantly affect the soil cyanobacterial community. In contrast, P addition increased soil P availability and soil pH, but decreased soil N availability and substantially changed the soil cyanobacterial community. P addition significantly decreased the abundance of soil cyanobacteria, especially abundant ones. P addition also increased cyanobacterial species richness and Shannon’s diversity, which might be explained by the decline in dominant species and the emergence of new species as nestedness and indicator species analyses suggest. We concluded that (1) soil cyanobacteria in tropical forests exhibits a greater sensitivity to elevated P availability compared to N; (2) an increase in soil P supply may mitigate the advantage held by dominant species, thus facilitating the growth of other species and leading to alterations in the soil cyanobacterial community. This study improves our understanding on how soil cyanobacterial communities in tropical forest responds to N and P addition.

Laboratory incubation experiments were conducted to investigate the effects of coal fly ash (FA) amendment (0%, 2.5%, 7.5%, and 15%) and moisture regimes (40%, 70%, and 100% water holding capacity (WHC)) on the mineralization of carbon (C) in an acidic agricultural soil. The results showed that the soil C mineralization intensity initially increased and subsequently decreased throughout the incubation period, with the mineralization dynamics well described by the first-order kinetic model (0.9633 ≤ R² ≤ 0.9972). Carbon mineralization increased with the application rate of FA, while moisture effect followed the order 70% WHC > 100% WHC > 40% WHC. Indicators showing highly significant correlations with total C mineralization amount included FA application rate, pH, water-soluble organic carbon, (WSOC) and cellulase (CEL) activity. Specific bacterial (Acidobacteriota, Gemmatimonadota, Pseudomonadota, and Actinobacteriota) and fungal phyla (Chytridiomycota, Glomeromycota, and Olpidiomycota) exhibited stronger correlations with C mineralization. The microbial taxa exhibiting significant responses to FA and moisture conditions were not consistent. Although the addition of high proportions of FA, especially with adequate moisture conditions, can enhance soil microbial activity and C mineralization, the potential risks of soil C loss and the accumulation of toxic elements necessitate the prudent implementation of elevated FA application rates in practical scenarios.

Enhancing soil nutrient content is fundamental to the ecological restoration of degraded soils. The application of microalgae represents a sustainable approach for soil remediation, as it contributes to environmental CO₂ sequestration while recycling nutrients into degraded ecosystems. Through a 105-day laboratory incubation experiment, this study investigated the impact of applying a mixed microalgal suspension containing active/inactive Chlorella ZJ and Anabaena azotica on the C and N fractions of an alkaline, degraded purple soil. The results showed that both active and inactive microalgae treatments (AM and IM) significantly decreased soil pH and increased soil moisture content (SMC). The AM treatment notably increased the proportion of large soil aggregates and enhanced soil structure. Both treatments significantly enhanced soil C and N fractions: dissolved organic carbon/nitrogen (DOC/DON) increased by 6.41/5.81 times (AM) and 4.22/4.76 times (IM) that of the control (without microalgae application); total organic carbon (TOC) rose by 147.07% (AM) and 138.73% (IM); and the contents of coarse particulate and mineral-associated organic C and N were also significantly elevated. Total nitrogen (TN) significantly increased only under the AM treatment. Soil C and N mineralization capacities were enhanced by 1.01–1.34 times and 7.56–8.43 times that of the control, respectively, indicating a more pronounced stimulation of N mineralization. Fluorescence analysis revealed that both AM and IM treatments increased the complexity and humification of dissolved organic matter. The application of microalgae significantly improved the soil structure and chemical characteristics of the degraded soil and enhanced the C/N pools, thereby creating favorable conditions for soil restoration.