Arsenic stress on soil microbial nutrient metabolism interpreted by microbial utilization of dissolved organic carbon.

In a 120-day microcosm incubation experiment, we investigated the impact of arsenic contamination on soil microbial nutrient metabolism, focusing on carbon cycling processes. Our study encompassed soil basal respiration, key enzyme activities (particularly, ß-1,4-N-acetylglucosaminidase and phosphatases), microbial biomass, and community structure. Results revealed a substantial increase (1.21-2.81 times) in ß-1,4-N-acetylglucosaminidase activities under arsenic stress, accompanied by a significant decrease (9.86%-45.20%) in phosphatase activities (sum of acid and alkaline phosphatases). Enzymatic stoichiometry analysis demonstrated the mitigation of microbial C and P requirements in response to arsenic stress. The addition of C-sources alleviated microbial C requirements but exacerbated P requirements, with the interference amplitude increasing with the complexity of the C-source. Network analysis unveiled altered microbial nutrient requirements and an increased resistance process of microbes under arsenic stress. Microbial carbon use efficiency (CUE) and basal respiration significantly increased (1.17-1.59 and 1.18-3.56 times, respectively) under heavy arsenic stress (500 mg kg-1). Arsenic stress influenced the relative abundances of microbial taxa, with Gemmatimonadota increasing (5.5-50.5%) and Bacteroidota/ Nitrospirota decreasing (31.4-47.9% and 31.2-63.7%). Application of C-sources enhanced microbial resistance to arsenic, promoting cohesion among microorganisms. These findings deepen our understanding of microbial nutrient dynamics in arsenic-contaminated areas, which is crucial for developing enzyme-based toxicity assessment systems for soil arsenic contamination.

The distribution of arsenic fractions and alkaline phosphatase activities in different soil aggregates following four months As(V) ageing.

Soil as a heterogeneous mass is composed of different size aggregates. The distribution of different arsenic (As) fractions in soil aggregates is vital to assess the potential risk of As pollution. In this study, soil samples were aged for 4 months with different arsenate [As(V)] concentrations. Dry sieving method was used to obtain five different size aggregates and the content of As in these fractions was determined. The results showed that P4 (0.1-0.25 mm) contained the highest organic matter (OM) than other size aggregates. After 4 months of ageing, available phosphorus (AP) content increased with the increase of As(V) concentration among 5 aggregates. The distribution of different arsenic fractions among 5 aggregates was similar. The relative contents of water-soluble (F1), exchangeable (F2) and carbonate (F3) fractions increased with the increase in As concentration, while the residual fraction (F7) decreased sharply. Humic-bound (F4), and Fe and Mn oxide bound fractions (F5) were about 35% and 20% respectively, after 4 months of As(V) ageing. Generally, the alkaline phosphatase (ALP) activities of P4 were lowest among five aggregates under each concentration of As(V). Moreover, F2 and F3 exhibited a strong inhibition of ALP activity. This study demonstrates that not only water-soluble and exchangeable arsenic but also humic-bound fraction should be considered when assessing As bioavailability and toxicity.