ARSENIC SOIL CONTAMINATION AFFECTS MICROBIAL CARBON MINERALIZATION AND ORGANIC CARBON USE EFFICIENCY

Labile organic carbon transformation plays a central role in soil organic matter formation and depends on carbon inputs, microbial activity, and environmental factors such as metal presence. Arsenic is a metalloid widely recognized as a soil contaminant, acting as a stress factor for soil microbiota and interfering with organic carbon mineralization. This study hypothesized that increasing arsenic contamination alters soil microbial metabolism, affecting organic carbon mineralization and carbon use efficiency. A microcosm experiment was conducted using 100 g of soil amended with 0.5 g of soybean extract as a labile carbon source and realgar as an arsenic source at doses ranging from 17 to 500 µg.g⁻¹ soil. Soil moisture was adjusted to 50% of water-holding capacity, and samples were incubated for 21 days, during which microbial respiration was quantified by CO₂ release. After incubation, microbial biomass carbon (MBC) was estimated using the irradiation–extraction method, and the metabolic quotient (qCO₂) and carbon use efficiency (CUE) were calculated. Arsenic addition increased microbial respiration by 8 35% with increasing doses, indicating enhanced mineralization of labile carbon. In contrast, MBC remained relatively stable among treatments, ranging from 370 to 404 µg C.g⁻¹ dry soil, with no linear trend across realgar doses, suggesting resilience of the soil microbiota to arsenic stress. The qCO₂ increased with arsenic doses, indicating elevated metabolic stress, while CUE decreased, demonstrating reduced efficiency in labile carbon utilization. Overall, these results confirm that arsenic alters microbial carbon mineralization dynamics and metabolic efficiency, leading to increased microbial CO₂ efflux from soil.