Microbial and Biogeochemical Responses to Drought in Soil Carbon Cycling Systems

Soil is the largest terrestrial carbon reservoir, with microorganisms playing a pivotal role in organic matter decomposition and carbon stabilization. Drought, intensified by climate change, alters microbial dynamics and soil carbon cycling. This study investigates the effects of drought-induced stress on microbial community composition, carbon allocation, and soil biochemical properties in agricultural and forest soils in Bangladesh. Soils from a drought-prone maize field and a semi-natural forest site in Kushtia, Bangladesh, were incubated in mesocosms under control (25% WHC) and drought (10% WHC) conditions, with and without carbon amendments. Microbial activity and structure were assessed through PLFA analysis and 16S rRNA sequencing. Soil CO₂ emissions, microbial biomass carbon (MBC), dissolved organic carbon (DOC), and soil nutrients were also analyzed. Drought significantly reduced microbial biomass (45.3 to 32.3 nmol PLFA g⁻¹, p = 0.021), MBC (215.7 ± 12.4 to 128.5 ± 10.7 µg C g⁻¹, p < 0.01), and CO₂ emissions (3.85 ± 0.21 to 2.16 ± 0.18 µg CO₂-C g⁻¹ day⁻¹). DOC decreased from 12.4 ± 1.2 to 7.1 ± 0.9 mg C g⁻¹ (p = 0.012). Gram-negative bacteria declined by 41.2%, while Gram-positive taxa and fungi increased, indicating a shift to drought-tolerant communities. Carbon use efficiency slightly increased (56.0–59.5%). Drought suppresses microbial activity and alters community structure, reducing soil carbon and nutrient availability. These findings underscore the need for drought-resilient soil management in climate-sensitive regions like Bangladesh.