Warming-induced unstable microbial community metabolically lowers straw-carbon sequestration in paddy soils.

Introduction: Growing academic attention has been given to the crucial role of soil microorganisms in the net loss of soil organic carbon (SOC) under climate warming and the effectiveness of straw-C sequestration to replenish the SOC stock. However, the lack of empirical investigations in anaerobic paddy soils hinders accurate estimation of the global soil C-climate feedback and development of countermeasures.

Objectives: This study aimed to unravel the impact of warming on the complexity of the microbial community network of the paddy soil in response to warming, and correspondent changes of microbial metabolic functions relevant to the transformation of straw-C in SOC pools.

Methods: We added C/N-labeled rice straw into a long-term paddy soil and incubated under three temperature treatments (25, 35 and 45 °C) for 140 days to quantify straw-C sequestration in various SOC fractions, and further deployed metagenomic sequencing and solid-state C NMR analyses to explore relevant biochemical mechanisms.

Results: Warming (35 °C and 45 °C vs. 25 °C) enhanced SOC decomposition, but straw amendment did not replenish the loss C in mineral-associated C, a major SOC fraction of this soil, especially at 45 °C. Compared to 25 °C, temperature increases to 35 °C and 45 °C led to decreases in microbial diversity indices by an average of 19 % and 43 %, respectively. Warming also destabilized the microbial community network with less connectivity and keystone nodes in the paddy soil. Furthermore, warming decreased the abundances of organic C- and N-mineralization genes. Those genes encode enzymes involved in the degradation of both labile and recalcitrant organic compounds, including starch, cellulose, hemicellulose, chitin, pectin and aromatics, as well as in N mineralization, such as glutamate dehydrogenase and glutamate synthase. A subsequent deficiency in the synthesis of those enzymes appeared to suppress the transformation of straw-C and N, thereby reducing their sequestration efficiency in the mineral-associated C fraction in the paddy soil.

Conclusion: The detrimental impact of warming on the microbial metabolic profiles lowered the role of straw amendment in sustaining SOC stability under warming. An improved understanding of the warming-induced loss of microbial community diversity and correspondent weakening metabolic functions for the turnover of exogenous C should be accounted for global mitigation practices in paddy fields under climate warming.