An evaluation on contribution of organo–metal complexes to enhanced soil CO2 release during drying–rewetting cycles using buried humic horizon soils

Under global warming, precipitation is becoming less frequent but with more intense events, with little change in total precipitation amounts. These shifts have raised concerns about drying–rewetting cycles (DWCs) and their impact on soil organic matter decomposition and carbon dioxide (CO 2 ) release. Microbially derived carbon (C), such as intracellular compounds released by cell lysis and osmolyte turnover, has been considered a primary C source of DWC-induced CO 2 release. However, C released from organo–metal complexes disrupted by rapid rewetting has emerged as a significant contributor. To evaluate this contribution, we conducted incubation experiments with a single DWC. Here, we used two buried humic layer soils characterized by abundant organo–metal complexes but low microbial biomass, with two surface soils for comparison. In both buried and surface soils, the DWC consistently led to higher CO 2 release rates during incubation compared to the control. The magnitude of the increase in CO 2 release from buried humic layers exceeded the measured decreases in microbial biomass C, which represented ≤ 40% of the increase, while the amount of pyrophosphate-extractable organic C associated with organo–metal complexes was at least 90-fold greater than the DWC-induced increase. A re-analysis including data from this study and our previous study showed that the relative CO 2 release increase at DWC from control was significantly correlated with the amounts of pyrophosphate-extractable aluminum in the soils. We also observed a shift in bacterial communities in the buried soils toward a nutrient acquisition strategy adapted for rapid growth after rewetting, which may have contributed to the enhanced CO 2 release. In the present study, we suggest that organo–metal complexes, traditionally considered a stable C pool, can substantially contribute to DWC-induced CO 2 release from buried humic horizons.