Gradual and abrupt increase in atmospheric CO2 concentrations trigger divergent responses of microbial necromass accumulation in paddy soils

Microbial necromass C (MNC) plays a critical role in promoting soil organic C (SOC) formation and stabilization, particularly in the context of climate change . Most investigations on the impact of elevated CO 2 concentrations (eCO 2 ) on MNC have been performed by exposing ecosystems to an abrupt increase in CO 2 . However, the atmospheric CO 2 increase is a gradual process, and knowledge about the response of necromass to gradual increase in CO 2 is lacking. We tested the hypothesis that microbial necromass would show different responses to abrupt and gradual CO 2 increases. Furthermore, it is still unknown whether eCO 2 will trigger similar responses between surface and subsurface soil layers . Here, we determined the MNC concentrations and their proportions in SOC in surface and subsurface soil layers via open-top chambers. CO 2 treatments included ambient control, abrupt CO 2 increase by 200 ppm above control, and gradual CO 2 increase by 40 ppm each year until reaching 200 ppm over five years. Overall, compared with the ambient control, abrupt eCO 2 induced a significantly stronger decrease in necromass (20.3 %) when averaged across three soil layers, while gradual eCO 2 led to insignificant variations in necromass (6.7 %). The necromass proportion in SOC decreased with depth under both eCO 2 treatments with a significant decline occurring in abrupt eCO 2 treatment. The observed greater magnitude of eCO 2 effects on fungal necromass relative to bacterial necromass in subsurface layers illustrates a distinct microbial community response to climate change. Taken together, abrupt and gradual eCO 2 approaches differed in their impact on MNC, and its response to eCO 2 may be somewhat overestimated by abrupt eCO 2 approach. Furthermore, MNC in subsurface soil witnessed a more sensitivity or vulnerability to eCO 2 than that of topsoil . We suggest vigilant attention will need to be paid to the feedback of necromass C in deep soil poised by future climate change.