[M-IS22] Biogeochemistry

[MIS22-11] Effects of repeated dry-wet cycles on soil carbon and nitrogen dynamics in humid temperate forests

*Masataka Nakayama^1,3, Yuri Suzuki^2,3, Yukiko Abe³, Takeshi Taniguchi¹, Mariko Atarashi-Andoh³, Jun Koarashi³, Hirohiko Nagano² (1.Tottori University, 2.Niigata University, 3.Japan Atomic Energy Agency)

Keywords:Soil dry-wet, Soil microbial biomass, Nitrogen mineralization, Microbial respiration

Global warming has altered precipitation patterns in recent decades, leading to less frequent rainfall events. This change may trigger soil drying and rewetting even in humid areas. Studies in at least seasonally dry regions have shown that rewetting dried soil stimulates microbially driven carbon and nitrogen cycling processes, resulting in pulse carbon dioxide (CO₂) emissions from soils and nitrogen mineralization. However, there is limited understanding of the effects and factors influencing them in humid temperate forests. Here, we aimed to reveal the impact of repeated soil dry-wet cycles (DWC) on soil carbon and nitrogen mineralization and the factors regulating the effect sizes in humid temperate forests by incubating soils under five DWCs (DW) or constant moisture conditions (CL). We measured CO₂ emissions from incubated soils throughout the incubation period, except during drying with silica gel. The nitrogen mineralization rates (net nitrification and mineralization) were measured during five days following the first, third, and fifth rewetting. We also measured soil extractable organic carbon (EOC) and nitrogen (EON), as well as microbial biomass carbon (MBC) and nitrogen (MBN), three hours and five days after the first, third, and fifth rewetting. Rewetting of dried soil caused pulse increases in CO₂ emissions and net nitrogen mineralization, although the size of these pulses decreased with increasing the numbers of DWCs. Although the nitrate concentration in CL at the end of the incubation tended to be higher than that in DW, no significant differences in the net nitrification rates among treatments were observed. MBC and MBN in DW tended to decrease continuously during the incubation. The concentration of EOC in DW increased following rewetting but declined during the subsequent wet period. An increase in EON in DW was noted only after the first rewetting. According to path analysis, EOC during the five days after rewetting (wet period) and MBC three hours after rewetting had significant negative and positive correlations with CO₂ emission during the wet period in DW, respectively. Likewise, MBN three hours after rewetting and change in EON during the five days after rewetting positively and negatively correlated with the net nitrogen mineralization rate after rewetting, respectively. In general, the microbes in the soil that historically experienced drought tend to have a higher tolerance for the drought than those in the soil of humid ecosystems. Therefore, these results indicated that the stress tolerance of soil microbial communities and microbial survivability for the novel dry-wet stress is the key to determining the response of CO₂ emission and nitrogen mineralization to chronic soil DWC in humid temperate forest ecosystems where severe drought and subsequent rapid rewetting rarely occurred.