Response of soil organic matter turnover to soil salinization and climate warming

Climate change is turning soil salinization into a global problem due to the increasing frequency and severity of coastal salt and brackish water ingress. How increasing salinity affects microbial metabolic activity and its consequences for matter and energy turnover under climate warming remain unclear. Thus, we conducted a lab incubation experiment to explore the interactive effects of salinization and warming on microbial and enzymatic functional traits related to the CO₂ (matter) and heat (energy) losses in the course of glucose metabolism.

Soil from coastal grassland was artificially salinized to, middle (2.06 mS cm^-1) and high (3.45 mS cm^-1) levels by gradually adding salt solution, while the soil with ambient salinity (0.49 mS cm^-1) was defined as control. Effect of realistic warming (+2 ℃) on CO₂ emission and heat release from soil amended with glucose was estimated by the respirometer Respicond V and microcalorimeter TAM Air, respectively. Energy and carbon use efficiency, calorespirometric ratio, microbial growth parameters and enzyme kinetics were determined in the salinity gradient.

Despite cumulative CO₂ emission and heat release were not affected by soil salinity, we observed gradual delay in glucose induced respiration (GIR) and heat release with the increasing salinity level. In contrast, warming facilitated both GIR and heat release, and increased the cumulative CO₂ by 8-14%, but had no effect on the cumulative heat.

Before glucose addition, high salinity greatly reduced the C-acquiring enzyme activities (β-D-glucosidase, cellobiohydrolase) by 17-39% compared with control, while an activity of the P-acquiring enzyme (acid phosphomonoesterase) notably increased by 24 and 82% under middle and high salinity, respectively. In soil activated with glucose, high salinity greatly increased the activities of both C- and P-acquiring enzymes up to 74 and 30%, respectively, compared with control. Surprisingly, irrespectively of microbial activation by glucose, the N-acquiring enzyme activity (leucine aminopeptidase) was not affected by salinity.

The interactive effect of soil salinity and climate warming on the fate of soil organic matter, energy and carbon use efficiency, calorespirometric ratio and microbial community will be discussed in our presentation.