Linking bacterial life strategies with soil organic matter accrual by karst vegetation restoration

Vegetation restoration affects belowground microbial diversity and trait-based life strategies, as well as soil organic matter (SOM) accumulation. Despite the growing focus on microbial diversity, the relationship between SOM accrual and life strategies following vegetation restoration remains unclear. We used three independent but complementary approaches to relate six soil parameters to SOM accrual and further connected them with microbial diversity and life strategies. Sites with two vegetation restoration strategies (after 15 years of vegetation recovery): i) actively planted forests and ii) passive naturally regenerating forests, were compared with croplands and established 60-year-old secondary forests. Data from seven sites along a large climatic gradient (Δ temperature >9 °C) in subtropical karst regions showed that the average index of SOM accrual increased by 47% in plantations and by 60% in natural regeneration forests compared to that in croplands, but remained lower than that of secondary forests. Related soil parameters (water holding capacity, organic carbon and total nitrogen contents, and bacterial and fungal biomasses) were comparable. Compared to croplands, vegetation restoration decreased the sensitivity of the soil parameters to climate warming. Vegetation restoration reduced bacterial diversity and shifted the community towards K-strategy, as evidenced by i) lower 16S rRNA operon copy number and ii) higher ratios of phyla classified a priori as oligotrophic versus copiotrophic bacteria. These changes were mainly attributed to the decline in labile nutrient content and increase in carbon stability in calcium-rich karst soils after vegetation restoration. Bacterial diversity was negatively associated with SOM accrual at low levels of functioning, whereas the prevalence of K-strategists showed a strong positive association with it, especially at near-peak capacity. Consequently, i) bacterial rather than fungal diversity and life strategies are associated with SOM accrual, and ii) linking bacterial life strategies with SOM accrual is important to deepen the understanding of soil-microbial interactions.