Yumei Pan, Xiaojuan Liu, Michael Staab, and Naili Zhang, Beijing Forestry University in China, discuss their article: Soil carbon sequestration: Facilitated effect of extrafloral nectary trees in a diverse subtropical forest
Carbon: Crucial role in climate change mitigation
Soil organic matter dynamics and carbon sequestration are critical for mitigating climate change. Forest soils, which account for 16–26% of the global soil carbon pool, represent the largest carbon pool in terrestrial ecosystems. Understanding the factors that influence soil carbon processes is essential for evaluating the carbon sequestration potential of soils. Collaborators from Germany demonstrated that the mutualistic relationships between extrafloral nectary (EFN) trees and defence-related arthropods enhances the growth and defence of adjacent non-EFN trees, thereby improving community-level carbon capture and multiple other ecosystem functions. Additionally, microbial communities, with immense diversity and various functions, can significantly influence soil carbon cycling and the forms in which carbon is stored. However, the dynamics of soil carbon processes under the canopy of EFN trees in subtropical forests, particularly in the context of declining tree diversity, remain poorly explored.
Our study: Unravelling the impact of EFN trees
To investigate the effects of EFN trees on soil carbon and nitrogen fractions in forest communities with varying levels of tree species richness (TSR), we conducted a field experiment at the BEF-China experimental sites in Xingangshan, Jiangxi Province, southeast China. We focused on five levels of TSR (one-, two-, four-, eight-, and 16-tree species). Three EFN tree species were selected: Triadica cochinchinensis (Euphorbiaceae), Ailanthus altissima (Simaroubaceae), and Idesia polycarpa (Flacourtiaceae). Damaged and healthy leaf samples and soil were collected from these trees and their neighbouring non-EFN trees. To distinguish transient plant-derived C from persistent microbially processed compounds, the soil samples were analysed for total carbon and nitrogen contents, microbial biomass carbon and nitrogen contents, particulate organic matter (POM), and mineral-associated organic matter (MAOM) fractions. We also analysed soil and leaf microbiomes to explore fungal-mediated mechanisms in the effects of TSR and EFN trees. We hypothesized that EFN trees may promote soil carbon and nitrogen cycling, further modifying the effect of TSR on carbon and nitrogen contents in different soil fractions. We further hypothesized that fungal communities would mediate the impacts of EFN trees and TSR on soil fractions.
Our key findings: EFN tree-modified TSR effects
We found higher contents of carbon relative to nitrogen in different fractions under a higher TSR, indicating that the increase in tree diversity may facilitate soil carbon accumulation. Forests with higher proportions of EFN trees exhibited enhanced soil carbon sequestration through decreases in the POM-to-MAOM ratio, while simultaneously improving soil quality by decreasing the carbon-to-nitrogen ratios in different soil fractions. Interestingly, the peak soil carbon-to-nitrogen ratios across all fractions, as well as the POM-to-MAOM ratio, shifted from four-tree species to eight-tree species under the canopy of EFN trees and their neighbouring non-EFN trees. We also found that the effects of TSR and EFN trees on soil C and N fractions were mediated by the phyllosphere–soil fungal linkage, which included alpha diversity, complexity, and stability of the fungal co-occurrence network, as well as keystone taxa. The changes in fungal communities are likely driven by the interaction between EFN trees and herbivorous insects.
Significance of our study
Extrafloral nectary (EFN) trees, which are widely distributed in tropical and subtropical forests, may contribute to modulating the biodiversity–ecosystem functioning relationship, as suggested by recent studies. Our findings demonstrate a positive response of soil carbon sequestration to tree diversity under EFN tree canopies, suggesting that establishment of EFN trees in a plant community can enhance tree species richness (TSR) effects on soil C sequestration. By highlighting the significance of EFN tree–phyllosphere/soil fungi associations and their role in shaping the effect of tree diversity, this study contributes to a comprehensive understanding of the mechanisms by which aboveground–belowground synergies govern soil C sequestration in a subtropical forest.
