Ph: A Predictor of Interaction Networks Among Soil Organo-Mineral Fractions

Mineral-associated organic matter (MAOM) acts as the largest and longest-turnover carbon pool in soils, and understanding its dynamics and feedback with respect to climate change is of great significance for future climate change mitigation. MAOM is expected to have relatively little sensitivity to climate change due to mineral protection, but its persistence involves several organo-mineral fractions. The uncertainty in the response of specific organo-mineral fractions to climate change hampers the reliability of predictions of MAOM preservation in the future. Here, we applied a sequential chemical fractionation method integrated with network analysis to investigate MAOM stabilization mechanisms across five alpine ecosystems: alpine desert, alpine steppe, alpine meadow, alpine wetland, and alpine forest. The following results were obtained. Seven extractable organic matter (OM) fractions in MAOM were grouped into three OM clusters: a weak linkages cluster consisting of the water-soluble organic matter (WSOM) and weakly adsorbed fractions (7.5-21.8% TOC); an Fe and Al complex-strong linkages cluster comprising Fe and Al complexes and a massive fraction with strong linkages (9.1-21.2% TOC); and a Ca complex-mineral cluster composed of Ca complexes, carbonates and Fe oxyhydroxides (5.7-19.3% TOC). The relative percentages of OM in the three OM clusters exhibited distinct pH dependence patterns: with increasing pH, the weak linkages cluster, Fe and Al complex-strong linkages cluster and Ca complex-mineral cluster showed U-shaped, hump-shaped and monotonically increasing patterns, respectively. Moreover, the contribution of metal cations to MAOM stabilization was closely related to soil pH; that is, Ca contributed the most in alkaline soils, and Fe and Al contributed the most in acidic soils. Organo-mineral fractions and metal cations in MAOM constructed a complex network with pH as the central node. Precipitation not only altered vegetation type and microbial biomass but also regulated soil pH, which is balanced by specific metal cations, thus resulting in particular pH preferences of specific OM clusters. These findings demonstrate that soil pH plays a central role in modulating MAOM dynamics and that its climatic feedback can thus serve as a good predictor of soil organo-mineral fractions across ecosystems.