Multiple element coupling and molecular-chemical diversity of organic matter control how much energy is retained in soils in mountain ecosystems

Organic matter is the primary repository of chemical energy driving biogeochemical cycles and maintaining ecological functions in soils. Therefore, revealing factors controlling energy in soils is critical but has less been studied so far. In this study, we investigated energy stored in soils and litter biomass, 12 elements level (macroelements including C, N, P, Fe, and K, microelements including Cu, Li, Mg, Mn, Ni, Pb, and Zn), and molecular-chemical composition of soil organic matter (SOM) among four vegetation zones along the elevation in the Changbai Mountain. The calorific values in soils (Qs) and litters (Ql) were determined using oxygen bomb calorimetry. The complexity of SOM was reflected by molecular diversity of pyrolytic products of SOM (SHID), and relations among 12 elements were represented by defining coupling extent (MEC). Potential impacts of MEC and SHID on energy stored in soils were discussed. Results indicated that the average Qs and Ql were 565 cal/g and 3799 cal/g, respectively. The proportion of Qs to Ql (Qr) ranged from 6.28 % to 43.65 % with a mean of 14.74 %. Significant differences in Qs and Ql were observed among four vegetation zones along the elevation. Significantly positive relations were observed between elevation and Ql, between MEC and Qr or Qs, and between Qs and SHID. The VPA analysis indicated that MEC itself could explain 19 % variation of Qs, and the exploration raised to 60 % when MEC was in conjunction with elevation, pH, and SHID. The structure equation model confirmed positive effects of MEC and SHID on Qs. In conclusions, MEC was primarily associated with energy holding by mountain soils, proving that a conceptual advance was required to link element biogeochemical cycle and energy flow in ecosystems, thereby bridging the potential gap between matter and energy in ecological research.