Energy retention from plant leaves by soil along an elevational gradient in arid mountain ecosystems

Plant photosynthetic products are microbially and chemically transformed and retained as soil organic matter (SOM), which is essential for ecosystem functioning, particularly in the detritusphere. The energy quantity (EQ) of an organic substance, defined as the net energy release per gram of the sample, depends on its OM content and the energy density (ED, the net energy release per gram of OM). During the transformation of plant leaves to SOM, both the energy quantity retention ratio (EQSoil/Leaf, the ratio of soil’s EQ to leaves’ EQ) and the OM retention ratio (OMSoil/Leaf, the ratio of SOM to leaves’ OM) are consistently less than one, reflecting the loss of OM as CO2 and coupled heat dissipation. However, how the energy density conversion ratio (EDSOM/Leaf-OM) changes and its influence on EQSoil/Leaf ratio remain unknown. Here, we 1) examined the EDSOM/Leaf-OM ratio along an elevation gradient (481–3035 m), where environmental conditions shift from warm-dry to cold-wet, and 2) explored how the EDSOM/Leaf-OM ratio was influenced by chemical composition (e.g., the H/C and C/N ratios of plant leaves, the ratio of stable-to-labile OM thermal pool (Exo2/Exo1)), climate, and edaphic factors. The EDSOM/Leaf-OM ratio was < 1 (0.7 ± 0.04) in warm-dry areas, indicating the decrease in OM’s energy density from plant leaves to SOM, whereas it remained 1 (1.0 ± 0.1) in cold-wet areas. In warm-dry areas, the EDSOM/Leaf-OM ratio decreased with the ratio of themal pools Exo2/Exo1 of SOM due to the protection of low-energy–density OM by Fe/Al oxides. Conversely, in cold-wet areas, EDSOM/Leaf-OM declined with increasing Exo2/Exo1 ratio of leaf, which was featured by low H/C and high C/N ratio. The EDSOM/Leaf-OM ratio modulates energy retention as OMSoil/Leaf in cold-wet areas, underlining the dynamics of SOM energy beyond mass loss. This study highlights that energy retention during SOM formation is environment-sensitive, mediated by two mechanisms: mineral protection in warm-dry areas versus biochemical transformation in cold-wet areas. We recommend integrating energy and mass flux assessments to better understand OM decomposition, stabilization, and ecosystem carbon storage.