Divergent controls of exchangeable calcium and iron oxides in regulating soil organic carbon content across climatic gradients in arid regions

Elevational gradients offer powerful natural experiments for testing how soil organic carbon (SOC) responds to environmental changes (e.g., climate, plant inputs, and soil geochemistry) across a wide range. Exchangeable calcium (Caex) emerges as an important geochemical driving factor of SOC content in arid regions, yet its significance in regulating SOC content across climatic gradients remains poorly understood. Here, we assessed the relative importance of climate variables, plant inputs, and geochemical properties in controlling SOC content along a 2,500-m elevational gradient in the Tianshan Mountains. We explored the roles of Caex in regulating the interactions between iron (Fe)-(hydro)oxides and SOC content. We found that soil Caex (i.e., Na2SO4-extracted), likely controlled by biological cycling, was the major contributor to SOC content in cold and wet climatic conditions (1,400–3,035 m a.s.l.), where soils had relatively high Feo and low pH (5.8‒7.9). In contrast, amorphous Fe, not Caex, was the most important regulator for SOC content in warm and dry climatic conditions (481–1,240 m a.s.l.) with Ca-rich and high pH soils (7.6‒9.0). Moreover, the results of partial correlation analysis suggested that Caex competed with Fe-(hydro)oxides in modulating SOC content in cold and wet climatic conditions due to low soil pH, but facilitated SOC stabilization by Fe-(hydro)oxides in warm and dry climatic conditions. Although climatic variables and plant inputs were significantly associated with SOC content, our findings indicated that Caex and Fe-(hydro)oxides had divergent controls over SOC content across climatic gradients in arid regions, depending on soil pH level. Our study reveals new insights on the potential role of cations in complexation of organic matter with minerals across a wide range of environmental conditions in the field.