Long-term conservation tillage and straw return affect thermal stability of soil organic matter

The equilibrium between energy persistence and microbial accessibility of soil organic matter (SOM) determines its contribution to climate mitigation and soil health. Labile SOM maintains the activity of soil microorganisms, which provide nutrients for crops and increase yields. Stable SOM is crucial to long-term carbon (C) storage and sequestration in soils. No-till and subsoiling combined with straw return increase SOM content and stocks, yet their specific impacts on the thermodynamic equilibrium between SOM stability and availability remain unclear. We studied the effects of 22 years of no-till and subsoiling with straw return on SOM thermal stability in loamy Calcaric Cambisols. SOM thermal stability was assessed based on the balance between the energy density (ED) in labile, stable, persistent, and refractory SOM and their activation energy (E_a) for thermal oxidation. The energy properties of SOM were evaluated via differential scanning calorimetry and thermogravimetry of soils from six treatments (3 tillage practices (no-till, subsoiling, and conventional tillage as a control) with 2 straw management practices (straw return and straw removal) at 0-12 cm, 12-30 cm and 30-35 cm depths. Compared with conventional tillage, long-term no-till and subsoiling increased the content and thermal stability of SOM, with these effects being amplified by straw return. Conservation tillage with straw return contrastingly affects SOM pools: 1) E_a of labile SOM decreased by 5 %, increasing energy availability for microorganisms; and 2) E_a of recalcitrant SOM increased by 10 %, which was sixfold higher E_a than thermally labile SOM. Subsoiling preferentially raised the content of energy-rich SOM pools, whereas no-till demonstrated superior efficacy in thermodynamic stabilization of SOM compared to subsoiling. Conservation tillage with straw return increased both highly stable organic C and energy-rich available C. Concluding, conservation tillage (no-till and subsoiling) increased content of available energy in soil via contribution to labile SOM, and intensified C stabilization through increased energetic constraints of persistent SOM. This makes conservation tillage with straw return a promizing strategy for climate-smart soil management.