Can Alternative Tillage Sequester Carbon Deeper? A Global Synthesis of Deep-Soil Carbon Stocks and Fractions

Alternative tillage (ALT) methods present promising strategies for enhancing soil organic carbon (SOC) sequestration, though their effectiveness varies across soil layers and carbon fractions. This global meta-analysis systematically quantifies the effects of various ALT practices on SOC dynamics in topsoil (0–30 cm), subsoil (> 30 cm), and whole profiles, with a focus on microbial biomass carbon (MBC), particulate organic carbon (POC), and mineral-associated organic carbon (MAOC). Our findings reveal that while all ALT practices increased whole-profile SOC stocks compared to conventional tillage, their impacts on specific soil layers varied. Deep tillage (DT; +6.3%) and no-tillage (NT; +6.1%) were more effective than reduced tillage (RT; 4.9%) at the whole-profile scale. Notably, assessing SOC changes solely within the topsoil layer may underestimate SOC accrual under ALT, given the positive contributions observed in deeper soil layers (DT: 10.3%; NT: 2.4%). Environmental and management factors also shaped the efficacy of these practices. Soil texture and climate were the primary determinants of SOC responses to NT or RT, whereas DT effectiveness was mainly influenced by nitrogen input and soil texture. Furthermore, ALT practices also altered the concentration of the carbon fractions, with MBC increasing across all ALT practices (22.9%–33.4%). Both NT and RT enhanced MAOC dominance, reflected in a strong MAOC-SOC correlation (r = 0.96). Changes in MBC and POC contributed similarly to SOC stock (r = 0.80 and r = 0.77, respectively) under NT. These findings underscore the importance of whole-profile SOC assessments, offering key insights for optimizing climate-smart tillage strategies to maximize long-term soil health and carbon sequestration.