Effects of lime and gypsum application on soil organic matter stabilization processes

Soil organic matter (SOM) management is critical for climate change mitigation. While liming and gypsum are essential for productivity in weathered soils, their impact on SOM stabilization remains debated due to the trade-off between cation-mediated organo-mineral protection (Ca²⁺/Mg²⁺) and enhanced microbial activity. We evaluated SOM dynamics in a Haplic Ferralsol (sandy-loam) undergoing a greenhouse experiment using Urochloa brizantha cv. Marandu, organized in a 3x2 factorial randomized design. Factors included lime (none, calcitic, and dolomitic) and gypsum (none and gypsum) across four soil depths (0-40 cm). Compared to the Control, amendments increased soil pH_KCl (from 4.16 to 5.01 in the Calcitic+Gypsum treatment), Ca (up to 6x) and Mg levels (up to 10x). Extractions showed a significant reduction in Al-NaPPi (from 65.0 to 54.1 mmol kg^-1) and an increase in Ca-NaPPi (from negligible levels to 11.1 mmol kg^-1), especially in Calcitic+Gypsum. Beta-glucosidase activity rose in all treatments relative to the Control, peaking with a 31% increase in Dolomitic+Gypsum (0-10 cm)—yet correlated negatively with Al-NaPPi (r = -0.62***) and Fe-NaPPi (r = -0.52***), suggesting that organometallic associations provide biochemical protection by restricting enzymatic access to stabilized carbon. Furthermore, C-MAOM concentrations increased by 38% alongside Ca levels in Lime+Gypsum treatments, correlating positively with C-NaPPi (r = 0.32***) and the MAOM C/N ratio (r = 0.25**). This indicates that stable carbon accumulation in the MAOM fraction is linked to organometallic interactions and plant-derived compound preservation. Overall, results show that liming and gypsum promote SOM sequestration by favoring mineral protection over microbial turnover.