Warming affects the direction and magnitude of the priming effect

Climate change induced warming may alter soil organic matter decomposition and intensify the priming effect (PE). This study aimed to evaluate the impact of temperature increase on the direction and magnitude of PE in soils under long-term no-till system. Four soil treatments derived from two long-term no-till experiments on a clayey (CY) and a sandy (SY) soil, each with high (H) and low (L) C input crop rotations, were evaluated in a laboratory experiment. ¹³C-labeled vetch (Vicia sativa L.) residues (5 Mg ha^-1) were added to the soil surface of soils and incubated at 10, 20, 30, and 40°C for 170 days. Apparent and real residue mineralization were quantified by continuous measurement of CO₂ and ¹³CO₂ emissions, and PE was calculated from these data. In the short term (0-3 days), PE was positive in all treatments, with greater magnitude at 20°C, especially in clayey soils (average +92.6 mg C kg^-1 soil), and the lowest values at 10°C (<13 mg C kg^-1 soil). After 170 days, temperatures of 10 and 20°C maintained positive PE in all soils. Except for SY-L, which exhibited the highest PE at 40 °C, the other soils showed a progressive decline in PE with increasing temperature, including changes in direction. In clayey soils, temperatures higher than 30°C caused negative PE, and in SY-H, PE was negative at 40°C. Overall, PE was highly sensitive to temperature, with moderate warming maintaining positive PE, while higher temperatures tended to reduce its magnitude, potentially changing its direction.