High-quality litter exerts a greater effect on soil carbon gain in unrestored than restored pine plantations.

Vegetation restoration of degraded land affects litter quality by changing the composition of tree species, providing direct effects on regulating the dynamic of soil organic C (SOC) through the priming effect (PE). However, it is unclear how the combined effects caused by vegetation restoration and input of different quality litters on PE-related C loss and gain. Here, we collected soils from an unrestored site and a site restored for 20 years, adding C-labeled low-quality (with high C/nitrogen [N] and lignin/N) and high-quality (with low C/N and lignin/N) litters to the soil, respectively. Our results revealed that adding high- and low-quality litter in two sites produced positive PEs after 150-day laboratory-based incubation. The PE induced by high-quality litter was lower than that of low-quality in two sites, which can be interpreted as low-quality litter has higher C/N that aggravates the nutrient imbalance of microorganisms and enhances their demand for N, prompting microorganisms to accelerate the mineralization of SOC through the "N mining". High-quality litter inputs can boost microbial C use efficiency and alleviate soil C loss due to PE in unrestored and restored pine forests. Moreover, high-quality litter input has a greater positive effect on SOC gain in unrestored lands than in restored lands, suggesting that litter with higher nutrient availability or fertilization is especially needed for the restoration of degraded soil fertility and C formation. Taken together, this study highlights the importance of tree species producing high-quality litter in mediating SOC decomposition and formation during degraded lands restoration, which is beneficial for the restoration of degraded lands and the enhancement of soil C sequestration.