Pinus sylvestris had no positive effect on topsoil organic carbon concentration in artificial shrublands

Afforestation in mobile sand dunes, mainly occurring in large arid and semi-arid regions of China, is expected to improve plant diversity and ecosystem function through vegetation restoration processes, which in turn modify SOC and associated soil properties. However, there is a limited understanding of how sand-binding vegetation alters topsoil organic carbon (SOC) sequestration through the vegetation-soil coupling mechanism in semi-arid regions. We compared the differences in soil properties, leaf litter traits, root traits and microbial traits of shrublands and after introducing Pinus sylvestris , evaluated the relative influence weights and driving mechanisms of different modules and key factors on SOC in Horqin Sandy Land. The results showed that SOC in Salix gordejevii shrublands and Caragana korshinskii shrublands was significantly higher than that in their mixed shrublands, and introducing P. sylvestris significantly decreased SOC. The hierarchical partitioning analysis indicated that soil properties, root traits, microbial traits and leaf litter traits explained 42.60 %, 30.84 %, 18.60 % and 7.96 % of the variance in SOC, respectively. The contribution of herb root traits to SOC was greater than that of shrub root traits. Multiple regression analysis showed that the introduction of P. sylvestris into shrublands significantly changed the effects of key factors on SOC, mostly negatively. The structural equation model illustrated that soil properties had direct and significant positive effect on SOC, whereas herbal root biomass indirectly affected SOC through soil properties. Our results indicate that the decrease in SOC was due to the change in litter composition after the introduction of P. sylvestris , which reduced the litter quality and slowed the decomposition rate by microorganisms. In addition, P. sylvestris inhibited the growth of herbaceous plants under the canopy and reduced the biomass of herb roots, which directly reduced the available input source of SOC, thus slowed the accumulation of topsoil organic carbon. We present evidence that positive SOC sequestration by the afforestation of shrubs in mobile sand dunes may be weakened by introducing P. sylvestris , which should be considered in future vegetation restoration in sandy areas.