Contrasting responses of above- and belowground multifunctionality to increasing soil salinealkaline conditions in a semi-arid grassland in Northern China

Saline alkaline conditions in soils represent a critical environmental stressor that threatens the sustainability of semi-arid grassland ecosystems. However, a systematic understanding how salinization affects ecosystem multifunctionality (EMF), particularly the coupled above- and belowground responses, is currently lacking. This study investigated a semi-arid saline alkaline grassland using 37 quadrats along a natural saline alkaline gradient spanning low, moderate, and high saline alkaline levels. By measuring key indicators of plant community biomass, dominant species, and soil nutrient content, we quantified the aboveground multifunctionality (AGMF) and belowground (soil chemical) multifunctionality (BGMF) as component indices of EMF. Along the gradient, EMF showed no significant overall difference, resulting from offsetting asymmetric responses of AGMF and BGMF. AGMF exhibited a unimodal pattern, driven by dominant species functional traits such as Ca²⁺ accumulation, whereas BGMF declined linearly with increasing pH and salinity, mainly due to reductions in soil organic carbon and suppression of the available nitrogen (NH₄⁺-N) pool. Soil pH acted as a bidirectional regulator, indirectly promoting AGMF by increasing dominant species Ca²⁺ content while directly inhibiting BGMF by decreasing soil NH₄⁺-N. Salinity exerted direct or indirect negative associations with both AGMF and BGMF. Although EMF did not differ significantly along the gradient, it showed a declining trend with increasing pH, suggesting that the sustained negative effects on belowground functions may dominate the long-term trajectory of ecosystem multifunctionality. This study emphasizes that coordinated above- and belowground strategies should be adopted in the management and restoration of semi-arid saline alkaline grasslands. The physiological characteristics of salt-tolerant dominant species can support aboveground productivity in the short term, but maintaining soil organic carbon pools and belowground ecological processes must be regarded as the core goal in curbing habitat degradation and achieving sustainable restoration. By systematically revealing the asymmetric response mechanisms of AGMF and BGMF, this study provides a scientific basis for shifting restoration practices from single vegetation targets to integrated plant soil system strategies.