Subsurface organic amendment of a saline soil increases ecosystem multifunctionality and sunflower yield

Salt stress poses a growing constraint to crop productivity in arid regions globally. Previous evidence indicates that organic amendment is a pivotal management practice for enhancing crop yield and soil fertility in agroecosystems. How organic amendment depth influences the interaction between soil health, ecosystem multifunctionality (EMF), and crop yield, however, still remains unclear. Thus, a 3-year field experiment was carried out to investigate the impacts of surface (0–15 cm) and subsurface (15–30 cm) applications of humic acid and manure on the soil quality index (SQI), enzyme activities, EMF, and crop yield on saline soils. Subsurface organic amendment improved the SQI (at the 0–45 cm layers) by 20–47 %, while the surface amendment improved the SQI at the 0–30 cm layer by 15–51 %. The higher soil quality under subsurface organic amendment was characterized by increases in soil organic carbon and available nutrients, and a decrease in electrical conductivity compared to surface organic amendment. The organic amendment increased microbial diversity and richness, stimulated enzyme activities, and ultimately improved soil EMF. The soil EMF increased by 122–214 % at the 0–30 cm layer under subsurface organic amendment and by only 178 % at the 0–15 cm layer under surface organic amendment. Pairwise comparisons further confirmed that electrical conductivity was negatively, and soil organic carbon positively, correlated with soil ecological functions within the 0–45 cm layer. The higher soil quality and microenvironment with better EMF under subsurface organic amendment increased sunflower yield by 16 % and 8 % as compared to inorganic fertilizer only and surface organic amendment, respectively. This relates to the considerable improvement in soil electrical conductivity, soil organic carbon, β-glucosidase activity, and diversity and richness of microbial communities resulting from deep organic amendment. Overall, subsurface organic amendment is an effective way to enhance soil EMF and crop yield on saline soils.