Soil water availability alters plant-soil feedback effects on invasive plant growth and foliar herbivory

Abstract Plant-soil feedback (PSF) effects of invasive plants are often regulated by abiotic factors, but whether soil water availability alters the impact of PSF on invasive plant growth and foliar herbivory remains unclear. We hypothesized that soil water content modifies PSF effects and then affects foliar herbivory. To test this, we established four soil water level treatments (soil surface elevated 0, 5, 10 or 15 cm above water) to examine their effects on PSF, growth traits, and herbivore resistance in the invasive weed Alternanthera philoxeroides. Results showed PSF was negative when the soil surface was elevated 5 cm above water, but it was positive in other treatments. Soil condition, water treatment and their interactions significantly affected total biomass, leaf and branch numbers. As soil water content decreased, leaf nitrogen content increased, while the leaf C/N ratio decreased. Root nitrogen and C/N ratios were also affected by water treatment. Leaf mass per area and leaf area consumption rate were significantly affected by water content, with foliar herbivory being lowest when water content was at its minimum. Importantly, the effects of water availability on invasive plant performance and foliar herbivore resistance appeared to be stronger than those mediated by soil feedback. These findings suggest that soil water content, as a critical role, modifies the PSF effects on invasive plant performance, thereby indirectly affecting foliar herbivory.