Additive and antagonistic interactions of drought and nutrient stress modulate fine root traits

Water and nutrient limitation typically co-occur in terrestrial ecosystems, exerting complex interactions on plants. However, the nature of these interactions on fine roots remains poorly understood. Here, we conducted a full-factorial experiment manipulating water and nutrient stress using seedlings of four tree species (Koelreuteria paniculata, Quercus variabilis, Acer truncatum and Prunus davidiana), focusing on the individual and interactive effects of drought (D) and nutrient stress (N) on fine roots. We found that both drought and nutrient stress induced a shift toward resource-acquisitive strategies in root traits, yet the magnitude of root responses differed between the two stressors. Specifically, nutrient stress exerted the strongest effects on root morphological traits, whereas the two stressors had relatively comparable impacts on root nutrient content. Under combined stress, this acquisitive shift was modulated, exhibiting a trend toward resource conservation. The interactive effects of D × N were highly trait-specific: additive effects dominated the responses of root morphological traits, whereas antagonistic effects were more prevalent in root nutrient content traits. Moreover, these interactions varied with treatment intensity and root order rather than with interspecific variation. Specifically, the antagonistic effect of D × N became stronger with increasing D and N intensity, as well as with ascending root order. Overall, this study provides critical insights into root adaptation strategies under complex environmental change, offering an empirical basis for refining predictive models and guiding the design of more ecologically relevant multi-factor experiments.