Driving effects of aridity on three dimensions of belowground components in grasslands change at aridity threshold

The belowground component of the grassland has abrupt changes with increasing aridity. However, the effects and driving pathways of aridification on different dimensions of the belowground component (such as specific root length, belowground biomass and soil organic carbon) before and after the aridity threshold have not been fully elucidated. This research gap is addressed by evaluating changes in soil and plant attributes with aridity along a 2600 km aridity gradient in the arid and semiarid grasslands of Inner Mongolia. Results showed an overall aridity threshold for grassland ecosystems of 0.67, where abrupt changes in belowground components were observed. Structural equation models results showed that the effect of aridity on specific root length was negative (-0.18) before the threshold and positive (0.24) after the threshold, due to a shift of plant strategies from drought tolerance to avoidance. The effect of aridity on belowground biomass was always negative and increased from -0.24 to -0.55 after the threshold without grass regulation. The effect of aridity on soil organic carbon exhibited a subtle change, but the driving pathway changed from soil loss to aridity and vegetation cover at plot scale. These findings highlight the changes in effect and dominant pathways of aridity on belowground components in grassland ecosystems before and after the aridity threshold, which provides a basis for understanding the impact of plant drought resistance strategies, vegetation type and spatial scale when protecting grasslands at different aridity levels.