From Buffering to Collapse: A Hump-shaped Rhizosphere Response to Shelterbelt Forest Degradation

Forest degradation is widely assumed to drive a monotonic decline in belowground functioning, yet plant-soil feedbacks may transiently buffer stress. We tested this idea by quantifying the rhizosphere effect (RE), the percent difference between rhizosphere and bulk soil, for soil carbon (C), nitrogen (N) and phosphorus (P) pools, enzymatic activities, and microbial biomass across four degradation stages in three types of shelterbelt forests. We found that RE generally increased or remained stable from undegraded to mild-moderate degradation stage and then declined sharply at severe degradation stage. This pattern was consistent across species but differed in amplitude, with Populus thevestina showing the largest early increases, Populus alba maintaining RE longer before decline, and Populus popularis sustaining higher RE for N-acquiring enzymes at early degradation stages. Early positive RE coincided with lower pH and higher water-soluble organic carbon (WSOC), soil water content (SWC), NH₄⁺, and NO₃⁻ in rhizospheres, conditions that stimulate microbial activities and nutrient turnover. As degradation intensified, RE contracted toward zero or negative values, reflecting reduced root exudation and weaker plant-microbe feedbacks. Random-forest and redundancy analyses highlighted rhizosphere P, rhizosphere N, bulk soil WSOC, rhizosphere SWC, and bulk-soil stoichiometry as the most influential factors, consistent with a transition from compensatory stimulation to functional collapse beyond a tipping zone. Our study provides the first field evidence that rhizosphere functioning responds nonlinearly to forest degradation. Recognizing this transient compensatory phase advances our understanding of ecosystem belowground resilience and can inform the intervention windows for dryland forest restoration.