Elevation threshold governs the decoupling of plant microbe feedbacks in restored grasslands 16S part2

Infrastructure development worldwide has caused severe grassland degradation, while conventional "one-time seeding and fertilization" practices rarely re-establishes plant cover due to persistent nutrient and microbial constraint, especially on the Qinghai Tibet Plateaus. Yet, the altitude dependent decoupling of aboveground-belowground ecological processes remains poorly understood. We investigated restoration trajectories of excavation-disturbed grasslands along an elevational gradient over five years. We found that total and available N and P in restored soils never reached natural levels and declined linearly with elevation. Plant cover averaged 32%, 21% and 24% in years 1, 3 and 5, respectively, versus 73% in natural grassland. Microbial richness did not decline, yet community composition progressively diverged. A critical elevational threshold of 4000 m emerged. Below this point, the response ratio of coverage decreased gradually with elevation, whereas bacterial and fungal richness increased. Above it, both plant coverage and microbial richness declined sharply, indicating a shift in plant-microbe interactions. In contrast, total and available soil nitrogen and phosphorus declined linearly with elevation without a detectable breakpoint. Partial least squares path modeling revealed that elevation indirectly reduced plant recovery in plant-soil-microbe interactions, with above 4000 m by weakening the positive contributions of bacterial communities, whereas elevation had no significant direct effect on plant recovery below 4000 m, but elevation indirectly influenced plant recovery by attenuating the positive effects of bacterial communities. Our findings demonstrate that alpine grassland recovery is altitude-dependent, governed by a breakpoint-driven uncoupling of plant and microbial responses: i < 4000 m re-activate soil nutrients and re-inoculate native mutualists; ii > 4000 m use cold-adapted plant cultivars plus functional microbial consortia. The 4000 m line is a practicable tipping point for deciding whether nutrient or microbial interventions should dominate restoration budgets on the world's high plateaus.