How do stand conversion approaches influence biodiversity and soil multifunctionality in Picea crassifolia plantations in the Northeastern Tibet Plateau?

Over the past few decades, the Grain-for-Green Program on the Northeastern Tibet Plateau has established extensive monoculture plantations of Picea crassifolia , which play an irreplaceable role in regional ecological security. However, large-scale monocultures often suffer from low biodiversity and land degradation, thereby weakening ecosystem services. This study aimed to investigate the effects of monoculture and mixed plantations on biodiversity and soil multifunctionality (SMF). This study focused on Picea crassifolia monocultures and three mixed plantations ( Picea crassifolia - Hippophae rhamnoides, Picea crassifolia - Larix principis-rupprechtii , and Picea crassifolia - Betula platyphylla ) on the Northeastern Tibet Plateau. We investigated the relationships between soil microbial species abundance and soil physicochemical properties, analyzed differences in biodiversity and SMF. The influencing factors of SMF and biodiversity, as well as the driving mechanisms of SMF, have been identified. The results showed that: (1) Mixed plantations of Picea crassifolia altered soil microbial community structure and composition, with soil fungi may be more responsive to environmental changes. (2) Picea crassifolia - Hippophae rhamnoides mixed plantation significantly enhanced soil microbial diversity, while Picea crassifolia - Hippophae rhamnoides and Picea crassifolia - Betula platyphylla mixed plantations significantly increased herbaceous diversity. (3) Key environmental factors affecting soil microbial diversity mainly included soil pH, nitrate nitrogen (NO₃⁻-N), available phosphorus (AP), total nitrogen (TN), and Clay contents; soil pH and AK were the primary factors influencing herbaceous diversity. (4) Picea crassifolia - Larix principis-rupprechtii mixed plantation significantly improved SMF. Soil pH and microbial biomass directly affected SMF, while soil texture indirectly influenced it by modulating microbial biomass. This study provides an assessment of the ecosystem services of Picea crassifolia monoculture and different mixed plantations following the implementation of the Grain-for-Green Program on the Northeastern Tibet Plateau. The findings offer potential insights that may informing forest ecosystem restoration and management decisions in alpine regions.