Forestation of abandoned and farmed lands enhances soil microbial biomass carbon, nitrogen and phosphorus compared to grassed and forested lands: A meta‐analysis

Abstract Forestation, including reforestation (restoring tree cover to previously forested lands) and afforestation (establishing tree cover on non‐forested lands), is a key strategy for ecological restoration and climate change mitigation. Soil microbial biomass and stoichiometry are critical indicators of the quality of ecological restoration, yet the global scale impacts of afforestation and reforestation remain poorly understood. We conducted a meta‐analysis of 1091 paired observations from 103 peer‐reviewed articles of globally distributed studies to quantify the impacts of afforestation and reforestation on soil microbial biomass and stoichiometry and to identify key influencing drivers. We found that forestation increased soil microbial biomass carbon (MBC) by 71.6%, microbial biomass nitrogen (MBN) by 80.0%, microbial biomass phosphorus (MBP) by 76.6%, and the MBN:MBP ratio by 53.3%. Preceding land use type was an important determinant of soil MBC, MBN and MBP. Afforestation of abandoned lands increased soil MBC, MBN and MBP by 28.4%, 125.5%, and 196.4%, respectively, while afforestation of farmed lands led to increases of 168.2%, 232.7% and 91.5%, respectively. Additionally, after forestation, the soil MBN:MBP ratio decreased with elevation. Synthesis and applications . We highlight the importance of considering farmed and abandoned lands in forestation programmes to enhance microbial biomass. Forestation at low elevations may increase P limitation for microorganisms. These findings are crucial for optimising forestation strategies and management, ultimately contributing to improved ecosystem‐level soil health and functioning.