Effects of afforestation on soil microbial diversity and enzyme activity: A meta-analysis

Highlights

• Afforestation enhanced soil fungal diversity and enzyme activity.

• Degraded land afforestation increased soil microbial diversity and enzyme activity.

• Soil bacterial diversity increased significantly with afforestation duration.

• Soil microbial richness changed greater after broad-leaved species afforestation.

Abstract

Afforestation is thought to be one of the key measures for mitigating climate change by capturing atmospheric carbon. However, despite the importance of afforestation in ecosystem functioning, its effects on soil microbial diversity and enzyme activity remain unclear. In this study, we conducted a meta-analysis of these effects, using a newly compiled dataset of soil microbial diversity and enzyme activity before and after afforestation collected from 80 sites worldwide. Soil fungal diversity and soil enzyme activities increased significantly after afforestation, but soil bacterial diversity did not change significantly. Among these soil enzymes, the activities of β-1,4-glucosidase (BG), urease (UREA), alkaline phosphatase (AP), dehydrogenase (DEH), and catalase (CAT) increased by 104.7%, 84.0%, 101.7%, 199.2%, and 58.3%, respectively. The responses of soil microbial diversity and enzyme activities varied across afforestation durations, climate zones, prior land use types, and species. Specifically, soil bacterial diversity and the activity of BG increased significantly with afforestation duration, and the increase in BG activity was higher in tropical than in temperate zones. In degraded sites, both soil microbial diversity and enzyme activities significantly increased after afforestation. In addition, structural equation models showed that soil carbon content, nitrogen content, and soil pH value were significant driving factors for the soil microbial community diversity and soil enzyme activities. Overall, our results provided a comprehensive understanding of the changes in soil microbial diversity and enzyme activity under different afforestation conditions, as well as scientific bases for locally adapted afforestation in the future.

Introduction

Afforestation is recognized as an effective way to mitigate climate change (IPCC, 2014, Bastin et al., 2019). In recent decades, the area of planted forests has continuously increased and currently accounts for 7% of the total global forest area (FAO, 2020). Afforestation influences the composition of soil microbial community that plays an important role in various ecological processes (Lamb et al., 2011, Deng et al., 2016). Many studies have suggested that the increase in vegetation coverage after afforestation leads to changes in soil environmental conditions (Farley et al., 2005, Peng et al., 2014), as attributed to changes in soil microbial community composition and the activities of relevant hydrolytic enzymes (Lauber et al., 2008, Sinsabaugh et al., 2008). Forest management activities such as soil disturbance from site preparation and timber machinery harvesting also affect the soil physical structure as well as soil air and water circulation, regulating microbial growth and community composition (Wardle, 1992, Ilstedt et al., 2007, Hartmann et al., 2012, Hui et al., 2018, Cui et al., 2020). Soil microbial diversity and enzyme activity change with afforestation, but there is little consensus on the direction of the change (Chodak et al., 2009, Kang et al., 2018, Gonzalez-Polo et al., 2019, Xu et al., 2020, Zhong et al., 2020, Chen et al., 2021, Ding and Wang, 2021). Moreover, quantitative synthesis clarifying the impacts of afforestation on soil microorganisms in different afforestation regions and conditions has not been conducted.

Soil microbial diversity and enzyme activity are commonly used as indicators of the degree of ecosystem restoration after afforestation (Fu et al., 2015, Gunina et al., 2017, Liu et al., 2017, Liu et al., 2018). In terrestrial ecosystems, changes in soil microbial diversity may alter the diversity, productivity, and environmental adaptability of plants, litter decomposition and nutrient cycling (van der Heijden et al., 2008, Bardgett and van der Putten, 2014, Wagg et al., 2014). Alterations in soil enzyme activities reflect changes in soil microbial nutrient requirements and metabolic activity (Sinsabaugh et al., 2008, Moorhead et al., 2012, Kang et al., 2018). For example, β-1,4-glucosidase (BG), urease (UREA), and alkaline phosphatase (AP) are involved in the carbon, nitrogen, and phosphorus cycles, respectively (Sinsabaugh et al., 2008), and oxidase enzymes such as catalase (CAT) and polyphenol oxidase (POX) are indispensable for the redox process of the macromolecules contained in soil organic matter (Garcı́a-Gil et al., 2000). Therefore, exploring the responses of soil microbial diversity and enzyme activity to afforestation is helpful not only for understanding the changes in the structure and metabolic activity of the soil microbial community but also for further understanding the functioning and stability of the plantation ecosystem. Nevertheless, previous studies on soil microbial properties after afforestation have mostly focused on soil microbial biomass (Zhang et al., 2017) or the community structure and metabolism of microorganisms through traditional methods, such as the phospholipid fatty acid method and Biolog microplate method (Macdonald et al., 2009, Zhao et al., 2019). Such methods cannot provide accurate details of soil microbial diversity and community composition compared with next-generation sequencing technology. In recent years, next-generation sequencing technology has promoted the study of soil microbial diversity, enriching research on the response of soil microbial diversity to afforestation (Garcia-Franco et al., 2015, Zhao et al., 2019, Xu et al., 2020).

To explore changes in soil microbial community after afforestation, we selected soil microbial diversity and enzyme activity as indicators of the composition and metabolic activity of soil microorganisms. We collected 260 observations from 80 sites worldwide to establish a global database of soil microbial diversity and enzyme activity before and after afforestation and then conducted a meta-analysis. We attempted to answer the following questions: (1) how do soil microbial diversity and soil enzyme activity change after afforestation? (2) how do the changes differ with afforestation duration, climate zone, land use type, and plant species (broad-leaved vs. coniferous)? and (3) what are the underlying mechanisms for these changes?