Introduction
Forest ecosystems are crucial for the planet’s health and sustainability by supporting an extensive range of biodiversity and ecosystem services, including carbon storage, primary production, water and nutrient cycling (Bardgett and Wardle 2011; Van Der Heijden, Bardgett, and Van Straalen 2008; Wagg et al. 2014). The potential of carbon storage within a forest depends on the interactions with the environment and the dominant management practices (Erb et al. 2013). Especially carbon source-sink dynamics are significantly influenced by the interactions between soil microbes and understory plants (S. Xu et al. 2020).
Microbial-driven decomposition of organic matter and nutrient cycling is essential for maintaining ecosystem productivity in many different biomes (Delgado-Baquerizo et al. 2016; Gottschall et al. 2019; Gougoulias et al. 2014; Van Der Heijden et al. 2008). Microbes are the primary drivers of belowground carbon storage in forests (Schmidt et al. 2011). They transform organic carbon into stable soil organic matter through processes like aggregation or accumulation of microbial necromass (Buckeridge et al. 2020; Miltner et al. 2012; Wang et al. 2021). Thus, this stabilisation of the forest carbon pool provides tools to mitigate climate change (Bastin et al. 2019; Lewis et al. 2019). Understanding the drivers of belowground carbon storage and its relationship with biodiversity is crucial for effective forest management and carbon sequestration (Messier et al. 2022). In particular, soil microbial biomass and respiration could serve as a proxy for nutrient cycling and soil organic matter turnover (Crowther et al. 2019) and were shown to be correlated with soil carbon sequestration (Beugnon, Bu, et al. 2023; Lange et al. 2015). Therefore, these soil microbial properties together can provide important information on multiple soil ecosystem functions (Eisenhauer et al. 2018).
Microbial properties generally vary between soil layers due to lower resource availability (e.g. nutrients and oxygen) in the deeper soil layers leading to reduced microbial diversity and biomass (Goebes et al. 2019; Jobbágy and Jackson, 2000). However, rhizodeposition can increase microbial activity at deeper soil layers (Lopez et al. 2020), potentially leading to different drivers of microbial activity and biomass across soil layers (Blume et al. 2002; Loeppmann et al. 2016).
Tree diversity was shown to enhance soil microbial diversity, abundance, and functioning, leading to improved nutrient cycling, organic matter decomposition, and carbon storage (Beugnon et al. 2021; Gamfeldt et al. 2013; Gottschall et al. 2019; Li et al. 2019; Pei et al. 2016); primarily due to higher diversity of substrates from litterfall and rhizodeposition as well as possible increased belowground interactions with tree species-specific soil microbes (Beugnon, Eisenhauer, et al. 2023; Huang et al. 2017). However, other studies showed that the tree diversity impact on soil microbial functions is non-significant, varies across functional groups such as bacteria and fungi (Cesarz et al. 2022; Rivest, Whalen, and Rivest 2019), or is less important than tree identity effects or abiotic conditions (Cesarz et al. 2022; Tedersoo et al. 2016; Yamamura, Schwendenmann, and Lear 2013). There are now empirical pieces of evidence that spatio-temporal dynamics along tree diversity gradients can drive soil microbial functions (Gottschall et al. 2022), which vary with the tree neighbourhood (Trogisch et al. 2021).
Forest soils’ spatial structure and processes can become highly heterogeneous due to the spatial distribution of roots and root inputs. Soil respiration, for instance, was shown to be higher at the base of mountain birch trees compared to 150 cm away, indicating ’hot-spots’ of soil microbial activity close to the tree (Parker et al. 2017). This spatial distribution of soil functions is crucial when considering interactions between trees or with the understory vegetation (Kuzyakov and Blagodatskaya 2015; Mao et al. 2015). Microbial communities were found to be more active and diverse when surrounded by neighbouring trees than close to an isolated tree (Habiyaremye et al. 2020). Especially, the effects of tree-tree interactions are expected to be maximised in the interaction zone between the trees (Trogisch et al. 2021). This highlights the role of the neighbouring trees on the functioning of soil microbes in forest ecosystems, especially in the context of highly diverse forests. However, information on the spatial distribution of soil processes (e.g. soil respiration) at finer spatial scales is missing (Friggens et al. 2020).
In this study, we aimed to understand the effects of tree-tree interactions on soil microbial biomass and respiration and their spatial distribution. We set up small-scale transects in tree neighbourhoods in a Chinese subtropical forest experiment (BEF-China), where we tested the following hypotheses: (H1) In monospecific tree pairs, we expect decreasing microbial biomass and respiration with increasing distance from the trees and with increasing soil depth, due to lower resource availability in greater distances. (H2) Due to higher complementarity as well as quantity and diversity of resource inputs between hetero-specific tree pairs, we expect overall higher microbial biomass and respiration than in mono-specific pairs. (H3) We expect the interaction between trees to be maximised in the interaction zone between the two trees; thus, soil microbial biomass and respiration are highest in the topsoil in the middle of the transect between two adjacent trees.