Spatial distribution of soil microbial biomass and respiration
(H1)
Soil microbial biomass decreased with increasing distance from the trees
in monospecific tree pairs. This could be explained on the one hand by
higher water availability due to stemflow near the tree base, which can
leach and transport nutrients and microorganisms from the canopy layer
to the soils (Bittar et al. 2018). Soil moisture was shown to be
important in many studies before (Cesarz et al. 2022; Schimel 2018;
Serna-Chavez, Fierer, and van Bodegom 2013). High levels of soil
moisture can increase soil enzyme activities, fluxes of soil nutrients,
and oxygen availability (Brockett, Prescott, and Grayston 2012; Stark
and Firestone 1995), and higher soil humidity can furthermore buffer
possible negative changes in soil pH, suggesting it to be a key driver
of soil microbial biomass (Cesarz et al. 2022). On the other hand it
could also be explained by a higher rhizodeposition closer to the trees
(Parker et al. 2017). Our findings would suggest the importance of
forest density in modulating soil functioning.
As expected, we also found a negative effect of soil depth on both
microbial properties. This is in line with previous findings where a
lower amount of carbon and nutrients was found in deeper soil layers, as
the main decomposition happens in the leaf litter cover and top soil
layers (Goebes et al. 2019; Jobbágy and Jackson, n.d.; Prescott and
Grayston 2013). Additionally, deeper soil layers often have a decreased
amount of oxygen, soil water content, and contain less plant root
biomass (Engelhardt et al. 2018; Fall et al. 2012; Serna-Chavez, Fierer,
and van Bodegom 2013). Thus, the present results at the small scale are
in line with previous findings at the larger scales, where soil organic
carbon decreased with increasing soil depth and distance to trees
(Rabearison et al. 2023). These similar results suggest that
understanding interaction effects at small scales have the potential to
be upscaled.
Spatial distribution of belowground overyielding (H2 - H3)
Our study showed no overyielding across all soil core positions and
depth layers. However, we found significant differences between the soil
layers for both soil microbial functions. Microbial biomass showed, on
average, higher overyielding in the shallower soil (0-5 cm), whereas
microbial respiration showed higher overyielding in the deeper soil
(5-10 cm). The BEF-China experimental Site A was established in 2009
after a clear-cut of the previous plantation (Yang et al. 2013). Plant
diversity effects on soil organic matter have been shown to become
stronger in the topsoil layer over time (Lange et al. 2023). These
findings from experimental grasslands could suggest that the soil
microbial biomass in forests is strongly related to the organic matter
content in the topsoil layer (Beugnon, Eisenhauer, et al. 2023) and that
positive plant diversity effects could get stronger with stand age
(Huang et al. 2018; Perles-Garcia et al. 2021). The increased microbial
respiration in the deeper soils could suggest increased carbon
sequestration by adding soil microbial necromass to the carbon pool
(Buckeridge et al. 2020; Schmidt et al. 2011).
We expected heterospecific tree-tree interactions effects to be
maximised in the middle between the planted trees. Contrary to our
hypothesis (H3), we found an overyielding of soil microbial biomass and
respiration close to L. formosana and underyielding close toS. saponaria , showing that microbes in mixed pairs perform better
than expected close to L. formosana but less well than expected
close to S. saponaria. The gradient from over- to underyielding
of microbial respiration was less pronounced in the shallower soil than
in the deeper soil. This could indicate that the presence of S.
saponaria had a positive effect on microbes close to L.
formosana and it is stronger in the deeper soil layer (5-10 cm),
possibly through fine root exudates (Zheng, Wei, and Zhang
2017). Microbial respiration was less affected from the mixture than
microbial biomass in the topsoil layer (0-5 cm). It was shown that the
balsam of L. formosana contains acidic compounds, which were
reported to be inhibitory for fungi (Chien et al. 2013). These could
also be present in the leaf litter or root exudates (Öztürk et al. 2008)
and inhibit microbial respiration more than microbial biomass. Together
with a spatial distribution of litter in the heterospecific pairs
(Beugnon, Eisenhauer, et al. 2023), this might lead to a small-scale
change of soil pH. Soil pH was found to be a strong driver of microbial
growth (Fierer and Jackson 2006), and additional pH measurements should
be performed in future studies to better understand the opposing species
identity effects of S. saponaria and L. formosana . It was
shown that soil fungi and bacteria react differently to changes in soil
pH: bacterial growth decreased with a more acidic pH, whereas fungal
growth was shown to increase (Rousk, Brookes, and Bååth 2009). This
might also explain the significant negative effect of L.
formosana on microbial respiration in this experiment (Suppl. S2: Fig.
S2). To better understand distribution patterns of microbial properties,
belowground tree traits (e.g. specific root length, root diameter)
should be taken into account. Recent studies could link them to carbon
exudation and fine root density (Bergmann et al. 2020; Sun et al. 2021),
as well as soil organic matter decomposition (Adamczyk et al. 2019).
The positive tree-tree interaction effect of the heterospecific tree
pair on soil microbial biomass and respiration shows that neighbourhood
effects are acting at small spatial scales, which could explain the
inconsistencies of BEF relationships reported in previous forest studies
(Beugnon et al. 2021; Cesarz et al. 2022; Li et al. 2019; Pei et al.
2016). Our results stress the need to standardize sampling methods by
considering small-scale interactions to understand the mechanisms behind
tree-soil interactions. In addition, measurements of soil microbial
properties across a wider range of species transects are now needed to
better understand tree-tree interactions in space and their biological
drivers.