Predictors of soil functions and aboveground-belowground
interactions
Plant functional groups (PFGs) emerged as the dominant predictors of
soil functions, explaining approximately 81% of the variance. Plant
species richness and PFGs collectively accounted for the majority of the
explained variance across four of the five soil functions we examined.
However, microbial diversity primarily explained variance in PHOS
activity in pre-drought measurements (Figure 2a). These findings were
robustly supported by our statistical models (Table 1), underscoring the
predominant role of PFGs, particularly N-fixing plant group, in driving
all soil functions, both independently and in synergy with microbial
diversity, and independent from drought disturbance. Importantly, it is
essential to note that the contribution of PFGs extends beyond
functional group diversity in non-monoculture contexts. Conversely,
microbial diversity exerted a comparatively low influence on soil
functions associated with nitrogen cycling during pre-drought sampling.
However, its importance increased notably during the drought period,
where it shared equal explained variance with PFGs (Figure 2b). These
results were further validated by regression models, which identified
interactions between PFGs and microbial diversity as the best-fit models
with the lowest AIC values (Table 1). While drought had a negligible
direct effect, accounting for less than 1% of the variance, it had a
significant impact on microbially driven PHOS activity. Following a
month of recovery from the drought, we observed the soil functions
displaying signs of recuperation. Approximately 78.9% of the variance
in these recovered soil functions was attributed to PFGs, with plant
species richness and microbial diversity contributing to approximately
12.9%, and 8.1% respectively (Figure 2c).
Relationship
between soil functions and plant species richness across microbial
dilution gradient
We examined the intricate relationships between microbial and plant
diversity by assessing correlations between plant species richness and
soil functions within individual soil dilutions. Additionally, we
explored associations between microbial communities and soil functions
at various levels of plant richness. Notably, we observed a negative
correlation between plant species richness and ammonium concentrations
in D0 and D2 (ρ = -0.32, p < 0.05), which were
statistically insignificant in D6 (ρ = -0.16, p >
0.05) (Figure 3). Conversely, plant species richness exhibited a
consistent negative relationship with both ammonium and nitrate levels
across all soil diversity levels (Figure 4i). The impact of drought on
nitrogen cycling processes was particularly pronounced in D6. This
resulted in a shift in the relationship between plant richness and
nitrogen levels, with higher microbial diversity in D0 and D2
stabilizing the effect (Figure 4(ii)). Importantly, after the recovery
period, the relationships largely returned to their pre-drought negative
associations (Figure 4(iii)). Plant species richness showed a
significant positive correlation with phosphate only in soils with
higher microbial diversity (D0 and D2). However, this relationship was
absent in D6 soils (R2 =0.02, p =0.24) (Figure 4c(i)).
While plant species richness had no discernible effect on PHOS activity
during the plant establishment stage (ρ = -0.04, p >
0.05; Figure 3i, and Figure 4e(i)), the drought phase resulted in a
significant positive correlation across all soil diversity levels
(Figure 4e(ii)). In case of N mineralization, increasing plant species
richness had a positive influence (ρ = 0.35, p < 0.05;
Figure 3i, and Figure 4d(i)) during pre-drought conditions. However, the
drought period led to a decoupling of this relationship, which was
further influenced by the soil biodiversity level (Figure 4d(ii)). Given
that higher plant species richness likely corresponds with increased
nutrient demand, a negative correlation (ρ = -0.37, p <
0.05) with nitrate levels was observed (Figure 3i). Overall, the
presence of plants significantly influenced the relationships between
microbial diversity and soil functions during the plant establishment
stage (pre-drought). Components of the nutrient pools were notably
affected with increasing plant species richness (Figure 3i and 4).
Plant functional
groups as the main predictor of soil functions during drought and
recovery
Individual PFGs played a significant role in shaping the intricate
relationships between microbial diversity and soil functions (Figure 3
and Table 1). To gain a more nuanced understanding of their
contributions, we employed multiple linear regression models to dissect
the unique roles of each PFG in multitrophic plant-microbial
interactions (Table 2, and Figure 5). During the early flowering stage
(pre-drought), all PFGs and their combinations exhibited similar impacts
on soil functions (Figure 3). However, when drought was induced,
extractable NH4, NO3, and N
mineralization became significantly influenced by PFGs (Table 1). This
influence was primarily driven by the presence of N-fixing plants, both
in isolation (p < 0.05) and in combination with other
species (p < 0.05) (Figure 5). Notably,
NH4 exhibited a negative correlation with N-fixing
plants, whereas NO3 and N mineralization displayed a
positive relationship with N-fixing plants (Table 2 and Figure 5).
Following the recovery from drought, many of these effects of PFGs on
soil functions also rebounded. The relationships between soil
extractable phosphate and PHOS activity with microbial richness remained
largely unaffected by PFGs, with the exception of the presence of C4
plants during the recovery phase (Table 2 and Figure 5).
DISCUSSION
This study represents a substantial advance in our understanding of the
intricate relationships among microbial and plant diversity,
particularly of plant functional groups (PFGs) and their impacts on
essential soil functions under water-limiting conditions. Our results
are supported by previous research, emphasizing the potential adverse
effects of microbial and plant diversity loss on ecosystem functions ,
while advancing the discipline by revealing that the composition of
plant communities, particularly their functional groups, could be a
dominant predictor of soil functions that could also affect microbial
response to environmental stressors. Crucially, our findings contribute
further evidence to the growing body of research that highlights the
sensitivity of biotic interactions between plant and microbial diversity
to biodiversity loss . Of note, our results underline that the
composition of plant communities, with a special focus on functional
groups, significantly impacts the rate and resilience of key soil
functions . This highlights the imperative need for holistic evaluations
that encompass both above- and belowground biodiversity loss, coupled
with explicit consideration of functional groups, in the development of
effective management and conservation policies.
One of the intriguing aspects we observed is the dynamic nature of the
effects of PFGs and diversity on soil functions over time and in
relation to various microbial processes. At the early pre-planting
stage, we detected a negative impact of soil microbial diversity loss on
soil nitrogen cycling and inorganic nitrogen concentrations. The
decrease in soil bacterial diversity was associated with a significant
reduction in nitrate accumulation in low-diversity soil (D6), suggesting
a direct negative relationship between the rate of nitrification and
microbial diversity . The loss of the nitrifier bacterial community,
responsible for converting ammonium into nitrate, coincided with
declining microbial diversity . This effect was likely amplified in the
absence of plants at the pre-planting stage. Similarly, we observed low
mineralization rates due to the higher microbial consumption of ammonium
and excess accumulation of nitrate, where no competition occurred
between different microbial communities and no plant nutrient demand was
present .
During the early flowering stage (pre-drought), the impact of plants on
soil functions, particularly the N cycle, was predominant, explaining
approximately 98% of the variance. This effect was not surprising, as
it was primarily attributed to the presence of N2-fixing plants, known
for their influence on N pool dynamics . More novel however was
identifying the fundamental role of PFGs in elucidating how ecosystem
functions respond to climate change . For example, PFGs emerged as the
primary factor accounting for the majority of the effects of plant
species richness on soil functions during drought. In our statistical
analyses, it is important to acknowledge the potential confounding
effect between PFGs and plant species richness, where the presence of
both variables in the model may make it challenging to disentangle their
individual contributions to observed outcomes.
Specifically, our findings align with the concept that the influence of
plant diversity on ecosystem functions is intricately linked to the
presence of greater plant functional diversity . This is exemplified by
a positive correlation we observed between the rate of N mineralization
and microbial richness in the presence of legumes during drought
conditions. Legumes increase soil N availability by N2fixation with microbial symbionts, and the mineralization of N-rich
legume litter . Therefore, presence of legumes increases N
mineralization rates, but soil biodiversity loss may lead to decrease in
legume performance and hence soil
functions . For example, a recent study reports the detrimental effect
of soil biodiversity loss, which decreased the plant yield under drought
conditions . However, a different pattern emerged in the case of the
P-cycle. Plant species richness had minimal impact on
phosphate across all microbial
diversity levels. Still, it did strengthen the relationship of PHOS
enzyme activity with plant species richness and showed a significant
positive correlation with microbial richness. Notably, plants themselves
play a role in P cycling as plant roots exude phosphatases as well as
carboxylic acids that can solubilize PO4 . In
particular, N2-fixers have a high P demand, possibly
explaining why an increase in PHOS activity was observed, without an
increase in P. This finding is consistent with a previous study which
observed that biodiversity strengthens plant and soil feedback and hence
increases P cycling . These findings significantly advance our
understanding of how environmental stressors alter the relative
contributions of microbial communities to soil functions . Moreover,
global change drivers, including warming, drought, land use
intensification, and other anthropogenic activities, pose threats to
both plant and soil biodiversity . In line with our hypothesis, the
results indicate that the strength and direction of plant diversity’s
effects on soil functions are influenced by microbial diversity during
drought.
Specifically, under drought conditions, microbial diversity become one
of the most important drivers of function. The likely reduction in plant
metabolic and growth rates during drought, combined with the microbial
ability to sustain these functions under stress, can elucidate this
shift in relative contribution . Microbes engage in dynamic interactions
with plants and enhance their resilience during changing environmental
stress. As an example, microbes are known to release extracellular
polymeric substances into the soil, enhancing water retention capacity .
While a previous study reported no significant impact of microbial
diversity loss on primary production , our findings in this study
indicate a positive influence of microbial diversity in reinforcing the
effects of plant diversity on soil functions. These effects are relative
and context-dependent. Therefore, these varying responses could be
attributed to differences in resource availability and other abiotic
conditions , indicating a context-dependent relationship between
plant-soil biodiversity and functions under climate change. This is
further supported by the finding that fungal richness may have been
indirectly influencing plant composition importance in predicting
aboveground functions (.
Most importantly, our results suggest that PFGs play a pivotal role in
driving context dependency in the BEF relationships supporting and
helping explain prior observations. For example, our results are
consistent with the few studies that examine plant functional group
identity and richness independently of species richness . Our findings
also imply that the effects of biodiversity on ecosystem processes
exhibit variation based on the types and traits of plants involved both
within and among PFGs . This is consistent with evidence that increasing
the number of PFGs in grassland plots increased primary productivity ,
especially if the PFGs differed in their resource use strategies, such
as nitrogen fixation or rooting depth and even when holding species
richness constant . Similarly, a meta-analysis by showed that the impact
of biodiversity on ecosystem functioning was stronger when PFGs were
defined based on traits related to resource acquisition and allocation,
rather than taxonomic affiliation. These reports exemplify how PFGs can
modulate the BEF relationship by influencing the complementarity and
redundancy of species within ecosystems.
In conclusion, this study employed an innovative experimental design to
provide mechanistic insights into the relative significance of plant and
microbial diversity, PFGs, and their interplay in shaping essential soil
functions amid environmental stressors. The results clearly illustrate
that while plant diversity can enhance soil functions, increasing plant
species richness cannot fully compensate for the loss of microbial
diversity. Moreover, the decline in microbial diversity directly
influences the magnitude of plant diversity impact on soil functions.
The significance of soil microbial diversity was particularly pronounced
in the under and environmental stress, where PFGs mediate microbial
responses to drought. Hence, both plant species richness and PFGs play
pivotal roles in shaping soil functions . These findings underscore the
vital need to conserve both microbial and plant diversity, paying close
attention to PFGs, in order to maintain the stability of multiple soil
functions in a changing world and advances our understanding of the
intricate interactions between above- and belowground biodiversity and
their combined effects on soil functions. It provides a new line of
future investigation to unravel the specific mechanisms governing these
interactions and elucidate their broader implications for sustainable
agriculture and ecosystem management. Such knowledge is essential for
the development of effective strategies to preserve both above- and
belowground biodiversity, ensuring the long-term health and productivity
of our ecosystems.