Discussion
Many studies have explored the responses of carbon and nitrogen
processes to increasing plant diversity from both experiments and
meta-analysis (Tilman et al. 2001; Roscher et al. 2004; Ma
and Chen 2016; Chen et al. 2019; Chen et al. 2020). Three
aspects of our study distinguish it from previous syntheses studies.
First, our study is the first to compare across a large number of carbon
and nitrogen processes. Second, our study attempts to reconcile the
effects of experimental type by exploring the differences between field
and greenhouse experiments. Third and most importantly, we show that
there are important interactions between plant diversity and
experimental age in field experiments, with the effects of plant
diversity becoming more pronounced with time.
Carbon and nitrogen cycle changed significantly in plant
mixtures
We generally found support for the hypothesis that more diverse plant
mixtures positively affect the provision of many carbon and nitrogen
ecosystem services in grasslands. Overyielding in aboveground biomass
(AGB), belowground biomass (BGB), and total biomass (TB) in plant
mixtures has been commonly observed due to complementary plant
interactions (Tilman et al. 1997; Ma & Chen 2016; Wang et
al. 2020); the strong general effects of diversity found here
demonstrate that these mechanisms are broadly important in grassland
ecosystems. Further, we observed positive relationships between the soil
carbon pool (SCP) response and BGB, suggesting that the higher SCP
frequently observed in plant mixtures can be caused by complementary in
BGB (Fig. S1a). Higher microbial biomass (MB), fungal and bacterial
biomass, and heterotrophic respiration (Rh) in plant mixtures are likely
also attributable to the effects of higher productivity on the carbon
and nutrient inputs to the soil ecosystem (Bartelt-Ryser et al.2005; Eisenhauer et al. 2010). Soil respiration (Rs) combines Rh
and root metabolism (Luo & Zhou 2008), so there is likely both Rh and
plant BGB impacts in mixtures on Rs.
Positive plant diversity effects were also observed for nitrogen
attributes. These can be attributed to the positive relationship between
aboveground nitrogen pool (ANP) and AGB (Fig. S1b), and again
demonstrate the general importance of diversity complementary effects in
grasslands. Positive relationships were found between soil nitrogen
stock (SNP) and SCP (Fig. S1c), and thus the higher SNP in plant
mixtures may be induced by overyielding in above- and belowground
biomass. Larger carbon accumulations in soils could provide more
exchange sites for ammonium (Mueller et al. 2013), which may
cause the observed higher soil ammonium under plant mixtures. Soil
nitrate and nitrogen leaching in plant mixtures was lower than
monocultures probably due to greater N uptake capacity in a diverse
mixtures (Mueller et al. 2013; Leimer et al. 2016).
Stoichiometric theory shows that net N mineralization was positively
related to root N concentration (Manzoni et al. 2008), and
reduction in root N concentration (Mueller et al. 2013) may lead
to a decrease in soil nitrogen mineralization in plant mixtures.
Divergent response between field and greenhouse
experiments
Our comparison of field and greenhouse experiments demonstrates that
diversity effects were generally stronger in field experiments than in
the greenhouse. Duration and experimental size constraints in greenhouse
experiments likely explain much of the difference. The maximum diversity
for individual measures in the greenhouse was 3-16 species compared to
16-60 in the field. Similarly, greenhouse experiments older than 3 years
were rare (Fig. S2) while many field experiments had run for a decade or
more. Given the experimental age by diversity interactions identified in
the field experiments (discussed below), it is not surprising that field
experiments show stronger effects than greenhouse studies. Mechanisms
important in longer experiments that would not be detected in the
greenhouse could include the accumulation of dead plant material with
time (Bartelt-Ryser et al. 2005; Eisenhauer et al. 2010;
Chen et al. 2019). Further, we found no significant diversity –
climate interactions, suggesting that the stronger responses in field
experiments was not induced by the greater range of climate conditions
in the field.
Species richness - experimental age
interactions
We show that the effects of plant diversity on carbon and nitrogen
processes increased with experimental age in the field experiments.
Increases in the magnitude of complementary effects with time have been
observed (Cardinale et al. 2007;
Ravenek et al. 2014; Wang et
al. 2020). The likely mechanism behind these time effects is the
accumulation of plant biomass and soil carbon with time (Khlifa et
al. 2017). Positive interactions between plant diversity and
experimental age on ANP and SNP may also be caused by the enhancement of
complementary effects with time. However, There is a time lag in
microbial processes response to changes in plant communities due to the
accumulation of dead plant materials was needed before their response
(Bartelt-Ryser et al. 2005; Eisenhauer et al. 2010), and
thus the effects of plant diversity on microbial biomass, respiration,
soil nitrate, and soil nitrogen mineralization, which are likely caused
by the accumulation of primary production (Janssens et al. 2010;
Mueller et al. 2013), was more pronounced at longer experimental
duration.
Long-term impacts of diversity loss in
grasslands
Our model shows that a 10% (from 100 to 90%) decrease in plant species
richness could cause a small decline in SCP and SNP over one year, but
cumulatively much larger declines are likely. The diversity of many
plant communities worldwide is thought to be declining due to factors
including global warming (Tilman & Lehman 2001; Tylianakis et
al. 2008). The negative effects of plant diversity loss on carbon
storage may generate positive feedbacks that could accelerate global
warming. Nitrogen limitation of primary production in grasslands (Fayet al. 2015; Wieder et al. 2015), the positive interactive
effects of plant diversity loss and time on SNP could aggravate the
deficiency of nitrogen in grasslands, which substantially exacerbate the
shortage of forage production.