Introduction
Grasslands play a critical role in global carbon and nitrogen cycles
(Scurlock & Hall, 1998;
Suttie et al. 2005). Increasing
grassland plant diversity can have positive effects on a wide range of
ecosystem functions including plant productivity
(Tilman et al. 2001;
Ravenek et al. 2014;
Oram et al. 2018;
Prieto et al. 2015), soil carbon
and nitrogen storage (Lange et al.2015; Weisser et al. 2017; Leloupet al. 2018; Yang et al.2019), and soil respiration (Chen et al. 2019). Declines in
grassland plant diversity through grazing mismanagement, land
conversion, and climate change will thus likely change many aspects of
the carbon and nitrogen cycles of grassland ecosystems
(Reich et al. 2012;
Isbell et al. 2013;
Hautier et al. 2018).
Numerous studies have shown close relationships between plant diversity
and carbon and nitrogen cycling processes in grasslands
(Hector et al. 2010; Oelmannet al. 2011; Mueller et al.2013; Craven et al. 2016). Above-
and belowground biomass, soil organic carbon, and soil respiration have
all been observed to increase significantly with species richness
(Tilman et al. 2001;
Cowles et al. 2016; Chen & Chen
2019; Yang et al. 2019) including
a number of meta-analyses showing that these are very general patterns
(Weisser et al. 2017; Chenet al. 2019; Wang et al. 2020). We expect, therefore, that
carbon cycles in grasslands will be changed significantly under
biodiversity loss scenarios. Given that the nitrogen cycle is closely
coupled with carbon cycling (Zaehle 2013;
Zaehle et al. 2014), we expect
similar impacts on nitrogen cycling processes (Schmid et al.2009; Weisser et al. 2017; Faninet al. 2018).
Numerous field and controlled environment experiments have examined
plant diversity impacts on the provision of ecosystem services (Fridley
2002; Prieto et al. 2015; Craven et al. 2016). Plant
diversity impacts on carbon and nitrogen cycles in these experiments may
be sensitive to experimental type (i.e. field versus controlled
environment) (Roscher et al. 2004; Thompson et al. 2005;
Malchair et al. 2010; Kreyling et al. 2017), experimental
duration (Chen et al. 2019; Chen et al. 2020), and, in the
case of field experiments, environmental conditions (Duffy et al.2017; Chen et al. 2019). For example, after controlling for
environmental covariates, increases in biomass with biodiversity are
stronger than has previously been documented in experiments (Duffyet al. 2017). Interactions between plant diversity and
experimental duration on carbon processes have also been reported. For
example, Tilman et al. (2001)
found that the diversity effects on productivity can become
progressively stronger due to the enhancement of complementarity effects
with time. Recent meta-analyses have found that the positive effects of
plant diversity on soil organic carbon, microbial biomass, and microbial
respiration are also enhanced with experimental duration (Chen et
al. 2019; Chen et al. 2020). Finally, a long-term grassland
experiment suggests that the close relationships between the carbon and
nitrogen cycles (Zaehle et al. 2014) may enhance the effects of
plant diversity on nitrogen processes with time
(Mueller et al. 2013). Synthesis
studies of these diversity experiments have not, however, explored
either the effects of plant diversity across multiple carbon and
nitrogen processes, or the interactive effects of plant diversity and
experimental type (field and greenhouse experiments) and duration.
Here we compile data from 73 studies examining the effects of plant
diversity in grasslands (Fig. 1). These studies report on nine ecosystem
carbon processes (aboveground biomass, belowground biomass, total
biomass, soil carbon pool, soil respiration, heterotrophic respiration,
microbial biomass, bacterial biomass, and fungal biomass) and six
nitrogen processes (aboveground nitrogen pool, soil nitrogen pool, soil
ammonium nitrogen, soil nitrate nitrogen, soil nitrogen leaching, and
soil nitrogen mineralization). We specifically test 1) how grassland
plant mixtures impact carbon and nitrogen processes relative to
monocultures, 2) whether diversity effects differ between field and
greenhouse experiments, and 3) how plant diversity impacts change with
experimental age in field experiments.