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.