INTRODUCTION
Soil carbon is highly relevant for climate change mitigation (Canadellet al. 2007) because it constitutes the largest carbon pool in
terrestrial ecosystems (Batjes 1996). However, there is still much
uncertainty regarding the relationship between soil carbon storage and
ecosystem biota, including the role of biodiversity patterns and human
activities (Schulze 2006; Soussana & Lemaire 2014; Ali & Yan 2017).
According to current conceptual models, ecosystem goods and services
-counting soil carbon sequestration- are expected to depend strongly on
biodiversity components (Hooper et al. 2005; Diaz et al.2007; Hector & Bagchi 2007; Kirwan et al. 2009; Suter et
al. 2015; Connolly et al. 2018). However, most evidence comes
from diversity function experiments, which might suggest important
underlying mechanisms (Fornara & Tilman 2008; Prommer et al.2019), but do not encompass the whole complexity encountered in real
ecosystems. Although small-scale experiments indicate that the
relationship between biodiversity and several ecosystem services might
exist (Hector et al. 1999; Ribas et al. 2015), evidence
about a positive association between soil carbon storage and plant
diversity in nature is scarce (Hollingsworth et al. 2008; Wardle
2016).
In recent years, attempts have been made to clarify how plant taxonomic
and functional diversity drive soil carbon at broad scales (Manninget al. 2015; Chen et al. 2018). In addition, plant
functional types (PFT, Steneck, 2001; Blondel, 2003) were also found
relevant for explaining soil carbon content in experiments (Fornara &
Tilman 2008; Lange et al. 2015) and local studies (Wiesmeieret al. 2019).
There is an unsolved discussion about the relevance of plant functional
types versus plant traits (or functional diversity) on ecosystem
function (Lavorel & Garnier 2002; Ricotta et al. 2016).
Furthermore, some authors postulate that ecosystem function depends on
the dominant plant species or functional type (Grime 1977) (e.g.,
grasses in grassland ecosystems (Strömberg & Strömberg 2011). However,
several studies indicate an important role of PFT diversity and
non-dominant plant functional types on grassland functioning (Debouket al. 2015), including legumes (Spehn et al. 2002), and
even in the absence of legumes (Cong et al., 2014).
Plant functional types can be described as guilds when defined in terms
of resource use (Sebastià 2007) sensu Root (Root 1967), for
animals). Furthermore, resource use can also be considered a relevant
plant functional trait. Although every plant species can be considered
to occupy a specific and unique point in the multivariate space of
functional traits (Wayne & Bazzaz 1991), and therefore their role in
the ecosystem to be unique, plant guilds provide good summary
representations for biodiversity-function analysis. Grasses are
efficient in terms of light capture because their leaves are at vertical
angles (Sebastià 2007), while the architecture of their roots made them
efficient capturing soil N. Legumes can have access to
symbiotically-fixed atmospheric N. Non-legume forbs present a variety of
ports and (flat to obtuse) leaf angles, and cannot fix atmospheric N
(Canals & Sebastià 2000; Sebastià 2007; Sebastià & Puig 2008).
However, recently, legumes were found to be more active in terms of
CO2 exchange per biomass unit compared with other
guilds, including highly dominant grasses, and non-legume forbs (Ibañezet al. 2020).
Plant guild effects on soil carbon storage need to be studied at broad
scales to understand how they work independently of broad scale abiotic
variables, as it is being done for plant taxonomical and functional
diversity (Manning et al. 2015; Carol Adair et al. 2018).
Ecologists and modelers need this information to validate conceptual
paradigms and generate new hypotheses contributing to the refinement of
global mechanistic models. Land managers and policy-makers need it to
establish priorities for conservation objectives.
Here, we aim to disentangle plant guild effects on soil organic carbon
(SOC) in the Pyrenees, by modelling data from an extensive database
generated from a survey of 98 natural grasslands. The survey included a
variety of climates; different landscape positions; and a range of
grazing management regimes. We applied the diversity-interaction
modelling (DIM) approach (Kirwan et al. 2009), which allowed us
to separate the identity and interaction effects of plant guilds. Taking
into consideration that previous studies, carried out at narrower
scales, found a crucial but contrasting role of legumes within the plant
guilds (p.e. Fornara & Tilman, 2008b; Lange et al., 2014), we aim at
answering the following questions:
Do the effects of plant guilds on SOC in natural grasslands mirror those
found in experimental systems?
Are enhancement effects of legumes on SOC stable across the range of
proportions commonly found in natural grasslands (10-50%)?
Do the effects of legume proportions depend on other plant guild
proportions, including forbs and grasses?