Soil moisture and species richness interactively affect multiple ecosystem functions in a microcosm experiment of simulated shrub encroached grasslands
Graphical abstract
Introduction
Global climate changes and human disturbances are altering terrestrial community processes, reducing biological diversity, and consequently changing community composition and landscape, affecting ecosystem functions finally (Cardinale et al., 2012; Petchey et al., 1999). Therefore, relationships between biodiversity and ecosystem functions (BEF) and the way in which ecosystem functions respond to disturbances and changing global conditions have attracted more and more attention in the past decades (Koutroulis, 2019; Maestre et al., 2012; van der Plas, 2019). However, how climatic/soil condition, community species diversity and composition interactively affect ecosystem functions is poorly understood, which limits our understanding of the ecological consequences of global climatic changes, especially in degrading regions (Mbaabu et al., 2020).
In earlier studies on BEF, researchers have focused on the relationships between plant species richness and a single ecosystem function that is usually biomass production, and a positive BEF relationship is often found in either stress such as drought or non-stress conditions (García-Palacios et al., 2018; Pennekamp et al., 2018; Tilman and Downing, 1994). The insurance hypothesis proposes that diverse plant communities have a higher chance to contain species that are well adapted to a certain environment (Pommerening et al., 2021; Yachi and Loreau, 1999). Besides, the performance of a positive biodiversity effect on biomass production is known as overyielding effect that is usually explained by complementarity effect and/or sampling effect (Loreau and Hector, 2001; Turnbull et al., 2013). Recently, an increasing number of studies have focused on multiple ecosystem functions or multifunctionality instead of a single function (Garland et al., 2021; Maestre et al., 2012; van der Plas, 2019). The responses of multiple ecosystem functions to the changes of environmental conditions sometimes differ, increase/decrease simultaneously or trade off with one another (Meyer et al., 2018; Zavaleta et al., 2010). Therefore, it is essential to explore the relationships of pairwise ecosystem functions and the mechanisms by which each ecosystem function responds to changing environmental conditions for the better conservation of degraded ecosystems (Gamfeldt et al., 2008).
The effects of plant diversity on ecosystem functions are largely driven by variation in plant species composition and functional traits. Plant functional traits are the plant features (morphological, physiological, phenological) that represent ecological strategies and determine how plants respond to environmental factors and affect ecosystem properties (Pérez-Harguindeguy et al., 2013). Moreover, community functional structure, particularly community-weighted mean trait values (CWM) and functional dispersion (FDis), can not only explain the variation in ecosystem functions but also be affected by environmental conditions through limiting similarity and habitat filtering (Abrams, 1983; Keddy, 1992). Therefore, a functional-based response-effect framework has been proposed to explore the ecosystem consequences of changes in biodiversity in response to ongoing global environmental changes (Díaz et al., 2007). The influential ‘mass ratio hypothesis’ (Grime, 1998) considers that the dominant trait values in the community (often calculated as CWM) largely determine the effects of plant communities on ecosystem functions. The niche complementarity hypothesis (Díaz et al., 2007; Grime, 1998) considers that high FDis may reflect an increase in complementarity of resource use between species, thus improving ecosystem functions.
Grassland is one of the major biomes where the BEF studies have been well-conducted (van der Plas, 2019), and it provides multiple ecosystem services such as livestock production. However, during the past decades, shrub encroachment has widely happened in grasslands across the world due to global climatic changes such as drought and over-grazing, resulting in considerable differences in species composition and landscape compared with non-shrub encroached grasslands (Eldridge et al., 2011). It is generally believed that the dominant shrubs in shrub encroached grasslands would not disappear without significant energy input (Friedel, 1991). In the semiarid steppe of Inner Mongolia, China, C. microphylla, a legume shrub, can expand into degrading grasslands and become the dominant species in some areas due to its strong root system that can better adapt to arid environments and disturbances (Peng et al., 2013). As a N-fixing species, C. microphylla can significantly influence the soil N cycle (Zhang et al., 2018). Furthermore, a trade-off relationship of biomass between C. microphylla and its neighbors has been found, and the inhabitation of neighbors on C. microphylla seedling increases with the increasing neighbor richness (Dong et al., 2020). To our knowledge, few studies have focused on the relationships of BEF in shrub-encroached grasslands facing ongoing climatic changes, even less on the mechanisms accordingly. Ignoring such information would restrict our ability to predict the ecological consequences of global climatic changes in shrub-encroached grasslands, in turn, to better manage shrub-encroached ecosystems that the majority of pastoralism depends on.
In this study, we carried out a soil moisture × species richness microcosm experiment with C. microphylla as the dominant and common species in every microcosm (See Dong et al., 2020) to explore how soil moisture, plant species richness affected multiple ecosystem functions (soil C-, N-, P cycles as well as community biomass) directly or indirectly by regulating community functional structure, and estimated the potential mechanisms by which neighbor species richness affecteed biomass production. Specifically, we proposed three hypotheses. First, we expected that a complementarity effect would mainly contribute to the overyielding of neighbor species richness on biomass because there would be a great niche difference among a diverse plant community (Loreau and Hector, 2001). Secondly, we expected that species richness would positively affect FDis that would show significant effects on ecosystem functions because the increase of species richness usually promotes the complementarity of species spatial and temporal resources utilization (Díaz et al., 2007; Grime, 1998). Thirdly, we expected that soil moisture would affect ecosystem functions directly or indirectly via regulating CWM because the changes of soil conditions could affect the values of plant functional traits and the relative abundance of species with specific traits (Valencia et al., 2015; Wang et al., 2019).
Section snippets
Study materials
C. microphylla, a N-fixing and drought-tolerant shrub, was used as the dominant and common species for each microcosm. Thirteen native grassland species in the semi-arid Inner Mongolia steppe (43°38′N, 116°42′E) were collected as neighbor species, including six perennial grasses (Achnatherum sibiricum, Agropyron cristatum, Cleistogenes squarrosa, Koeleria macrantha, Leymus chinensis, Stipa grandis), five perennial forbs (Allium ramosum, Anemarrhena asphodeloides, Potentilla acaulis, P. bifurca,
Effects of soil moisture and species richness on community functional structure and ecosystem functions
The effect of the interaction between soil moisture and species richness was significant on CWMPC1 (Table 1). Soil moisture positively affected CWMPC1 and FDis; while species richness negatively affected CWMPC1 but positively affected FDis (Table 1; Fig. 1).
The effects of the interaction between soil moisture and species richness were significant on all the soil functions observed in this study (Table 1). Soil moisture significantly and positively affected total shoot biomass (Table 1; Fig. 2a
Soil moisture and specie richness interactively affected ecosystem functions
Our findings that soil moisture and species richness interactively affected soil functions (Table 1; Fig. 1) are consistent with majority of studies (Reich et al., 2001; Soliveres et al., 2014). For example, in a study on natural woody dryland, Soliveres et al. (2014) have found that the relationship between species richness and ecosystem multifunctionality shifts with drought conditions. Furthermore, these soil functions except soil P cycle were not different between soil moisture conditions
Conclusion
Our study provides strong empirical support that soil moisture and species richness interactively affect ecosystem functions and that the effects of soil moisture/species richness on ecosystem functions are direct or indirect via regulating community functional structure. Interestingly, the overyielding effects of neighbor species richness are common either under the control condition or under the drought condition, and are independent of the presence of C. microphylla seedlings (Yang et al.,
CRediT authorship contribution statement
Yujuan Xu and Ke Dong conceived the ideas and designed the study; Yujuan Xu, Ke Dong, Man Jiang, Yulin Liu and Luoyang He compiled the data; Nianxi Zhao, Yujuan Xu and Jinlong Wang analyzed the data; Nianxi Zhao, Yujuan Xu and Yubao Gao wrote the paper. All authors commented on the manuscript.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was supported by the National Natural Science Foundation of China (32171522, 31770505). All authors have approved this manuscript, and declared no competing interests.
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Xu Yujuan and Dong Ke should be considered joint first author.