Stoichiometric N:P flexibility and mycorrhizal symbiosis favour plant resistance against drought

Summary

Drought induces changes in the nitrogen (N) and phosphorus (P) cycle but most plant species have limited flexibility to take up nutrients under such variable or unbalanced N and P availability. Both the degree of flexibility in plant N:P ratio and of root symbiosis with arbuscular mycorrhizal fungi might control plant resistance to drought‐induced changes in nutrient availability, but this has not been directly tested.

Here, we examined the role of plant N:P stoichiometric status and mycorrhizal symbiosis in the drought‐resistance of dominant and subordinate species in a semi‐natural grassland.

We reduced water availability using rainout shelters (control vs. drought) and measured how plant biomass responded for the dominant and subordinate species. We then selected a dominant (Paspalum dilatatum) and a subordinate species (Cynodon dactylon), for which we investigated the N:P stoichiometric status, mycorrhizal root colonization and water‐use efficiency.

The biomass of all dominant plant species, but not subordinate species, decreased under drought. Drought increased soil available nitrogen, and thus increased soil N:P ratio, due to decreasing plant N uptake. The dominant P. dilatatum showed a high degree of plant N:P homeostasis and a considerable reduction in biomass under drought. At the opposite, the more flexible subordinate species C. dactylon increased its N uptake and water‐use efficiency, apparently due to stronger symbiosis with mycorrhizae, and maintained its biomass.

Synthesis. We conclude that the maintenance of N:P homeostasis in dominant species, possibly because of a large root nutrient foraging capacity, becomes inefficient when water stress limits N mobility in the soil. By contrast, we demonstrate that higher stoichiometric N:P flexibility coupled with stronger mutualistic association with mycorrhizae allow subordinate species to better withstand drought perturbations. Using a stoichiometric approach in a field experiment, our study provides for the first time clear and novel understandings of the mechanisms involved in drought‐resistance within the plant‐mycorrhizae‐soil system.

Introduction

Water availability is a main driver of net primary production and changes in precipitation play an important role in the dynamic and functioning of grassland ecosystems (Ciais et al. 2005; Engler et al. 2011). Thus, extended periods of drought, expected to increase in intensity and severity in many areas of the world (Handmer et al. 2012), are putting the sustainability of forage production and other ecosystem services at stake (Buttler et al. 2012). Beside direct water stress effects on plant survival and growth (Tezara et al. 1999), reduction in soil moisture also affects plant mineral nutrition by reducing nitrogen (N) and phosphorus (P) availability (i.e., through decrease in mineralization rates and mobility) and/or uptake (Lambers, Chapin & Pons 2008; Sanaullah et al. 2012; Sardans & Peñuelas 2012; He & Dijkstra 2014; Jiao et al. 2016). Co‐limitation in N and P is an important factor influencing plant productivity (Harpole et al. 2011) and plants require both nutrients in specific ratios for optimal growth (Sterner & Elser 2002; Cernusak, Winter & Turner 2010). However, most plant species have limited flexibility to take up nutrients under conditions of variable or unbalanced N and P availability (Güsewell 2004; Elser et al. 2007; Dijkstra et al. 2012; Li, Niu & Yu 2016). Therefore, drought‐induced changes in the N and P cycle are likely to affect plant community composition and productivity, especially in nutrient‐limited systems.


Plants can respond in multiple ways to resource limitations that are ultimately reflected by the degree of homeostasis of a species (Hessen et al. 2004). For example, the ability of a plant species to maintain a constant N:P ratio, independently of changes in N:P supply, is known as stoichiometric N:P homeostasis, which varies greatly among species (Sterner & Elser 2002; Güsewell 2004). Plant species with a high degree of stoichiometric N:P homeostasis may be more successful at maintaining plant growth when the availability of nutrients decreases. Such ability has been linked to higher root scavenging and forage functions allowing plants to sustain the metabolism and production of above‐ground biomass against environmental variations (Yu et al. 2010). The degree of stoichiometric N:P homeostasis has therefore been used to predict plant dominance and stability in grassland communities (Yu et al. 2010, 2011). Yu et al. (2010) showed that dominant plant species had higher stoichiometric N:P homeostasis than subordinate species, supporting the idea that stoichiometric homeostasis is an important characteristic determining community structure. Differences in stoichiometric N:P homeostasis between dominant and subordinate species relate to niche differentiation and complementarity for resource use (von Felten et al. 2009). Indeed, dominant species generally control the majority of the resources (i.e., exploitative species) and account for a high proportion of the community biomass, thus driving ecosystem functioning (Grime 1998). By contrast, subordinate species (i.e., conservative species) benefit from the remaining resources and from unexploited ecological niches (Mariotte 2014), relying more on ecophysiological adaptations to resource limitation (Daßler et al. 2008).


According to previous studies, dominant species are expected to better sustain growth under drought as these homeostatic species can maintain a constant N:P ratio, independently of drought‐induced changes in N and P supply, mobility and uptake (Yu et al. 2010, 2011). Among three dominant species, those with higher stoichiometric homeostasis for N responded the least to decreased soil water availability (Yu et al. 2015). Conversely, in recent field experiments it was shown that dominant species are the most sensitive species to water stress while subordinate species can maintain or increase their biomass under drought (Kardol et al. 2010; Mariotte et al. 2013), thus promoting grassland stability (Mariotte et al. 2015, 2016). These findings suggest that higher stoichiometric N:P homeostasis does not necessarily enhance plant resistance against drought, especially when contrasting dominant with subordinate species. On the contrary, higher stoichiometric N:P flexibility (i.e., lower stoichiometric N:P homeostasis) might be advantageous under stress, as the maintenance of stoichiometric homeostasis is energetically expensive for plants (Sterner & Elser 2002), while stoichiometric N:P flexibility allows for opportunistic uptake of nutrients that are in greatest supply.


As previously mentioned, drought generally decreases soil N and P availability and/or mobility. Under such conditions, maintaining a constant N:P ratio is energetically demanding for plants, as nutrient foraging and uptake by roots become challenging (Sardans & Peñuelas 2012). Independently of the degree of stoichiometric N:P homeostasis, investing in mycorrhizal symbiosis might also significantly improve plant nutrient uptake under nutrient limitation. Indeed, fungal hyphae can explore a greater volume of soil than roots and considerably expand the surface area of nutrient absorption and water uptake (Augé 2001; Allen et al. 2003; Khalvati et al. 2005; Rapparini & Peñuelas 2014). Plant carbon (C) is generally in excess when soil resources, such as N and P, limit photosynthesis, and the excess C is therefore exchanged with the arbuscular mycorrhizal (AM) fungi for N and P (Allen et al. 2003; van der Heijden et al. 2015). Interestingly, due to lower competitive abilities for nutrient uptake, subordinate species tend to accumulate more C than dominant species and have been shown to exert stronger associations with AM fungi (van der Heijden et al. 1998; Urcelay & Díaz 2003; Mariotte et al. 2013, 2015). Fungi are known to tolerate water stress (de Vries et al. 2012; Barnard, Osborne & Firestone 2013) and root colonization by AM fungi often increases under drought (Augé 2001), thus benefiting the survival and growth of the host plant (Worchel, Giauque & Kivlin 2013). Previous studies also demonstrated that the degree of mycorrhizal symbiosis is intimately linked to plant water‐use efficiency (WUE; Querejeta et al. 2003; Zhang, Chang & Anyia 2016). WUE is defined as the amount of C acquired per unit of water transpired, reflecting the trade‐off between C gain and water loss. Increasing WUE is one of the main attributes allowing plants to cope with drought, and mycorrhizal plants have been shown to increase their WUE more than non‐mycorrhizal plants, in both trees (Querejeta et al. 2003; Hobbie & Colpaert 2004) and herbaceous species (Al‐Karaki 1998; Omirou, Ioannides & Ehaliotis 2013; Zhao et al. 2015; Zhang, Chang & Anyia 2016). Beneficial associations with AM fungi might explain the resistance of subordinate species to drought but this has not been directly tested.


In this study, we examined the role of plant stoichiometric N:P homeostasis and mycorrhizal symbiosis in the resistance to water stress of dominant and subordinate species in a semi‐natural grassland of Eastern Australia. We manipulated water availability using rainout shelters that continuously excluded half of the precipitation and we measured the plant biomass responses of dominant and subordinate species. We then selected two widely common C4 species, one dominant (Paspalum dilatatum) and one subordinate (Cynodon dactylon), for which we investigated the stoichiometric N:P status, AM fungal colonization and WUE in ambient and drought plots. Based on previous studies, we hypothesized that the dominant species possesses a high degree of N:P homeostasis while the subordinate species is more flexible in its tissue N:P ratio. Our primary objective was to determine if the stoichiometric N:P status could be related to plant drought resistance of the dominant and the subordinate species. We also tested the role of mycorrhizal symbiosis in nutrient uptake and WUE of both species, predicting that subordinate species would benefit from greater AM fungal colonization to better withstand water stress conditions.