Introduction
The Stipa steppe is one of the most common steppe types continuously distributed in the Eurasian grassland (Kang et al. 2007; Lv et al. 2019b). Most Stipa species are xerophytes and the dominant species in the arid and semiarid grassland of Inner Mongolia (Hu et al. 2014; Hu et al. 2013; Qi et al. 2010), while S. breviflorapopulations are more widely distributed than other Stipa species (Zhang et al. 2012). This species has a wider range of ecological adaptations to diverse environments as compared to other Stipaspecies (Kang et al. 2007; Zhang et al. 2012). S. breviflora is not only one of the dominant species in the desert steppe of China (Kang et al. 2007), but also is a codominant species with S. bungeanaand S. krylovii in typical steppe (Kang et al. 2007; Zhang et al. 2012).
During the long evolutionary process of plants adapting to heterogenous environments, root systems of terrestrial plants have a functional importance in resource acquisition and storage (Doussan et al. 2003). In the arid or semiarid steppe ecosystems, water deficit is a severe environmental constraint to plant growth and distribution (Buxbaum and Vanderbilt 2007; Knapp et al. 2017). Reducing water loss by limiting transpiration rate and increasing water uptake with improved roots extension and proliferation are two main mechanisms for plants to adapt to drought environments (Farooq et al. 2009). A structured root system can allow plants to maintain a balance level of water status, which can ensure optimum shoot growth and development (Sánchez-Blanco et al. 2014). Since roots are the only organ to acquire water from soil, therefore, they are the main organs to respond, perceive and maintain the plant growth under drought stress (Wasaya et al. 2018). In general, the morphological variations of root system, its length, density, and proliferation can directly reflect responses of plants to drought stress (de Vries et al. 2016; Wasaya et al. 2018; Zhou et al. 2018). Normally, plants with deeper root systems, longer root length, larger root surface area and more lateral roots can extract and absorb more water from soil (Fitz Gerald et al. 2006; Sánchez-Blanco et al. 2014; Wasaya et al. 2018). Under drought stress environment, plants would allocate more biomass to roots than shoots, enhancing water uptake and decreasing water loss synchronously (de Vries et al. 2016; Zhou et al. 2018). During this procedure of root biomass accumulation under drought condition, it is a result of storing more carbohydrates in root systems and ensure the plants regeneration after drought stress (Farooq et al. 2009). However, it is observed from previous studies that plants would choose to reduce C demand for reducing root elongating rate or the number of lateral roots (Bengough et al. 2011), and displayed a resource conservation strategy with low specific root length (SRL) and high root tissue density (RTD) under drought stress (Chapin III et al. 1993; de Vries et al. 2016; Pérez-Ramos et al. 2013). Simply, water is a key factor limiting root growth, while it is difficult to explain the wide distribution of S. breviflora in arid and semi-arid areas by studying root growth strategies solely under water stress. In fact, plant growth is always influenced by the synergy of water and soil nutrients under drought condition (Farooq et al. 2009; Wasaya et al. 2018; Xue et al. 2017). For example, water and nutrient absorption and availability may be limited in plants under drought condition (Stasovski and Peterson 1991; Xue et al. 2017). And an inappropriate water-nutrient application can degrade soil physicochemical properties and influencing plant growth (Farooq et al. 2009).
In semi-arid or arid steppe ecosystems, besides water, nitrogen (N) and phosphorus (P) are two main nutrient elements limiting plant growth (Gong et al. 2011; Vitousek et al. 2010; Yahdjian et al. 2011). In general, there would be strong interaction between water and nutrients for plant growth (Rouphael et al. 2012; Stasovski and Peterson 1991; Wasaya et al. 2018). However, most studies focused on assessing the effect of single stressful condition on plant root growth, it is rarely considered interactions of soil water and nutrients (Hill et al. 2006; Jin et al. 2005; Song et al. 2010; Walch-Liu et al. 2006). Soil water shortage limits the microbial mineralization for organic matter, and then negatively affect N, P availability, uptake and transport, which impact nutrient absorption by plant root growth (Rouphael et al. 2012; Stasovski and Peterson 1991; Wasaya et al. 2018). In this sense, under drought condition, the more severe the drought the lower the water and nutrient flow and the more limited is the availability of nutrients for absorption and transportation by the root system (Farooq et al. 2009; Rouphael et al. 2012). While other studies pointed that when there was sufficient water applied under drought condition, plants are capable of making the best use of soil nutrients, and N or P is easier to be limited factors during plant growth (Gong et al. 2011). However, Likewise, N addition also can improve water availability in root systems for plants, which can increase photosynthetic rate, and then promote plants growth (Zhang and Han 2008). Meanwhile, the root system architecture has been evolved to be responsive and extremely adaptive to the heterogenous soil habitats (Doussan et al. 2003; Wasaya et al. 2018), which greatly affects plants growth and development under drought condition (Farooq et al. 2009; Gong et al. 2011; Rouphael et al. 2012).
S. breviflora as a tussock species, its intact clusters are easily fragmented from stem base independently in nature or under environmental or human interferences, and then form multiple sub-clusters with the identical genetic information to enlarge their populations, this process is defined as cluster fragmentation (Lv et al. 2019a). The cluster fragmentation allows S. breviflora to make full use of patchy resources and thus enhance their tolerance to stressful environments (Wang et al. 2017). Whereas the resource acquisition and efficiency are different based on the size of fragmented sub-clusters. It is generally accepted that the size of sub-cluster is smaller, the fewer nutrients or water needed during the plant growth. Here, we want to confirm (1) whether soil water, N or P is the limiting factor for S. breviflora root growth; (2) how root traits of S. breviflora responds to soil water, N and P additions; and (3) the cluster fragmentation influences the root growth of S. breviflora . During the process of plants adapting to the environment, it is the key to quantify the root traits in response to the treatments of soil water, N, P addition and cluster fragmentation of S. breviflora , and revealed their primary adaptation strategies in arid or semiarid areas.