Effects of nutrient additions on root growth
N and P are the two essential nutrient elements for plant growth, whereas they also are two main limited elements in grassland ecosystem (Gong et al. 2011; Vitousek et al. 2010). Generally, N or P deficiency can limit root branching or elongating (Gruber et al. 2013; Han et al. 2014; López-Bucio et al. 2003; Walch-Liu et al. 2006). Variations of the number of root branching not only influenced total root biomass, total root length and total root surface area, but also the availability of soil nutrients for plants (Paez-Garcia et al. 2015). In general, N and P contents are closely associated with root architectures formation. In our research, we found that S. breviflora ’s roots proliferated with larger root surface area, bigger root volume and more root tips at high–level P addition than that at low–level P addition respectively. This is in agreement with previous reports that low P supply decreased root length and root branching (Han et al. 2014). By contrast, in our study, high–level N addition restrained their root growth with significant lower values of the total root length, root surface area and root tips than those in the condition of low–level N addition for S. breviflora , which was inconsistent with most of former studies (Hodge et al. 1999; Walch-Liu et al. 2006). Hodge (2004) pointed that, whether plants’ roots could proliferate in some nutrient – rich environment, not only depends on the content of nutrient in environments, but also depends on demand for the nutrient by themselves, and they may have a specific threshold of nutrients to induce root morphological traits or biomass changing. For example, though high – level N addition could induce root elongation and branching, sometimes the high internal nitrate/N may inhibit lateral roots growth, and influence the architecture or biomass of root system (Walch-Liu et al. 2006). Therefore, in our research, 120 mg N / kg may have exceeded the threshold value of N addition for promoting the root growth of S. breviflora , which cause its root growth restricted.