4.2. Modification of root traits
To date, the effects of biochar application on the root traits remains controversial and highly variable. Root biomass may increase, decrease or remain relatively stable under biochar application (Prendergast‑Miller et al., 2014; Xiang et al., 2017; Van De Voorde et al., 2014). In the current study, root biomass was increased after biochar amendment application. According to the optimal partitioning theory, plants allocate their biomass to the organ that has the most limited resources (Bonifas et al., 2005; McCarthy and Enquist, 2007). The expression of such optimization is the change in allocation between the biomasses of shoots and roots in response to the nutrient availability. When the water supply or nutrient availability increases, plants allocate less mass to their roots because less effort is required to acquire these resources (Bonifas et al., 2005; Głąb et al., 2014). However, this study did not confirm this theory. The straw and biochar amendments increased the RLD and RDMD. There were also other reports with similar relationships. According to a meta-analysis by Xiang et al. (2017), biochar application significantly increased the RDMD, RVD, RSA, MRD, RLD, and the number of root tips. This relationship was confirmed for annual crops (De Giorgio and Fornaro, 2012) and perennial plants (Fageria and Moreira, 2011; Głąb, 2013). The RSR was not significantly altered by the biochar application, while the SRL was significantly enhanced. These beneficial effects were higher in the annual plants than in the perennial plants. In the current research, the differences for most root morphometric parameters were noticed mainly between the control, without fertilizer and any amendments, and treatments where the straw or biochar were applied. However, there were no differences between the particular treatments where wheat and miscanthus straw and biochar were applied.
In the present study, the relationship between the MRD and the plant yields was significant. This confirmed the thesis that the MRD treatment was beneficial for belowground biomass accumulation, whereas the RLD and RDMD are usually assumed to be proportional to the water or nutrient acquisition. Since grasses have a greater RDMD, they are effective competitors for nutrients, potentially decreasing the proportion of legumes in grass-legume mixtures under nutrient-limiting conditions (Oram et al., 2014). On the other hand, Kulmatiski et al. (2017) suggested that root systems are foraged independently for different resources. The RSR parameter was found to be a very useful parameter for characterizing the plant reactions to the applied soil amendments. Because of the variations in the root and shoot growth due to various soil and climatic conditions among the regions and during different periods of crop growth, the RSR can vary over a wide range; for perennial grasses, it ranges from 0.18‑6.25 (Sainju et al., 2017). In this study, the RSR ranged from 0.61 to 0.98. Lower values of RSR resulted from the predominant percentage of the grass species in the botanical composition.