Si Supply Could Alter N Uptake and Assimilation of Saplings—A 15N Tracer Study of Four Subtropical Species

Si availability may be altered by bamboo expansion when other trees are replaced by bamboo due to the influence of plant communities on the quantity of phytoliths and Si accumulation. It has been shown that Si availability can modify nutrient-use efficiency (e.g., N and P) of some Si-accumulating plants. However, it is unclear how Si availability might alter N uptake and assimilation between Si-accumulating plants such as bamboo compared to other species, particularly for different chemical forms such as ammonium (NH4+) and nitrate (NO3−). To explore the influences of Si availability on uptake and assimilation rates for different forms of inorganic N between bamboo and other trees, we selected one-year-old seedlings of bamboo (Phyllostachys pubescens) and three other native subtropical species, namely Phoebe bournei, Schima superba, and Cunninghamia lanceolata. We applied three levels of Si and 15N tracers in a pot experiment and then measured the concentrations of Si (total Si, soluble Si, and exchangeable Si), C, N (total N, NH4+-N, and NO3−-N), and N uptake and assimilation rates for both roots and leaves. We found that there were higher inorganic N root uptake and assimilation rates for bamboo compared to other species, likely due to higher biomass accumulation and quicker turnover of fine roots. Moreover, Si supply did not change the uptake preference for N forms or overall uptake and assimilation rates in most species; however, a high concentration of the Si supply slightly increased NO3−-N uptake and assimilation rates in fine roots and leaves of P. bournei, particularly immediately following the addition of Si. These results have implications for predicting the coexistence and competition between bamboo and other trees through the uptake and assimilation of different forms of inorganic N (i.e., high Si-accumulating plants compared to other plants), particularly when Si availability is altered in ecosystems.