Biochar addition affects root morphology and nitrogen uptake capacity in common reed (Phragmites australis)

Nitrogen (N) is a key factor that limits plant growth in most terrestrial ecosystems, and biochar reportedly improves soil characteristics and grain yields. However, the effects of biochar on plant N uptake in wetland ecosystems and the underlying mechanisms of these effects remain unclear. Therefore, our study sought to characterise the effects of biochar addition on Phragmites australis N absorption rates at two different N deposition conditions [30 and 60 kg N hm^-2 yr^-1; i.e., "low" and "high" N treatments, respectively]. Our results demonstrated that biochar significantly promoted root biomass growth in P. australis in the high N treatment group. In contrast, the low N treatment group exhibited an increased proportion of fine roots and a decrease in the average P. australis root diameter. The N absorption rate of P. australis in the low N treatment group significantly increased with biochar addition and ammonium N became the preferred N source. The absorption rates of both ammonium and nitrate N were negatively correlated with the average P. australis root diameter. Therefore, our findings indicate that biochar may affect the N uptake strategy of P. australis by altering root morphogenesis, thereby providing new insights into potential restoration strategies for wetland vegetation.