Plant-soil hydraulic interaction and rhizosphere bacterial community under biochar and CO₂ enrichment

The increasing atmospheric CO₂ concentration is a global concern that affects the plant-bacteria-soil system. Previous studies have investigated plant growth and bacteria activity under CO₂ enrichment. However, the effects of coupled elevated CO₂ and biochar amendment on the interactions of soil and medicinal plants are not well understood. This study aims to investigate the medicinal plant-soil hydraulic interactions and rhizosphere bacteria communities under coupled CO₂ enrichment and biochar conditions. Two levels of CO₂ concentration (400, 1000 ppm) and two biochar dosages (3%, 5% by mass) were considered. Pseudostellaria heterophylla was used as the tested medicinal plant. During plant growth, coupled CO₂ enrichment and biochar at 3% and 5% dosage increased the volumetric water content at a matric suction of 33 kPa by 97% and 82% respectively, which indicates enhanced water retention. The transpiration rate of P. heterophylla was slightly reduced by 11-30% with an increase in biochar dosage due to higher total suction, while it was significantly reduced by up to 57% due to CO₂ enrichment. In the rhizosphere of P. heterophylla, elevated CO₂ (1000 ppm) coupled with 3% biochar dramatically increase the relative abundance of Thaumarchaeota, which played an important role in C and N cycles. Moreover, coupled CO₂ enrichment and biochar addition resulted in the highest bacterial richness, while 3% biochar at ambient CO₂ induced the highest bacterial diversity. This study provides a basis for understanding the medicinal plant-bacteria-soil system under CO₂ enrichment and biochar conditions.