Soil water and nutrient status are both of major importance for plant appearance and growth performance.The objective of this study was to understand the effect of biochar (1.5％) and a biochar-compost mixture (1.5％ biochar + 1.5％ compost) on the performance of Phragmites karka plants grown on a synthetic nutrient-poor sandy clay soil (50％ sand,30％ clay,and 20％ gravel).Indicators of plant performance,such as growth,lignocellulosic biomass,water status (leaf water potential,osmotic potential,and turgor potential),mineral nutrition status,leaf gas exchange,and chlorophyll fluorescence,and soil respiration (carbon dioxide (CO2) flux) were assessed under greenhouse conditions.Biochar-treated plants had higher growth rates and lignocellulosic biomass production than control plants with no biochar and no compost.There was also a significant increase in soil respiration in the treatments with biochar,which stimulated microbial interactions.The increase in soil water-holding capacity after biochar amendment caused significant improvements in plant water status and plant ion (K+,Mg2+,and Ca2+) contents,leading to an increase in net photosynthesis and a higher energy-use efficiency of photosystem II.Biochar-treated plants had lower oxidative stress,increased water-use efficiency,and decreased soil respiration,and the biochar-compost mixture resulted in even greater improvements in growth,leaf turgor potential,photosynthesis,nutrient content,and soil gas exchange.Our results suggest that biochar and compost promote plant growth with respect to nutrient uptake,water balance,and photosynthetic system efficiency.In summary,both the soil amendments studied could increase opportunities for P.karka to sequester CO2 and produce more fodder bio-active compounds and biomass for bio-energy on nutrient-poor degraded soils.