Effects of biochar application on root traits: a meta-analysis

Roots are the interfaces between biochar particles and growing plants. Biochar application may alter root growth and traits and thereby affect plant performance. However, a comprehensive understanding of the effects of biochar on root traits is lacking. We conducted a meta-analysis with 2108 paired observations from 136 articles to evaluate the responses of root traits associated with 13 variables under biochar application. Overall, biochar application increased root biomass (+32%), root volume (+29%) and surface area (39%). The biochar-induced increases in root length (+52%) and number of root tips (+17%) were much larger than the increase in root diameter (+9.9%); this result suggests that biochar application benefits root morphological development to alleviate plant nutrient and water deficiency rather than to maximize biomass accumulation. Biochar application did not change root N concentration but significantly increased root P concentration (+22%), particularly when combined with N fertilization. Biochar application also affected root-associated microbes and significantly increased the number of root nodules (+25%). The responses of root traits to biochar application were generally greater in annual plants than in perennial plants and were affected by soil texture and pH values. Moreover, it appears that biochar production process (pyrolysis temperature and time) plays a more important role in regulating root growth than does biochar source. Together, findings obtained from this meta-analysis may have significant implications for the future sustainable development of biochar management to improve plant growth and functioning.