The role of biochar in alleviating soil drought stress in urban roadside greenery

Soil structure degradation is a major obstacle to vegetation growth in urban roadside greenery, particularly under drought conditions. Biochar application can improve soil structure and water retention; however, the mechanisms linking changes in soil aggregation with those in pore size distribution, and how they interactively influence plant growth remain unclear. In this study, we investigated the role of biochar in improving soil structure and water retention under drought stress in urban roadside greenery. In a field study, plots (2 m × 2 m) were established on roadside greenery in Suwon, Korea, in which 2.5% wt bochar was mixed with surface soil (<10 cm depth) (BC_field). During the eight-month experiment, drought conditions prolonged, and soil water content was continuously higher in BC_field than in CON_field. For a more mechanistic understanding, a 100-day greenhouse experiment was conducted on Rudbeckia hirta planted in sandy soil, either mixed with 4% wt biochar (BC_greenhouse) or without biochar (CON_greenhouse). Drought conditions were simulated by maintaining soil water content below 40% of the water-holding capacity. In the biochar-added soil, macro-aggregates (250–1000 μm) increased significantly after 60 days, probably due to biochar particles themselves acting as the same-sized aggregates. In addition, biochar can act as a binding agent for forming macro-aggregates, thereby preventing their disintegration into smaller-sized aggregates. Enhanced macro-aggregation in biochar-added soil, therefore, is a potential mechanism for the increased formation of meso-pores. These pores could retain more soil water for plant uptake, eventually increasing plant biomass and water use efficiency in the BC_greenhouse, by 39%, when compared with that in the CON_greenhouse under drought conditions. Our results indicate that biochar application is a potential management strategy for improving soil physical structure in urban roadside greenery, which would, in turn increase plant resistance and resilience to drought stress.