Biochar and Nitrogen Fertilizer Promote Alfalfa Yield by Regulating Root Development, Osmoregulatory Substances and Improve Soil Physicochemical Properties

In artificial grassland systems, the extensive use of inorganic nitrogen (N) fertilizers has greatly enhanced grassland yields but also caused significant environmental issues. The combined use of biochar and N fertilizer is recognized as an effective and sustainable approach to reducing environmental risks while boosting crop production. However, the specific impacts of biochar and N on alfalfa yield, soil properties, and root morphology remain unclear. This study examined the effects of three biochar application rates (0, 10, 20 t hm−2) and four N application levels (0, 47, 94, 188 kg N hm−2 yr−1) on alfalfa growth and soil characteristics. Results revealed that biochar notably promoted root development and increased osmoregulatory substance content. It enhanced root biomass by improving root nodule count, root neck bud formation, and root neck diameter, while N application reduced root nodule numbers. Biochar and N application reduced soil bulk density by 0.8–10.5%, with biochar further increasing available phosphorus and potassium levels. Additionally, their combined use significantly elevated soil nitrate and ammonium concentrations. Overall, the synergy of biochar and nitrogen application enhances alfalfa yield by fostering better root growth and improving soil fertility.