De-ashed biochar enhances nitrogen retention in manured soil and changes soil microbial dynamics

[Display omitted] •De-ashing improved surface area and intensity of functional groups on biochar.•De-ashed biochar (DB) enhanced N-sorption in manured soil.•DB altered N-cycling enzymes activities to regulate N-retention.•DB increased the abundance of N-cycling16S rRNA genes to regulate N retention. Enhancing soil N retention is important to mitigate the negative environmental impacts caused by the loss of applied N fertilizers. Therefore, the use of modified biochar in the soil is a novel approach to enhance the sorption of N ions. Biochars derived from bamboo, cow, and pig manures were de-ashed and their properties were characterized. The de-ashed biochars were comparatively assessed with raw biochars on N-retention, and how this process is regulated by microbial activities, diversity, and abundance in a manured soil. De-ashing increased the intensity of biochar functional groups and their surface structure. In manured soils, the application of de-ashed bamboo biochar (PDBB) and de-ashed cow manure biochar (PDCB) led to an increase in NH4+-N retention of 34.5% and 6.8% relative to the amendment with raw biochars. Similarly, increased retention of 20.9%, 14.8%, and 19.1% of NO3−-N, and an increase of 21.1%, 11.1% and 11.5% in total N were observed in PDBB, PDCB and de-ashed pig manure biochar (PDPB) respectively, compared to their raw biochars after 8 weeks. The modified surface properties of de-ashed biochars led to higher bacterial colonization, with an increase in the diversity and relative abundance of bacterial 16S rRNA genes, compared to the raw biochar treatments. This spike in microbial groups stimulated protease activities and hence, an increased abundance of N-cycling organisms. The increased microbial abundance was associated with a reduction in the release of urease by these microbial groups for the hydrolysis of urea, thereby ensuring soil N retention. De-ashing increased the intensity of biochar functional groups and improved its surface properties, which resulted in increased N-sorption, and immobilization by stimulating microbial activities and diversity in soil.