Biochar and its combination with nitrogen fertilization altered soil organic matter, humic substances, and soil structure: Short-term vs. long-term changes

Biochar (B), as well as its combination with nitrogen (N) fertilization, can influence soil quality and fertility. Humus formation and aggregation is a long-term process in soils and the impact of combined biochar-N fertilization on its formation remains underexplored for long term studies. The aim of this study was to determine the extent to which combined biochar-N fertilization on the soil organic matter (SOM) content, quality of humic substances (HS), and soil structure. We also aimed at quantifying changes in the relationships between HS and soil structure. Silty loam Haplic Luvisol was sampled from the field after 1- and 9- years from the incorporation of biochar (0, 10, and 20 t ha ^− 1 of biochar marked as B0, B10, B20) combined with N fertilization (N0, N1, and N2). The results showed that B + N fertilization moderately increased the soil organic carbon (Corg) content in the soil after 1 year of incorporation. After 9 years, the Corg content in the soil was relatively balanced among the treatments. Only in B20N2 did the HS content significantly increase compared to B0N0. In B20N2, the content of microaggregates significantly decreased compared to B0N0 after 9 years. Significant changes in correlations between SOM, HS, and aggregate size fractions suggest potential shifts in their relationships over the decade. The gradual strengthening and changes in the intensity of positive or negative relationships between them suggests the aging of biochar may have long term effects on crop productivity and soil health.