Soil acidification threatens N availability, but the effects of maize straw and biochar on organic N fractions in acidic black soil are unclear. We examined the effects of straw (1.5%, 3%) and biochar (1%, 2%) with fertilizer on soil acidity, N fractions, and enzyme activities in a pot experiment with soybean. All amendments raised soil pH, but total exchangeable acidity decreased only at high doses (S3, B2). High-dose straw (S3) increased amino acid N by 21.74% and decreased amino sugar N by 33.33%, with urease (+30.1%) and invertase (+29.8%), indicating accelerated organic N mineralization. For inorganic N, S3 raised NH₄⁺-N by 47.0%, while B2 raised NO₃⁻-N by 30.4%, but low doses (S1.5, B1) reduced inorganic N. B2 increased total N (+5.6%) and total acid-hydrolysable N (+2.9%), with unknown N up by 11.9%, but suppressed catalase and dehydrogenase, suggesting N stabilization. Amino acid N correlated positively with available N and invertase, while amino sugar N correlated negatively with available N. Thus, straw promotes labile N, and biochar stabilizes N; high doses are recommended for acidic black soil fertility.