Aggregate-size functional differentiation of potential nitrogen turnover in a subtropical Ultisol across Eucalyptus rotation histories

Under intensive short-rotation forestry, nitrogen (N) concentrations and potential transformation rates can differ disproportionately, but their aggregate-scale relationships remain unclear. We compared first- to third-rotation Eucalyptus plantations (G1–G3) in subtropical China, with five independent six-year-old stands per rotation. Organic and inorganic N fractions and their net changes were measured in 30-day incubations of separated aggregates and matched bulk soils from 0 to 20 and 20 to 40 cm. Relative to G1, G3 had 28% and 51% lower total N concentrations, but 81.3% and 72.4% lower measured bulk-soil potential net mineralization in topsoil and subsoil, respectively. Smaller fractions generally had higher inorganic N and acid-hydrolyzable ammonium, amino sugar and amino acid N concentrations, whereas >2 mm aggregates had the highest mean net mineralization and nitrification rates. Descriptive comparisons of group means showed the largest proportional G3–G1 declines in net mineralization and nitrification rates in the >2 mm fraction. Their mean amino acid N depletion rates were 88% and 80% lower, compared with 44% and 67% lower concentrations in topsoil and subsoil, respectively. Of the mass-weighted totals from separated fractions, >2 mm aggregates accounted for 71% and 73% of net mineralization in G1 topsoil and subsoil, respectively, versus 57% and 61% in G3. Mean reconstructed mineralization rates exceeded matched bulk-soil measurements in all groups. These findings show differences in aggregate-scale functional partitioning across rotation histories, with a smaller but persistent majority share of the >2 mm fraction in reconstructed potential net N mineralization.