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Aims: Initial substrate chemical characteristics are the most important factor in the regulation of fine root decomposition. However, it remains unclear how nitrogen (N) deposition changes the decomposition process by affecting initial substrate chemical characteristics with different fine root diameter sizes. Methods: We compared the root decomposition processes across three diameter sizes (very fine roots, < 0.5 mm; intermediate fine roots, 0.5–1.0 mm; largest fine roots, 1.0–2.0 mm) of Pinus tabulaeformis treated with N addition (control, low, medium, high N are 0, 3, 6, and 9 g N m−2 y−1 respectively) for two years. Results: (1) The root decomposition rates, which were mainly determined by initial N, phosphorus (P), cellulose and lignin concentrations, and carbon (C)/N and lignin/N ratios, increased with the root diameters. (2) The effect of N addition on fine root decomposition rate was not significant (P > 0.05), but low N addition enhanced the correlation coefficients between initial chemical indexes and decomposition rates. (3) Low N addition increased the release rates of C and cellulose in the very fine roots but not intermediate fine and largest fine roots, while the medium and high N addition decreased the release rates of N, P, cellulose and lignin in the very fine and intermediate fine roots by affecting the initial C, N, P, starch, cellulose and lignin concentrations. (4) Release of compounds from large diameter fine roots is less responsive to N addition than that from the small ones. Conclusions: The initial substrate chemistry plays an important role during the N addition affecting fine root decomposition and release of chemical compounds. Our results suggest that N deposition may change the biogeochemical processes of forest ecosystems by affecting the release of compounds from fine roots. [ABSTRACT FROM AUTHOR]

Background and aims: Global nitrogen deposition alters grassland ecosystems. Whether added nitrogen changes root production and turnover by root orders is unclear. Methods: We compared the root dynamics across four root orders of Bothriochloa ischaemum treated with nitrogen addition (0-10 g N m year). Results: The higher order roots exhibited lower production, turnover, number, length, and biomass, indicating a hierarchical system of B. ischaemum. At whole root system level, nitrogen addition increased length production, biomass production and turnover. At root order level, nitrogen addition increased length production, biomass production, and turnover of the first two order roots but not of the third- and fourth-order roots. Nitrogen addition reduced root biomass, and the belowground to aboveground biomass ratio, supporting the functional equilibrium hypothesis. The increased root production, turnover and decreased root number, length and biomass were mainly attributed to the increasing ammonium and nitrate nitrogen. Conclusions: Nitrogen addition increased the length production (7-30%), biomass production (10-34%) and turnover (8-35%) of the first two order roots but not of the higher order roots compared with the control pots. The discrepancy in root characteristics and their responses to nitrogen availability among root orders should be considered in establishing root dynamic models . [ABSTRACT FROM AUTHOR]