Objective  This study aims to investigate the response characteristics of soil aggregate distribution and carbon (C) and nitrogen (N) contents under different N addition conditions in order to provide data support for soil C sequestration mechanism in alpine meadow under the background of N deposition.

  Method  In 2014, a long-term N addition platform was established in an alpine meadow on the Qinghai-Tibet Plateau of western China. A completely randomized block design was used to simulate N deposition with six N addition levels: 0 g/(m²·year) (N0, control), 2 g/(m²·year) (N2), 4 g/(m²·year) (N4), 8 g/(m²·year) (N8), 16 g/(m²·year) (N16), 32 g/(m²·year) (N32). Macroaggregates (0.25−2 mm), microaggregates (0.053−0.25 mm), silt and clay (< 0.053 mm) were obtained by wet sieving method, and the organic C and total N contents of soil aggregates were determined.

  Result  The mass proportion of macroaggregates (79%) was significantly higher than that of silt and clay (13%) and microaggregates (8%) (P < 0.05). There was no significant difference in the distribution of soil aggregates under different N addition gradients (P > 0.05), whereas the mean mass diameter of soil aggregates did not change with N addition (P > 0.05), which could attribute to the trade off between the positive effect of N addition on root biomass and the decreased microbial activity. Moreover, N addition decreased the organic C content of macroaggregates and microaggregates, but increased the organic C content of silt and clay (P > 0.05). Compared with the control, nitrogen addition reduced the total nitrogen content of microaggregates and clay particles by 2% and 12%, respectively (P > 0.05). The C/N ratio of all aggregates significantly decreased under N addition (P < 0.05).

  Conclusion  The C/N ratios of different size of soil aggregate decreased, indicating the acceleration of SOC mineralization in alpine meadow under increased N deposition.