Nitrogen deposition impacts on the amount and stability of soil organic matter in an alpine meadow ecosystem depend on the form and rate of applied nitrogen

Summary The effects of atmospheric nitrogen (N) deposition on carbon (C) sequestration in terrestrial ecosystems are controversial. Therefore, it is important to evaluate accurately the effects of applied N levels and forms on the amount and stability of soil organic carbon ( SOC ) in terrestrial ecosystems. In this study, a multi‐form, small‐input N addition experiment was conducted at the Haibei Alpine Meadow Ecosystem Research Station from 2007 to 2011. Three N fertilizers, NH 4 Cl , ( NH 4 ) 2 SO 4 and KNO 3 , were applied at four rates: 0, 10, 20 and 40 kg N ha −1 year −1 . One hundred and eight soil samples were collected at 10‐cm intervals to a depth of 30 cm in 2011. Contents and δ 13 C values of bulk SOC were measured, as well as three particle‐size fractions: macroparticulate organic C ( MacroPOC , > 250 µm), microparticulate organic C ( MicroPOC , 53–250 µm) and mineral‐associated organic C ( MAOC , < 53 µm). The results show that 5 years of N addition changed SOC contents, δ 13 C values of the bulk soils and various particle‐size fractions in the surface 10‐cm layer, and that they were dependent on the amounts and forms of N application. Ammonium‐N addition had more significant effects on SOC content than nitrate‐N addition. For the entire soil profile, small additions of N increased SOC stock by 4.5% (0.43 kg C m −2 ), while medium and large inputs of N decreased SOC stock by 5.4% (0.52 kg C m −2 ) and 8.8% (0.85 kg C m −2 ), respectively. The critical load of N deposition appears to be about 20 kg N ha −1 year −1 . The newly formed C in the small‐input N treatment remained mostly in the > 250 µm soil MacroPOC , and the C lost in the medium or large N treatments was from the > 53 µm POC fraction. Five years of ammonium‐N addition increased significantly the surface soil POC : MAOC ratio and increased the instability of soil organic matter ( SOM ). These results suggest that exogenous N input within the critical load level will benefit C sequestration in the alpine meadow soils on the Qinghai–Tibetan Plateau over the short term.