Contrasting effects of NH4+ and NO3− amendments on amount and chemical characteristics of different density organic matter fractions in a boreal forest soil

Publication date: 1 May 2017
Source:Geoderma, Volume 293
Author(s): Shulan Cheng, Shun He, Huajun Fang, Jiangzhou Xia, Jing Tian, Guirui Yu, Jing Geng, Guangxia Yu
Elevated nitrogen (N) deposition variously affects the soil carbon (C) cycle. It is not clear how deposited NH4 ⁺ and NO3 ⁻ divergently affect the amount and stability of soil organic C (SOC) in the N-limiting forests. A multi-form N addition experiment was conducted in a boreal forest in the Great Khingan mountain in 2010. Three fertilizers, NH4Cl, KNO3 and NH4NO3, were applied at four rates of 0, 10, 20, and 40kgNha⁻¹ yr⁻¹. Solid-state ¹³C nuclear magnetic resonance spectroscopy and elemental analysis were used to determine the chemical structure and C contents in bulk soils and/or two density fractions (<1.70gcm⁻³ light fraction and >1.70gcm⁻³ heavy fraction). NH4Cl addition significantly decreased the SOC contents in the organic layer by 28.35% to 54.23%, but KNO3 addition significantly increased the SOC contents in the organic layer and mineral layer by 26.53% and 87.44%, respectively. Nitrogen addition rates significantly influenced the degradability (alkyl-C/O-alkyl-C) and hydrophobicity ((alkyl C+aromatic C)/(O-alkyl C+carboxyl C)) of light and heavy fractions, whereas N addition forms only impacted their aromaticity (aromatic C/(aromatic C+alkyl C+O-alkyl C)). The movement between residue-C and stable humin fraction, as well as the chemical stability of SOM could profoundly affect the storage of SOC under N enrichment. Overall, added NH4 ⁺ and NO3 ⁻ differently affect C sequestration in the N-limiting forest soils. This should be differentiated in the biogeochemical models of CN cycle coupling.