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

Elevated nitrogen (N) deposition variously affects the soil carbon (C) cycle. It is not clear how deposited NI-It 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 40 kg N ha(-1) yr(-1). Solid-state C-13 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 (<1.70 g cm-3 light fraction and>1.70 g cm(-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 C N cycle coupling. (C) 2017 Elsevier B.V. All rights reserved.