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

- Reduced NH4+ and oxidized NO3− have contrasting effects on the SOC content.
- Nitrogen addition rates and forms differently influenced the stability of SOM fractions
- Ammonium-N fertilizer addition moves more residue into humin relative to nitrate-N fertilizer.
- Chemical stability of SOM largely dominates the storage of SOC under N enrichment.

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 40 kg N ha− 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 (< 1.70 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 CN cycle coupling.