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Research article - Peer-reviewed, 2021

Decadal nitrogen addition alters chemical composition of soil organic matter in a boreal forest

Hasegawa, Shun; Marshall, John; Sparrman, Tobias; Nasholm, Torgny

Abstract

Boreal forests store approximately 470 Pg of carbon (C) in the soil, and rates of soil C accumulation are significantly enhanced by long-term nitrogen (N) enrichment. Dissecting the compositional profile of soils could help better understand the potential mechanisms driving changes in C cycling under enriched N conditions.We examined the impacts of long-term N addition on the chemical composition of soil organic matter (SOM) in a mature boreal forest. Two large experimental plots (15 ha each) were established: a control and a fertilised plot. The latter received NH4NO3 fertilizer at an average rate of 75 kg N ha(-1) year for 12 years. While the centre of this plot received the prescribed amounts of fertilizer, the year-to-year variation in distribution of fertilizer around the designated edges of the plot created a gradient in N-loading. Along this gradient, a compositional shift in SOM in the organic horizon was assessed using two methods: pyrolysis-gas chromatography/mass spectrometry (GC/MS) and solid-state C-13 nuclear magnetic resonance spectroscopy (C-13 NMR).Both of these methods revealed that the chemical composition of SOM changed with increasing N loading, with an increased fraction of lignin derivatives (i.e., aromatic, methoxy/N-alkyl C) relative to that of carbohydrate (i.e., 0-alkyl C), accompanied by increased soil C mass (kg m(-2)) at the fertilised plot. Also, the relative abundance of N compounds in the pyrolysis products increased with the N loading, mainly due to increased methyl N-acetyl-alpha(D)-glucosaminide in the F/H horizon, plausibly of microbial origin. Microbial N processing likely contributed to soil accumulation of fertilizer-derived N.Our results corroborate the hypothesis that addition of inorganic N suppresses enzymatic white-rot decomposition relative to non-enzymatic brown-rot oxidation. Taken together, our study suggests that N enrichment leads to a selective accumulation of lignin-derived compounds and points to a key role of such compounds for N-induced SOM accumulation.

Keywords

Boreal forest; Soil organic matter; Nitrogen enrichment; Carbon accumulation; Pyrolysis; NMR

Published in

Geoderma
2021, volume: 386, article number: 114906

Authors' information

Swedish University of Agricultural Sciences, Department of Forest Ecology and Management
Marshall, John D (Marshall, John)
Swedish University of Agricultural Sciences, Department of Forest Ecology and Management
Sparrman, Tobias
Umea University
Swedish University of Agricultural Sciences, Department of Forest Ecology and Management

UKÄ Subject classification

Soil Science

Publication Identifiers

DOI: https://doi.org/10.1016/j.geoderma.2020.114906

URI (permanent link to this page)

https://res.slu.se/id/publ/109534