The effects of long-term nitrogen addition on the composition and sequestration of SOM in a boreal forest

Dr Shun Hasegawa1, Professor John Marshall1, Professor Torgny Näsholm1

1Swedish University Of Agricultural Sciences, Umeå, Sweden

Boreal forests are responsible for large terrestrial carbon (C) stores. They are typically nitrogen (N)-limited, such that the intense use of fertilisers for forest management in this biome has drawn great attention to the long-term impacts of N additions on biogeochemical processes, especially, decomposition and sequestration of SOMs. We investigated the impacts of N addition on SOMs both qualitatively and quantitatively in a mature Scots pine forest located in Northern Sweden. Two experimental plots were established: reference and fertilised plots. The latter has received the total amount of 950 kg N/ha over the past 13 years (c. 50-100 kg N/ha/yr), resulting in an N gradient in the soil adjacent to this plot. We established soil sampling transects along the gradient and assessed the relationships between the N level and SOMs.

Soils were collected from the litter and humus layers and assessed for 1) the composition of C compounds using solid-state carbon-13 nuclear magnetic resonance (NMR) spectroscopy and pyrolysis-GC/MS and 2) total C mass. NMR demonstrated decreases in O-alkyl relative to N-alkyl/methoxyl C with the N levels. These compounds were derived from carbohydrate and lignin components, respectively. This shift in C compounds was consistent with pyrolysis-GC/MS showing that carbohydrate: lignin ratios were negatively correlated with N. The total C mass was 2.09±0.43 and 1.7±0.30 kg C/m2 (Mea±95% confidence interval) in the fertilised and reference plots, respectively. This treatment difference corresponded to C sequestration of 30 g C/m2/yr. Furthermore, C mass in humus was positively related to the N level. Thus, our study suggests that an N addition in this pine forest alters the composition of C compounds by decreasing carbohydrate-derived compounds relative to lignin and also may increase C sequestration in the organic layer. Our results may help us to disentangle the potential mechanisms of C decomposition/sequestration in N-limited boreal forests.

I am a postdoc researcher at Swedish University of Agricultural Sciences with a broad research interest in the impacts of climate and environmental changes on terrestrial biogeochemistry and plant-soil interactions. My current work investigates the effects of nitrogen (N) addition on the molecular composition of soil organic matters (SOMs) in boreal forests with an aim of discerning the mechanisms determining the balance between decomposition and accumulation of SOMs under the elevated N world. In this study, a whole molecular picture of SOMs is meticulously developed using two techniques: Nuclear magnetic resonance (NMR) and Pyrolysis GC-MS.

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