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Doctoral thesis, 2016

The effect of simulated anthropogenic nitrogen deposition on the net carbon balance of boreal soils

Maaroufi, Nadia

Abstract

Anthropogenic activities have globally increased nitrogen (N) deposition and carbon dioxide (CO₂) gas emissions. It is proposed that anthropogenic N deposition may increase the size of boreal forest CO₂ sink, because boreal ecosystems are N limited. Despite studies that have helped to clarify the magnitude by which N deposition enhances carbon (C) sequestration in the vegetation, there remains a paucity of studies evaluating how soils respond. This thesis aims to clarify the magnitude to which C sequestration in boreal forests responds to N enrichments, including rates that realistically simulated N deposition (≤ 12.5 kg N ha⁻¹ yr⁻¹). This work was conducted in two long-term experiments in northern Sweden. The N treatments consisted of ambient, low N addition (3-12.5 kg N ha⁻¹ yr⁻¹) and high N addition (50 kg N ha⁻¹ yr⁻¹) rates, in a Norway spruce and a Scots pine forest, maintained since 1996 and 2004, respectively. The organic soil C pool positively responded to N enrichment, especially at the high N addition level. This increase corresponded to a relationship between C sequestration and N addition of 10 kg C kg⁻¹ N. Further, low N addition treatments had no effect on microbial biomass and soil respiration (i.e. soil C outputs, microbial activity), while high N addition decreased total microbial, ectomycorrhizal fungal biomasses and soil respiration. The actinomycetes were the only microbes showing an increase with N addition. Annual litter production showed a minor impact on aboveground litter C inputs. Only mosses were the only major litter component showing significant quantitative and qualitative changes in response to N additions. Further, litter quality mediated by N enrichment was not the main driver of litter decomposition, while shifts in soil microbes strongly influence the early stages of litter decomposition. Low N addition rates had little effect on litter and humus decomposition, whereas high N addition rates impeded the early stage of decomposition of both substrates. The decline of litter decomposition appeared to be mediated by shifts in the abundance or community structure of saprophytic organisms, while the decrease of humus decomposition was likely the result of reduced ectomycorrhizal fungi. Altogether, the results of this thesis show that long-term N inputs simulating current atmospheric N deposition in the boreal region are likely to have subtle effects on the soil C balance and therefore on soil C accumulation.

Keywords

boreal forest; carbon sequestration; soil respiration; litter; humus; PLFA; ectomycorrhizae; ecological stoichiometry; root-exclosure; nitrogen addition

Published in

Acta Universitatis Agriculturae Sueciae
2016, number: 2016:98
ISBN: 978-91-576-8698-5, eISBN: 978-91-576-8699-2
Publisher: Department of Forest Ecology and Management, Swedish University of Agricultural Sciences

Authors' information

Swedish University of Agricultural Sciences, Department of Forest Ecology and Management

UKÄ Subject classification

Ecology
Soil Science
Forest Science

URI (permanent link to this page)

https://res.slu.se/id/publ/77444