The biogeochemical consequences of litter transformation by insect herbivory in the Subarctic: a microcosm simulation experiment

Jeppe A Kristensen,Daniel B Metcalfe,Johannes Rousk

doi:10.1007/s10533-018-0448-8

Jeppe A Kristensen, Daniel B Metcalfe + Show 1 more

Open Access

https://doi.org/10.1007/s10533-018-0448-8

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Journal: Biogeochemistry	Publication Date: May 1, 2018
Citations: 19	License type: open-access

Affiliation: Lund University, Umeå University

Abstract

Warming may increase the extent and intensity of insect defoliations within Arctic ecosystems. A thorough understanding of the implications of this for litter decomposition is essential to make predictions of soil-atmosphere carbon (C) feedbacks. Soil nitrogen (N) and C cycles naturally are interlinked, but we lack a detailed understanding of how insect herbivores impact these cycles. In a laboratory microcosm study, we investigated the growth responses of heterotrophic soil fungi and bacteria as well as C and N mineralisation to simulated defoliator outbreaks (frass addition), long-term increased insect herbivory (litter addition at higher background N-level) and non-outbreak conditions (litter addition only) in soils from a Subarctic birch forest. Larger amounts of the added organic matter were mineralised in the outbreak simulations compared to a normal year; yet, the fungal and bacterial growth rates and biomass were not significantly different. In the simulation of long-term increased herbivory, less litter C was respired per unit mineralised N (C:N of mineralisation decreased to 20 ± 1 from 38 ± 3 for pure litter), which suggests a directed microbial mining for N-rich substrates. This was accompanied by higher fungal dominance relative to bacteria and lower total microbial biomass. In conclusion, while a higher fraction of foliar C will be respired by insects and microbes during outbreak years, predicted long-term increases in herbivory linked to climate change may facilitate soil C-accumulation, as less foliar C is respired per unit mineralised N. Further work elucidating animal-plant-soil interactions is needed to improve model predictions of C-sink capacity in high latitude forest ecosystems.

Highlights

High latitude ecosystems contain substantial amounts of terrestrial soil C (Tarnocai et al 2009), but great uncertainties still remain about how the soil processes governing this C-storage will respond to environmental changes (Conant et al 2011; Sistla et al 2013; Crowther et al 2016)
We investigated how insect herbivore transformation of litter affects its use by the soil microbial community, and its subsequent impact on soil biogeochemistry and bioavailability, in a Subarctic birch forest soil
Notable exceptions were the inorganic ammonium content, which was an order of magnitude higher at the higher site compared to the lower (F2,3 = 21.74, p \ 0.01)

Summary

Introduction

High latitude ecosystems contain substantial amounts of terrestrial soil C (Tarnocai et al 2009), but great uncertainties still remain about how the soil processes governing this C-storage will respond to environmental changes (Conant et al 2011; Sistla et al 2013; Crowther et al 2016). Much of the Arctic is dominated by low productivity ecosystems, but Scandinavian mountain birch forests constitute a relatively productive ecosystem storing substantial amounts of C (Sjogersten and Wookey 2009) Despite their potentially important role in high-latitude C-storage, relatively little research has focused on the biogeochemistry of these systems compared to other Arctic ecosystem types. Outbreaks by Geometrid moths (primarily Eppirita autumnata and Operophtera brumata), are the largest natural disturbance in the Nordic mountain birch forests (Tenow et al 2005, 2013; Bjerke et al 2014; Olsson et al 2017) During these outbreaks, insect deposits (including excreta/frass, cadavers, moults, pupae, greenfall) constitute considerable nutrient fluxes from the canopy to the soil, with the largest single constituent being frass (Hunter 2001; Arnold et al 2016). Understanding how these outbreaks influence element cycling and plant growth is essential for improving predictions of C and N cycling (Throop et al 2004; Metcalfe et al 2016) and plant community composition (Jepsen et al 2013) across the Scandinavian Arctic, as the extent and intensity of outbreaks appear to be increasing due to climate warming (Jepsen et al 2008; Kozlov and Zvereva 2017)

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