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Discussion papers
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.
© Author(s) 2019. This work is distributed under
the Creative Commons Attribution 4.0 License.

Submitted as: original research article 29 Jul 2019

Submitted as: original research article | 29 Jul 2019

Review status
This discussion paper is a preprint. A revision of the manuscript is under review for the journal SOIL (SOIL).

Strong warming of subarctic forest soil deteriorated soil structure via carbon loss – Indications from organic matter fractionation

Christopher Poeplau1, Páll Sigurðsson2, and Bjarni D. Sigurðsson2 Christopher Poeplau et al.
  • 1Thünen Institute of Climate-Smart Agriculture, Bundesallee 68, 38116 Braunschweig, Germany
  • 2Agricultural University of Iceland, Hvanneyri IS-311, Borgarnes, Iceland

Abstract. Net loss of soil organic carbon (SOC) from terrestrial ecosystems is a likely consequence of global warming and this may affect key soil functions. Strongest changes in temperature are expected to occur at high northern latitudes, with boreal forest and tundra as prevailing land-cover types. However, specific ecosystem responses to warming are understudied. We used a natural geothermal soil warming gradient in an Icelandic spruce forest (0–17.5 °C warming intensity) to assess changes in SOC content in 0–10 cm (topsoil) and 20–30 cm (subsoil) after 10 years of soil warming. Five different SOC fractions were isolated and the amount of stable aggregates (63–2000 µm) was assessed to link SOC to soil structure changes. Results were compared to an adjacent, previously investigated warmed grassland. Soil warming had depleted SOC in the forest soil by −2.7 g kg−1 °C−1 (−3.6 % °C−1) in the topsoil and −1.6 g kg−1 °C−1 (−4.5 % °C−1) in the subsoil. Distribution of SOC in different fractions was significantly altered, with particulate organic matter and SOC in sand and stable aggregates being relatively depleted and SOC attached to silt and clay being relatively enriched in warmed soils. The major reason for this shift was aggregate break-down: topsoil aggregate mass proportion was reduced from 60.7 ± 2.2 % in the unwarmed reference to 28.9 ± 4.6 % in the most warmed soil. Across both depths, loss of one unit SOC caused a depletion of 4.5 units aggregated soil, which strongly affected bulk density (R2 = 0.91 when correlated to SOC and R2 = 0.51 when correlated to soil mass in stable aggregates). The proportion of water extractable carbon increased with decreasing aggregation, indicating an indirect SOC protective effect of aggregates > 63 µm. Topsoil changes in total SOC and fraction distribution were more pronounced in the forest than in the adjacent warmed grassland soils, due to higher and more labile initial SOC. However, no ecosystem effect was observed in the response of subsoil SOC and fraction distribution. Whole profile differences across ecosystems might thus be small. Changes in soil structure upon warming should be studied more deeply and taken into consideration when interpreting or modelling biotic responses to warming.

Christopher Poeplau et al.
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Christopher Poeplau et al.
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Short summary
Global warming leads to increased mineralization of soil organic matter, inducing a positive climate-carbon cycle feedback loop. Loss of organic matter can be associated with loss of soil structure. Here we use a strong geothermal gradient to investigate soil warming effects on soil organic matter and structural parameters in subarctic forest and grassland soils. Strong depletion of organic matter caused a collapse of aggregates highlighting the potential impact of warming on soil functions.
Global warming leads to increased mineralization of soil organic matter, inducing a positive...