Abstract
Abstract. Most forest soils are characterised by a steep carbon gradient from the forest floor to the mineral soil, indicating that carbon is prevented from entry into the soil. Bioturbation can facilitate the incorporation of litter-derived carbon into the mineral soil. Wild boar are effective at mixing and grubbing in the soil and wild boar populations are increasing in many parts of the world. In a 6-year field study, we investigated the effect of simulated wild boar bioturbation on the stocks and stability of soil organic carbon in two forest areas. Regular bioturbation mimicking grubbing by wild boar was performed artificially in 23 plots, and the organic layer and mineral soil down to 15 cm depth were then sampled. No significant changes in soil organic carbon stocks were detected in the bioturbation plots compared with non-disturbed reference plots. However, around 50 % of forest floor carbon was transferred with bioturbation to mineral soil carbon, and the stock of stabilised mineral-associated carbon increased by 28 %. Thus, a large proportion of the labile carbon in the forest floor was transformed into more stable carbon. Carbon saturation of mineral surfaces was not detected, but carbon loading per unit mineral surface increased by on average 66 % in the forest floor due to bioturbation. This indicates that mineral forest soils have non-used capacity to stabilise and store carbon. Transfer of aboveground litter into the mineral soil is the only rate-limiting process. Wild boar may speed up this process with their grubbing activity.
Highlights
The stability of soil organic carbon (SOC) strongly depends on its association with soil minerals (Lehmann and Kleber, 2015; von Lützow et al, 2006)
The invasion of European earthworms into some North American forests has led to complete disappearance of the forest floor due to bioturbation, but most of this forest floor C was found in the underlying mineral soil (Alban and Berry, 1994)
The proportion of mineral SOC stocks increased by 26 %, and correspondingly, forest floor C stocks decreased by 40 % due to bioturbation
Summary
The stability of soil organic carbon (SOC) strongly depends on its association with soil minerals (Lehmann and Kleber, 2015; von Lützow et al, 2006). Organic carbon (C) that enters forest soil as aboveground litter is retained in an organic layer, with little opportunity for long-term stabilisation. Carbon in the organic layer is vulnerable to disturbances such as wildfire, windthrow or forest clear-cutting, since it is present in non-stabilised organic matter and exposed at the soil surface (Zakharova et al, 2014; Jandl et al, 2007). The organic layer in German forests stores on average 19 Mg C ha−1, which may reduce the available amount of SOC that enters the mineral soil below and can be stabilised (Grüneberg et al, 2014). Besides downward movement of dissolved organic matter, bioturbation is a major process that brings organic matter from the forest floor into the mineral soil. Bioturbation by earthworms has been found to be very effective in translocating litter C from the surface to mineral soil (Don et al, 2008). Most earthworms are restricted to non-sandy, non-acidic soils (Curry, 2004), while bioturbation by other animals such as termites, small rodents and ants can be found in many soils irrespective of soil texture
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