Abstract
Deadwood forms a significant carbon pool in forest systems and is a potential source of dissolved organic carbon (DOC) input to soil, yet little is known about how deadwood effects forest soil carbon cycling. Deadwood DOC inputs to soil may be retained through sorption or may prime microbial decomposition of existing organic matter to produce additional DOC. To determine impacts of deadwood on soil C cycling, we analysed surface soil from beneath deadwood or leaf litter only, along chronosequences of stands of lowland oak and upland Sitka spruce. The concentration and quality (by optical indices) of water-extracted soil DOC (water-extractable organic carbon; WEOC), in situ decomposition ‘tea bag index’ (TBI) parameters and enzymatic potential assays (β-D-cellubiosidase, β-glucosidase, β-xylosidase, leucine aminopeptidase, phosphatase, phenol oxidase) were determined. Presence of deadwood significantly (p < 0.05) increased WEOC concentration (~ 1.5 to ~ 1.75 times) in the mineral oak soil but had no effect on WEOC in spruce soils, potentially because spruce deadwood DOC inputs were masked by a high background of WEOC (1168 mg kg−1 soil) and/or were not retained through mineral sorption in the highly organic (~ 90% SOM) soil. TBI and enzyme evidence suggested that deadwood-derived DOC did not impact existing forest carbon pools via microbial priming, possibly due to the more humified/aromatic quality of DOC produced (humification index of 0.75 and 0.65 for deadwood and leaf litter WEOC, respectively). Forest carbon budgets, particularly those for mineral soils, may underestimate the quantity of DOC if derived from soil monitoring that does not include a deadwood component.
Highlights
Forests are acknowledged as large and important carbon (C) sinks, storing carbon in soils and both living and dead biomass
Our finding of greater quantities of Water-extractable organic carbon (WEOC) under deadwood at Alice Holt is in accordance with other research that has found that concentrations of soil dissolved organic carbon (DOC) increase in forests over mineral soils where deadwood is present (Spears and Lajtha 2004; Hafner et al 2005; Kahl et al 2012), with increases of up to nine-fold when compared to soils with leaf litter only (Hafner et al 2005)
We initially hypothesized that the effects of deadwood on the soil DOC pool might depend on stand age due to differences in DOC fluxes from wood at different stages of decay
Summary
Forests are acknowledged as large and important carbon (C) sinks, storing carbon in soils and both living and dead biomass. Forests are estimated to hold 861 ± 66 Pg C, of which 119 ± 6 Pg are accounted for by temperate forest ecosystems (Pan et al 2011). The amount of carbon stored in forest soils and biomass varies with forest type. The remaining carbon is held within litter and deadwood and Pan et al (2011) estimated that 8% (73 ± 6 Pg) of the world’s forest C is held in deadwood. In the UK, forests store up to 1Pg C (Morison et al 2012), of which 74% is held within the soils down to 1 m depth (Vanguelova et al 2013). The remaining 26% C is split between tree biomass (22%) and litter and deadwood (4%) (Morison et al 2012). For lying deadwood, the decay classes range from 1 (least decomposed; intact bark, texture and structure)
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