Soil carbon stock change in the forests of Denmark between 1990 and 2008

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Soils of the forests in Denmark were sampled in permanent plots in 1990 (t1) and resampled (N = 124) in 2007–9 (t2). The soils were classified according to the carbon concentration in the uppermost mineral soil horizon (0–25 cm) at t1, and according to subsoil texture and presence or absence of CaCO3 in the subsoil. Soil organic carbon (SOC) stocks in forest floor + mineral soil (0–100 cm) at t2 had a median of 15.9 kg C m− 2 (range 4.1–68.9 kg C m− 2). There was no detectable overall change in SOC during the 18-year period, but different trends were observed within the subsoil texture classes. SOC stocks decreased with increasing initial SOC content (except calcareous mineral soils), consistently with reports from other inventories of SOC change. Unlike coarse and medium textured soils, fine-textured soils (> 10% clay in subsoil) with less than 4.1% C in the 0–25 cm layer gained 2.1 ± 1.1 kg C m− 2 (lsmean and 95% confidence limit) during the 18 year period (0.11 kg C m−2 year−1). With only two observation points in time, SOC changes could not be safely interpreted as true changes for subsets of the data, e.g. distinct soil types. Initially very C rich mineral soils and organic soils (C% > 12) on average lost 4 and 7 kg C m− 2. These C losses from very C rich mineral (4.1 < C% < 12) and organic soils are highly uncertain due to large sampling uncertainty and thus a possible effect of regression to the mean. However, decreased C concentrations in either topsoil or subsoil layers may also reflect net C mineralization as an adaptation to the current more aerobic drainage regime of soils that were frequently water saturated in previous centuries. Plots afforested after 1954 and ranging in stand age from 7 to 42 years had accumulated forest floors with an average stock of 0.3 ± 0.1 kg C m−2, which was still significantly lower than forest floor stocks in soils of the forests remaining forests. On afforested sites the mineral SOC stock did not show any significant increase between t1 and t2, but SOC redistribution due to deep ploughing was observed in a few sites. The minimum detectable difference for the national soil C stock in forests was estimated to 0.3 kg C m−2 over two decades equivalent to a change of 0.015 kg C m−2 year−1. We conclude that the average forest SOC stock remained unchanged over two decades for soils < 4.1% C in the top mineral soil, whereas the sink/source status of very carbon rich and organic soils remains uncertain.
Original languageEnglish
JournalGeoderma Regional
Pages (from-to)169-180
Number of pages12
Publication statusPublished - 2015

ID: 141954039