Abstract
The knowledge of tree species dependent turnover of soil organic matter (SOM) is limited, yet required to understand the carbon sequestration function of forest soil. We combined investigations of 13C and 15N and its relationship to elemental stoichiometry along soil depth gradients in 35-year old monocultural stands of Douglas fir (Pseudotsuga menziesii), black pine (Pinus nigra), European beech (Fagus sylvatica) and red oak (Quercus rubra) growing on a uniform post-mining soil. We investigated the natural abundance of 13C and 15N and the carbon:nitrogen (C:N) and oxygen:carbon (O:C) stoichiometry of litterfall and fine roots as well as SOM in the forest floor and mineral soil. Tree species had a significant effect on SOM δ13C and δ15N reflecting significantly different signatures of litterfall and root inputs. Throughout the soil profile, δ13C and δ15N were significantly related to the C:N and O:C ratio which indicates that isotope enrichment with soil depth is linked to the turnover of organic matter (OM). Significantly higher turnover of OM in soils under deciduous tree species depended to 46% on the quality of litterfall and root inputs (N content, C:N, O:C ratio), and the initial isotopic signatures of litterfall. Hence, SOM composition and turnover also depends on additional—presumably microbial driven—factors. The enrichment of 15N with soil depth was generally linked to 13C. In soils under pine, however, with limited N and C availability, the enrichment of 15N was decoupled from 13C. This suggests that transformation pathways depend on litter quality of tree species.
Highlights
Soils in forest ecosystems bear a high potential as carbon (C) sinks in the mitigation of climate change (Pan et al 2011)
The litterfall of coniferous species was characterized by significantly higher C:N ratios compared to deciduous forest stands
The isotopic signatures of roots in the forest floors did not differ from roots in deeper soil horizons but the tree species effect was pronounced in each soil depth
Summary
Soils in forest ecosystems bear a high potential as carbon (C) sinks in the mitigation of climate change (Pan et al 2011). Stoichiometric ratios like the carbon:nitrogen (C:N) ratio of soil organic matter (SOM) are influenced by the forest stand (Cools et al 2014; Lorenz and Thiele-Bruhn 2019). A combination of both approaches, ecological stoichiometry and stable isotopes, in soil depth gradients promises to get deeper insights into the turnover of SOM. To our knowledge, this has not been used before to characterize the turnover of tree species dependent organic matter (OM) in the soil
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