Tree Species Is the Major Factor Explaining C:N Ratios in European Forest Soils. Cools, N., Vesterdal, L., De Vos, B., Vanguelova, E., & Hansen, K. 311:3–16.
Tree Species Is the Major Factor Explaining C:N Ratios in European Forest Soils [link]Paper  doi  abstract   bibtex   
[Highlights] [::] C:N ratio in forest floor, peat and mineral topsoil mainly depend on tree species. [::] Most important differences in C:N ratios are seen between deciduous and evergreen species. [::] Either soil type, ecoregion or humus type was the second most important explanatory factor. [::] There was no consistent relationship between modelled deposition and C:N ratios. [Abstract] The C:N ratio is considered as an indicator of nitrate leaching in response to high atmospheric nitrogen (N) deposition. However, the C:N ratio is influenced by a multitude of other site-related factors. This study aimed to unravel the factors determining C:N ratios of forest floor, mineral soil and peat top soils in more than 4000 plots of the ICP Forests large-scale monitoring network. The first objective was to quantify forest floor, mineral and peat soil C:N ratios across European forests. Secondly we determined the main factors explaining this C:N ratio using a boosted regression tree analysis (BRT), including fifteen site and environmental variables. [\n] Ninety-five percent of the C:N ratios were between 16 and 44 in the forest floor, between 13 and 44 in the peat topsoil and between 10 and 32 in the mineral topsoil. Within the aerated forest floor and the mineral soil, the C:N ratios decreased with depth, while in the hydromorphic forest floor and the peats no clear trend with depth was observed. [\n] Tree species was clearly the most important explanatory variable for the C:N ratio in both forest floors and topsoils, while it was soil type in the deeper mineral soil layers. The lowest C:N ratios both in the forest floor and the top mineral soil were found in black locust (Robinia pseudoacacia L.) and black alder (Alnus glutinosa L.) stands, both N fixing tree species. While in the forest floor the highest C:N ratios were found in evergreen species like pine, cork oak (Quercus suber L.) and eucalyptus, the pine species and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) showed the highest C:N ratios in the mineral soil. The second most important explanatory variable in the forest floor and mineral topsoil was the biogeographical zoning (ecoregion). In the peat topsoil and in the deeper mineral soil layers it was the humus type. Deposition and climatic variables were of minor importance at the European scale. [\n] Further analysis for eight main forest tree species individually, showed that the influence of environmental variables on C:N ratios was tree species dependent. For Aleppo pine (Pinus halepensis Miller) and holm oak (Quercus ilex L.), both with a typical Mediterranean distribution, the relationship between N and S deposition and C:N ratio appeared to be positive. This study suggests that applying C:N ratios as a general indicator of the N status in forests at the European level, without explicitly accounting for tree species, is too simplistic and may result in misleading conclusions. [:Excerpt: Environmental variables explaining C:N ratios] [::Main tree species] In spite of the numerous potential variables affecting soil properties in the European-wide soil inventory, we found a striking effect from the main tree species. The importance of tree species for the C:N ratio is supported by Lovett and Rueth (1999) and Lovett et al., 2002 and Lovett et al., 2004 in the US as well as Hobbie et al., 2006 and Hansson et al., 2011 and Vesterdal et al., 2008 and Vesterdal et al., 2012 in Europe. These studies suggested that tree species influences microbial decomposition primarily via differences in litter lignin and N content, with high lignin and low N species decomposing most slowly, with subsequent influences on soil C:N ratio of the forest floor. Different nutrient cycling traits are indeed seen between broadleaves and conifers (Fassnacht and Gower, 1999) and among individual tree species within those species categories (Finzi et al., 1998 and Vesterdal et al., 2008). Our results did not directly confirm a general contrast in C:N ratio between conifers and broadleaves (Fig. 2), but rather indicated clear differences between deciduous and evergreen tree species, especially in the forest floor. Eucalyptus, cork oak and holm oak, which are evergreen broadleaved tree species showed C:N ratios in the same range as the pine and spruce species, while larch, a deciduous conifer, has C:N ratios below 25. In this respect, Douglas-fir was an exception as it also has low C:N ratios, at least in the forest floor. [\n] The fact that such large differences were found in C:N ratio among tree species at the European level supports a stratification of thresholds of critical C:N ratios according to tree species for the evaluation of risk of nitrate leaching. Previous evaluations of nitrate leaching across Europe only stratified tree species to conifers and broadleaves (Kristensen et al., 2004) while our results indicate that specific species differences in C:N ratio are poorly represented by these overall categories. When applying the N status classes proposed by Gundersen et al. (2006), large differences are indicated among individual conifer species, all of them deviating from the distribution computed for all conifers (Table 11). So stratification into a conifer-only group seems highly questionable. Presumably the same holds true for stratification into one common deciduous group. Our results support strong arguments to derive individual critical C:N thresholds for potential nitrate leaching for each of the main tree species. Therefore it is also essential to specify whether the OL layers are included in the calculations of the C:N ratio of the forest floor. [\n] The range of the C:N ratio in the topsoil was more narrow compared to the forest floor (Fig. 2) indicating a decreasing influence of the tree species on the C:N ratio with depth. This can be explained by decreasing organic matter content and decreasing tree root density with increasing soil depth. Decreasing influence of tree species on C:N ratio and other soil properties with increasing soil depth is supported by studies of D́ıaz-Pinés et al., 2011, Hansson et al., 2011 and Langenbruch et al., 2012 and Vesterdal et al. (2008). [\n] After elimination of the tree species effect on forest floor C:N ratio, the most important explanatory variable was the soil type. However, this was less pronounced for the tree species with the widest geographical distribution across Europe (Scots pine, Norway spruce and common beech) where the soil effect is diluted by a broad range of other environmental variables. A number of environmental variables had comparable relative influences (10-15%), namely climatic (temperature for Scots pine), ecoregion, parent material, soil and humus type and deposition characteristics. This result is not surprising given the challenge of identifying specific drivers for soil N status in large inventories across a multitude of environmental gradients. [\n] [...] [Conclusions] At the European scale, tree species was the most important discriminating variable in explaining C:N ratio in the forest floor as well as in the top mineral and peat soil. C:N ratios below 20 in the forest floor were found in black locust and alder stands, both N fixing tree species. The highest C:N ratios in the forest floor were found in pine, eucalypt and cork oak stands. In the forest floor, the largest differences were seen between deciduous and evergreen species, while in the mineral soil the largest differences were rather observed between the broadleaved and coniferous species categories. The second most important set of variables at the European level were related to geographic location (ecoregion) or related to soil and humus type, for both forest floor and topsoil. The effect of site variables differed considerably among the eight most common tree species on the ICP Forest Level I network, especially for the tree species with the widest geographical distribution. We consequently suggest that C:N ratio thresholds for N leaching should be evaluated for each tree species individually. [\n] This study clearly stated that a combination of variables should be investigated when the variables influencing soil C:N ratio are evaluated, of which the most important ones are tree species, ecological region, soil and humus types. Individual climatic variables did only occasionally play an important role. Mean annual temperature was important in explaining the C:N ratio in the forest floor of Scots pine but not for other tree species. Nitrogen deposition did not show any consistent direct impact on C:N ratios at the European scale. Within certain common tree species N deposition did explain a small part of the variation in C:N ratio, but these relationships were inconsistent among tree species and soil layers and therefore did not unequivocally support a positive effect of N deposition on soil N status. Finally, conclusions based on forest floor C:N ratios are often different from those drawn from topsoil C:N ratios. The mechanisms behind this deviance need further investigation. Meanwhile, we recommend C:N ratios to be systematically assessed both in the forest floor and topsoil.
@article{coolsTreeSpeciesMajor2014,
  title = {Tree Species Is the Major Factor Explaining {{C}}:{{N}} Ratios in {{European}} Forest Soils},
  author = {Cools, N. and Vesterdal, L. and De Vos, B. and Vanguelova, E. and Hansen, K.},
  date = {2014-01},
  journaltitle = {Forest Ecology and Management},
  volume = {311},
  pages = {3--16},
  issn = {0378-1127},
  doi = {10.1016/j.foreco.2013.06.047},
  url = {https://doi.org/10.1016/j.foreco.2013.06.047},
  abstract = {[Highlights]

[::] C:N ratio in forest floor, peat and mineral topsoil mainly depend on tree species.

[::] Most important differences in C:N ratios are seen between deciduous and evergreen species.

[::] Either soil type, ecoregion or humus type was the second most important explanatory factor.

[::] There was no consistent relationship between modelled deposition and C:N ratios.

[Abstract]

The C:N ratio is considered as an indicator of nitrate leaching in response to high atmospheric nitrogen (N) deposition. However, the C:N ratio is influenced by a multitude of other site-related factors. This study aimed to unravel the factors determining C:N ratios of forest floor, mineral soil and peat top soils in more than 4000 plots of the ICP Forests large-scale monitoring network. The first objective was to quantify forest floor, mineral and peat soil C:N ratios across European forests. Secondly we determined the main factors explaining this C:N ratio using a boosted regression tree analysis (BRT), including fifteen site and environmental variables.

[\textbackslash n] Ninety-five percent of the C:N ratios were between 16 and 44 in the forest floor, between 13 and 44 in the peat topsoil and between 10 and 32 in the mineral topsoil. Within the aerated forest floor and the mineral soil, the C:N ratios decreased with depth, while in the hydromorphic forest floor and the peats no clear trend with depth was observed.

[\textbackslash n] Tree species was clearly the most important explanatory variable for the C:N ratio in both forest floors and topsoils, while it was soil type in the deeper mineral soil layers. The lowest C:N ratios both in the forest floor and the top mineral soil were found in black locust (Robinia pseudoacacia L.) and black alder (Alnus glutinosa L.) stands, both N fixing tree species. While in the forest floor the highest C:N ratios were found in evergreen species like pine, cork oak (Quercus suber L.) and eucalyptus, the pine species and Douglas-fir (Pseudotsuga menziesii (Mirb.) Franco) showed the highest C:N ratios in the mineral soil. The second most important explanatory variable in the forest floor and mineral topsoil was the biogeographical zoning (ecoregion). In the peat topsoil and in the deeper mineral soil layers it was the humus type. Deposition and climatic variables were of minor importance at the European scale.

[\textbackslash n] Further analysis for eight main forest tree species individually, showed that the influence of environmental variables on C:N ratios was tree species dependent. For Aleppo pine (Pinus halepensis Miller) and holm oak (Quercus ilex L.), both with a typical Mediterranean distribution, the relationship between N and S deposition and C:N ratio appeared to be positive. This study suggests that applying C:N ratios as a general indicator of the N status in forests at the European level, without explicitly accounting for tree species, is too simplistic and may result in misleading conclusions.

[:Excerpt: Environmental variables explaining C:N ratios]

[::Main tree species]

In spite of the numerous potential variables affecting soil properties in the European-wide soil inventory, we found a striking effect from the main tree species. The importance of tree species for the C:N ratio is supported by Lovett and Rueth (1999) and Lovett et al., 2002 and Lovett et al., 2004 in the US as well as Hobbie et al., 2006 and Hansson et al., 2011 and Vesterdal et al., 2008 and Vesterdal et al., 2012 in Europe. These studies suggested that tree species influences microbial decomposition primarily via differences in litter lignin and N content, with high lignin and low N species decomposing most slowly, with subsequent influences on soil C:N ratio of the forest floor. Different nutrient cycling traits are indeed seen between broadleaves and conifers (Fassnacht and Gower, 1999) and among individual tree species within those species categories (Finzi et al., 1998 and Vesterdal et al., 2008). Our results did not directly confirm a general contrast in C:N ratio between conifers and broadleaves (Fig. 2), but rather indicated clear differences between deciduous and evergreen tree species, especially in the forest floor. Eucalyptus, cork oak and holm oak, which are evergreen broadleaved tree species showed C:N ratios in the same range as the pine and spruce species, while larch, a deciduous conifer, has C:N ratios below 25. In this respect, Douglas-fir was an exception as it also has low C:N ratios, at least in the forest floor.

[\textbackslash n] The fact that such large differences were found in C:N ratio among tree species at the European level supports a stratification of thresholds of critical C:N ratios according to tree species for the evaluation of risk of nitrate leaching. Previous evaluations of nitrate leaching across Europe only stratified tree species to conifers and broadleaves (Kristensen et al., 2004) while our results indicate that specific species differences in C:N ratio are poorly represented by these overall categories. When applying the N status classes proposed by Gundersen et al. (2006), large differences are indicated among individual conifer species, all of them deviating from the distribution computed for all conifers (Table 11). So stratification into a conifer-only group seems highly questionable. Presumably the same holds true for stratification into one common deciduous group. Our results support strong arguments to derive individual critical C:N thresholds for potential nitrate leaching for each of the main tree species. Therefore it is also essential to specify whether the OL layers are included in the calculations of the C:N ratio of the forest floor.

[\textbackslash n] The range of the C:N ratio in the topsoil was more narrow compared to the forest floor (Fig. 2) indicating a decreasing influence of the tree species on the C:N ratio with depth. This can be explained by decreasing organic matter content and decreasing tree root density with increasing soil depth. Decreasing influence of tree species on C:N ratio and other soil properties with increasing soil depth is supported by studies of D́ıaz-Pinés et al., 2011, Hansson et al., 2011 and Langenbruch et al., 2012 and Vesterdal et al. (2008).

[\textbackslash n] After elimination of the tree species effect on forest floor C:N ratio, the most important explanatory variable was the soil type. However, this was less pronounced for the tree species with the widest geographical distribution across Europe (Scots pine, Norway spruce and common beech) where the soil effect is diluted by a broad range of other environmental variables. A number of environmental variables had comparable relative influences (10-15\%), namely climatic (temperature for Scots pine), ecoregion, parent material, soil and humus type and deposition characteristics. This result is not surprising given the challenge of identifying specific drivers for soil N status in large inventories across a multitude of environmental gradients.

[\textbackslash n] [...]

[Conclusions]

At the European scale, tree species was the most important discriminating variable in explaining C:N ratio in the forest floor as well as in the top mineral and peat soil. C:N ratios below 20 in the forest floor were found in black locust and alder stands, both N fixing tree species. The highest C:N ratios in the forest floor were found in pine, eucalypt and cork oak stands. In the forest floor, the largest differences were seen between deciduous and evergreen species, while in the mineral soil the largest differences were rather observed between the broadleaved and coniferous species categories. The second most important set of variables at the European level were related to geographic location (ecoregion) or related to soil and humus type, for both forest floor and topsoil. The effect of site variables differed considerably among the eight most common tree species on the ICP Forest Level I network, especially for the tree species with the widest geographical distribution. We consequently suggest that C:N ratio thresholds for N leaching should be evaluated for each tree species individually.

[\textbackslash n] This study clearly stated that a combination of variables should be investigated when the variables influencing soil C:N ratio are evaluated, of which the most important ones are tree species, ecological region, soil and humus types. Individual climatic variables did only occasionally play an important role. Mean annual temperature was important in explaining the C:N ratio in the forest floor of Scots pine but not for other tree species. Nitrogen deposition did not show any consistent direct impact on C:N ratios at the European scale. Within certain common tree species N deposition did explain a small part of the variation in C:N ratio, but these relationships were inconsistent among tree species and soil layers and therefore did not unequivocally support a positive effect of N deposition on soil N status. Finally, conclusions based on forest floor C:N ratios are often different from those drawn from topsoil C:N ratios. The mechanisms behind this deviance need further investigation. Meanwhile, we recommend C:N ratios to be systematically assessed both in the forest floor and topsoil.},
  keywords = {*imported-from-citeulike-INRMM,~INRMM-MiD:c-13095631,~to-add-doi-URL,alnus-glutinosa,betula-pendula,deciduous,eucalyptus-spp,europe,evergreen,fagus-sylvatica,forest-resources,larix-decidua,nitrogen,picea-abies,pinus-halepensis,pinus-nigra,pinus-pinaster,pinus-sylvestris,quercus-ilex,quercus-robur,quercus-suber,robinia-pseudoacacia,soil-carbon,soil-resources,tree-species}
}
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