No abstract

No abstract

No abstract

Despite their importance for water uptake and transport, the xylem anatomical and hydraulic properties of tree roots have only rarely been studied in the field. We measured mean vessel diameter (D), vessel density (VD), relative vessel lumen area (lumen area per xylem area) and derived potential hydraulic conductivity (Kp) in the xylem of 197 fine- to medium-diameter roots (1–10 mm) in the topsoil and subsoil (0–200 cm) of a mature European beech forest on sandy soil for examining the influence of root diameter and soil depth on xylem anatomical and derived hydraulic traits. All anatomical and functional traits showed strong dependence on root diameter and thus root age but no significant relation to soil depth. Averaged over topsoil and deep soil and variable flow path lengths in the roots, D increased linearly with root diameter from ∼50 μm in the smallest diameter class (1–2 mm) to ∼70 μm in 6–7 mm roots (corresponding to a mean root age of ∼12 years), but remained invariant in roots >7 mm. D never exceeded ∼82 μm in the 1–10 mm roots, probably in order to control the risk of frost- or drought-induced cavitation. This pattern was overlain by a high variability in xylem anatomy among similar-sized roots with Kp showing a higher variance component within than between root diameter classes. With 8% of the roots exceeding average Kp in their diameter class by 50–700%, we obtained evidence of the existence of ‘high-conductivity roots’ indicating functional differentiation among similar-sized roots. We conclude that the hydraulic properties of small to medium diameter roots of beech are mainly determined by root age, rendering root diameter a suitable predictor of hydraulic functioning, while soil depth – without referring to path length – had a negligible effect.

Abstract. Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO2 fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30 cm), which represent a significant carbon (C) pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO2 production in subsoils. In a 2-year field study in a European beech forest in northern Germany, vertical CO2 concentration profiles were continuously measured at three locations, and CO2 production was quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO2 production in the three soil profiles, an isotopic labelling experiment, using 13C-enriched leaf litter, was performed. Additionally, radiocarbon measurements of CO2 in the soil atmosphere were used to obtain information about the age of the C source in the CO2 production. At the study site, it was found that 90 % of total soil respiration was produced in the first 30 cm of the soil profile, where 53 % of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO2 production below a depth of 10 cm. In the first 2 months after litter application, fresh litter-derived C contributed, on average, 1 % at 10 cm depth and 0.1 % at 150 cm depth to CO2 in the soil profile. Thereafter, its contribution was less than 0.3 % and 0.05 % at 10 and 150 cm depths, respectively. Furthermore CO2 in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO2 in the subsoil originated from young C sources despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils, in the form of roots and root exudates, are rapidly respired, and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of 13CO2 in the soil profile against the total CO2 gradient. This isotopic dependency should be taken into account when using labelled 13C and 14C isotope data as an age proxy for CO2 sources in the soil.

No abstract

Radiocarbon (¹⁴C) analysis is an important tool that can provide information on the dynamics of organic matter in soils. Radiocarbon concentrations of soil organic matter (SOM) however, reflect the heterogeneous mixture of various organic compounds and are affected by different chemical, biological, and physical soil parameters. These parameters can vary strongly in soil profiles and thus affect the spatial distribution of the apparent ¹⁴C age of SOM considerably. The heterogeneity of SOM and its ¹⁴C signature may be even larger in subsoil horizons, which are thought to receive organic carbon inputs following preferential pathways. This will bias conclusions drawn from ¹⁴C analyses of individual soil profiles considerably. We thus investigated important soil parameters, which may influence the ¹⁴C distribution of SOM as well as the spatial heterogeneity of ¹⁴C distributions in soil profiles. The suspected strong heterogeneity and spatial variability, respectively of bulk SOM is confirmed by the variable ¹⁴C distribution in three 185 cm deep profiles in a Dystric Cambisol. The ¹⁴C contents are most variable in the C horizons because of large differences in the abundance of roots there. The distribution of root biomass and necromass and its organic carbon input is the most important factor affecting the ¹⁴C distribution of bulk SOM. The distance of the soil profiles to a beech did not influence the horizontal and vertical distribution of roots and ¹⁴C concentrations. Other parameters were found to be of minor importance including microbial biomass-derived carbon and soil texture. The microbial biomass however, may promote a faster turnover of SOM at hot spots resulting in lower ¹⁴C concentration there. Soil texture had no statistically significant influence on the spatial ¹⁴C distribution of bulk SOM. However, SOM in fine silt and clay sized particles (< 6.3 µm) yields slightly higher ¹⁴C concentrations than bulk SOM particularly at greater soil depth, which is in contrast to previous studies where silt and clay fractions contained older SOM stabilized by organo-mineral interaction. ¹⁴C contents of fine silt and clay correlate with the microbial biomass-derived carbon suggesting a considerable contribution of microbial-derived organic carbon. In conclusion, ¹⁴C analyses of bulk SOM mainly reflect the spatial distribution of roots, which is strongly variable even on a small spatial scale of few meters. This finding should be considered when using ¹⁴C analysis to determine SOM.

<p><strong>Abstract.</strong> Large amounts of total organic carbon are temporarily stored in soils, which makes soil respiration one of the major sources of terrestrial CO₂ fluxes within the global carbon cycle. More than half of global soil organic carbon (SOC) is stored in subsoils (below 30&amp;thinsp;cm), which represent a significant C pool. Although several studies and models have investigated soil respiration, little is known about the quantitative contribution of subsoils to total soil respiration or about the sources of CO₂ production in subsoils. In a two-year field study in a European beech forest in northern Germany, vertical CO₂ concentration profiles were continuously measured at three locations and CO₂ production quantified in the topsoil and the subsoil. To determine the contribution of fresh litter-derived C to CO₂ production in the three soil profiles, an isotopic labelling experiment using ¹³C-enriched leaf litter was performed. Additionally, radiocarbon measurements of CO₂ in the soil atmosphere were used to obtain information about the age of the C source in CO₂ production. At the study site, it was found that 90&amp;thinsp;% of total soil respiration was produced in the first 30 cm of the soil profile where 53&amp;thinsp;% of the SOC stock is stored. Freshly labelled litter inputs in the form of dissolved organic matter were only a minor source for CO₂ production below a depth of 10&amp;thinsp;cm. In the first two months after litter application, fresh litter-derived C contributed on average 1&amp;thinsp;% at 10&amp;thinsp;cm depth and 0.1&amp;thinsp;% at 150&amp;thinsp;cm depth to CO₂ in the soil profile. Thereafter, its contribution was less than 0.3&amp;thinsp;% and 0.05&amp;thinsp;% at 10&amp;thinsp;cm and 150&amp;thinsp;cm depths respectively. Furthermore CO₂ in the soil profile had the same modern radiocarbon signature at all depths, indicating that CO₂ in the subsoil originated from young C sources, despite a radiocarbon age bulk SOC in the subsoil. This suggests that fresh C inputs in subsoils in the form of roots and root exudates are rapidly respired and that other subsoil SOC seems to be relatively stable. The field labelling experiment also revealed a downward diffusion of 13CO₂ in the soil profile against the total CO₂ gradient. This isotopic dependency should be taken into account when using labelled ¹³CO₂ and ¹⁴C isotope data as an age proxy for CO₂ sources in the soil.</p>