Remaining added leaf litter mass [%] with determined C and N concentrations [mg g⁻¹] and C/N ratios at the end of the experiment (31.05.2010) with the differences in comparison to the initial added leaf litter.

<p>Values were calculated for all pure treatments of unlabeled and labeled (*) leaf litter of beech (Be) and ash (As). Represented are mean values with their standard deviation in parenthesis. High-letters represent significant differences (Kruskal-Wallis test followed by Mann-Whitney U test, p<0.05) between the different litter types.</p><p>Remaining added leaf litter mass [%] with determined C and N concentrations [mg g⁻¹] and C/N ratios at the end of the experiment (31.05.2010) with the differences in comparison to the initial added leaf litter.</p

Calculated average daily release of litter-derived DOM (mean + standard error; n = 6) for the pure and mixed labeled (*) treatments of beech (Be) and ash (As) in the different winter (I: 16.12.08–30.03.09; II: 21.12.09–22.03.10) and summer periods (I: 20.04.09–30.11.09; II: 07.04.10–31.05.10).

<p>Calculated average daily release of litter-derived DOM (mean + standard error; n = 6) for the pure and mixed labeled (*) treatments of beech (Be) and ash (As) in the different winter (I: 16.12.08–30.03.09; II: 21.12.09–22.03.10) and summer periods (I: 20.04.09–30.11.09; II: 07.04.10–31.05.10).</p

Influence of Litter Diversity on Dissolved Organic Matter Release and Soil Carbon Formation in a Mixed Beech Forest

<div><p>We investigated the effect of leaf litter on below ground carbon export and soil carbon formation in order to understand how litter diversity affects carbon cycling in forest ecosystems. ¹³C labeled and unlabeled leaf litter of beech (<i>Fagus sylvatica</i>) and ash (<i>Fraxinus excelsior</i>), characterized by low and high decomposability, were used in a litter exchange experiment in the Hainich National Park (Thuringia, Germany). Litter was added in pure and mixed treatments with either beech or ash labeled with ¹³C. We collected soil water in 5 cm mineral soil depth below each treatment biweekly and determined dissolved organic carbon (DOC), δ¹³C values and anion contents. In addition, we measured carbon concentrations and δ¹³C values in the organic and mineral soil (collected in 1 cm increments) up to 5 cm soil depth at the end of the experiment. Litter-derived C contributes less than 1% to dissolved organic matter (DOM) collected in 5 cm mineral soil depth. Better decomposable ash litter released significantly more (0.50±0.17%) litter carbon than beech litter (0.17±0.07%). All soil layers held in total around 30% of litter-derived carbon, indicating the large retention potential of litter-derived C in the top soil. Interestingly, in mixed (ash and beech litter) treatments we did not find a higher contribution of better decomposable ash-derived carbon in DOM, O horizon or mineral soil. This suggest that the known selective decomposition of better decomposable litter by soil fauna has no or only minor effects on the release and formation of litter-derived DOM and soil organic matter. Overall our experiment showed that 1) litter-derived carbon is of low importance for dissolved organic carbon release and 2) litter of higher decomposability is faster decomposed, but litter diversity does not influence the carbon flow.</p></div

Average percent of litter-derived C (C_litter) in the different carbon pools for the pure and mixed labeled (*) treatments of beech (Be) and ash (As) at the end of the experiment (n = 6).

<p>The DOM pool (<0.2%) for the labeled beech treatments in the diagram is too small to be visible. No significant (p>0.05, n = 6, one-way ANOVA) differences between pure and mixed labeled ash and beech treatments for litter-derived C in the remaining litter, the O horizon and for the mineral soil were found. For the litter-derived carbon in DOM we found significant (p<0.01, n = 6, one-way ANOVA) differences only between labeled ash (As*, BeAs*) and beech (Be*, Be*As) treatments (<a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0114040#pone-0114040-g003" target="_blank">Fig. 3</a>).</p

Measured conductivities (mean values ± standard error) in soil water under treatments with only labeled beech (Be*; n = 6), labeled ash (As*; n = 6), mixed ((BeAs)*; n = 12) and unlabeled litter treatments (unlabeled; n = 18).

<p>The dashed lines subdivide the experiment into the two winter (I: 16.12.08–30.03.09; II: 21.12.09–22.03.10) and two summer periods (I: 20.04.09–30.11.09; II: 07.04.10–31.05.10).</p

Measured DOM concentrations (1a) and δ¹³C values of the DOM (1b) in the soil water (mean ± standard error) under treatments with only labeled beech (Be*; n = 6), labeled ash (As*; n = 6), mixed ((BeAs)*; n = 12) and unlabeled litter treatments (unlabeled; n = 18).

<p>The dashed lines subdivide the experiment into the two winter (I: 16.12.08–30.03.09; II: 21.12.09–22.03.10) and two summer periods (I: 20.04.09–30.11.09; II: 07.04.10–31.05.10).</p

Determined amounts (± standard error) of litter-derived DOM per added litter-carbon summarized over the whole experiment for treatments with only labeled beech (Be*), labeled ash (As*) and mixed litter treatments (Be*As, BeAs*).

<p>The litter-derived DOM was significantly lower in the labeled beech (Be*, Be*As) treatments in comparison to the labeled ash (As*, BeAs*) treatments (p<0.01, n = 6, one-way ANOVA followed by Tukey’s HSD post hoc test).</p

Above and below ground carbohydrate allocation differs between ash (<i>Fraxinus excelsior</i> L.) and beech (<i>Fagus sylvatica</i> L.)

<div><p>We investigated soluble carbohydrate transport in trees that differed in their phloem loading strategies in order to better understand the transport of photosynthetic products into the roots and the rhizosphere as this knowledge is needed to better understand the respiratory processes in the rhizosphere. We compared beech, which is suggested to use mainly passive loading of transport sugars along a concentration gradient into the phloem, with ash that uses active loading and polymer trapping of raffinose family oligosaccharides (RFOs). We pulse-labeled 20 four-year old European beech and 20 four-year old ash trees with ¹³CO₂ and tracked the fate of the label within different plant compartments. We extracted soluble carbohydrates from leaves, bark of stems and branches, and fine roots, measured their amount and isotopic content and calculated their turnover times. In beech one part of the sucrose was rapidly transported into sink tissues without major exchange with storage pools whereas another part of sucrose was strongly exchanged with unlabeled possibly stored sucrose. In contrast the storage and allocation patterns in ash depended on the identity of the transported sugars. RFO were the most important transport sugars that had highest turnover in all shoot compartments. However, the turnover of RFOs in the roots was uncoupled from the shoot. The only significant relation between sugars in the stem base and in the roots of ash was found for the amount (r² = 0.50; p = 0.001) and isotopic content (r² = 0.47; p = 0.01) of sucrose. The negative relation of the amounts suggested an active transport of sucrose into the roots of ash. Sucrose concentration in the root also best explained the concentration of RFOs in the roots suggesting that RFO in the roots of ash may be resynthesized from sucrose. Our results interestingly suggest that in both tree species only sucrose directly entered the fine root system and that in ash RFOs are transported indirectly into the fine roots only. The direct transport of sucrose might be passive in beech but active in ash (sustained active up- and unloading to co-cells), which would correspond to the phloem loading strategies. Our results give first hints that the transport of carbohydrates between shoot and root is not necessarily continuous and involves passive (beech) and active (ash) transport processes, which may be controlled by the phloem unloading.</p></div

Relationship between sucrose in compartments stem base and root in beech.

<p>The solid line shows the total ¹³C enrichment [mg ¹³C g^-1DW] and the dashed line shows the concentrations [mg g^-1DW]. The filled symbols represent the mean values based on the independent replicates (open symbols).</p

Enrichment of total ¹³C in the main carbohydrates sucrose and raffinose group (RFO) in different plant compartments of beech (<i>Fagus sylvatica</i>) and ash (<i>Fraxinus excelsior</i>) from 1 day (17.08.2011) to 60 days after labeling.

<p>Black dots refer to different replicates of each time point for sucrose in beech <b>(A)</b> sucrose in ash <b>(B)</b> and RFO in ash <b>(C).</b> Different letters indicate significant differences (p ≤ 0.05) among days after labeling for a given carbohydrate (repeated measure ANOVA followed by post-hoc Tukey HSD test). (see also supporting information <a href="http://www.plosone.org/article/info:doi/10.1371/journal.pone.0184247#pone.0184247.s001" target="_blank">S1 Table</a>).</p