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We propose a novel pragmatic approach of in situ 15N and 13C isotope labelling of trees for subsequent litter decomposition and turnover studies under field conditions. Using this method the labelling of even large trees under natural conditions is possible and compared to tree labelling under artificial conditions in greenhouses the in situ approach is less expensive. 13C and 15N labelling were carried out simultaneously via photosynthesis by tree gassing with 13CO2 and by stem injection of 15NH4 15NO3. The aims of this study were: (i) to produce a sufficient quantity of labelled plant material for subsequent field incubation studies and (ii) to investigate the effectiveness and distribution of in situ 15N (15NH4 15NO3) and 13C (13CO2) labelling of Podocarpus falcatus, Croton macrostachys, Prunus africana and Cupressus lusitanica. The following targets need to be achieved: (i) Assuming almost natural litter fall conditions, enough labelled plant material must be produced in situ for the turnover experiment; (ii) intra-plant tracer enrichment shall be homogeneous; (iii) tracer enrichment should be comparable for different tree species; and (iv) tracer enrichment must be sufficient for subsequent litter turnover studies using the stable isotope approach. Our results clearly demonstrated that several kilograms of labelled plant material can be produced in situ. For many ecosystems, this amount is sufficient for a long term litter turnover experiment on a field scale under almost natural litter fall conditions. However, intra-plant label uptake of 13C and 15N was heterogeneous so that only leaves (litter) should be used for the turnover study. It could be shown that only a part of the labile C and N fraction in the leaves was labelled. Nevertheless, label uptake was sufficient for subsequent litter turnover studies.

Amino acid turnover in soil is an important element of terrestrial carbon and nitrogen cycles. This study accounts for their driver - the microbial metabol

Obwohl Sorption an Mineraloberflachen als dominierender Prozess zur Erklarung des langsamen Umsatzes mineralassoziierter organischer Bodensubstanz (OBS) dient, widerspricht diese Idee einer zunehmenden Zahl an Inkubationsstudien, die zeigen, dass fur niedermolekulare Substanzen nicht nur die mikrobielle Aufnahme kompetitivier als die Sorption ist, sondern auch sorbierte Substanzen in hohem Mase desorbiert und mikrobiell verwertet werden konnen. Dabei wurde gezeigt, dass sich die Verstoffwechselung desorbierter Substanzen zugunsten eines erhohten Recyclings verschiebt. Dies wirft die Frage auf, ob Recycling von intakten Metaboliten, d.h. unter Erhalt des Kohlenstoffgerustes, generell ein bisher stark unterschatzter Prozess ist, der die relativ hohen 14C Alter der OBS teilweise erklaren kann. Nach Applikation hoher Toxindosen konnte nachgewiesen werden, dass die nachfolgende Reetablierung der mikrobiellen Gemeinschaft zu grosem Anteil auf Recycling der Nekromasskomponenten der Vorgeneration basiert. Intaktes Metabolitrecycling ist jedoch unter steady-state Bedingungen nur auserst schwierig von der direkten Stabiliserung zu unterscheiden. Um diesen Prozess im Fliesgleichgewicht nachzuweisen muss 1) ein Biomolekul untersucht werden, dessen Biosyntheseweg so aufwandig ist, dass Recycling einen deutlichen Vorteil fur die Zelle im Vergleich zur Neusynthese darstellt, 2) dieses Biomolkul in lebenden Zellen in einer anderen Form gebunden sein, als die zu recycelnde Einheit in der Bodenlosung, so dass beide Zustandsformen unterschieden werden konnen und 3) dieses Biomolekul positionsspezifisch isotopenmarkiert zugegeben werden, so dass uber einen identischen Einbau der Positionen die Intaktheit des Kohlenstoffgerustes nachgewiesen werden kann. Am Beispiel der Alkylketten von Fettsauren, die in mikrobiellen Zellen primar als Phospholipide in den Membranen gebunden sind, soll dieses Prinzip veranschaulicht werden. Eine erste Abschatzung des intakten Recyclings von Alkylketten durch Mikroorganismen in Boden zeigt, dass von den 0.03% der basierend auf Alkyl-Kohlenstoff neugebildeten PLFA mehr als 75% aus intaktem Recycling dieser Ketten hervor gingen. Obwohl der Beitrag des Recyclings intakter Metabolite zur Umsatzzeit der gesamten OBS aufgrund der geringen Anzahl bisher untersuchter Metabolite noch nicht final quantifiziert werden kann, untermauern die hier vorgestellten Ergebnisse jedoch die hohe Relevanz dieses Prozesses fur die Dynamik der OBS.

Most subsoil carbon (C) turnover occurs in biopore hotspots such as root channels and earthworm burrows. Biopores allocate large C amounts into the subsoil, where a vast capacity for long-term C sequestration is predicted. We hypothesise that organic matter (OM) cycling in biopores depends on their origin. Earthworm and root biopores were induced under field conditions and were sampled from the subsoil (45–75 and 75–105 cm) after two years of biopore formation. The effects of biopore formation on OM decomposition were studied by biomarkers: neutral sugars, cutin and suberin-derived lipids, lignin-derived phenols and free lipids. The degradation stage of OM was biopore type-specific but was only governed by the soil depth in root biopores. Degradation of OM increased from earthworm biopores to root biopores and bulk soil. Hemicelluloses (GM/AX ratio) were more strongly degraded than lignin side-chains (relative change from initial values). Two years of microbial processing during biopore formation increased the GM/AX ratio in earthworm biopores from 0.65 to 1.05 and in root biopores from 0.15 to 1.35 (both relative to source biomasses). Root biopores and bulk soil had the highest GM/AX ratios (1.2–1.3), hinting to rapid processing of plant residues and accumulation of microbial residues. The regular, frequent OM inputs by earthworms stimulated microbial growth and processing of mostly bioavailable OM and, thus, relatively enriched more persistent OM (e.g. lignin). Syringyl subunits of lignin underwent low (ratio changed from 0.35 to 0.55 relative to initial input) and vanillyl subunits underwent almost no processing in earthworm biopores indicating the preferential microbial utilisation of the easily available compounds frequently replenished by earthworm activity. After two years of decomposition of the root detritus, mainly structural plant material was enriched in root biopores. Short periods (6 months) of earthworm activity effectively recharged the highly processed OM in root biopores with fresh OM. In total, deep-rooting catch crops and short-term earthworm activities promote C accumulation in the subsoil followed by biopore-specific microbial processing predominantly governed by the C input frequency. As root biopores are up to 40 times more common than earthworm biopores, they dominate the OM input into subsoils. Such C inputs create several years lasting hotspots for preferential root growth and nutrient mobilisation in the subsoil. We conclude that root- and earthworm-derived biopores are vertical pathways for plant C from the soil surface into the subsoil and for intensive processing of litter C and sequestration of microbial necromass.

<p>Among the factors controlling root exudation, root hair proliferation and warming strongly influence exudate release, microbial substrate utilization and enzyme activities. The interactions of these two factors are important but poorly known in the rhizosphere. To clarify these interactions, two maize varieties – a wild type with root hairs and a hairless mutant – were grown at 20 and 30 °C for 2 weeks. We applied a unique combination of zymography to localize hotspots of β-glucosidase with microcalorimetry and substrate-induced respiration from soil sampled in hotspots. This approach enabled monitoring exudate effects on microbial growth strategy, enzyme kinetics (V<sub>max</sub> and K<sub>m</sub>), heat release and CO<sub>2</sub> production in the hotspots in response to warming.</p><p>Root hair effects on enzyme activity and efficiency were pronounced only at the elevated temperature: i) β-glucosidase activity of the wild type at 30 °C was higher than that of the hairless maize; ii) temperature shifted the microbial growth strategy, whereas root hairs (i.e. C input) promoted the fraction of growing microbial biomass; iii) K<sub>m</sub> and the activation energy for β-glucosidase under the hairless mutant was lower than that under wild maize. These results suggest that microorganisms inhabiting hotspots of the wild type synthesized more enzymes to fulfill their higher energy and nutrient demands than those of the hairless mutant. In contrast, at higher temperature the hairless maize produced an enzyme pool with higher efficiencies rather than higher enzyme production, enabling metabolic needs to be met at lower cost. These changes in enzyme kinetics and metabolic shifts confirmed evolutionary theory on tradeoffs of enzyme structure–function and thermal–substrate under warming at the soil hotspot level. We conclude that, if microbial and enzymatic activities are stimulated by more substrate input under warming, then this shift in the microbial community and in enzyme systems to a lower efficiency could offset C losses.</p>

Faeces incorporation can alter the concentration patterns of stanols, stanones, Δ5-sterols and bile acids in soils and terrestrial sediments. A joint quantification of these substances would give robust and specific information about the faecal input. Therefore, a method was developed for their purification and determination via gas chromatography–mass spectrometry (GC–MS) based on a total lipid extract (TLE) of soils and terrestrial sediments. Stanols, stanones, Δ5-steroles and bile acids were extracted by a single Soxhlet extraction yielding a TLE. The TLE was saponified with KOH in methanol. Sequential liquid–liquid extraction was applied to recover the biomarkers from the saponified extract and to separate the bile acids from the neutral stanoles, stanones and Δ5-steroles. The neutral fraction was directly purified using solid phase extraction (SPE) columns packed with 5% deactivated silica gel. The bile acids were methylated in dry HCl in methanol and purified on SPE columns packed with activated silica gel. A mixture of hexamethyldisilazane (HMDS), trimethylchlorosilane (TMCS) and pyridine was used to silylate the hydroxyl groups of the stanols and Δ5-sterols avoiding a silylation of the keto groups of the stanones in their enol-form. Silylation of the bile acids was carried out with N,O-bis(trimethylsilyl)trifluoroacetamide (BSTFA) containing N-trimethylsilylimidazole (TSIM). TLEs from a set of soils with different physico-chemical properties were used for method evaluation and for comparison of amounts of faecal biomarkers analysed with saponification and without saponification of the TLE. Therefore, a Regosol, a Podzol and a Ferralsol were sampled. To proof the applicability of the method for faecal biomarker analyses in archaeological soils and sediments, additional samples were taken from pre-Columbian Anthrosols in Amazonia and an Anthrosol from a site in central Europe settled since the Neolithic. The comparison of the amounts of steroids in combination with and without saponification of the TLE showed that high amounts of faecal biomarkers occur bound to other lipids and were liberated by saponification. The method was evaluated by standard addition. The standard contained 5β-stanols, 5β-stanones and their 5α-isomers together with Δ5-sterols and bile acids (19 substances). The standard addition revealed mean recoveries of individual substances ≥85%. The recoveries of biomarkers within each biomarker group did not differ significantly. Precisions were ≤0.22 (RSD) and quantification limits were between 1.3 and 10ngg−1 soil. These data showed that the method can be applied for quantification of trace amounts of faecal steroids and for the analyses of steroid patterns to detect enhanced faeces deposition in soils and sediments.

Abstract. The mean residence times (MRT) of different compound classes of soil organic matter (SOM) do not match their inherent recalcitrance to decomposition. One reason for this is the stabilisation within the soil matrix, but recycling, i.e. the reuse of "old" organic material to form new biomass may also play a role as it uncouples the residence times of organic matter from the lifetime of discrete molecules in soil. We analysed soil sugar dynamics in a natural 30 years old labelling experiment after a~wheat-maize vegetation change to determine the extent of recycling and stabilisation in plant and microbial derived sugars: while plant derived sugars are only affected by stabilisation processes, microbial sugars may be subject to both, stabilisation and recycling. To disentangle the dynamics of soil sugars, we separated different density fractions (free particulate organic matter (fPOM), light occluded particulate organic matter (≤1.6 g cm−3; oPOM1.6), dense occluded particulate organic matter (≤2 g cm−3; oPOM2) and mineral-associated organic matter (>2 g cm−3; Mineral)) of a~silty loam under long term wheat and maize cultivation. The isotopic signature of sugars was measured by high pressure liquid chromatography coupled to isotope ratio mass spectrometry (HPLC/IRMS), after hydrolysis with 4 M Trifluoroacetic acid (TFA). While apparent mean residence times (MRT) of sugars were comparable to total organic carbon in the bulk soil and mineral fraction, the apparent MRT of sugars in the oPOM fractions were considerably lower than those of the total carbon of these fractions. This indicates that oPOM formation was fuelled by microbial activity feeding on new plant input. In the bulk soil, mean residence times of the mainly plant derived xylose (xyl) were significantly lower than those of mainly microbial derived sugars like galactose (gal), rhamnose (rha), fucose (fuc), indicating that recycling of organic matter is an important factor regulating organic matter dynamics in soil.