The capability of coastal wetlands to store large amounts of soil organic carbon (SOC) has been increasingly recognised. Stabilisation mechanisms (e.g. aggregation or mineral association) and the stability of organic matter (OM) (recalcitrant vs. labile) are important features for the long-term storage of SOC. In estuarine marshes, SOC storage is dominated by a complex and dynamic interaction of abiotic conditions such as tidal inundation or changes in salinity. Little is known on OC stabilisation and stability in these transitional ecosystems and how they are affected by system-specific characteristics. Therefore, our aim was to assess the effect of flooding and salinity on (i) OC stabilisation by aggregation and mineral association, (ii) the stability of OC and (III) dissolved organic carbon (DOC) formation in estuarine marsh soils.
We analysed topsoil (0 – 10 cm) and subsoil (10 – 30 cm) samples from 9 marsh zones along the salinity gradient (salt, brackish and freshwater) and flooding gradient (pioneer zone, low and high marsh) of the Elbe Estuary for total SOC contents, OC stabilisation mechanisms (density fractionation), aggregate fractionation, OC stability (incubation with one- and two-compartment model fits), DOC concentrations and microbial biomass.
Total SOC contents were positively correlated with mineral-associated OC (CMAOM). Proportion of CMAOM was highest in topsoils of freshwater marshes and decreased towards higher salinities in topsoils of high marshes and pioneer zones. The OM protection by aggregation (CoPOM) increased in topsoils of high marshes. The proportion of pedogenically unprotected particulate organic matter (CfPOM) correlated positively with the potential mineralisable C (Cpot) and labile C (Clabile) and negatively with the recalcitrant C pool (Crecalcitrant) that were derived from the one- and two-compartment models. Labile C, Cpot and Crecalcitrant were strongly influenced by the CN ratio of the aboveground biomass. Moreover, Crecalcitrant was linked to the proportion of CMAOM. Concentrations of DOC increased with total SOC and Cpot but decreased with CoPOM.
We conclude that SOC stabilisation in the Elbe Estuary is mainly related to mineral association of OM. With increasing terrestrial influence, the contribution of physical protection in aggregates increases – which reduces DOC formation. Besides these pedogenic stabilisation mechanisms, recalcitrance is strongly determined by vegetation characteristics.

Cover crops are mostly grown after the harvest of a primary crop and can increase soil organic carbon content, mainly by providing additional inputs of biomass carbon to the soil. Their cultivation also has additional benefits, such as improved water and nutrient retention, erosion prevention and increased biodiversity. Studies have shown that root biomass generally contributes more to soil organic carbon accumulation in soils than above-ground biomass. However, to date, little data is available on root biomass and root to shoot ratios of cover crop species.
In November 2021 and again in November 2022, we sampled root biomass of 14 cover crop species and a range of mixtures, and from three different sowing times, in field trials run by the seed producer KWS. Absolute root biomass and root to shoot ratios were strongly depended on the cover crop species. Root biomass decreased by up to about 80% between the first sawing date in the beginning of August and the last sawing date in mid-September. Interestingly, also the root to shoot ratio depended on the sawing date, suggesting changes in resource allocation between above- and below-ground. Based on this data, we are going to estimate the potential to increase soil carbon accumulation by selecting cover crop species with high root biomass. We are also analysing root traits of the cover crops, such as carbon and nitrogen content, specific root length, root diameter and rooting depth, as these traits are also relevant to soil carbon dynamics. Additionally, we are going to test if root traits can be used to predict root biomass at different sowing times.
Our results allow us to give recommendations for the selection of cover crops with the potential to increase soil carbon sequestration. This data is also useful for more accurate mechanistic modelling of the soil carbon content under different management scenarios.

Wenn Totholz im Wald verbleibt, kann dies die Kohlenstoffspeicherung in Waldböden erhöhen, wenn die aus dem Totholz freiwerdende organische Substanz in den Boden gelangt und dort stabil gespeichert wird. Wie sich Totholz zersetzt und welche Faktoren diesen Prozess behindern oder beschleunigen ist schon umfangreich erforscht. Über den Verbleib des aus dem Totholz freigesetzten Kohlenstoffs (C) gibt es bislang aber nur unzureichende Kenntnisse, vor allem bezüglich der Standortabhängigkeit. Bei der Holzzersetzung wird C infolge mikrobieller Abbauprozesse als CO2 freigesetzt, in Form fester Partikel durch Bodentiere in den Boden eingearbeitet oder in gelöster Form mit dem Niederschlagswasser in den Boden transportiert. Während die CO2-Freisetzung eine C-Quelle in der Treibhausgasbilanz darstellt, kann der Eintrag von gelöstem oder partikulärem C in den Boden die C-Senkenfunktion erhöhen. Unter welchen Bedingungen der in den Boden eingetragene C aber tatsächlich langfristig stabil gespeichert wird, ist bislang kaum erforscht. Um zu verstehen, unter welchen Bedingungen totholzbürtiger C bevorzugt im Boden gespeichert wird und somit die C-Senkenfunktion verbessert oder über die Bodenwasser- und -gasphase ausgetragen wird, sind kontinuierliche Messungen der Bodengas- und Bodenwasserflüsse essentiell.
In dem vorgestellten Projekt werden zur Untersuchung dieser C-Flüsse auf verschiedenen Messplots Bodenwasserflüsse mittels Saugkerzen und CO2-Flüsse mittels Kammermessungen im Umfeld von liegendem Totholz erfasst. Erste Ergebnisse zeigen eine erhöhte Konzentration von gelöstem organischem Kohlenstoff (DOC) unter Totholz im Vergleich zum nicht von Totholz beeinflussten Boden. Ebenso waren im Umfeld von Totholz die CO2-Flüsse im Vergleich zum unbeeinflussten Boden erhöht. Die C-Flüsse unterscheiden sich systematisch zwischen den untersuchten Baumarten (Fichte, Buche) und variieren mit dem standörtlichen Wasserhaushalt. Mit Hilfe der Daten sollen C-Bilanzen im Umfeld von liegendem Totholz erstellt und mit Untersuchungen der Kohlenstoffgehalte in der Bodenfestphase verknüpft werden.

The investigations for carbon sequestration in Plaggic Anthrosols is most often informed by bulk soil carbon inventories, without considering the form (particulate or mineral-associated) in which carbon is stored, its capacity, and the chemical composition. Here, we focus on the unusual high organic carbon (OC) accumulation in sandy Plaggic Anthrosols and their reference soils under agriculture use. In these soils, the fine-sized particles (fraction ≤ 20 µm), commonly assumed to be the major factor for OC stabilization, are very low in mass proportion. Soil organic matter (SOM) physical fractionation was done to evaluate the quantity and quality of OC in the topsoils (Ap horizon). For the resulting fine fraction ≤ 20 µm, we measured the OC concentration, the radiocarbon concentration, calculated the OC storage capacity and contribution, and analyzed the chemical composition of the carbon units by solid-state 13C NMR spectroscopy. This highly sandy (80−90%) soils showed an accumulation of OC much higher than the conventionally calculated saturation level controlled by the proportion of the fine fraction ≤ 20 µm. However, there was no correlation between the topsoil OC concentration of the medium, fine silt− and clay-sized fraction and the mass percentage of the particles ≤ 20 µm. Both Plaggic Anthrosols and the reference soils were comparably similar with respect to fractional concentration of OC, radiocarbon age, and its OM composition. The fine fractions ≤ 20 µm are characterized by a high mean OC concentration (reference soils: 226 ± 66.5 mg g−1, Plaggic Anthrosols: 202 ± 59.0 mg g−1). The SOM composition of the fine fraction was specifically rich in alkyl−C, with minor proportion of O-alkyl−C and low percentages of aryl−C. The radiocarbon concentration and the conventional radiocarbon age might indicate that fine fraction OM accumulated in topsoils received low inputs of OC derived from recent photosynthesis and is stored for long time periods with high mean radiocarbon ages not only for Plaggic Anthrosols (F14C: 0.92 ± 0.04; 14C: 639 yBP), but also for the reference soils (F14C: 0.93 ± 0.04; 14C: 575 yBP). It is not clear, if the OM inherited is stable under present-day soil and management conditions. Our data indicate that there may be specific mechanisms generally operative in sandy OM-rich agriculture soils for storing high amounts of OM that go beyond a mechanistic explanation just by association of OM with fine mineral surfaces.