Jena Soil Model (JSM v1.0; revision 1934): a microbial soil organic carbon model integrated with nitrogen and phosphorus processes

Plant–soil interactions, such as the coupling of plants' below-ground biomass allocation with soil organic matter (SOM) decomposition, nutrient release and plant uptake, are essential to understand the response of carbon (C) cycling to global changes. However, these processes are poorly represented in the current terrestrial biosphere models owing to the simple first-order approach of SOM cycling and the ignorance of variations within a soil profile. While the emerging microbially explicit soil organic C models can better describe C formation and turnover, at present, they lack a full coupling to the nitrogen (N) and phosphorus (P) cycles with the soil profile. Here we present a new SOM model – the Jena Soil Model (JSM) – which is microbially explicit, vertically resolved and integrated with the N and P cycles. To account for the effects of nutrient availability and litter quality on decomposition, JSM includes the representation of enzyme allocation to different depolymerisation sources based on the microbial adaptation approach as well as of nutrient acquisition competition based on the equilibrium chemistry approximation approach. Herein, we present the model structure and basic features of model performance in a beech forest in Germany. The model reproduced the main SOM stocks and microbial biomass as well as their vertical patterns in the soil profile. We further tested the sensitivity of the model to parameterisation and showed that JSM is generally sensitive to changes in microbial stoichiometry and processes

Zeitliche Änderung des Gehaltes und der Isotopie von Kohlenstoff und Stickstoff in Böden des Hainich Nationalparks

Wälder gelten derzeit als wichtige Senke für das Treibhausgas CO2, wobei Düngung durch N-Deposition und steigende CO2 Konzentrationen in der Atmosphäre eine Rolle spielen. Ob dies zu einer langfristigen Änderung der C-Speicherung im Boden führen wird ist ungeklärt und wird davon abhängen, in welcher Form der zusätzlich über die Biomasse in den Boden gelangende Kohlenstoff dort verbleibt. Die Sorption von OC an Mineraloberflächen gilt derzeit als wichtigster Mechanismus zur Stabilisierung des Kohlenstoffs in Böden, doch auch er hat Teile mit schnellen Umsatzzeiten. Um herauszufinden, wie sich die Kohlenstoff- und Stickstoffspeicherung in Böden naturnaher Wälder entwickelt, haben wir an einem Standort im Hainich Nationalpark in den Jahren 2004, 2009 und 2016 an jeweils 10 Bodenbohrkernen von 0-50 cm Tiefe die Gehalte an C, N und 14C untersucht. Zusätzlich wurde der Boden in den Tiefen von 0-5 cm und 10-20 cm mittels Dichtefraktionierung in die leichte und freie partikuläre (FPOM), die okkludierte partikuläre (OPOM) und die mineralgebundene (MOM) Fraktion des OC separiert und analysiert (OC, N, 14C). Erste Ergebnisse zeigen, dass der ∆14C-Gehalt des Gesamtbodens in 0-5 cm von 2004 bis 2016 von 86±12‰ auf 24±7‰ abgenommen hat. Die durchschnittliche jährliche Abnahme von 5,2‰ liegt damit in einer ähnlichen Größenordnung wie die des atmosphärischen CO2 (4.6‰ von 2004 bis 2014). Mit zunehmender Bodentiefe nehmen absolute 14C-Gehalte und deren zeitliche Änderungen bis in 10-20 cm Tiefe ab, was auf reduzierte Umsatzzeiten des Gesamtbodens und einen geringeren Anteil an aktivem OC im Unterboden schließen lässt. Während ein Teil der starken Abnahme im 14C-Gehalt in 0-5 cm auch durch die leicht geringere OC Konzentrationen im Jahr 2016 erklärt werden kann, trifft dies auf die überraschend großen beobachteten 14C-Abnahmen unterhalb von 20 cm nicht zu, was darauf hinweist, dass auch im Unterboden Umsatzprozesse stattfinden können, die wir noch nicht verstehen. Dazu gehört auch, dass Die Ergebnisse der Dichtefraktionierung werden zeigen, in welchem Umfang die beobachteten zeitlichen Änderungen im Gesamtboden auf Änderungen der Anteile der Fraktionen und deren Umsatzzeiten zurückzuführen sind

Editorial: Carbon storage in agricultural and forest soils

International audienc

Impacts of Drying and Rewetting on the Radiocarbon Signature of Respired CO2 and Implications for Incubating Archived Soils

The radiocarbon signature of respired CO2 (∆14C-CO2) measured in laboratory soil incubations integrates contributions from soil carbon pools with a wide range of ages, making it a powerful model constraint. Incubating archived soils enriched by “bomb-C” from mid-20th century nuclear weapons testing would be even more powerful as it would enable us to trace this pulse over time. However, air-drying and subsequent rewetting of archived soils, as well as storage duration, may alter the relative contribution to respiration from soil carbon pools with different cycling rates. We designed three experiments to assess air-drying and rewetting effects on ∆14C-CO2 with constant storage duration (Experiment 1), without storage (Experiment 2), and with variable storage duration (Experiment 3). We found that air-drying and rewetting led to small but significant (α < 0.05) shifts in ∆14C-CO2 relative to undried controls in all experiments, with grassland soils responding more strongly than forest soils. Storage duration (4–14 y) did not have a substantial effect. Mean differences (95% CIs) for experiments 1, 2, and 3 were: 23.3‰ (±6.6), 19.6‰ (±10.3), and 29.3‰ (±29.1) for grassland soils, versus −11.6‰ (±4.1), 12.7‰ (±8.5), and −24.2‰ (±13.2) for forest soils. Our results indicate that air-drying and rewetting soils mobilizes a slightly older pool of carbon that would otherwise be inaccessible to microbes, an effect that persists throughout the incubation. However, as the bias in ∆14C-CO2 from air-drying and rewetting is small, measuring ∆14C-CO2 in incubations of archived soils appears to be a promising technique for constraining soil carbon models

Storage and stability of organic carbon in soils as related to depth, occlusion within aggregates, and attachment to minerals

Conceptual models suggest that stability of organic carbon (OC) in soil depends on the source of plant litter, occlusion within aggregates, incorporation in organomineral complexes, and location within the soil profile. Density fractionation is a useful tool to study the relevance of OC stabilization in aggregates and in association with minerals, but it has rarely been applied to full soil profiles. We aim to determine factors shaping the depth profiles of physically unprotected and mineral associated OC and test their relevance for OC stability across a range of European soils that vary in vegetation, soil types, parent material, and land use. At each of the 12 study sites, 10 soil cores were sampled to 60 cm depth and subjected to density separation. Bulk soil samples and density fractions (free light fractions - fLF, occluded light fractions - oLF, heavy fractions - HF) were analysed for OC, total nitrogen (TN), δ13C, and Δ14C Bulk samples were also incubated to determine CO2 evolution per g OC in the samples (specific mineralization rates) as an indicator for OC stability. Depth profiles of OC in the light fraction (LF-OC) matched those of roots for undisturbed grassland and forest sites, suggesting that roots are shaping the depth distribution of LF-OC. Organic C in the HF declined less with soil depth than LF-OC and roots, especially at grassland sites. The decrease in Δ14C (increase in age) of HF-OC with soil depth was related to soil pH as well as to dissolved OC fluxes. This indicates that dissolved OC translocation contributes to the formation of subsoil HF-OC and shapes the Δ14C profiles. The LF at three sites were rather depleted in 14C, indicating the presence of fossil material such as coal and lignite, probably inherited from the parent material. At the other sites, modern Δ14C signatures and pos sit tive correlations between specific mineralization rates and fLF-OC indicate the fLF is a potentially available energy and nutrient source for subsurface microorganisms throughout the profile. Declining specific mineralization rates with soil depth confirm greater stability of OC in subsoils across sites. The overall importance of OC stabilization by binding to minerals was demonstrated by declining specific mineralization rates with increasing contributions of HF-OC to bulk soil OC, and the low Δ14C values of HF-OC. The stability of HF-OC was greater in subsoils than in topsoils; nevertheless, a portion of HF-OC was active throughout the profile. While quantitatively less important than OC in the HF, consistent older ages of oLF-OC than fLF-OC suggest that occlusion of LF-OC in aggregates also contributes to OC stability in subsoils. Overall, our results indicate that association with minerals is the most important factor in stabilization of OC in soils, irrespective of vegetation, soil type, and land use. © Author(s) 2013.European Unio