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

How well does ramped thermal oxidation quantify the age distribution of soil carbon? Assessing thermal stability of physically and chemically fractionated soil organic matter

Carbon (C) in soils persists on a range of timescales depending on physical, chemical, and biological processes that interact with soil organic matter (SOM) and affect its rate of decomposition. Together these processes determine the age distribution of soil C. Most attempts to measure this age distribution have relied on operationally defined fractions using properties like density, aggregate stability, solubility, or chemical reactivity. Recently, thermal fractionation, which relies on the activation energy needed to combust SOM, has shown promise for separating young from old C by applying increasing heat to decompose SOM. Here, we investigated radiocarbon (C-14) and C-13 of C released during thermal fractionation to link activation energy to the age distribution of C in bulk soil and components previously separated by density and chemical properties. While physically and chemically isolated fractions had very distinct mean C-14 values, they contributed C across the full temperature range during thermal analysis. Thus, each thermal fraction collected during combustion of bulk soil integrates contributions from younger and older C derived from components having different physical and chemical properties but the same activation energy. Bulk soil and all density and chemical fractions released progressively older and more C-13-enriched C with increasing activation energy, indicating that each operationally defined fraction itself was not homogeneous but contained a mix of C with different ages and degrees of microbial processing. Overall, we found that defining the full age distribution of C in bulk soil is best quantified by first separating particulate C prior to thermal fractionation of mineral-associated SOM. For the Podzol analyzed here, thermal fractions confirmed that similar to 95 % of the mineral-associated organic matter (MOM) had a relatively narrow C-14 distribution, while 5 % was very low in C-14 and likely reflected C from the < 2 mm parent shale material in the soil matrix. After first removing particulate C using density or size separation, thermal fractionation can provide a rapid technique to study the age structure of MOM and how it is influenced by different OM-mineral interactions