Resolving the conundrum of highly stable organic matter in Plaggic Anthrosols
Final Report Abstract
The project aimed to understand long-term soil organic matter (SOM) accumulation in sandy Plaggic Anthrosols with a focus on the relevant stabilization mechanisms for SOM. During the first part of the project, we provided an inventory of the OC concentrations and stocks for 8 sites (three replicate profiles) for Plaggic Anthrosols and adjacent reference soils. All soils are under agricultural use and were sampled in West Central Europe. Compared to the ~ 30 cm thick topsoils of the reference soils, the Plaggic Anthrosols are characterized by topsoil horizons of 53 to 124 cm thickness (plaggic topsoil). In comparison to the reference soils, the increased topsoil volume, and the slightly higher OC concentrations in top- and subsoil horizons of Plaggic Anthrosols resulted in higher OC stocks. Special attention was paid to the role of Fe(hydr)oxides and phyllosilicates for the formation of stable SOM. However, the results did not indicate associations with fine-sized minerals as we did not find any correlation between the content of clay and Fe oxides/hydroxides and the concentration of OC. Stabilization by organo-mineral associations seems not of major relevance in the investigated Plaggic Anthrosols. The results obtained here rather relate to a self-sustaining effect in the Plaggic Anthrosols: the thick topsoil horizon with high OC concentrations improve plant growth and yields by higher availability of water and nutrients resulting in sustaining high inputs of OM. The results from soil carbon fractionation in the second part of the project showed that both Plaggic Anthrosols as well as adjacent reference soils are similar with respect to their fractional concentration of OC, radiocarbon concentration, and SOM composition. The isolated fine fraction (≤ 20 µm particle size) contained, on average, 81% of the total soil OC in only 9% of the corresponding soil mass. The OM composition of the fine fraction was specifically rich in alkyl carbon and the high mean conventional radiocarbon ages indicated low inputs of OC derived from recent photosynthesis. Overall, the findings of the project showed that very sandy agricultural soils are able to accumulate OM despite of their low proportion of fine fraction material. This points to specific OC accumulation processes in the investigated soils (and probably also other sandy agricultural soils in West Central Europe) that leads to storage of OM for long time periods, but which goes beyond a mechanistic explanation just by association of OM with fine mineral surfaces. In the third part, two soil incubation experiments were conducted to investigate the stability of soil OC and the age of the carbon source in Plaggic Anthrosols, as well as the impact of different source materials combined with animal excrements on the formation of stable plaggic OM. The six-month soil incubation experiment revealed that CO2 in the plaggic horizons originated from the mineralization of older C sources compared to those in recent topsoil horizons. Despite higher carbon contents, CO2 production was two to three times lower in the plaggic horizons than in the topsoil horizons, indicating the long-term stability of organic matter (OM) in plaggic horizons. The second incubation experiment revealed that recalcitrant plant residues, such as Calluna vulgaris, produced less CO2 compared to more labile residues like oat litter, supporting the hypothesis that slow-degrading plant materials are essential for the formation of stable SOM in Plaggic Anthrosols. These findings underscore the potential for carbon sequestration in sandy agricultural soils, even a century after plaggen agriculture system has ceased.
Publications
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Legacy of plaggen agriculture: High soil organic carbon stocks as result from high carbon input and volume increase. Geoderma, 406, 115513.
Urbanski, Livia; Schad, Peter; Kalbitz, Karsten; van Mourik, Jan; Gehrt, Ernst & Kögel-Knabner, Ingrid
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Unexpected high alkyl carbon contents in organic matter-rich sandy agricultural soils of Northwest Central Europe. Geoderma, 439, 116695.
Urbanski, Livia; Kalbitz, Karsten; Rethemeyer, Janet; Schad, Peter & Kögel-Knabner, Ingrid
