Resolving the conundrum of highly stable organic matter in Plaggic Anthrosols

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The proposed research project aims to elucidate why highly stable organic matter (OM) persists in Plaggic Anthrosols. The formation of Plaggic Anthrosols is linked with a former technique of arable land management - the plaggen agriculture. Plaggen agriculture probably started in the 10th century A.D. in NW Europe. To improve the fertility of nutrient-poor and acidic sandy soils (e.g., Podzols), sods from heather and grassland were cut, used for animal bedding in stables, partly mixed with household residues used for composting and subsequently applied to arable fields. Straw residues or forest floor materials were also used as plaggen material. Over centuries plaggen agriculture led to the formation of a plaggic horizon. The plaggic horizon is still enriched in soil organic matter (SOM), even though plaggen agriculture was terminated more than 100 years ago. These large organic carbon (OC) stocks illustrate the high stability of SOM in these soils, which might be used as model systems to understand long-term OC stabilization in sandy arable soils. Our proposal will elucidate why sandy Plaggic Anthrosols have higher SOM levels compared to adjacent sandy arable soils without plaggen history. We will investigate contents and stocks of OC in the bulk soil and SOM composition in soil fractions, particularly in organo-mineral associations. We will study the contribution of recalcitrant plant materials of sods, charcoal or the role of mineral-associated OM to stable SOM. Special attention will be paid to the role of Fe(hydr)oxides and phyllosilicates for the formation of stable SOM. Investigations of long-term mineralization processes of SOM by an incubation experiment will allow to analyse SOM stability in Plaggic Anthrosols. For the first time, we will simulate historic plaggen amendment by application of litter from (i) oat straw, (ii) oak, and (iii) 13C labeled litter from heather in combination with excrements to convert sandy arable soils into plaggic soil material. During incubation, the 14C content of the released CO2 will be determined to clarify if OM in Plaggic Anthrosols is more stable than in adjacent arable soils and if this is related to a specific OM fraction (e.g., mineral-associated OM). With this approach, we will be able to contribute to the understanding why highly stable OM persists in Plaggic Anthrosols and to elucidate the relevant mechanisms of OM stabilization. Using these data it will be possible to derive a conceptual model for OM storage in Plaggic Anthrosols, which may have implications for further soil management to improve carbon sequestration in sandy soils.