Linking soil architecture formation with changing permafrost regime to carbon turnover in high latitude soils at multiple spatial scales
Final Report Abstract
Our study revealed that permafrost thaw, results in a number of physicochemical and biological processes leading to a successional development of hierarchical vertical and lateral spatial structures on the (poly-)profile, the pedon, and the sub-aggregate scale. On the (poly-)profile and pedon scale, advancing permafrost thaw leads to a decreasing horizontal heterogeneity, especially in the topsoil that could be attributed to a more homogeneous distribution of plant derived organic matter and thus C input. The spatial correlations between C contents and root-specific suberin monomers on the one hand, and pedogenic iron- and aluminum(hydr-)oxides on the other hand, that were most pronounced in the subsoil, demonstrate, that both, sorption processes and the incorporation of belowground residues to SOM are important controlling factors for the changing C distribution in soil.The spatial distribution and weathering of apatite and other primary P-bearing minerals, but also the distribution of organic P sources may furthermore contribute to the patchiness of root derived C input with ongoing permafrost thaw, as indicated by significant spatial correlations between suberin monomers and plant available P fractions. As a whole these processes contribute to an increasing vertical stratification of the soil profile with a particular high capacity of subsoils rich in pedogenic iron- and aluminum(hydr-)oxides for further C storage. However, mineralization of SOM especially from thawed organic layers and topsoils is a much faster process in comparison to the potential subsoil stabilization processes as also indicated by results from the incubation study. On the subaggregate scale, decreasing permafrost impact lead to a pronounced differentiation between conceptual carbon pools, i.e. fPOM, oPOM and MOM, with respect to the state of degradation of org. C. This differentiation was not found for soils with high permafrost impact, which clearly shows, that thawing leads to a quick alteration and redistribution of C in soil pools. Again, the correlation between C contents and content of pedogenic iron(hydr)oxides stressed the increasing importance of sorption for C storage in thawing permafrost, at least for subsoils. The fact that this correlation was not found for top soils indicates that here C input is the most important factor as also indicated by spatial correlations from WP 3 & 4. This is also supported by extrem young C ages in top soils for NP soils which follows the conceptual chromatographic stripping model in which transport of DOM is followed by sorption on minerals, microbial alteration and desorption by younger DOM again. Here, our incubation studie shows that microbial communities in iron poor top and iron rich subsoils are highly differing in their C metabolism: While in top soils high losses of old and young carbon appear, the microbial community of subsoils has a much higher C use efficiency leading to C-losses that were by half lower as for top soils. We therefore conclude that 1) Thawing permafrost soils develop a hierarchical stratified vertical C distribution, which is triggered by the distribution of root-derived C input and the presence of pedogenic minerals. 2) The importance of these two factors depends on the status of thawing and the vertical profile position. While for topsoil C storage and distribution, plant input is most important, C sorption is more decisive in subsoils. 3) The loss of carbon is additionally depending on the present microbial community, which shows higher C turnover in topsoils, which are thus prone to high C losses. 4) Loss of C in thawing permafrost soils will thus be highest for top soils poor in pedogenic minerals but subsoils hold a high capacity for C sequestration especially when root development und thus C input reaches lower soil horizons. Beside these findings, the importance of C sequestration in subsoils is also depending on the proliferation of SOM along the permafrost table, which was indicated by a second C max. found in subsoils of WP3. We would thus like to emphasize that future research should also consider the tempo-spatial dynamics of DOM reactive transport in this highly variable zone to fully understand C dynamics in thawing permafrost soils.
Publications
- (2018) Small-scale spatial patterns of soil organic carbon and nitrogen stocks in permafrost-affected soils of northern Siberia. Geoderma 329 91–107
Evgrafova, Alevtina; La Haye, Tilman René de; Haase, Ina; Shibistova, Olga; Guggenberger, Georg; Tananaev, Nikita; Sauheitl, Leopold; Spielvogel, Sandra
(See online at https://doi.org/10.1016/j.geoderma.2018.05.014)
