Biogeosciences, 15, 13-29, 2018
https://doi.org/10.5194/bg-15-13-2018
© Author(s) 2018. This work is distributed under
the Creative Commons Attribution 3.0 License.
Research article
03 Jan 2018
Organic matter dynamics along a salinity gradient in Siberian steppe soils
Norbert Bischoff1, Robert Mikutta2, Olga Shibistova1,3, Reiner Dohrmann4, Daniel Herdtle1, Lukas Gerhard1, Franziska Fritzsche1, Alexander Puzanov5, Marina Silanteva6, Anna Grebennikova6, and Georg Guggenberger1 1Institute of Soil Science, Leibniz University Hanover, Herrenhäuser Straße 2, 30419 Hanover, Germany
2Soil Science and Soil Protection, Martin Luther University Halle-Wittenberg, Von-Seckendorff-Platz 3, 06120 Halle (Saale), Germany
3VN Sukachev Institute of Forest, Siberian Branch of the Russian Academy of Sciences, Akademgorodok 50, 660036 Krasnoyarsk, Russian Federation
4Federal Institute for Geosciences and Natural Resources, Stilleweg 2, 30655 Hanover, Germany
5Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences, Molodezhnaya Street 1, 656038 Barnaul, Russian Federation
6Faculty of Biology, Altai State University, Prospekt Lenina 61a, 656049 Barnaul, Russian Federation
Abstract. Salt-affected soils will become more frequent in the next decades as arid and semiarid ecosystems are predicted to expand as a result of climate change. Nevertheless, little is known about organic matter (OM) dynamics in these soils, though OM is crucial for soil fertility and represents an important carbon sink. We aimed at investigating OM dynamics along a salinity and sodicity gradient in the soils of the southwestern Siberian Kulunda steppe (Kastanozem, non-sodic Solonchak, Sodic Solonchak) by assessing the organic carbon (OC) stocks, the quantity and quality of particulate and mineral-associated OM in terms of non-cellulosic neutral sugar contents and carbon isotopes (δ13C, 14C activity), and the microbial community composition based on phospholipid fatty acid (PLFA) patterns. Aboveground biomass was measured as a proxy for plant growth and soil OC inputs. Our hypotheses were that (i) soil OC stocks decrease along the salinity gradient, (ii) the proportion and stability of particulate OM is larger in salt-affected Solonchaks compared to non-salt-affected Kastanozems, (iii) sodicity reduces the proportion and stability of mineral-associated OM, and (iv) the fungi : bacteria ratio is negatively correlated with salinity. Against our first hypothesis, OC stocks increased along the salinity gradient with the most pronounced differences between topsoils. In contrast to our second hypothesis, the proportion of particulate OM was unaffected by salinity, thereby accounting for only  <  10 % in all three soil types, while mineral-associated OM contributed  >  90 %. Isotopic data (δ13C, 14C activity) and neutral sugars in the OM fractions indicated a comparable degree of OM transformation along the salinity gradient and that particulate OM was not more persistent under saline conditions. Our third hypothesis was also rejected, as Sodic Solonchaks contained more than twice as much mineral-bound OC than the Kastanozems, which we ascribe to the flocculation of OM and mineral components under higher ionic strength conditions. Contrary to the fourth hypothesis, the fungi : bacteria ratio in the topsoils remained fairly constant along the salinity gradient. A possible explanation for why our hypotheses were not affirmed is that soil moisture covaried with salinity along the transect, i.e., the Solonchaks were generally wetter than the Kastanozems. This might cause comparable water stress conditions for plants and microorganisms, either due to a low osmotic or a low matric potential and resulting in (i) similar plant growth and hence soil OC inputs along the transect, (ii) a comparable persistence of particulate OM, and (iii) unaffected fungi : bacteria ratios. We conclude that salt-affected soils contribute significantly to the OC storage in the semiarid soils of the Kulunda steppe, while most of the OC is associated with minerals and is therefore effectively sequestered in the long term.

Citation: Bischoff, N., Mikutta, R., Shibistova, O., Dohrmann, R., Herdtle, D., Gerhard, L., Fritzsche, F., Puzanov, A., Silanteva, M., Grebennikova, A., and Guggenberger, G.: Organic matter dynamics along a salinity gradient in Siberian steppe soils, Biogeosciences, 15, 13-29, https://doi.org/10.5194/bg-15-13-2018, 2018.
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This study suggests that soil moisture significantly affects soil organic matter dynamics along a salinity gradient in semiarid steppe soils. The covarying moisture gradient along the salinity gradient serves as an explanatory factor for (i) the increasing soil organic carbon (OC) stocks with increasing salinity, (ii) the constant proportion and stability of particulate OC along the transect, and (iii) a similar fungi : bacteria ratio in the topsoils along the studied gradient.
This study suggests that soil moisture significantly affects soil organic matter dynamics along...
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