Limited protection of macro-aggregate occluded organic carbon in
Siberian steppe soils
Norbert Bischoff1, Robert Mikutta2, Olga Shibistova1,3, Alexander Puzanov4, Marina Silanteva5, Anna Grebennikova5, Roland Fuß6, and Georg Guggenberger1,31Institute of Soil Science, Leibniz Universität Hannover, Hannover, 30419, Germany 2Soil Science and Soil Protection, Martin-Luther University Halle-Wittenberg, Halle, 06120, Germany 3VN Sukachev Institute of Forest, Siberian Branch of the Russian Academy of Sciences, Krasnoyarsk, 660036, Russian Federation 4Institute for Water and Environmental Problems, Siberian Branch of the Russian Academy of Sciences, Barnaul, 656038, Russian Federation 5Faculty of Biology, Altai State University, Barnaul, 656049, Russian Federation 6Institute of Climate-Smart Agriculture, Johann Heinrich von Thünen Institute, Braunschweig, 38116, Germany
Received: 28 Nov 2016 – Accepted for review: 16 Jan 2017 – Discussion started: 16 Jan 2017
Abstract. Macro-aggregates especially in agricultural steppe soils are supposed to play a vital role for soil organic carbon (OC) stabilization at a decadal time scale. While most research on soil OC stabilization in steppes focused on North American prairie soils of the Great Plains with information mainly provided by short-term incubation experiments, little is known about the agricultural steppes in south-western Siberia, though they belong to the greatest conversion areas in the world and occupy an area larger than that in the Great Plains. To quantify the proportion of macro-aggregate protected OC under different land-use and as function of land-use duration and intensity in Siberian steppe soils, we determined OC mineralization rates of intact (25–2000 µm) and crushed (< 250 µm) macro-aggregates in long-term incubations over 401 days (20 °C; 60 % water holding capacity) along two agricultural chronosequences in the Siberian Kulunda steppe. Additionally we incubated bulk soil (< 2000 µm) to determine the effect of land-use change (LUC) and subsequent agricultural use on a fast and a slow soil OC pool (labile vs. more stable OC), as derived from fitting exponential decay models to incubation data. We hypothesized that (i) macro-aggregate crushing leads to increased OC mineralization due to an increasing microbial accessibility of a previously occluded labile macro-aggregate OC fraction, and (ii) bulk soil OC mineralization rates and the size of the fast OC pool are higher in pasture than in arable soils with decreasing bulk soil OC mineralization rates and size of the fast OC pool as land-use duration and intensity increase. Against our hypothesis, OC mineralization rates of crushed macro-aggregates were similar to those of intact macro-aggregates under all land-use regimes. Macro-aggregate protected OC was almost absent and accounted for < 1 % of the total macro-aggregate OC content and to maximally 8 ± 4 % of mineralized OC. In accordance to our second hypothesis, highest bulk soil OC mineralization rates and sizes of the fast OC pool were determined under pasture, but mineralization rates and pool sizes were unaffected by the duration and intensity of land-use. However, mean residence times of the fast and slow OC pool tended to become shorter along one chronosequence. We conclude, that the tillage-induced break-down of macro-aggregates has not reduced the OC contents in the soils under study. The decline of OC after LUC is probably attributed to the faster soil OC turnover under arable land as compared to pasture at a reduced plant residue input.
Bischoff, N., Mikutta, R., Shibistova, O., Puzanov, A., Silanteva, M., Grebennikova, A., Fuß, R., and Guggenberger, G.: Limited protection of macro-aggregate occluded organic carbon in
Siberian steppe soils, Biogeosciences Discuss., doi:10.5194/bg-2016-518, in review, 2017.