Organic matter dynamics along a salinity gradient in Siberian steppe soils
Authors: Bischoff Norbert , Mikutta Robert , Shibistova Olga , Dohrmann Reiner , Herdtle Daniel , Gerhard Lukas , Fritzsche Franziska , Puzanov Alexander , Silanteva Marina , Grebennikova Anna , Guggenberger Georg 
Published: January 2018

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.