149-10 Is the Physico-Chemical Environment of Small Pores in Soils Accurately Reflected in Current Soil Biogeochemical Models?.

Poster Number 1004

See more from this Division: SSSA Division: Soil Biology & Biochemistry
See more from this Session: Soil Biology & Biochemistry: II
Monday, November 3, 2014
Long Beach Convention Center, Exhibit Hall ABC
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Tobias Karl David Weber, Technische Universität Braunschweig, Institute of Geoecology, Department Soil Science and Soil Physics, Braunschweig, GERMANY and Thomas Riedel, ICBM-MPI-Brckengruppe fr Marine Mikrobiologie, Carl-von-Ossietzky Universitt, Oldenburg, Germany
Chemical reactions and biological activity in earths upper crust depend on free water. The void volume between the solid compounds provides space for water, air, and organisms that thrive on the consumption of minerals and organic matter thereby regulating soil carbon turnover. However, not all water in the pore space in soils and sediments is in its liquid state, because small pores and charged mineral surfaces reduce its activity through adhesive forces. This water has a lower tendency to react chemically in solution as this additional binding energy lowers its activity. In this work, we estimated the amount of soil pore water that is thermodynamically different from a simple aqueous solution. The quantity of soil pore water with properties different from liquid water (here defined by water activity < 1) was found to systematically increase with increasing clay content. The thermodynamic state of water was therefore apparently affected by the grain size. This implies that current methods to determine the amount of water content, traditionally determined from bulk density or gravimetric water content after drying at 105C overestimates the amount of free water in a soil especially at higher clay content. Our findings have consequences for biogeochemical processes in soils. From water activity measurements on a set of various soils with 0 to 100 wt-% clay, we can show that 5 to 130 mg H2O per g of soil can generally be considered as unsuitable for microbial respiration. These results may therefore provide a unifying explanation for the grain size dependency of organic matter preservation in sedimentary environments and call for a revised view on the biogeochemical environment in soils and sediments.
See more from this Division: SSSA Division: Soil Biology & Biochemistry
See more from this Session: Soil Biology & Biochemistry: II