Abstract

Abrupt increases in the temperature sensitivity of soil respiration below 0 degrees C have been interpreted as a change in the dominance of other co-dependent environmental controls, such as the availability of liquid-state water. Yet the relationship between unfrozen water content and soil respiration at sub-zero temperatures has received little attention because of difficulties in measuring unfrozen water contents. Using a recently-developed semi-solid H-2 NMR technique the unfrozen water content present in seasonally frozen boreal forest soils was quantified and related to biotic CO2 efflux in laboratory microcosms maintained at temperatures between -0.5 and -8 degrees C. In both soils the unfrozen water content had an exponential relationship with temperature and was increased by addition of KCl solutions of defined osmotic potential. Approximately 13% unfrozen water was required to release the dependence of soil respiration on unfrozen water content. Depending on the osmotic potential of soil solution, this threshold unfrozen water content was associated with temperatures down to -6 degrees C; yet if temperature were the predictor of CO2 efflux, then the abrupt increase in the temperature sensitivity of CO2 efflux was associated with -2 degrees C, except in soils amended with -1500 kPa KCl which did not show any abrupt changes in temperature sensitivity. The KCl-amendments also had the effect of decreasing Q(10) values and activation energies (Ea) by factors of 100 and three, respectively, to values comparable with those for soil respiration in unfrozen soil. The disparity between the threshold temperatures and the reductions in Q(10) values and activation energies after KCl amendment indicates the significance of unfrozen water availability as an environmental control of equal importance to temperature acting on sub-zero soil respiration. However, this significance was diminished when soils were supplied with abundant labile C (sucrose) and the influences of other environmental controls, allied to the solubility and diffusion of respiratory substrates and gases, are considered to increase. 2010 Elsevier Ltd. All rights reserved.

Keywords

C-13 natural abundance; Activation energy; Boreal forest; Frozen soil; Organic matter quality; Osmotic potential; Q(10); H-2 semi-solid state NMR; Sucrose

Published in

Soil Biology and Biochemistry
2010, Volume: 42, number: 9, pages: 1396-1407
Publisher: Elsevier

SLU Authors

• Öquist, Mats

  • Department of Forest Ecology and Management, Swedish University of Agricultural Sciences
    

UKÄ Subject classification

Soil Science


Publication identifier

DOI: https://doi.org/10.1016/j.soilbio.2010.04.018

Permanent link to this page (URI)

https://res.slu.se/id/publ/30324