Investigating the role of moisture as an environmental constraint in the decomposition of shallow and deep mineral soil organic matter of a temperate coniferous soil Carrie-Ellen Gabriel a, b, * , Lisa Kellman a,1 a St. Francis Xavier University, Department of Earth Sciences, 5009 Chapel Square, Room 2052, Physical Sciences Centre, Antigonish, Nova Scotia B2G 2W5, Canada b Dalhousie University, Earth Sciences Department, 1459 Oxford Street, Room 3006, Life Sciences Centre, Halifax, Nova Scotia B3H4R2, Canada article info Article history: Received 6 March 2013 Received in revised form 30 September 2013 Accepted 4 October 2013 Available online 22 October 2013 Keywords: Forest soil carbon Soil organic matter Decomposition Heterotrophic respiration Moisture Temperature sensitivity Q 10 Mineral soil abstract Temperature and moisture are primary environmental drivers of soil organic matter (SOM) decompo- sition, and an improved understanding of how they interact to control SOM cycling processes in temperate forest soils is needed. Intact soil cores from shallow (0e25 cm) and deep (25e50 cm) mineral soils were incubated under constant and/or diurnal temperature regimes, and subjected to a series of moisture manipulations within a climate-controlled facility. Soil temperature, moisture, and CO 2 efflux were monitored daily for all intact cores and monitored continuously on a subset of cores in order to establish predictive relationships between these variables. Moisture constraints upon the decomposition of SOM were observed below 0.20 and above 0.60 water filled pore space (WFPS) for all shallow mineral soils, and below 0.40 WFPS for shallow soils that had been recently rewet. These thresholds were also evidenced in phase lags between respiration and temperature at 5 cm at high moisture contents, sug- gesting that while biological responses drive soil respiration at low moisture contents, diffusivity limits the response at high soil moisture. While the shallow mineral soil dominated the contribution to CO 2 flux and consistently generated short term responses to rewetting events, deep mineral soil layers respired an order of magnitude lower than shallow layers (per gram C) with no short term measured response to rewetting events. The temperature sensitivity, measured using a Q 10 function, was close to 2 for all soil cores, regardless of soil depth or (steady state) moisture content. The exclusion of fluxes collected following precipitation events from field-derived estimates of CO 2 fluxetemperature relationships, improved these relationships. This study provides insights into how we consider the role of moisture in evaluating SOM decomposition-temperature responses for temperate forest soils. Ó 2013 Elsevier Ltd. All rights reserved. 1. Introduction Soil comprises the largest terrestrial store of carbon (C), housing more than two thirds of terrestrial C (Hibbard et al., 2005). The terrestrial-atmospheric surface flux of carbon dioxide (CO 2 ) arising from the decomposition of soil organic matter (SOM) through heterotrophic respiration drives soil organic carbon (SOC) losses in many soils (Raich and Schlesinger, 1992). It is generally accepted that the majority of respired CO 2 from SOM decomposition is derived from a small fast cycling labile C pool (Trumbore, 2000), with the production of CO 2 highest at the surface organic layers and declining with depth (Fang and Moncrieff, 2005; Risk et al., 2008b). Although carbon concentrations decline with soil depth in profiles, the total contribution from subsurface soil layers can be 50% of the total organic C in a 1 m profile (Batjes, 1996). Subsurface soils are composed of SOM that is generally considered unavailable for decomposition through physical separation (Xiang et al., 2008), such as inaccessibility within aggregate structures (Denef et al., 2001), or due to inherent chemical recalcitrance (Giardina and Ryan, 2000; Ågren and Bosatta, 2002; von Lutzow and Kögel- Knabner, 2010), although several recent publications challenge this traditional view (Kemmitt et al., 2008; Kleber et al., 2010; Schmidt et al, 2011; Dungait et al., 2012). The primary role of temperature in driving SOM decomposition has been well established in soils (Kirschbaum, 2000; Davidson and Janssens, 2006), although considerable effort is currently aimed at * Corresponding author. St. Francis Xavier University, Department of Earth Sci- ences, 5009 Chapel Square, Room 2052, Physical Sciences Centre, Antigonish, Nova Scotia B2G 2W5, Canada. Tel.: þ1 9028673318; fax: þ1 9028672414. E-mail address: cgabriel@stfx.ca (C.-E. Gabriel). 1 Tel.: þ1 9028675086; fax: þ1 9028672414. Contents lists available at ScienceDirect Soil Biology & Biochemistry journal homepage: www.elsevier.com/locate/soilbio 0038-0717/$ e see front matter Ó 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.soilbio.2013.10.009 Soil Biology & Biochemistry 68 (2014) 373e384