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

Temperature and moisture are primary environmental drivers of soil organic matter (SOM) decomposition, and an improved understanding of how they interact to control SOM cycling processes in temperate forest soils is needed. Intact soil cores from shallow (0–25 cm) and deep (25–50 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 CO2 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, suggesting 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 CO2 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 Q10 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 CO2 flux–temperature 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.