Study of soil heterotrophic respiration as a function of soil moisture under different land covers
Nishadini Widanagamage, Eduardo A. Santos, Charles W. Rice, Andres Patrignani
Abstract
The relationship between soil heterotrophic respiration ( R h ) and soil moisture has been often studied using disturbed soil samples and simple gravimetric and volumetric soil moisture indicators. The objective of this study was to investigate the relationship between R h and soil moisture in terms of water-filled porosity (WFP), matric potential ( Ψ m ), and relative soil gas diffusivity ( D p / D o ) using undisturbed soil cores obtained under different land covers . Soil CO 2 efflux, WFP, and Ψ m were measured in undisturbed soil samples (250 cm 3 ) collected in the 0–5 cm soil layer (without any vegetation or living roots) under laboratory conditions by combining a CO 2 gas analyzer, a scale, and precision mini-tensiometers. For each site and land cover, we also measured soil chemical properties , soil physical properties , and soil microbial composition using phospholipid fatty acid analysis. Grassland soils had the largest total microbial biomass (6275 ng g −1 ), followed by soils from riparian (5327 ng g −1 ), and cropland (2745 ng g −1 ) sites. Bacteria were the dominant group representing 46% (SD = 5%) of the total microbial biomass across all sites and land covers. Maximum R h was 1.88 (SD = 0.40) μmol CO 2 m −2 s −1 in grassland, 1.64 (SD = 0.82) μmol CO 2 m −2 s −1 in riparian, and 0.94 (SD = 0.56) μmol CO 2 m −2 s −1 in cropland soils. Considering all land cover and soil types, our observations revealed that peak R h occurred at mean WFP = 0.81 , Ψ m = −6 kPa, and D p / D o = 0.003. Thus, we recommend avoiding the traditional field capacity definition of −33 kPa for representing peak microbial activity . Water-filled porosity was a more consistent predictor of R h than Ψ m or D p / D o across soils with contrasting organic matter content, total microbial biomass, soil texture , and soil structure .