Relating the biological stability of soil organic matter to energy availability in deep tropical soil profiles |
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| Institution: | 1. Department of Environmental Sciences & Engineering, Government College University Faisalabad, Allama Iqbal Road, 38000 Faisalabad, Pakistan;2. Department of Food Engineering/National Institute of Food Science and Technology, University of Agriculture Faisalabad, Faisalabad, Pakistan;3. Department of Forestry and Range Management, University of Agriculture Faisalabad, Faisalabad, Pakistan;4. Institute of Soil Science, Chinese Academy of Sciences, 71 # East Beijing Road, Nanjing 210008, China;5. Institute of Soil and Environmental Sciences, University of Agriculture, Faisalabad, Pakistan;6. King Abdulaziz University, Center of Excellence in Environmental Studies, P.O Box 80216, Jeddah 21589, Saudi Arabia;7. COMSATS Institute of Information Technology, Department of Environmental Sciences, 61100 Vehari, Pakistan;1. Unit of Sustainable Forest Management (Soil Science and Agricultural Chemistry), University of Santiago de Compostela, 27002 Lugo, Spain;2. Department of Applied Physics, University of Santiago de Compostela, 27002 Lugo, Spain;3. Centro de Investigación Forestal de Lourizán, Consellería de Medio Rural, Xunta de Galicia, P.O. Box 127, 36080, Pontevedra Spain;1. Department of Agroecology, Aarhus University, Blichers Allé 20, 8830 Tjele, Denmark;2. Department of Sustainable Agriculture Sciences, Rothamsted Research, Harpenden, Hertfordshire AL5 2JQ, United Kingdom;3. Department of Plant and Environmental Sciences, University of Copenhagen, Thorvaldsensvej 40, 1821 Frederiksberg, Denmark |
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| Abstract: | Tropical subsoils contain large reservoirs of carbon (C), most of which is stored in soil organic matter (SOM). Subsoil OM is thought to be particularly stable against microbial decomposition due to various mechanisms and its position in the soil profile, potentially representing a long-term C sink. However, few experiments have explicitly investigated SOM stability and microbial activity across several orders of magnitude of soil C concentrations as a function of soil depth. The objective of this study was to evaluate the biological stability of SOM in the upper 1.4 m of tropical forest soil profiles. We did so by measuring CO2 evolution during a 90-day laboratory incubation experiment on a sample set that was previously characterized for C and nutrient concentrations and microbial biomass. We concurrently measured the energy content of SOM using differential scanning calorimetry (DSC) as an index of the energy available for microbial metabolism, with the hypothesis that the biological stability of SOM would be inversely related to the energy contained within it. Cumulative CO2 evolution, mean respiration rates, and the energy density of SOM (energy released during combustion normalized to soil C) all declined with soil depth (P < 0.01). Biological indices of C stability were well correlated with measures of SOM energy. There was no change in the mean respiration rate as a function of depth when normalized to soil C, and a trend toward increased respiration per-unit microbial biomass (P = 0.07). While reduced microbial respiration in subsoils suggests an increase in the biological stability of SOM, we suggest this is driven principally by concurrent declines in energy availability as measured by DSC and the size of the microbial biomass pool. On a per-unit biomass basis, subsoil OM may be as prone to decomposition and destabilization as surface SOM. |
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| Keywords: | Soil organic matter Stability Tropical Deep soil Incubation Thermal analysis |
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