Application of Thermal Analysis and Isotope Ratio Mass Spectrometry to Determine the Stability and Function of Soil Organic Matter in Forest Systems

Abstract

The application thermal analysis and stable isotope mass spectrometry together to the study of soil organic matter (SOM) has received limited attention to quantify its heterogeneous nature and function within an ecosystem context. We analysed surface soils (0–30 cm) from forest plantations in Northern California with thermogravimetry-differential scanning calorimetry (TG-DSC) and 13C- and 15N-labelled litters combined with long-term incubations to quantify affects of diverse litter and fertiliser inputs on SOM energetic properties and the stability of SOM. For the thermal analysis, replicate sample data was processed to develop summary models of DSC thermograms for the range of sampling depths and treatments along with subsoils with comparable particle size and mineralogy. The models estimate ‘mineral-free’ SOM energy density and the proportion of energy from the three primary regions of exothermic activity corresponding to carbohydrates and lipids (150–350°C), aromatic and condensed polymers (400–460°C) and refractory/mineral associated C (500–550°C). Calculated energy budgets show soils that receive diverse litter inputs (pine and evergreen hardwood litter & roots) have greater SOM energy density than soils with pine only inputs. Fertiliser (NPK) additions to these systems enhanced these signals with ‘energetic enrichment’ in fertilised-mixed litter soil and ‘energy depletion’ in fertilised-pine litter soils. Long-term incubations following the fate of 13C/15N-labelled litters showed that increased diversity in the form of more understory shrub species increased the stability of recently added litter C and N. The treatment effects show changes in the stability of SOM with reduced diversity leading to less energy and less C being stabilised where understory was removed, a common practice in the forest industry. The results suggest plant diversity increases the stability of SOM showing that diverse species affect the stability and likely the function of SOM.