Monitoring decomposion of organic matter in soil
Decomposition of plant residues and older soil organic matter is important for ecosystem functioning. Studying decomposition processes is often obscured by interactions between freshly added plant materials and soil organic matter. The added plant materials may either have “priming” or “conserving” effects on soil organic matter. Plant residues with a different (enriched or depleted) d13C signature provide a fine technique to separate the contributions of different sources in the total CO2 respired from soil.
Stable Isotope Solution : Stable isotope enriched or depleted plant residues
Plant components (leaves, roots) enriched (Coûteaux et al, 2001) or depleted (Van Vuuren et al, 2000) with the stable isotopes 13C are applied to soils in order to follow decomposition. Van Vuuren (2000) used 13C-labelling to test the technique’s accuracy in distinguishing CO2 originating from different organic pools. By following 13C mineralization through measuring d13C values in the respired CO2, the dynamics of the decomposition processes could be analysed and the separate contributions of plant residues and soil organic matter could thus be sorted out.
Figure 1 below shows how CO2 originating from wheat roots and soil organic matter can easily be separated from each other.
Figure 1. C mineralization in soil amended with 13C depleted wheat roots. (C,D): 13C values (‰) of mineralized C. (E,F): Amounts of C mineralized (mg per plant) from soil organic matter (SOM; open symbols) and from 13C-labelled roots (closed symbols). Graphs (C,E) are for plants labelled at 350 ppm CO2; (D,F) are for plants labelled at 700 ppm CO2 (Van Vuuren et al, 2000).
The authors concluded that the potential accuracy of the 13C method was demonstrated and13C/12C fractionation during root decomposition was negligible. By using larger differences in delta-values – higher enriched plant residues – the resolution of the method may be strongly improved. When uniformly 13C-labelled materials are used, even decomposition pathways can be tracked using HR-MAS NMR spectroscopy and micro-organisms involved determined by RNA-SIP.
Coûteau MM, P Bottner, JM Anderson, B Berg, T Bolger, P Casals, J Romanyà, JM Thiéry, V Ramon. 2001.
Decomposition of 13C-labelled standard plant material in a longitudinal transect of coniferous forests: Differential impact of climate on the decomposition of soil organic matter compartments.
Biogeochemistry 54: 147 – 170.
Van Vuuren MMI, D Robinson, CM Scrimgeourb, JA Ravena, AH Fitter. 2000.
Decomposition of 13C-labelled wheat root systems following growth at different CO2 concentrations.
Soil Biology and Biochemistry 32: 403 – 413.