Application of carbon isotopic techniques in the study of soil organic matter dynamics

Abstract

Soil organic matter (SOM) is the earth’s largest terrestrial reservoir of carbon which contributes to a variety of biological, chemical and physical properties of soil and is essential for good soil health. The amount (stock) of organic matter in a given soil can increase or decrease depending on numerous factors. But even when stocks are at equilibrium, SOM is in a continual state of flux; new inputs cycle - via the process of decomposition - into and through organic matter pools of various qualities and replace materials that are either transferred to other pools or mineralized. For the functioning of a soil ecosystem, this “dynamics” of SOM is probably more significant than the sizes of SOM stocks. The different carbon isotopic techniques used to study SOM dynamics are: use of organic matter labeled with 14C or 13C, use of natural variation in 13C in organic matter, use of 14C (radiocarbon dating), bomb 14C technique. 14C pulse labeling can be used to trace 14C allocation to shoot and roots, root and rhizomicrobial respiration in grassland communities under drought stress. Based on δ13C signature of C pools, it is possible to disentangle the contribution of 13C fractionation and preferential substrate utilization of recent (C4) versus old carbon (C3) to microbial turnover. 13C isotopic analysis after application of straw is a convenient approach for quantifying carbon-flux partitioning during methanogenic degradation of straw and SOM. The 13C method is generally used in medium-term observations or experiments (5--50 yr); hence, this method gives an estimate of turnover dominated by relatively recent inputs and C pools that cycle within the time frame of the experiment. In contrast, the oldest and most recalcitrant C pools dominate estimates by radiocarbon dating because of the long-term time frame (200-40,000 yr) that this method measures. In future the focus should be on resolving molecular characteristics of organic matter in organo-mineral associations with the knowledge of isotopic signatures of specific molecules. Also the potential of combining isotopic analyses of soil respiration rates with other methods (nutrient uptake in plants) is to be explored.