Scharpenseel, H. W., Pfeiffer, E.-M., Becker-Heidmann, P. (2001): Ecozone and Soil Profile Screening for C-Residence Time, Rejuvenation, Bomb 14C and Photosynthetic 13C Changes. – In: Lal, R., Kimble, J. M., Follett, R. F., Stewart, B. A. (Hg.): Assessment methods for soil carbon, S. 207–220. Advances in soil science. CRC Press, Boca Raton, Fla.
Several major parameters are decisive in assessing the formation of humic substance and the size of the SOM pool:
- Living biomass and annual biomass input of the ecozones;
- Rate of biomass turnover into SOM;
- SOM residence time;
- Amount/concentration of SOM in the ecozones.
Biomass formation varies, depending on climate factors (mainly temperature and rainfall), atmospheric CO2 concentration, nutrient availability, type of vegetation and its protein -N content, soil pH, and soil texture, including HAC or LAC clay mineral dominance. Table 1 shows rough estimates of biomass for major ecozones. The rate of turnover of biomass into SOM in many cases can be derived from decomposition studies with uniformly 14C-labelled plant substance (Scharpenseel and Pfeiffer, 1997). Commonly used standard methods can analyze the SOM concentration in an ecozone rather exactly. According to the Century Model (Parton et al., 1987), SOM fractions vary considerably in different landscapes (Woomer et al., 1994). Post et al. (1982) and Degens (1989) indicate that 24.5% of the total Soil-C is in wetlands, 13.7% in tundra, 12.0% in croplands and agricultural areas, 9.5% in wet boreal forests, 9.6% in tropical woodland and savanna, and 8.6% in cool 14 temperate steppes. The 375 million ha of peats contain an additional C-pool of ca 1500 PG C.
14C-dating can detect the mean residence time (MRT) of SOM-C. However, some factors such as rejuvenation by leaching of modem C and bomb-14C entering the soil profile from atomic testing complicate these measurements.
δ13C measurement reveals changes in vegetation involving different photosynthetic mechanisms – C3, C4, and CAM (δ13C of C3 = ca -25 to 27‰, δ13C of C4 = ca – 12 to 14‰, δ13C of CAM = ca – 17‰) in the course of the dated period and earlier.
Undoubtedly, if one can afford it, a (thin) layer sampling and 14C dating and δ13C MS measurement of the soil profile is superior to single or few sample tests. Mainly respiratory biotic δ13C and eventually also δ18O.