Scharpenseel, H.-W., Pfeiffer, E.-M., Becker-Heidmann, P. (1995): Soil organic matter studies and nutrient cycling. – In: IAEA (Hg.): Nuclear techniques in soil-plant studies for sustainable agriculture and environmental preservation, S. 285–305. IAEA, Vienna, Austria
A short account is given of biomass and soil organic matter (SOM) development in the geological past, from Hadean via Archaean and Proterozoic, to present day Phanerozoic, with its much reduced atmospheric CO2, enriched O2, a stratospheric ozone belt that supports the evolution of terrestrial life and, currently, a carbon sink (by photosynthesis, chemical weathering, precipitation in oceans) that exceeds the carbon source (by respiration, volcanism, ejection from subduction zones). Three major photosynthetic mechanisms reflect the changes with time in the availability of CO2 for biomass production, with a present net photosynthesis for biomass production of about 60 Pg C a-1. The distance of the earth from the sun and the current 30% albedo of the incoming radiation from the sun are only in a narrow margin of change tolerable to the earth’s biotic system. SOM-C, with about 1550 Pg, is the largest accessible C compartment (mineable fossil C in oil, gas and coal is only about 1000 Pg C). A continuous blanket of SOM has probably existed since the end of Permian glaciation. The annual biomass-C and SOM-C emissions from anthropogenic activities such as forest clearing/ ’slash and burn‘ seem to have been overcompensated for by biomass gains due to CO2 fertilization. To sequester the 3 Pg C added annually to the atmosphere, about 465 · 106 ha expansion of current forest cover would be required. Sustainable land use with resilience against system degradation requires a reduction in greenhouse forcing trace gas emissions, partly from biological systems (mitigation of CO2, CH4, N2O and tropospheric O2). Radiation and isotope use in studies of SOM and nutrient cycling relies mainly on: (1) Scanning the turnover of uniformly 14C or 13C labelled biomass. (2) Measurement of the SOM-C residence time by natural 14C tests. (3) Use of the whole set of labelled plant nutrients, especially 32P, 33P and 42K for tracer studies, e.g. for A or L value tests. (4) Most importantly, use of 15N labelled nitrogen fertilizer and natural 15N to reveal the N dynamics in the soil-plant system as well as to assess, via isotope fractionation, the contribution of atmospheric N in the diazotrophic system’s nitrogen collection. (5) Thin layer scanning of soil profdes for natural 14C and 13C, the latter to indicate the changes in photosynthesis resulting from climate or land use changes (C3 forest with a 13C value of -25‰, – C4 savannah grasses with a 13C value of -10‰). The 13C and 18O measurements in semi-arid calcretes (about 1000 Pg C) reflect the mechanism of formation (ad ascensum – ad descensum-catena). (6) Owing to 13C enrichment in residual SOM, 13C in wetlands is used to indicate emission of methane, which is depleted in 13C to -60‰. (7) Nitrogen-15 labelling of nitrification and denitrification is used to reveal the N2O emission from soil related sources, about 7 Tg a-1. The NH3-N emission from soil + fertilizer + livestock + motor vehicles amounts to 22-35 Tg a-1. The N release from fires, shifting agriculture, firewood and agricultural wastes is about 15-46 Tg a-1 (C:N 100). (8) Studies of zinc deficiency in ricelands, where 65Zn is great importance.