Thermal stability of soil organic matter pools and their turnover times calculated by δ13C under elevated CO2 and two levels of N fertilisation†

Abstract

Soil from Free-Air Carbon dioxide Enrichment (FACE) plots (FAL, Braunschweig) under ambient air (375 ppm; δ13C–CO2−9.8‰) and elevated CO2 (550 ppm; for six years; δ13C–CO2−23‰), either under 100% nitrogen (N) (180 kg ha−1) or 50% N (90 kg ha−1) fertilisation treatments, was analysed by thermogravimetry. Soil samples were heated up to the respective temperatures and the remaining soil was analysed for δ13C and δ15N by Isotope Ratio Mass Spectrometry (IRMS). Based on differential weight losses, four temperature intervals were distinguished. Weight losses in the temperature range 20–200 °C were connected mostly with water volatilisation. The maximum weight losses and carbon (C) content were measured in the soil organic matter (SOM) pool decomposed at 200–360 °C. The largest amount of N was detected in SOM pools decomposed at 200–360 °C and 360–500 °C. In all temperature ranges, the δ13C values of SOM pools were significantly more negative under elevated CO2 versus ambient CO2. The incorporation of new C into SOM pools was not inversely proportional to its thermal stability. 50% N fertilisation treatment gained higher C exchange under elevated CO2 in the thermally labile SOM pool (200–360 °C), whereas 100% N treatment induced higher C turnover in the thermally stable SOM pools (360–500 °C, 500–1000 °C). Mean Residence Time of SOM under 100% N and 50% N fertilisation showed no dependence between SOM pools isolated by increasing temperature of heating and the renovation of organic C in those SOM pools. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C. Revised version of a paper presented at the 30th Annual Meeting of the German Association for Stable Isotope Research (GASIR), 8–10 October 2007, Bayreuth, Germany.