Thermal stability of soil organic matter pools and their turnover times calculated by δ¹³C under elevated CO₂and two levels of N fertilisation†

Soil from Free-Air Carbon dioxide Enrichment (FACE) plots (FAL, Braunschweig) under ambient air (375 ppm; delta(13)C-CO(2)-9.8 per thousand) and elevated CO(2) (550 ppm; for six years; delta(13)C-CO(2)-23 per thousand), 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 delta(13)C and delta(15)N by Isotope Ratio Mass Spectrometry (IRMS). Based on differential weight losses, four temperature intervals were distinguished. Weight losses in the temperature range 20-200 degrees 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 degrees C. The largest amount of N was detected in SOM pools decomposed at 200-360 degrees C and 360-500 degrees C. In all temperature ranges, the delta(13)C values of SOM pools were significantly more negative under elevated CO(2) versus ambient CO(2). 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 CO(2) in the thermally labile SOM pool (200-360 degrees C), whereas 100% N treatment induced higher C turnover in the thermally stable SOM pools (360-500 degrees C, 500-1000 degrees 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.