Effects of atmospheric CO2 enrichment on δ 13C, δ 15N values and turnover times of soil organic matter pools isolated by thermal techniques

Abstract

CO2 applied for Free-Air CO2 Enrichment (FACE) experiments is strongly depleted in 13 C and thus provides an opportunity to study C turnover in soil organic matter (SOM) based on its δ 13 Cv alue. Simultaneous use of 15 N labeled fertilizers allows N turnover to be studied. Various SOM fractionation approaches (fractionation by density, particle size, chemical extractability etc.) have been applied to estimate C and N turnover rates in SOM pools. The thermal stability of SOM coupled with C and N isotopic analyses has never been studied in experiments with FACE. We tested the hypothesis that the mean residence time (MRT) of SOM pools is inversely proportional to its thermal stability. Soil samples from FACE plots under ambient (380 ppm) and elevated CO2 (540 ppm; for 3 years) treatments were analyzed by thermogravim- etry coupled with differential scanning calorimetry (TG- DSC). Based on differential weight losses (TG) and energy release or consumption (DSC), five SOM pools were distinguished. Soil samples were heated up to the respective temperature and the remaining soil was analyzed for δ 13 Ca ndδ 15 Nb y IRMS. Energy con- sumption and mass losses in the temperature range 20- 200°C were mainly connected with water volatilization. The maximum weight losses occurred from 200-310°C. This pool contained the largest amount of carbon: 61% of the total soil organic carbon in soil under ambient treatment and 63% in soil under elevated CO2, respectively. δ 13 C values of SOM pools under elevated CO2 treatment showed an increase from −34.3‰ of the pool decomposed between 20-200°C to −18.1‰ above 480°C. The incorporation of new C and N into SOM pools was not inversely proportional to its thermal stability. SOM pools that decomposed between 20-200 and 200-310°C contained 2 and 3% of the new C, with a MRT of 149 and 92 years, respectively. The pool decomposed between 310-400°C contained the largest proportion of new C (22%), with a MRT of 12 years. The amount of fertilizer-derived N after 2 years of application in ambient and elevated CO2 treatments was not significantly different in SOM pools decomposed up