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

Abstract

CO 2 applied for Free-Air CO 2 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 C value. 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 CO 2 (540 ppm; for 3 years) treatments were analyzed by thermogravimetry 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 C and δ 15 N by IRMS. Energy consumption 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 CO 2 , respectively. δ 13 C values of SOM pools under elevated CO 2 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 CO 2 treatments was not significantly different in SOM pools decomposed up Plant Soil (2007) 297: [15][16][17][18][19][20][21][22][23][24][25][26][27][28] to 480°C having MRT of about 60 years. In contrast, the pool decomposed above 480°C contained only 0.5% of new N, with a MRT of more than 400 years in soils under both treatments. Thus, the separation of SOM based on its thermal stability was not sufficient to reveal pools with contrasting turnover rates of C and N.

How to link soil C pools with CO<sub>2</sub> fluxes?

“…A similar application that provides a new isotopic signature is the Free Air Carbon dioxide Enrichment (FACE) studies, which add 13 C depleted CO 2 (Andrews et al, 1999;Van Kessel et al, 2000;Hoosbeek et al, 2004;Dorodnikov et al, 2007a). In addition, the combination of C 3 -C 4 vegetation change and FACE approaches was used to increase the differences in isotopic signature of the C input and SOM (Ineson et al, 1996).…”