C and N in soil organic matter density fractions under elevated atmospheric CO2: Turnover vs. stabilization

Turnover of C and N in an arable soil under Free Air Carbon Dioxide (FACE) experiment was studied by the use of 13C natural abundance and 15N-labeled fertilizers. Wheat was kept four growing seasons under ambient and elevated CO2 concentrations and fertilized for three growing seasons. Density fractionation of soil organic matter (SOM) allowed to track 13C and 15N in free particulate organic matter (fPOM; <1.6 g cm−3), particulate organic matter occluded within aggregates with two densities (oPOM 1.6, oPOM 1.6–2.0 g cm−3), and in mineral-associated organic matter (>2.0 g cm−3) fractions. Elevated CO2 and N fertilization did not significantly affect C and N contents in the bulk soil. Calculated mean residence time (MRT) of C and N revealed the qualitative differences of SOM density fractions: (i) the shortest MRTC and MRTN in fPOM confirmed high availability of this fraction to decomposition. Larger C/N ratio of fPOM under elevated vs. ambient CO2 indicated an increasing recalcitrance of FACE-derived plant residues. (ii) There was no difference in MRT of C and N between lighter and heavier oPOMs probably due to short turnover time of soil aggregates which led to oPOM mixing. The increase of MRTC and MRTN in both oPOMs during the experiment confirmed the progressive degradation of organic material within aggregates. (iii) Constant turnover rates of C in the mineral fraction neither confirmed nor rejected the assumed stabilization of SOM to take place in the mineral fraction. Moreover, a trend of decreasing of C and N amounts in the Min fraction throughout the experiment was especially pronounced for C under elevated CO2. Hence, along with the progressive increase of CFACE in the Min fraction the overall losses of C under elevated CO2 may occur at the expense of older “pre-FACE” C.