3rd day since the start of incubation (Fig. 4a). The δ13C of CO2 ranged from −20.2 ‰ to −22.2 ‰. The CO2 was considerably enriched in comparison to C3 soil emission (the soil collected in the area neighboring the experimental plot) due to the new C4-SOC input. According to Eq. (5), the contribution of recent C4 soil to CO2 flux during the 7 days of incubation was significantly higher than that of the C3 soil. The fresh organic matter of C4 eroded soil was easier consumed by microbial biomass. The C4 contribution to CO2 flux decreased during incubation period due to the preferential microbial utilization of fresh versus old C (Fig. 4b); and decreased along the plot from the upper to the lower tram (Fig. 4b). On the other hand, the ratio between the CO2 emissions and SOC was highest in the lower tram of the plot in comparison to the upper tram for the C4 portion of the plot and was higher for C4 than for the C3 portion (Fig. 5). Therefore, the soil in the lower tram of the plot emitted more enriched CO2 in comparison to the upper and middle
trams of the plot; this finding indicated the C4-SOC at the lower tram of the plot was more easily decomposed by soil microbial biomass and