II. Material and methods
3.3 Translocation of plant-derived carbon in plant-soil microcosms
Figure 3.6 │Total emission of methane from plant-soil microcosms with different inocula during reproductive plant stage.
Different letters indicate significant difference (mean ± SD, n = 3).
Short chain fatty acids of the pore water like propionate and acetate, as well as CO2, were assumed to be potential precursors for methane formation. Enrichment of pore water propionate and acetate in microcosms with mixed inoculum was lower compared to the other inocula, indicating a higher conversion of initial soil organic carbon than root derived carbon. Microcosms based on digested sludge showed a higher labeling of propionate and acetate than the mixed inocula samples. For pore water CO2, both soil-systems were in the same range of 13C enrichment in comparison to the 100 % soil control. Compared with the other soil-systems, microcosms based on synthetic rice paddy soil showed by far the highest enrichment of 13C in the pore water substances of propionate, acetate and CO2. The 13C values of these microcosms were slightly lower for acetate compared with the control, while for propionate and CO2 enrichment was far higher than in the control. Also the 13C labeling of emitted methane was highest for microcosms based on synthetic rice paddy soil, while those of other microcosms were in the same range. No
13C enrichment occurred for formate or pyruvate in any of the microcosms.
Figure 3.7 │Enrichment of 13C in different carbon pools of plant-soil microcosms with different inocula, during the reproductive growth stage. These C-pools were above ground plant biomass (ag), plant roots, rhizospheric soil (rs), pore water substances of propionate, acetate, CO2 as well as emitted CH4. Different letters indicate significant difference within a carbon pool (mean ± SD, n = 3).
Beside the amount, 13C enrichment rates (13C enrichment per time) for propionate, acetate, CO2, and CH4 were also dependent on the soil-system of the different microcosms (Figure 3.8). While labeling of propionate was at its maximum after 8 h in digested sludge and the mixed inoculum, it lasted 32 h in the 100 % soil control and 80 h in the rice paddy soil microcosms to reach the maximum state. Nevertheless, 13C enrichment rates of propionate were higher for rice paddy soil and digested sludge compared with mixed inoculum and the control. The same enrichment time of 8 h could be determined for pore water CO2 of digested sludge and mixed inoculum, while maximum labeling was reached after 56 h in the rice paddy soil microcosms and 80 h in the control. The 13C enrichment rates for CO2 were within the same range for all soil-systems and higher compared to the control. Maximum labeling of acetate occurred after 8 h in rice paddy soil, mixed inoculum and the control, while the maximum was reached after 32 h in digested sludge. For acetate, 13C enrichment rates were higher in rice paddy soil and the control compared to digested sludge and mixed inoculum. The 13C enrichment of emitted CH4 was at its maximum after 32 h for
ag root rs propionate acetate CO2 CH4
0 1 2 3 4
5 rice paddy soil_20 % digested sludge_20 % mixed_20 %
rice paddy soil_100 %
13C [atom % excess]
a a a
a
a a a a
a a a a a
b
c
bc ab a
bc c
a
b b b
a
b b b
CO2 CH4
all soil-systems, in comparison to the control where it lasted 80 h to reach maximum labeling. The
13CH4 enrichment rate for microcosms based on rice paddy soil was shown to be higher compared with the other soil-systems and the control.
Summarizing for all soil-systems, the measured 13C enrichment rates indicated that microbial conversion of rhizodeposits to propionate occurred at higher rates in microcosms based on rice paddy soil and digested sludge, while conversion to CO2 and CH4 occurred most rapidly in rice paddy soil. The 13C values of CO2 and CH4 in rice paddy soil microcosms were stable after enrichment, indicating some availability of rhizodeposits in the preceding carbon pools. The same could be detected for propionate, acetate, CO2, and CH4 in the control.
Figure 3.8 │Enrichment of 13C in carbon pools of the pore water as well as the emitted methane, of microcosms with different soil-systems (rps: rice paddy soil, ds: digested sludge, mix: mixed inoculum), set in relation to time after initial labeling.
0 20 40 60 80 100 120
0 1 2 3 4
propionate_rps_20 % propionate_ds_20 % propionate_mix_20 % propionate_rps_100 % acetate_rps_20 % acetate_ds_20 % acetate_mix_20 % acetate_rps_100 %
CO2_rps_20 % CO2_ds_20 % CO2_mix_20 % CO2_rps_100 %
CH4_rps_20 % CH4_ds_20 % CH4_mix_20 % CH4_rps_100 %
hours after labeling
13 C [atom % excess]
The emission of 13CH4 was assumed to be originating from recently plant-assimilated carbon. Since not the entire root released carbon was labeled, the emission of methane based on rhizodeposition might be underestimated by this presumption. However, 13CH4 emission rates, as well as the contribution of recently plant-assimilated carbon to the emission of total methane Rm, was dependent on the soil-system (Figure 3.9). Microcosms with soil-systems based on rice paddy soil and digested sludge showed highest 13CH4 emission rates, while those of the mixed inoculum were lower compared to the 100 % soil control. For rice paddy soil microcosms, Rm was also higher than in other microcosms. Although digested sludge microcosms showed a high production of 13CH4, as well as a high formation of total methane, Rm was lower in microcosms based on digested sludge compared to those with rice paddy soil. Rm of microcosms based on mixed inoculum was within the same range as digested sludge systems, whereas both showed Rm values below the control.
Therefore, methane formed in microcosms with mixed inocula was almost entirely formed by conversion of initial soil organic carbon.
Rm 10.42 ± 3.07 % 1.33 ± 0.04 % 1.30 ± 0.01 % 1.69 ± 0.03 %
Figure 3.9 │Emission of methane originating from plant root derived carbon in plant-soil microcosms with different inocula during the time of highest 13C-labeling of the methane pool. Different letters indicate significant difference (mean
± SD, n = 3). Rm: contribution of recently plant-assimilated carbon to total CH4 emission.