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Three-source partitioning of CO2 efflux from soil planted with maize by C natural abundance fails due to inactive microbial biomass |
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| Authors: | M Werth I Subbotina |
| Institution: | a Institute of Soil Science and Land Evaluation, University of Hohenheim, Emil-Wolff-Str. 27, 70593 Stuttgart, Germany b Department of Soil Science and Agrochemistry, State University of Rostov on Don, 344006, Rostov on Don, Russia c Department of Agroecosystem Research, University of Bayreuth, 95440 Bayreuth, Germany |
| Abstract: | A theoretical approach to the partitioning of carbon dioxide (CO2) efflux from soil with a C3 vegetation history planted with maize (Zea mays), a C4 plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ13C values of SOM, roots, microbial biomass, and total CO2 efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ13C values and on assumptions made in other studies showed that RR, RMR, and SOMD amounted to 91%, 4%, and 5%, respectively. Two assumptions were thoroughly examined in a sensitivity analysis: the absence of 13C fractionation and the conformity of δ13C of microbial CO2 and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO2 efflux from unplanted soil was enriched in 13C by 2.0‰ compared to microbial biomass. The consideration of this 13C fractionation in the mass balance equation changed the proportions of RR and RMR by only 4% and did not affect SOMD. A calculated δ13C value of microbial CO2 by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO2 partitioning to the measured and literature data. The active microbial biomass in the rhizosphere amounted to 37% to achieve an appropriate ratio between RR and RMR compared to measured data. Therefore, the three-source partitioning approach failed due to a low active portion of microbial biomass, which is the main microbial CO2 source controlling the δ13C value of total microbial biomass. Since fumigation-extraction reflects total microbial biomass, its δ13C value was unsuitable to predict δ13C of released microbial CO2 after a C3-C4 vegetation change. The second adjustment to the CO2 partitioning results in the literature showed that at least 71% of the active microbial biomass utilizing maize rhizodeposits would be necessary to achieve that proportion between RR and RMR observed by other approaches based on 14C labelling. The method for partitioning total below-ground CO2 efflux into three sources using a natural 13C labelling technique failed due to the small proportion of active microbial biomass in the rhizosphere. This small active fraction led to a discrepancy between δ13C values of microbial biomass and of microbially respired CO2. |
| Keywords: | δ13C Root respiration Rhizomicrobial respiration SOM decomposition 13C isotopic fractionation CO2 efflux from soil Microbial biomass 13C natural abundance |
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