Three-source partitioning of CO 2 efflux from soil planted with maize by 13C natural abundance fails due to inactive microbial biomass

Abstract

A theoretical approach to the partitioning of carbon dioxide (CO 2) efflux from soil with a C 3 vegetation history planted with maize ( Zea mays), a C 4 plant, into three sources, root respiration (RR), rhizomicrobial respiration (RMR), and microbial soil organic matter (SOM) decomposition (SOMD), was examined. The δ 13C values of SOM, roots, microbial biomass, and total CO 2 efflux were measured during a 40-day growing period. A three-source isotopic mass balance based on the measured δ 13C 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 δ 13C of microbial CO 2 and that of microbial biomass. This approach strongly overestimated RR and underestimated RMR and microbial SOMD. CO 2 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 δ 13C value of microbial CO 2 by a mass balance equation including active and inactive parts of microbial biomass was used to adjust a hypothetical below-ground CO 2 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 CO 2 source controlling the δ 13C value of total microbial biomass. Since fumigation–extraction reflects total microbial biomass, its δ 13C value was unsuitable to predict δ 13C of released microbial CO 2 after a C 3–C 4 vegetation change. The second adjustment to the CO 2 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 CO 2 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 δ 13C values of microbial biomass and of microbially respired CO 2.