Root and rhizomicrobial respiration: A review of approaches to estimate respiration by autotrophic and heterotrophic organisms in soil

Partitioning the root-derived CO2 efflux from soil (frequently termed rhizosphere respiration) into actual root respiration (RR, respiration by autotrophs) and rhizomicrobial respiration (RMR, respiration by heterotrophs) is crucial in determining the carbon (C) and energy balance of plants and soils. It is also essential in quantifying C sources for rhizosphere microorganisms and in estimation of the C contributing to turnover of soil organic matter (SOM), as well as in linking net ecosystem production (NEP) and net ecosystem exchange (NEE). Artificial-environment studies such as hydroponics or sterile soils yield unrealistic C-partitioning values and are unsuitable for predicting C flows under natural conditions. To date, several methods have been suggested to separate RR and RMR in nonsterile soils: 1) component integration, 2) substrate-induced respiration, 3) respiration by excised roots, 4) comparison of root-derived (CO2)-C-14 with rhizomicrobial (CO2)-C-14 after continuous labeling, 5) isotope dilution, 6) model-rhizodeposition technique, 7) modeling of (CO2)-C-14 efflux dynamics, 8) exudate elution, and 9) delta C-13 of CO2 and microbial biomass. This review describes the basic principles and assumptions of these methods and compares the results obtained in the original papers and in studies designed to compare the methods. The component-integration method leads to strong disturbance and non-proportional increase of CO2 efflux from different sources. Four of the methods (5 to 8) are based on the pulse labeling of shoots in a (CO2)-C-14 atmosphere and subsequent monitoring of (CO2)-C-14 efflux from the soil. The model-rhizodeposition technique and exudate-elution procedure strongly overestimate RR and underestimate RMR. Despite alternative assumptions, isotope dilution and modeling of (CO2)-C-14-efflux dynamics yield similar results. In crops and grasses (wheat, ryegrass, barley, buckwheat, maize, meadow fescue, prairie grasses), RR amounts on average to 48 +/- 5% and RMR to 52 +/- 5% of root-derived CO2. The method based on the C-13 isotopic signature of CO2 and microbial biomass is the most promising approach, especially when the plants are continuously labeled in (CO2)-C-13 or (CO2)-C-14 atmosphere. The difference methods, i.e., trenching, tree girdling, root-exclusion techniques, etc., are not suitable for separating the respiration by autotrophic and heterotrophic organisms because the difference methods neglect the importance of microbial respiration of rhizodeposits.