Carbon dioxide (CO2) efflux from soils is one of the most important fluxes of the global carbon (C) cycle. CO2 efflux measurements from soil are commonly used to investigate short-term soil organic matter turnover. Carbon recently assimilated by plants and released into the soil by exudation is very important in this short-term turnover, since it is a readily available C source for microorganisms, which becomes decomposed to CO2 within a few hours to days. Root respiration represents a further important contribution to the total CO2 efflux from soil, which must be considered separately from soil organic matter turnover. Various techniques based on 13C labelling, 14C labelling, and 13C natural abundance are used to separate these CO2 flows and to quantify plant mediated C input into the soil as well as root-derived CO2 fluxes. The separation techniques based on 13C natural abundance assume absence of 13C fractionation by root respiration and exudation, but this assumption was not thoroughly proven.
Coupling 13C natural abundance and 14C pulse labelling enabled us to investigate dependence of 13C fractionation on assimilate partitioning between maize shoots, roots, exudates, and root-respired CO2. The amount of recently assimilated C in these four pools was controlled by three levels of nutrient supply: full nutrient supply (NS), ten times diluted nutrient supply (DNS), and deionised water (DW). Every container with one maize plant was sealed between root and shoot. Thus, root respiration could be examined in a closed system: air was pumped through the nutrient solution, CO2 from root respiration was trapped in 1 M NaOH solution, and the resulting CO2-free air was again pumped through the nutrient solution. The experiment consisted of three cycles started on days 14, 19, and 24. Each cycle included: 1) supply of the plants with full NS for recovery from DNS or DW for one day before labelling (CO2 and exudates were collected during this period before second and third labelling), 2) labelling of shoots in a 14CO2 atmosphere for 1.5 hours, and 3) trapping of CO2 in NaOH and of exudates released into NS, DNS, or DW for four days. Total C, 14C activity, and d13C were determined in NaOH, in exudates, and in roots and shoots.
Increasing amounts of recently assimilated C in the roots (from 8 to 10% of recovered 14C in NS and DNS treatments) led to a 0.3‰ 13C enrichment with decreasing nutrient supply. A further increase of C allocation in the roots (from 10 to 13% of recovered 14C in DNS and DW treatments) resulted in an additional enrichment of the roots by 0.3‰. d13C of CO2 evolved by root respiration was similar to that of the roots in DNS and DW treatments. However, if the amount of recently assimilated C in root respiration was reduced, the respired CO2 in the NS treatment became 0.7‰ depleted in 13C compared to roots. Increasing amounts of recently assimilated C in the CO2 from NS via DNS to DW treatments resulted in a 1.6‰ d13C increase of root respired CO2. Thus, for both pools, i.e. roots and root respiration, increasing amounts of recently assimilated C in the pool with decreasing nutrient supply led to a d13C increase. In DW and DNS plants there was no 13C fractionation between roots and exudates. However, high-nutrient supply decreased the amount of recently assimilated C in exudates compared to the other two treatments and led to a 5.3‰ 13C enrichment in exudates compared to roots.
We conclude that 13C discrimination between plant pools and within processes such as exudation and root respiration cannot be accepted as fixed but strongly depends on the amount of C in the respective pool and on partitioning of recently assimilated C between plant pools.