Carbon pulse-labeling

Several authors have applied in situ pulse labeling of plants (grasses and crops) with C-CO2 under field conditions with the objective of quantifying the gross annual fluxes of carbon (net assimilation, shoot and root turnover, and decomposition) in production grasslands and so assess the net input of carbon (total input minus root respiration minus microbial respiration on the basis of rhizodeposition and soil organic matter) and carbon fixation in soil under ambient climatic conditions in the field. 

Recently, pulse labeling has frequently been applied to determine the fate of carbon in crops such as barley and wheat and the losses from roots and subsequent microbial transformations. In general, the results indicate that 15-25% of the net " C assimilation is transferred to the roots and that there are seasonal differences in the distribution of assimilated carbon. Meharg and Killham (25) measured the C distribution in perennial ryegrass (L perenne). At 8 days after the pulse with... 

A. A. Meharg and K. Killham, Compari.son of carbon flow from pre-labelled and pulse-labelled plants. Plant Soil 112 225 (1988). 

P. J. Gregory and B. J. Atwell, The fate of carbon in pulse-labelled crops of barley and wheat. Planl Soil /.J6 205 (1991). 

Plants grown for longer periods in solid supports such as sand or soil repre-.sent the next level of complexity and, although other techniques are available, carbon flow is most frequently estimated using C labeling experiments. In the laboratory, COt can be supplied to shoots either as a short pulse or continuously, and the carbon flow can be monitored. In the field, due to technical limitations, only COi pulse labeling procedures are possible. A final approach, termed crop studies, involves the measurement of components of crop growth from which... 

Other sand-based systems using COi pulse-chase procedures have been used to produce carbon budgets for Festuca ovina and Plantago lanceolata seedlings (30) and white lupin (Lupimis albiis) (31). Significantly, CO2 pulse labeling of proteoid roots of white lupin under phosphate-deficient conditions showed that high levels of dark fixation of COi by the roots took place and that 66% of this root-fixed carbon was exuded from the roots (31). Clearly, dark fixation of CO2 by roots and subsequent rhizodeposition is an area that deserves further study in the future. 

T. Shepherd and H. V. Davies, Carbon loss from the roots of forage rape (Brassica napus L.) seedlings following pulse-labelling with CO, Ann. Boi. 72 155 (1993). 

J. Swinnen, J. A. van Veen, and R. Merckx. C pulse-labelling of field-grown spring wheat an evaluation of its use in rhizosphere carbon budget estimations. Soil Biol. Biochem. 25 161 (1994). 

J. A. Palta and P. J. Gregory. Drought affects the fluxes of carbon to roots and soil in C pulse-labelled plants of wheat. Soil Biol. Biochem. 29 1395 (1997). 

Johnson, D., Leake, J. R., Ostle, N., Ineson, P. Read, D. J. (2002). In situ (C02)- C pulse-labeling of upland grassland demonstrates a rapid pathway of carbon flux from arbuscular mycorrhizal mycelia to the soil. New Phytologist, 153, 327-34. 

Leake, J. R., Donnelly, D. P., Saunders, E. M., Boddy, L. Read, D. J. (2001). Rates and quantities of carbon flux to ectomycorrhizal mycelium following C pulse labeling of Pinus sylvestris seedlings effects of litter patches and interaction with a wood-decomposer fungus. Tree Physiology, 21, 71-82. 

Isotope pulse-labelling experiments have been the main technique by which carbon can be traced through the plant mycorrhizal soil system in order to quantify its allocation to AMF. By providing plants with... 

Ostle, N., Ineson, P., Benham, D. Sleep, D. (2000). Carbon assimilation and turnover in grassland vegetation using an in situ C02 pulse labelling system. Rapid Communications Mass Spectrometry, 14, 1345—50. 

As stated before a mycorrhizal mycelium might connect several plants of the same species but also plants of different species Fig (6.9). It has been shown that this also happens in nature and that all kinds of nutrients can flow between the plants including carbon fixed by photosynthesis. If a plant is linked to other plants by a common mycorrhiza it has been experimentally shown that carbon can flow from a plant in the sun to a shaded plant. These experiments have only been done by radioactive pulse-labelling and it is still debated if this flow is of any significance to the plant systems. There is thus a possibility that plant ecosystems are much more dependent on the below-ground hyphal network than we have traditionally thought. 

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