A simple chamber technique for the in situ labelling of pasture sward with carbon (14C)

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Abstract
Information on carbon (C) flows and transformations in the rhizosphere provides a basis for understanding the functioning of the system. However, the sophisticated growth cabinet facilities required for collecting quantitative data, with 14C labelling, generally limit their application under field conditions. We determined the feasibility of ‘pulse‐labelling’ pasture swards with 14C [exposing the plants to a single large 14C‐carbon dioxide (CO2) pulse] to monitor C transformations under field conditions using a simple chamber modified to form a sealed hemisphere over an area of pasture. The 14C‐CO2 was introduced into the hemisphere to 14C label the plant material. Assimilation of 14C‐CO2 was checked by taking samples of the chamber atmosphere. Any leakage of 14C‐CO2 from the chamber was also checked by taking air samples from around and outside the chamber during the assimilation period. The chamber was subsequently removed, and the pasture was opened to natural conditions. Cores were taken periodically from the treated area. Herbage, roots and soils were separated and analyzed for 14C. Incorporation of 14C‐CO2 into the pasture sward was rapid and the variability was non‐limiting. Up to 78% of the calculated 14C‐CO2 produced in the syringe and injected into the chamber could be accounted for in the plant/soil system four hours after labelling. The fate of the 14C label was monitored after an allocation period of 4 hour to 35 days in the plant/soil system using well established methods of analysis. This simple chamber technique appears to be useful for studying C transfers through the pasture plant/soil system and for understanding C dynamics in the field.