Rhizodeposition by field-grown winter barley exposed to 14CO 2 pulse-labelling

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Winter barley ( Hordeum vulgare L. cultivar ‘Marinka’) was grown in the field in 30 stainless steel cylinders arranged in ten triplets. At four growth stages (Feekes Stage 4, 6–7, 10.3–4 and 10.5.4–11.1), the shoots of two or three triplets were exposed to 14CO 2 for 8 – 10 h using a closed, temperature-regulated cabinet. Five days after labelling, the barley was cut at ground level and the cylinders retrieved for analyses. Below-ground transfer of 14C was divided into macro-roots (roots isolated by soil washing), macro-root-free soil, and rhizosphere (root and microbial) respiration. Additional labelled cylinders were left in the field and retrieved at crop maturity on 2 August or 11 October. At tillering 83% of the macro-root C was in the top 0–15 cm soil layer decreasing to 69% at maturity. At tillering, 43.2% of total 14C recovered was translocated below-ground, 10.1%, 6.0% and 27.0% being recovered in macro-roots, macro-root-free soil, and as rhizosphere respired 14C, respectively. The corresponding values at early grain filling were 1.9, 9.7 and 2.3%. From tillering to early grain filling the proportion of labelled C in the 0–15 cm soil layer decreased from 88 to 69% for macro-roots and from 66 to 36% for root-soil systems. The half life of 14C in macro-root-free soil was 4–48 days depending on root growth intensity. At crop maturity, grain, straw and macro-roots yielded 292.5 g C m −2, 448.5 g C m −2 and 79.7 g C m −2, respectively. The total below-ground transfer of C during the growth of winter barley was estimated to be 237.2 g C m −2 (2372 kg C ha −1), of which rhizosphere respired C and macro-root C at maturity accounted for 39.5% and 23.7–43.8%, respectively. Below-ground transfer of C corresponded to one-third of the above-ground C harvested at maturity. Excluding root respiration, the rhizodeposition totalled 174.8 g C m −2 of which 66.9% was retained below-ground at crop maturity. The in situ decomposition rates of root and root-derived C were low following crop maturity.