Effect of nitrogen fertilisation on below‐ground carbon allocation in lettuce

The aims of this study were to investigate the effect of nitrogen (N) fertilisation on the below‐ground carbon (C) translocation by lettuce and the CO 2 efflux from its rhizosphere. Two N fertilisation levels (80 and 160 kg N ha −1 ) and two growth stages (43 and 60 days) were tested. 14 C pulse labelling of shoots followed by 14 C monitoring in the soil, roots, microbial biomass and CO 2 efflux from the soil was used to distinguish between root‐derived and soil organic matter‐derived C. The 14 C allocation in the below‐ground plant parts was 1.5–4.6 times lower than in the leaves. The total quantity of C translocated into the soil was much lower than in the case of cereals and grasses, amounting to 120 and 160 kg C ha −1 for low and high N respectively. N fertilisation diminished the proportion of assimilated C translocated below ground. About 5–8% of the assimilated C was respired into the rhizosphere. Root‐derived CO 2 (the sum of root respiration and rhizomicrobial respiration) represented about 15–60% of the total CO 2 efflux from the planted soil. Two peaks were measured in the 14 CO 2 efflux: the first peak (4–5 h after labelling) was attributed to root respiration, whilst the second peak (12 h after labelling) was attributed to microbial respiration of exudates. Twelve days after labelling, 0.15–0.25% of the assimilated C was found in the microbial biomass. The higher microbial activity in the lettuce rhizosphere doubled the soil organic matter decomposition rate compared with unplanted soil. © 2002 Society of Chemical Industry