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

Kuzyakov, Y., Siniakina, S.V., Ruehlmann, J., Domanski, G., Stahr, K.
Journal of the science of food and agriculture 2002 v.82 no.13 pp. 1432-1441
carbon, carbon dioxide, developmental stages, fertilizer rates, grains, leaves, lettuce, microbial activity, microbial biomass, nitrogen, nitrogen fertilizers, organic soils, rhizosphere, roots, shoots, soil organic matter
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 CO2 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. 14C pulse labelling of shoots followed by 14C monitoring in the soil, roots, microbial biomass and CO2 efflux from the soil was used to distinguish between root-derived and soil organic matter-derived C. The 14C 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 CO2 (the sum of root respiration and rhizomicrobial respiration) represented about 15–60% of the total CO2 efflux from the planted soil. Two peaks were measured in the 14CO2 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.