Carbon partitioning and below-ground translocation by Lolium perenne

Carbon (C) balance, rhizodeposition and root respiration during development of Lolium perenne were studied on a loamy Gleyic Cambisol by 14CO2 pulse labeling of shoots in a two-compartment chamber under controlled laboratory conditions. The losses from shoot respiration were about 36% of the total assimilated C. The highest respiration intensity was measured in the first night after the labeling, and diminishes exponentially over time. Total 14CO2 efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first eight days after the 14C pulse labeling increased with plant development from 2.7 to 11% of the total 14C assimilated by plants. A model approach used for the partitioning of rhizosphere respiration showed that measured root respiration was between 1.4 and 3.5% of assimilated 14C, while microbial respiration of easily available rhizodeposits and dead root residues was between 0.9 and 6.8% of assimilated C. Both respiration processes increased during plant development. However, only the increase in root respiration was significant. The average contribution of root respiration to total 14CO2 efflux from the soil was approximately 46%. Total CO2 efflux from the soil was separated into plant-derived and soil-derived CO2 using 14C labeling. Additional decomposition of soil organic matter (positive priming effects) in rhizosphere was calculated by subtracting the CO2 efflux from bare soil from soil-derived CO2 efflux from soil with plants. Priming effects due to plant rhizodeposition reach 60 kg of C ha-1 d-1. 14C incorporated in soil micro-organisms (extraction-fumigation) amounts to 0.8-3.2% of assimilated C. The total below-ground transfer of organic C by Lolium perenne was about 2800 kg of C ha-1. The C input into the soil consists of about 50% of easily available organic substances.