Contribution of Lolium perenne rhizodeposition to carbon turnover of pasture soil

Carbon rhizodeposition and root respiration during eight development stages of Lolium perenne were studied on a loamy Gleyic Cambisol by (CO2)-C-14 pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total (CO2)-C-14 efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first 8 days after C-14 pulse labelling decreased during plant development from 14 to 6.5% of the total C-14 input. Root respiration accounted for was between 1.5 and 6.5% while microbial respiration of easily available rhizodeposits and dead root remains were between 2 and 8% of the C-14 input. Both respiration processes were found to decline during plant development, but only the decrease in root respiration was significant. The average contribution of root respiration to total (CO2)-C-14 efflux from the soil was approximately 41%. Close correlation was found between cumulative (CO2)-C-14 efflux from the soil and the time when maximum (CO2)-C-14 efflux occurred (r=0.97). The average total of CO2 efflux from the soil with Lolium perenne was approximately 21 mu g C-CO2 d(-1) g(-1). It increased slightly during plant development. The contribution of plant roots to total CO2 efflux from the soil, calculated as the remainder from respiration of bare soil, was about 51%. The total C-14 content after 8 days in the soil with roots ranged from 8.2 to 27.7% of assimilated carbon. This corresponds to an underground carbon transfer by Lolium perenne of 6-10 g C m(-2) at the beginning of the growth period and 50-65 g C m(-2) towards the end of the growth period. The conventional root washing procedure was found to be inadequate for the determination of total carbon input in the soil because 90% of the young fine roots can be lost.