Decrease of humus decomposition during growth of Lolium perenne — negative priming effect

Carbon rhizodeposition and root respiration at eight developmental stages of Lolium perenne were studied on a loamy Gleyic Cambisol by 14C02 pulse labelling of shoots in a two compartment chamber under controlled laboratory conditions. Total 14C02 efflux from the soil (root respiration, microbial respiration of exudates and dead roots) in the first eight days after labelling decreased during plant de­ velopment from 14 to 6.5 % of 14C input. The average of total CO2 efflux from the soil with Lolium perenne was approximately 21 J.lg C-COi(d . g). It increased during plant development. The contribution of plant roots to total CO2 efflux from the soil increased from 10-20 % at the beginning and to more than 90 % towards the end of plant development. The root exudates led to a enormous reduction in humus decomposition. This interaction can be termed as 'negative priming effect' caused by Lolium perenne.