Carbon partitioning in plant and soil, carbon dioxide fluxes and enzyme activities as affected by cutting ryegrass

The effect of a single cut (simulated grazing) and regrowth of Lolium perenne on CO2 efflux from soil (loamy Haplic Luvisol), on below-ground C translocation and on the distribution of plant C among different soil particle size fractions was investigated under controlled conditions with and without N fertilization by pulse labelling with 14C 7 times (four before and three after the cutting). The amount of 14C respired from the rhizosphere of Lolium decreased by a factor of about 3 during 1 month of growth. At the same time the amount of 14C stored in soil increased. Cut and non-fertilized plants respired less C in the rhizosphere compared to the uncut plants and cut fertilized plants. About 80% of the root-derived CO2 efflux originated from the C assimilated after defoliation, and 20% originated from the C assimilated before cutting. N fertilization decreased the below-ground C losses (root respiration and exudation) during regrowth. The shoot is the main sink of assimilated C before and after the defoliation. N fertilization led to higher C incorporation into the shoot parts growing after defoliation compared to unfertilized plants. A lower incorporation of 14C was observed in the roots of N fertilized plants. The relative growth rates (expressed as 14C specific activity) of roots and stubble were minimal and that of shoot parts growing after defoliation was maximal. Twelve percent of 14C was found in the newly grown leaves after regrowth; nevertheless, 4.7% and 2.4% of 14C in the new shoot parts were translocated from the root and shoot reserves of unfertilized and fertilized plants, respectively. Most of the C retranslocated into the new Lolium leaves originates from the stubble and not from the roots. Between 0.5% and 1.7% of 14C recovered in shoots and below-ground C pools was found in the soil microbial biomass. Cutting and fertilization did not change 14C incorporation into the microbial biomass and did not affect xylanase, invertase, and protease activities. Tracing the assimilated C in particle size fractions revealed maximal incorporation for the sand and clay fraction.