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 C-14 7 times (four before and three after the cutting). The amount of C-14 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 C-14 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 C-14 was observed in the roots of N fertilized plants. The relative growth rates (expressed as C-14 specific activity) of roots and stubble were minimal and that of shoot parts growing after defoliation was maximal. Twelve percent of C-14 was found in the newly grown leaves after regrowth; nevertheless, 4.7% and 2.4% of C-14 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 C-14 recovered in shoots and below-ground C pools was found in the soil microbial biomass. Cutting and fertilization did not change C-14 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.