Effect of clipping and shading on C allocation and fluxes in soil under ryegrass and alfalfa estimated by 14C labelling

Photosynthesis of higher plants drives carbon (C) allocation below-ground and controls the supply of assimilates to roots and to rhizosphere microorganisms. To investigate the effect of limited photosynthesis on C allocation, redistribution and reutilization in plant and soil microorganisms, perennial grass Lolium perenne and legume Medicago sativa were clipped or shaded. Plants were labelled with three 14C pulses to trace allocation and reutilization of C assimilated before clipping or shading. Five days after the last 14C pulse, plants were clipped or shaded and the total CO2 and 14CO2 efflux from the soil was measured. 14C in above- and below-ground plant biomass and bulk soil, rhizosphere soil and microorganisms was determined 10 days after clipping or shading. After clipping, 2% of the total assimilated 14C originating mainly from root reserves were detected in the newly grown shoots. This corresponded to a translocation of 5 and 8% of total 14C from reserve organs to new shoots of L. perenne and M. sativa, respectively. The total CO2 efflux from soil decreased after shading of both plant species, whereas after clipping, this was only true for L. perenne. The 14CO2 efflux from soil did not change after clipping of both species. An increased 14CO2 efflux from soil under shading for both plants indicated that lower assimilation was compensated by higher utilization of the reserve C for root and rhizomicrobial respiration. We conclude that C stored in roots is an important factor for plant recovery after limiting photosynthesis. This stored C is important for shoot regrowth after clipping, whereas after shading, it is utilized mainly for maintenance of root respiration. Based on these results as well as on a review of several studies on C reutilization for regrowth after clipping, we conclude that because of the high energy demand for nitrogen fixation, legumes use a higher portion (9–10%) of stored C for regrowth compared to grasses (5–7%). The effects of limited photosynthesis were of minor importance for the exudation of the reserve C and thus, have no effect on the uptake of this C by microorganisms.