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

Abstract

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 C-14 pulses to trace allocation and reutilization of C assimilated before clipping or shading. Five days after the last C-14 pulse, plants were clipped or shaded and the total CO2 and (CO2)-C-14 efflux from the soil was measured. C-14 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 C-14 originating mainly from root reserve's were detected in the newly grown shoots. This corresponded to a translocation of 5 and 8% of total C-14 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 (CO2)-C-14 efflux from soil did not change after clipping of both species. An increased (CO2)-C-14 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. (C) 2012 Elsevier B.V. All rights reserved.