C and N allocation in soil under ryegrass and alfalfa estimated by 13C and 15N labelling

Background and Aims Below-ground translocated carbon (C) released as rhizodeposits is an important driver for microbial mobilization of nitrogen (N) for plants. We investigated how a limited substrate supply due to reduced photoassimilation alters the allocation of recently assimilated C in plant and soil pools under legume and non-legume species. Methods A non-legume (Lolium perenne) and a legume (Medicago sativa) were labelled with N-15 before the plants were clipped or shaded, and labelled twice with (CO2)-C-13 thereafter. Ten days after clipping and shading, the N-15 and C-13 in shoots, roots, soil, dissolved organic nitrogen (DON) and carbon (DOC) and in microbial biomass, as well as the C-13 in soil CO2 were analyzed. Results After clipping, about 50 % more C-13 was allocated to regrowing shoots, resulting in a lower translocation to roots compared to the unclipped control. Clipping also reduced the total soil CO2 efflux under both species and the C-13 recovery of soil CO2 under L. perenne. The N-15 recovery increased in the shoots of M. sativa after clipping, because storage compounds were remobilized from the roots and/or the N uptake from the soil increased. After shading, the assimilated C-13 was preferentially retained in the shoots of both species. This caused a decreased C-13 recovery in the roots of M. sativa. Similarly, the total soil CO2 efflux under M. sativa decreased more than 50 % after shading. The N-15 recovery in plant and soil pools showed that shading has no effect on the N uptake and N remobilization for L. perenne, but, the N-15 recovery increased in the shoot of M. sativa. Conclusions The experiment showed that the dominating effect on C and N allocation after clipping is the need of C and N for shoot regrowth, whereas the dominating effect after shading is the reduced substrate supply for growth and respiration. Only slight differences could be observed between L. perenne and M. sativa in the C and N distribution after clipping or shading.