Summary• The aims of the study were to determine group specificity in microbial utilization of root-exudate compounds and whole rhizodeposition; quantify the proportions of carbon acquired by microbial groups from soil organic matter and rhizodeposition, respectively; and assess the importance of root-derived C as a driver of soil microbial community structure.• Additions of 13 C-labelled root-exudate compounds to organic soil and steady-state labelling of Lolium perenne , coupled to compound-specific isotope ratio mass spectrometry, were used to quantify group-specific microbial utilization of rhizodeposition.• Microbial utilization of glucose and fumaric acid was widespread through the microbial community, but glycine was utilized by a narrower range of populations, as indicated by the enrichment of phospholipid fatty acid (PLFA) analysis fractions. In L. perenne rhizospheres, high rates of rhizodeposit utilization by microbial groups showed good correspondence with increased abundance of these groups in the rhizosphere.• Although rhizodeposition was not the quantitatively dominant C source for microbes in L. perenne rhizospheres, relative utilization of this C source was an important driver of microbial group abundance in organic soil.

A 13 C/ 12 C mass spectrometer was interfaced with a open gas exchange system including four growth chambers to investigate CO 2 exchange components of perennial ryegrass ( Lolium perenne L.) stands. Chambers were fed with air containing CO 2 with known .8 to ----2.5‰, and increased by ~ 10‰ following a shift from ----2.6 to ----46.7‰ due to isotopic disequilibria between photosynthetic and respiratory fluxes. Isotopic imbalances were used to assess (non-photorespiratory) respiration in light and the replacement of the respiratory substrate pool(s) by new photosynthate. Respiration was partially inhibited by light, but increased during the light period and decreased in darkness, in association with temperature changes. The labelling kinetics of respiratory CO 2 indicated the existence of two major respiratory substrate pools: a fast pool which was exchanged within hours, and a slow pool accounting for ~ 60% of total respiration and having a mean residence time of 3.6 d.

We assessed the contribution of pre-anthesis reserve C to protein and carbohydrate deposition in grains of wheat (Triticum aestivum L.) using a new approach comprised of steady-state 13 C/ 12 C labeling and separation of the protein and carbohydrate fractions of mature grains. Experiments were performed with two spring wheat cultivars (Kadett and Star) grown with differential N fertilizer supply over 2 years. Pre-anthesis reserves contributed between 30% and 47% of the C in protein and 8% to 27% of the C in carbohydrates of grains. Partitioning of pre-anthesis C among the grain fractions was strongly dependent on the C/N (w/w) ratio in mobilized preanthesis biomass (r 2 ‫؍‬ 0.92). There appeared to be no significant exchange of pre-anthesis C between amino acids and carbohydrates during redistribution. The mean apparent efficiency of mobilized carbohydrate-C use in grain filling (ME CHO , estimated as the mass of pre-anthesis C deposited in grain carbohydrates per gram of preanthesis C mobilized from carbohydrates in vegetative plant parts) was 0.72, whereas that of protein-C (ME P ) was 0.56. However, ME P and ME CHO varied among treatments. ME CHO increased with increasing contributions of water-soluble carbohydrates to total preanthesis carbohydrate mobilization. ME P decreased with increasing residence time of protein in vegetative biomass. Possible causes for variability of ME P and ME CHO are discussed.

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Significant mobilization of protein and carbohydrates in vegetative plant parts of wheat regularly occurs during grain filling. While this suggests a contribution of reserves to grain filling, the actual efficiency of mobilized assimilate conversion into grain mass (ME) is unknown. In the present study the contribution of pre-anthesis C (C fixed prior to anthesis) to grain filling in main stem ears of two spring wheat (Triticum aestivum L.) cultivars was determined by 13 C/ 12 C steady-state labelling. Mobilization of pre-anthesis C in vegetative plant parts between anthesis and maturity, and the contributions of water-soluble carbohydrates (WSC) and protein to pre-anthesis C mobilization were also assessed. Experiments were performed with two levels of N fertilizer supply in each of 2 years. Pre-anthesis reserves contributed 11-29% to the total mass of C in grains at maturity. Pre-anthesis C accumulation in grains was dependent on both the mass of pre-anthesis C mobilized in aboveground vegetative plant parts (r 2 = 0·87) and ME (defined as g pre-anthesis C deposited in grains per g pre-anthesis C mobilized in above-ground vegetative plant parts; r 2 = 0·40). ME varied between 0·48 and 0·75. The effects of years, N fertilizer treatments and cultivars on ME were all related to differences in the fractional contribution of WSC to preanthesis C mobilization. Multiple regression analysis indicated that C from mobilized pre-anthesis WSC may be used more efficiently in grain filling than C present in proteins at anthesis and mobilized during grain filling. Possible causes for variability of ME are discussed.