Abstract/Summary

Fluxes of rhizodeposit carbon (C) to soil stimulate microbial activity affecting soil organic matter (SOM)decomposition and, in turn, nutrientfluxes in soil. In agricultural soils, residues from previous crops alsohave major impacts on SOM and nutrient cycling, and their turnover by microbes is likely to be indirectlyimpacted by rhizodeposition. However, the combined effects of rhizodeposit C and inputs of C from deadplant materials in soil on native SOM decomposition are unclear. In this study, we assessed (i) the in-dividual and combined effects of barley rhizodeposition and ryegrass root residue inputs (as a model forresidue input from previous crop) on SOM mineralization, (ii) the intraspecies variation within barley inimpacting residue mineralization, and (iii) whether genotypes that stimulate high mineralization rates ofplant residues in soil also directly benefit through increased nutrient uptake from these residues. Wecontinuously applied13C depleted CO2to selected barley recombinant chromosome substitution lines(RCSLs) to trace theflow of barley root-derived C in surface soil CO2efflux, soil microbial biomass and soilparticle-size fractions. In addition,13C and15N enriched ryegrass root residues were mixed into soil totrace the mineralization of residue-derived C and the residue-derived nitrogen (N) uptake by plants. Ourresults show (i) genotype-specific variation in impacting total soil CO2efflux and its component sources:SOM-derived C, barley root-derived C and/or ryegrass residue-derived C, (ii) residue effects on total C andSOM-derived C respired as CO2, (iii) genotype-residue combined effects on SOM primed C, that were verysimilar to the sum of primed C caused by planting or residue addition alone (except for the last samplingdate), and (iv) that plant uptake of residue released N between genotypes was linked to genotype im-pacts on residue mineralization. These results suggest that impacts of plant rhizodeposition and residueinputs had additive effects on SOM priming. Furthermore, these results demonstrate, for thefirst time,genotype differences in impacting the mineralization of recent plant-derived organic materials in soil,and reveal that this process directly contributes to plant nutrition.