RHIZOSPHERE MICROBIAL COMMUNITIES AND CARBON PARTITIONING UNDER ZERO-TANNIN LENTIL GENOTYPES

Abstract

The decomposition of soil organic carbon (C) is primarily mediated by soil microorganisms. By partitioning C through anabolic and catabolic processes, soil microorganisms control the flow of C through terrestrial ecosystems. As microorganisms metabolize organic compounds to satisfy heterotrophic demands for C and energy, C partitioning should be related to both the physiology of the active microbial population and the biochemical quality of substrate. Zero-tannin (ZT) lentils have been selectively bred, for alterations in the phenylpropanoid pathway, to remove tannins from their seed coats. Any modification in a plant biochemical pathway has the potential to alter the tissue chemistry across the entire plant. The objective of this research was to examine soil microbial responses to ZT lentil genotypes both during plant growth and after, and to investigate how differences in the biochemical quality of aboveground (AG) and belowground (BG) plant tissues of ZT versus conventional tannin (TAN) genotypes affected soil C partitioning. Lentil plants were exposed to 13CO2 during plant growth and harvested at flowering to coincide with peak rhizodeposition. Carbon isotope ratios in phospholipid fatty acids (PLFAs) of soil microorganisms revealed significant differences in microbial community structure and biomass between ZT and TAN genotypes. Further, microorganisms produced elevated levels of extracellular enzymes in the rhizosphere of ZT lentil genotypes. When AG and BG residues of each genotype harvested at maturity were incubated for a period of 107 days, microbial communities in microcosms incubated with BG residues produced proportionately more cell biomass per unit C degraded than microcosms incubated with AG residues. Further, unconstrained ordination by non-metric multidimensional scaling (NMDS) of Bray–Curtis dissimilarity revealed significant differences in decomposer community structure between AG and BG tissues but not between ZT and TAN genotypes. Extracellular enzyme activities were generally highest in control soils followed by BG soils, though no difference in enzyme activities were observed between genotypes. The results of this research suggest that biochemically complex compounds have the potential to be stabilized within the soil matrix via microbial residues. Moreover, differences in polyphenol content between TAN and ZT genotypes did not significantly affect respiration rates or cumulative C loss and may be an indication that C:N ratios are more important than the chemical composition of C compounds in regulating decomposition processes; while in the rhizosphere, small scale changes have the potential to alter soil process and C dynamics.