Rhizosphere Carbon Dynamics in Soils Under Zero-Tannin and Tannin Containing Genotypes of Lentil (Lens culinaris) and Faba Bean (Vicia faba).

Decomposition of crop residues and rhizodeposits play an essential role in carbon and nutrient cycling. The fate of the C released from these plant inputs depends on the chemical nature of the starting materials, with plant residue properties such as initial C:N, lignin or polyphenol content being particularly important, and the structure and function of the microbial community present in the soil. Polyphenols constitute a group of compounds derived from plant metabolism, however, some annual legume crops have been selectively bred to remove tannins (phenolic compounds) from seed coats. Tannins comprise a large group of plant phenolic secondary metabolites which can alter rhizosphere soil microbial communities, inhibit microbial enzyme activities important for nutrient cycling, reduce rates of nutrient mineralization, and alter decomposition. It is yet unclear how the elimination of seed tannins in faba bean and lentil cultivars will affect soil microbial processes and C dynamics. We conducted a greenhouse study using ¹³C isotope labelling to test the hypotheses that when compared to tannin containing cultivars 1) there will be a reduction in lignin and phenolic compounds in plant tissues of zero-tannin cultivars of lentil and faba bean and 2) this reduction will lead to differences in microbial community structure and function in the rhizosphere when compared to traditional tannin containing cultivars. Plant roots and rhizosphere soils will be harvested at flowering, coinciding with anticipated peak rhizodeposition, and hydrolytic and oxidative enzyme activities involved in C cycling will be assessed. Phospholipid fatty acid (PLFA) profiles will be characterized to determine microbial community composition and further analysis of ¹³C-PLFA will be used to determine whether there are differences in the composition of the microbial community that is actively assimilating root derived carbon under soils of zero tannin and tannin containing genotypes of lentil and faba bean.