Molecular-level phenolic composition of fine roots is modulated by soil resource environment and is decoupled from morphological and macro-elemental root traits
Fine roots (traditionally defined as <2mm diameter), are highly structured organs of plants, and account for 33-67% of terrestrial NPP. Although it is recognized that ~65% of C sequestered in soil is derived from fine roots, we know surprisingly little about the factors regulating the residence time of C in these organs. Traditional parameters such as C, nitrogen (N), and acid-unhydrolyzable-residue that predict decomposition of above-ground plant litter fail to predict the decomposition of fine roots. Furthermore, within the category of fine roots, the distal first- and second-order roots with smaller diameters and low C:N ratio decompose slower than third- and fourth-order roots, which are larger in diameter and have a higher C:N ratio. This challenges the traditional paradigm that associates higher proportion of tissue N with faster decomposition rates. Phenolic-matrix in plants that has an inherently slower decomposition rate and that fortify the cellulose matrix through cross-linkages could disproportionately influence the decomposition of root tissues. To understand the linkages between traditional macro-elemental and morphological traits of roots with its molecular-level carbon chemistry, we analyzed the seasonal variations in monomeric yields of the free-, bound-, and lignin-phenols in fine roots and leaves of Ardisia quinquegona. Also, the influence of soil resource availability on root chemistry was investigated in fine roots of Liquidambar styraciflua exposed to N-addition treatment.
Fine roots of Ardisia quinquegona contained two-fold higher levels of bound-phenols and three-fold higher levels of lignin-phenols than leaves. Within fine roots, the levels of free- and bound-phenols decreased with increasing root order, and seasonal variation in phenolic profile was more evident in the lower-order than in higher-order roots. The morphological and macro-elemental root-traits were decoupled from the quantity, composition and tissue-association of phenolic compounds, revealing the potential inability of these traditional parameters to capture the C-quality within the fine root architecture and between fine roots and leaves. The bound phenols in the fine roots of Liquidambar styraciflua developed in N-addition treatments was only marginally lower than those developed in low N environments, but the phenolic monomers that cross link polysaccharide to lignin was 41% lower in roots exposed to N-addition treatments, reflecting the role of soil resources in modulating finer-level root chemistry during the developmental stage. Our results highlight the molecular-level heterogeneity in C-composition within the fine root architecture, and imply that traits that capture molecular identity of the root-construct might better predict the decomposition dynamics within fine root orders.