Molecular-level carbon traits of fine roots: unveiling adaptation and decomposition under flooded conditions

Abstract. Fine roots are vital for plant development and carbon biogeochemical cycling in terrestrial ecosystems. Flooding is known to regulate the physiology and morphology in plant roots; however, its impact on molecular-level characteristics of carbon compounds (carbon traits) in fine roots remains largely unexplored, which limits our understanding of root adaptation and decomposition under changing environments. Here, we used a sequential extraction method, starting from nonpolar to polar solvents, in order to obtain dichloromethane- and methanol-extractable (FDcMe) fractions, base-hydrolyzable (FKOHhy) fractions, and CuO-oxidizable (FCuOox) fractions from fine roots of Dysoxylum binectariferum, which is naturally grown in soil and water. Subsequently, we characterized them using targeted gas chromatography–mass spectrometry and nontargeted Fourier transform ion cyclotron resonance mass spectrometry. Also, decomposition experiments were conducted on soil- and water-grown roots under aerobic and anoxic conditions. Results showed a consistent increase in the unsaturation degree and aromaticity of the analytes from FDcMe to FCuOox fractions. Both analyses were sufficiently sensitive to show that, compared to soil-grown roots, the water-grown roots developed more polyphenolics with a high unsaturation degree and aromaticity and had more nonstructural compositions. Furthermore, although flooding provided an anoxic condition that slowed down root decomposition, the adaptive strategy of developing more nonstructural labile components in water-grown roots accelerated root decomposition, thereby counteracting the effects of anoxia. This advances our understanding of biogeochemical processes in response to global environmental change.