Soil decomposition of wheat internodes of different maturity stages: relative impact of the soluble and structural fractions

The aim of this study was to clarify the importance of the soluble fraction on cell wall decomposition. Wheat plant was chosen as a model and was harvested at three stages of maturity: anthesis (A stage), 20 days after anthesis (B stage) and physiological maturity (PM stage). Wheat third internode (numbered down from the ear) were selected for this study. Internode age influenced the cumulative CO(2) kinetics with internodes from wheat stem harvested at anthesis mineralizing 62.1%+/-2.2 of added residue C whereas those harvested at the B and PM stages mineralized 58.8%+/-1.4 and 51.6%+/-1.7, respectively of the added C. Chemical analyses revealed that maturation of the selected internodes mainly altered residue quality by modifying the proportion of soluble to cell wall fractions rather than the quality of these fractions. The hexose to pentose ratios were good biomarkers of microbial sugars for both soluble and cell wall fractions, as were the uronic acids, which are not commonly determined in soil decomposition studies. This study clearly demonstrated that the contents of the internode soluble fraction did not affect the extent of cell wall C mineralization. Therefore, the soluble content of crop residues would not regulate the soil microbial populations able to mineralize cell wall C. However, this needs to be validated on a broader range of residue types with different nature of cell wall C or soluble compounds.