4.4 | Abiotic stress and soil polysaccharide composition
The saccharides secreted by plant roots are an important source of soil organic matter, microbial carbon, and energy, mainly hexoses and a small number of pentoses. This study shows that pentose content tends to increase under most stress conditions, but it is not synchronized with the change in hexose. Under N stress, the secretion of polysaccharides from S. miltiorrhiza roots was down-regulated, confirming that nitrogen stress significantly reduced the secretion of saccharides from maize roots (Carvalhais et al., 2010, Zhu et al., 2016). P, DTP, and DT stress significantly down-regulated polysaccharide secretion in S. miltiorrhiza roots, unlike phosphorus deficiency, which promoted sugar secretion in maize roots (Carvalhais et al., 2010), and drought stress, which promoted polysaccharide secretion in plant roots (Ulrich et al., 2022). The release of root exudates positively correlates with microbial growth (Bengtson et al., 2012), suggesting that this result and the literature results may be related to the lack of microbial involvement in rhizosphere activities. Further analysis shows that the content of polysaccharides secreted by roots is positively correlated with plant yield. Abiotic stress is not conducive to the accumulation of plant biomass and may also affect the distribution of plant polysaccharides, resulting in most energy substances, such as carbon source synthetic sugars, being supplied to plants, reducing the roots secrete polysaccharides that are not conducive to carbon sequestration in the soil. Thus, unfavorable environment affects plant growth and also affects the carbon stored in the soil by the plants.