Glucose addition promotes C fixation and bacteria diversity in C-poor soils, improves root morphology, and enhances key N metabolism in apple roots

The interaction between plant, soil and microorganism plays a crucial role in sustainable development of terrestrial ecosystem function and diversity. However, little information is known about how plant growth, soil organic carbon (C) fractions and microorganism respond to exogenous C addition in soils with low organic C content. Three levels of ¹³C-glucose (equal to 0, 100% and 500% of initial microbial biomass C) were added to non-sterilized (corresponding to treatment abbreviation of CK, Glu-1, Glu-2, respectively) and sterilized soils (corresponding to treatment abbreviation of SS, SS+Glu-1, SS+Glu-2, respectively) planted with apple rootstock (<i>Malus baccata</i> (L.) Borkh.) seedings. The objectives of this study were to analyse the dynamics of soil organic C (SOC) fractions and soil bacterial community diversity with glucose levels and soil sterilization, and to explore the morphology of roots and nitrogen (N) metabolism by plant after glucose addition to sterilized/non-sterilized soils. Results showed that the contents of labile organic C fractions were significantly varied (<i>P</i>&lt;0.05) with the levels of glucose addition and soil sterilization. SS+Glu-2 and Glu-2 treatments increased the contents of labile organic C fractions, on average, by 48.47% and 35.33% compared with no glucose addition, respectively. About 21.42% and 16.17% of glucose-C remained in sterilized and non-sterilized soils, respectively at the end of experiment (day 45). Regardless of soil sterilized or not, the glucose addition increased the richness and diversity indices of soil bacterial community compared with no-glucose addition. The glucose addition optimized root zone conditions, and enhanced root vitality, morphology and biomass. Both SS+Glu-2 and Glu-2 treatments significantly enhanced (<i>P</i>&lt;0.05) the contents of nitrate (NO₃^--N) and nitrite (NO₂^--N), but sharply decreased (<i>P</i>&lt;0.05) the ammonium (NH₄⁺-N) content compared with no glucose addition. Also, these two treatments significantly (<i>P</i>&lt;0.05) increased the enzymic activities and gene transcript levels involved in root N metabolism, which demonstrated that the high level of glucose addition promoted N assimilation and transformation into free amino acids by root. Overall, the addition of exogenous C to not only promotes its fixation and bacterial community diversity in C-poor soils, but also improves root morphology and N absorption by plant.<br>

植物、土壤与微生物之间的相互作用，对陆地生态系统功能与多样性的可持续发展具有关键作用。然而，针对有机碳含量较低的土壤，目前关于植物生长、土壤有机碳（soil organic carbon, SOC）组分以及微生物如何响应外源碳输入的相关研究仍较少。本研究设置3个浓度梯度的¹³C-葡萄糖（分别相当于初始微生物生物量碳的0、100%和500%），将其施加至种植平邑甜茶（<i>Malus baccata</i> (L.) Borkh.）实生苗的未灭菌土壤（对应处理缩写分别为CK、Glu-1、Glu-2）与灭菌土壤（对应处理缩写分别为SS、SS+Glu-1、SS+Glu-2）中。本研究旨在分析不同葡萄糖浓度与土壤灭菌处理下，土壤有机碳组分及土壤细菌群落多样性的动态变化，并探究外源葡萄糖施加至灭菌/未灭菌土壤后，植物根系形态与氮（nitrogen, N）代谢的响应特征。研究结果表明，活性有机碳组分含量随葡萄糖添加浓度与土壤灭菌处理发生显著变化（<i>P</i>&lt;0.05）。与未添加葡萄糖的对照组相比，SS+Glu-2与Glu-2处理的活性有机碳组分平均含量分别提升48.47%与35.33%。实验结束（第45天）时，约21.42%与16.17%的葡萄糖碳分别残留在灭菌土壤与未灭菌土壤中。无论土壤是否经过灭菌处理，添加葡萄糖均能提升土壤细菌群落的丰富度与多样性指数。葡萄糖添加还优化了根际微环境，提升了根系活力、形态指标与生物量。与未添加葡萄糖组相比，SS+Glu-2与Glu-2处理显著提升了硝态氮（NO₃^--N）与亚硝态氮（NO₂^--N）的含量，却显著降低了铵态氮（NH₄⁺-N）的含量（<i>P</i>&lt;0.05）。此外，这两组处理还显著提升了参与根系氮代谢的酶活性与基因转录水平（<i>P</i>&lt;0.05），表明高浓度葡萄糖添加可促进根系对氮的同化作用，并将其转化为游离氨基酸。总体而言，向贫碳土壤中添加外源碳，不仅可促进碳固持与细菌群落多样性的提升，还能改善植物根系形态并促进氮素吸收。