Response of soil microbial community and nutrients to low molecular weight organic acids in a Hongbaoshi pear orchard

【Objective】The LMWOAs in the rhizosphere can drive the changes of the rhizospheric microbial community by affecting pH and EC values, and promote the dissolution of some insoluble nutrient ions, thereby increasing the effective nutrient content in the rhizosphere. In addition, LMWOAs have strong chelating ability and form complexes with various metal ions, thereby increasing the absorption of metal ions by plants. As a microbial carbon source, LMWOAs attract beneficial microbe to the rhizosphere and promote the colonization and proliferation of probiotics in the rhizosphere to avoid the invasion of pathogenic bacteria. From the aspect of pear soil microbial community and nutrients, the relationship between microbial community and nutrient uptake and fruit quality after low molecular weight organic acid (LMWOAs) treatment was analyzed.【Methods】Using Hongbaoshi pear as the experimental material, 5% and 10% malic acid (LM and HM), citric acid (LC and HC) and oxalic acid(LO and HO) combined with NPK fertilizers were set (5% and 10% respectively represent 5% and 10% of the total amount of LMWOAs and NPK fertilizers), with only NPK fertilizers serving as a control to determine the effects of LMWOAs on soil nutrients and the functional diversity of microbial community.【Results】Soil EC values decreased with LMWOAs, in which citric acid and oxalic acid significantly decreased compared to the control. Also, soil pH were significantly decreased by citric acid and oxalic acid. Compared with the control, the organic matter content significantly decreased with citric acid and LO, while the organic matter content at 0-20 cm soil layer with LM and HM significantly decreased by 49.44% and 36.52%, respectively. Compared with the control, the nitrate nitrogen content at 0-20 cm soil layer with LC significantly decreased by 34.28%, while the nitrate nitrogen content at ＞20-40 cm soil layer with HM and LC significantly decreased by 54.46% and 44.22%. The ammonium nitrogen content at 0-20 cm soil layer significantly increased with LO and HO by 49.37% and 54.54%, respectively, while the ammonium nitrogen content at ＞20-40 cm soil layer significantly increased with LO by 51.84%. LMWOAs significantly decreased the soil available P content at 0-20 cm soil layer, among which the citric acid treatment (LC and HC) decreased it by 44.26% and 41.93%. The soil available P at ＞20-40 cm soil layer with LM and HO significantly increased by 31.31% and 25.18%, respectively, while the HC significantly decreased it by 31.67% compared with the control. Compared with the control, HM, citric acid and oxalic acid significantly decreased the available K content at 0-20 cm soil layer, while malic acid increased the available K content at ＞20-40 cm soil layer compared with the control, and LM significantly increased it by 37.23% compared with the control. The diversity index and carbon substrate utilization patterns of pear soil microbial community were changed by LMWOAs, and the 0- 20 cm microbial dominance index D, diversity index H’and richness index S significantly increased by HM. The carbon source utilization intensity of ＞20-40 cm microbial community was reduced by oxalic acid, and was significantly reduced by LO treatment compared with the control. The results showed that the correlations between microbial communities and soil nutrients at 0-20 cm soil layer were all significantly and negatively correlated, and microbial community index (except E) at ＞20- 40 cm soil layer was significantly and positively correlated with the contents of organic matter and available K, and was significantly and negatively correlated with the content of ammonium nitrogen. The results also showed that the significant correlation between microbial community at 0-20 cm soil layer and fruit vitamin C, fruit NPK content, yield and other indicators were all negatively and significantly correlated, while those with titratable acid content were significantly correlated. The significant correlations between the microbial community at ＞20-40 cm soil layer and the single fruit weight, fruit color, fruit potassium content and leaf nitrogen content were all positively and significantly correlated. 【Conclusion】Soil nutrient content and microbial community carbon characteristics in a pear orchard were changed by malic acid, citric acid and oxalic acid combined with NPK fertilizers. In addition, compared with 0-20 cm soil layer, the microbial community at ＞20-40 cm soil layer played a more important role in nutrient absorption and fruit quality improvement of pear trees.