| Guangxi is located in the subtropical karst geomorphological area,and the calcareous soil containing a large amount of calcium carbonate minerals is widely distributed.When highly water-soluble P fertilizer is applied into calcareous soil,it will be trapped by calcium carbonate in the soil and leads to a decrease in the current phosphate fertilizer utilization rate of crops.Fortunately,the plant rhizosphere can secrete organic acids to release the symptoms of P deficiency,and the calcite soil contains a phosphate-solubilizing bacteria,which can also secrete organic acids to convert insoluble P into a form that can be absorbed by plants.The complexity of soil microenvironment leads to many uncertainties in macroscopic research.The basic chemistry of how organic acids release insoluble P from soil is not well understood,which makes it difficult to understand the nature of soil P fixation and release.In view of this,this study identified the main types of low molecular weight organic acids secreted by phosphate-solubilizing bacteria in the calcareous soil of Wuming,Guangxi,and studied the interaction of low molecular weight organic acids with calcium carbonate and calcium phosphate from the perspective of crystallography.The specific content is as follows:1.The inorganic phosphorus forms in soil were analyzed and the low molecular weight organic acids secreted by phosphorus solubilizing bacteria in calcic soil were identified.Soil samples from 8 villages in Wuming District,Nanning,Guangxi Province were collected and the inorganic phosphorus forms were analyzed.The results showed that compared with acidic soil,the inorganic phosphorus in alkaline calcic soil accounted for 52.52% of total phosphorus,and more than 90% of inorganic phosphorus existed in the form of insoluble phosphorus.At the same time,the phosphorus solubilizing bacteria in the calcic soil of the village were isolated,and the organic acid types of the culture liquid were analyzed by HPLC.It was found that the phosphorus solubilizing bacteria in the calcic soil area could secrete oxalic acid,tartaric acid,formic acid,succinic acid,citric acid,malic acid,acetic acid and other organic acids with low molecular weight under low phosphorus stress.2.Effects of different low molecular weight organic acids on the crystallization of calcium carbonate.Calcite was used to investigate the effects of background electrolyte and low molecular weight organic acid functional groups on calcite crystallization.Equimolar CaCl2 and NaHCO3 solutions were mixed rapidly under ultrasonic conditions,and the effect of low molecular weight organic acids with similar structure on calcite crystallization was investigated under the condition of presence or absence of background electrolyte.By analyzing the morphology,crystal phase and adsorption behavior of organic acids of calcite,it is found that the carboxyl and hydroxyl groups of low molecular weight organic acids play different roles in the crystallization process of calcite.The influence of carboxyl group on the morphology and crystallinity of calcite is more significant than that of background electrolyte and hydroxyl group.Lactic acid（LA）,malic acid（MA）and citric acid（CA）,which contain one hydroxyl group,interact specifically with the calcite crystal core,changing the calcite from rhomboid to ellipsoid,needle and dumbbell morphologies.However,MA,succinic acid（SA）and tartaric acid（TA）containing dicarboxylic groups delay the phase transformation process of vaterite to calcite with the increase of the number of alcohol hydroxyl groups.3.Regulation of calcium phosphate nucleation dynamics by citric acid.In order to fully understand how organic acids regulate calcium phosphate crystals,the mechanism of citric acid regulation on supersaturated calcium phosphate crystals was explored using the idea of nucleation kinetics.By continuously monitoring the p H of supersaturated calcium phosphate solution,it is found that citric acid has a dual effect on the regulation of calcium phosphate crystallization,that is,low concentration promotes the crystallization of calcium phosphate and high concentration inhibits the crystallization of calcium phosphate.Combined with scanning electron microscopy（SEM）,transmission electron microscopy（TEM）,X-ray photoelectron spectroscopy（XPS）and Raman spectroscopy,it is found that calcium phosphate formed in the presence of low concentration of citric acid is flake,while calcium phosphate formed in the presence of no citric acid and high concentration of citric acid also contains a few spherical crystals.The amorphous calcium phosphate（ACP）phase with low crystallinity gradually transforms into calcium bisphosphate（DCPD）,anhydrous calcium bisphosphate（DCPA）and octadecalcium phosphate（OCP）,and the high concentration of citric acid affects the crystallization process of calcium phosphate mainly through adsorption. |
