| Summary: | 碩士 === 國立中興大學 === 土壤環境科學系所 === 105 === Phosphorus (P) is a major essential nutrient of plants, but P fertilization for plant growth is inevitable due to its low mobility and availability in soils. Phosphorite, a limited resource, will be exhausted in a few decades, and it is an important issue to increase the long-term P availability in the agricultural soils. In soils, P is commonly fixed by hydrous oxides of Fe and Al, especially for a poorly crystalline ferrihydrite. Ferrihydrite could be formed coprecipitates with organic matters, e.g. humic acid, in the environments, resulting in the effects of interactions between P and ferrihydrite on P availability. To increase the bioavailability of P in soils, plant roots would secrete some small molecular weight organic acids, e.g., citric acids and oxalic acids, to exchange or dissolve the fixed P from soil minerals. The previous studies were focused on the P availability in soils improved by organic acids, however, it is rare to understand the effects of organic acids on P sorption on the coprecipitates of ferrihydrite and humic acids (FH-HA). Therefore, the aims of this study were to investigate that the effects of citric acid sorption on the coprecipitates structure of FH-HA and to evaluate the P adsorption on FH-HA in presence of citric acid. According to our results, we found that the maximum adsorption capacity of citric acid on FH-HA decreased with the increasing ratio of humic acid in FH-HA coprecipitates. The Fe K-edge extended X-ray absorption fine structure (EXAFS) results indicated that citric acid sorption made the structures of FH-HA coprecipitates poorly, contributing to affect the anion adsorption capacities.
The PO4 adsorption capacities of FH-HA coprecipitates were greatly affected by the order of citric acid or PO4 addition. When PO4 was firstly added (entitled as P-C) into the systems, the PO4 adsorption capacities of FH-HA coprecipitates were reduced to about 80% of the maximum adsorption capacity of PO4 on FH-HA without citric acid, but the PO4 adsorption capacities were decreased to about 50% with the addition of citric acid prior to PO4 (C-P). As the ratio of humic acid in the FH-HA coprecipitates increased, the adsorption capacities of PO4 would be decreased, leading to the improvement of P availability. However, the addition order of PO4 had no significant effect on the adsorption capacities of citric acid on FH-HA coprecipitates. It suggested that citric acid would be the diffusion barrier of PO4 to the interior of FH-HA coprecipitates. We also found that the dissolved irons of the P-C treatment were more than those of the C-P treatment because a number of PO4 adsorbed on FH-HA developed to electrostatic repulsive forces making large particle FH-HA aggregate become small particles, and thus citric acid could rapidly dissolve more iron form FH-HA coprecipitates..
Based on the results of the C-P and P-C treatment changed with time, it showed that the PO4 addition in the C-P treatments had unobvious effects on the citric acid adsorption capacity of FH-HA. However, the PO4 adsorption capacity of FH-HA had obvious effects by citric acid addition on P-C treatment. The PO4 adsorption capacity was dropped during the addition of citric acid for 8 hours in the P-C treatment, and then PO4 would be adsorbed on FH-HA again. We hypothesized that: (1) new adsorption sites were exposed after iron being dissolved; (2) the dissolved Fe could be the bridging metal between organic substances and PO4, and so the formation of the organic anion-Fe-phosphate complex caused PO4 re-adsorbed on FH-HA. The dissolved iron increased results were in accord with the change of PO4 adsorption capacities on FH-HA in the C-P treatment, but PO4 re-adsorption would reduce the P availability due to the dissolved iron.
For the improvement of P availability, we suggested that the PO4 adsorption capacity of the FH-HA coprecipitates was the lowest as citric acid added firstly, the increased ratio of humic acid in the FH-HA coprecipitates would reduce the PO4 adsorption capacity and promote the dissolved Fe. Thus, we understood that citric acid improved the availability of PO4 for the sustainable use of phosphorus in soils, and of the appropriate dissolved Fe also have positive effects on crop growth and rhizophere soils.
Key words: Phosphate、Citric acid、Ferrihydrite、Humic acid、Coprecipitates
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