Phosphate ageing in soil and bioavailability of aged phosphates
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Phosphorus (P) is one of the most limiting plant nutrients. This limitation of P is due to strong retention of phosphate ions with soil particles. Thus, most of the applied P in soils becomes unavailable for plants. In acid soils, phosphate is adsorbed at the surfaces of Fe and Al oxides and then becomes occluded with time, termed phosphate ageing. The aged phosphate is highly unavailable to plants due to its very strong fixation. It is known that plant species have developed various adaptations to enhance P uptake from the soil under P starved conditions. One of these adaptations is the formation of cluster roots. The plant species with the cluster roots such as white lupin, release various exudates into the soil which may mobilize various P fractions in the soil. The objectives of this study were to better understand the process of the phosphate ageing by investigating the kinetics and relationship of the aged and applied P in the soil, and to investigate the bioavailability of the aged P. Soil incubation and plant growth experiments were carried out to achieve these objectives.In the first experiment, a Luvisol topsoil and a Ferralsol were incubated for 1 d, 3 months, and 6 months in a growth chamber at 25°C. There were two pH levels i.e. 7.2 and 5.5 of each soil and two P levels i.e. 0 (P-) and 100 (P+) mg P kg-1 soil. P fertilization had a significant effect on CAL-extractable-P (CAL-P) concentrations after 1 d, 3 months, and 6 months of soil incubation. The CAL-P concentrations were higher in the P+ treatments than in the P- treatments in both of the soils. The soils had a significant effect on the CAL-P concentrations. The CAL-P concentrations were higher in the Luvisol topsoil than in the Ferralsol. These were very low in the Ferralsol. In this soil, most of the applied P (more than 90%) was not extractable with the CAL method. Maize (Zea mays L. cv. Amadeo) and white lupin (Lupinus albus L. cv. Amiga) were grown in the 6 month-incubated soils. Plants grown in the Luvisol topsoil had a higher P content than those grown in the Ferralsol. The maximum P contents were in the P+ treatments. Maize had a higher P content than white lupin when it was grown in the Luvisol topsoil with P application (P+). White lupin had a higher P content than maize when grown in the Ferralsol.In the second experiment, the soils (the Luvisol topsoil and the Ferralsol) were incubated for 0.5 h, 1 h, 2 h, 4 h, 8 h, 12 h and 24 h in pots at 25°C in the growth chamber. P was applied at the rate 100 mg kg-1 soil. The CAL-P concentration data showed that most of the added P in the Ferralsol had become non-CAL-extractable after 1 h. The Luvisol topsoil did not adsorb phosphate in the P+ treatment immediately. After 8 h of incubation, the CAL-P concentration decreased. In the second part of this experiment, these soils were incubated for 24 h in pots with various P levels i.e. 0, 100, 150, 200, 250, 500 mg P kg-1 soil. The CAL P and aged P data showed that most of the P applied in the Ferralsol was aged, in contrast to the Luvisol topsoil.In the third experiment, a Luvisol subsoil was incubated in plastic buckets. Each bucket had 3 kg of soil. There were three pH levels, i.e. 7.2, 5.2, 4.6 and two P levels i.e. with P (P+) and without P (P-). In the P+ treatments, 200 mg P kg-1 soil were applied as KH2PO4. Goethite (Fe oxide) and Gibbsite (Al oxide) minerals were added as P adsorbents at the rate of 300 mmol Fe and Al kg-1 soil. The soils were incubated for 1 week, 3 months, and 6 months, respectively, at 25°C in a growth chamber. The results showed that the aged-P concentrations were affected by the P application, phosphate adsorbent and time. The aged-P concentrations increased after 3 months of incubation in the P+ treatments. The aged-P concentrations were increased where Al oxide was applied as P adsorbent. In the fourth experiment, maize (Zea mays L. cv. Amadeo) and white lupin (Lupinus albus L. cv. Amiga) were cultivated in the Luvisol subsoil in pots. Each pot had 1 kg of soil with one plant. Ten mg P kg-1 soil were applied as Al oxide-occluded P, Fe oxide-occluded P, and Ca(H2PO4)2. These occluded phosphates were synthesized before the start of the experiment. The data from the P contents in plants and the occluded-P concentrations in soil after the plant cultivation showed that white lupin mobilized the Fe oxide-occluded P but not the Al oxide-occluded P, while maize was unable to mobilize both occluded P forms.Verknüpfung zu Publikationen oder weiteren Datensätzen
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Giessen : VVB Laufersweiler