Available P in andosols cropped with Italian ryegrass [Lolium multiflorum]: Evaluation in the field by means of 33P isotopic dilution method
1995
Kato, N. (National Inst. of Agro-Environmental Sciences, Tsukuba, Ibaraki (Japan)) | Koyama, T. | Watanabe, K. | Kobayashi, Y. | Niimi, H.
Plant-available P in two Humic andosols with different P level (total soil P:1.53 or 8.64 g P kg(-1)) was evaluated by an A-value technique based on a 33P isotopic dilution method. Before the field study, a P leaching experiment using 32P-labeled fertilizer was carried out in a laboratory for safety assessment and it was predicted that the risk of radioactive contamination into groundwater was negligible. 33P, having longer half-life and lower maximum beta-ray energy than 32P, was used for the field study to reduce the amount of radioisotope intended to be applied to the soils and to decrease radiation exposure during the field experiment. Not only calcium phosphate solution labeled with 33P (33P carrier solution) but also 33P carrier-free solution was used to examine the effect of P fixation by the soils on plant-available P measurement by an isotopic dilution method (A-value). The 33P-labled solutions were incorporated in the soils cropped with Italian ryegrass at 6 cm in depth, and the ryegrass was harvested at 36 d after 33P application. The A-values of the soils obtained with 33P carrier solution were 300 kg P ha(-1) and 1420 kg P ha(-1) for a low P level soil and for a high P level soil, respectively, indicating that a large amount of plant-available P was accumulated in the soils. In the case of the low P level soil using 33P carrier-free solution, more than 74.4% of applied P was fixed by the soil during the experiment and plant-available P was overestimated, whereas the effect of P-fixing on the A-value was negligible in the high P level soil. Air radiation dose rate in the field was level with background throughout the experiment. Radioactive contamination in soils and plants nearby the test field was not observed. It was concluded that 33P was superior to 32P for field studies from an environmental point of view
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