Effects of rhizosphere pH on utilization of P in phytin and lecithin by maize

【Objectives】Soil organic phosphorus is accounted for a large proportion in total soil phosphorus, and is a kind of potential available phosphorus. Hydrolyzing by phosphatase is the only way obtaining phosphorus for the plants. There are mainly two forms of organic phosphorus in the soil, one is phosphomonoester, and the other one is phosphodiester. An axenic agar cultivation in Petri dish was conducted to study effects of pH on phosphomonoesterase or phosphodiesterase utilization in maize rhizosphere. 【Methods】The method of an axenic agar in Petri dish was used in this experiment. Two kinds of nitrogen forms (NO-3 and NH+4) and two kinds of organic phosphorus forms (phytin: phosphomonoester and lecithin: phosphodiester) were supplied in the maize rhizosphere. Different nitrogen forms will lead to the pH changes in rhizosphere, which can be used to evaluate the effects of pH on phosphomonoesterase or phosphodiesterase utilization in maize rhizosphere. 【Results】When theNH+4 form was supplied, rhizosphere pH was decreased from 5.5 to 4.0, while nitrogen was supplied in theNO-3 form, rhizosphere pH was increased to 6.6. Activity analysis showed that activity of phosphomonoesterase secreted by maize roots in the axenic agar is highest when pH is 6.0-7.0, and phosphodiesterase activity is highest when pH is 7.0-8.0. Whether phytin or lecithin was supplied, phosphomonoesterase or phosphodiesterase activity in the NO-3 treatment was higher than that in theNH+4 treatment. For the phosphorus was derived from organic phosphates of maize in each treatment, the hydrolyzation of organic phosphorus is controlled by phosphatase activity and availability of substrate. The hydrolysis of phytin was strongly influenced by rhizosphere pH, especially in the acid condition, the solubility of phytin increased and then the availability of phosphatase substrate increased, and the concentration of the organic phosphorus became higher than before, and led to that the maize could absorb more phosphorus. The maize absorbed more phosphorus from phytin in thetheNH+4 treatment than that from in theNO-3 treatment, although the phosphatase activity was not higher. In a similar way, when the NH+4 form nitrogen was supplied, the phosphorus use efficiency by the maize in the phytin treatment was higher significantly than that in the lecithin treatment, because the solubility of phytin was higher under this condition. Although phosphomonoesterase or phosphodiesterase activity was lower, the effectiveness of phosphatase substrate was higher, therefore, the maize absorbed more phosphorus. In the contrast, whenNO-3 was supplied, the solubility of phytin decreased and the activity of two kinds of phosphatase was increased, and led to that more phosphorus absorbed by maize in the lecithin treatment was higher than that in the phytin treatment. 【Conclusions】Hydrolyzation of organic phosphorus is influenced by phosphatase activity and substrate availablilty，and the changes of soil enzyme activity is closely related to pH environment. Activation of organic phosphorus in the soils must be transformed into the dissoluble form, which is a substrate of phosphatase and is hydrolyzed. Fertilizer application for a long time in China can result in acidification in large areas, and this acidification can be helpful to activation of organic phosphorus in soils and/or in organic residues，indicating that the practices of soil acidification should be scientifically considered in soil nutrient manegement in northern China.