根际pH对玉米利用磷酸单酯和双酯盐的影响

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

【目的】土壤有机磷在土壤全磷中占有很大比重,是植物潜在的有效磷源,但必须通过磷酸酶的水解作用释放出无机磷才能被植物利用。土壤中有机磷的主要形式为磷酸单酯和磷酸双酯。本研究中,我们探讨了无菌条件下不同形态的氮源引起的根际pH变化如何影响植物对这两种有机磷的活化利用。【方法】采用琼脂无菌培养体系种植玉米,向玉米植株供应两种形态的氮源和磷源,氮源为硝态氮和铵态氮,磷源为植酸钙和卵磷脂,植酸钙属于磷酸单酯盐,卵磷脂属于磷酸双酯盐。不同的供氮形态会导致根际pH变化,进而研究根际pH变化对磷酸单酯盐和磷酸双酯盐的活化利用所产生的影响。【结果】当给玉米供应铵态氮时,根际pH从5.5降至4.0;供应硝态氮时,根际pH升至6.6。测定玉米根际的琼脂中根系分泌的磷酸单酯酶和磷酸双酯酶活性发现,磷酸单酯酶活性在pH 6.0~7.0之间最高,磷酸双酯酶活性在pH 7.0~8.0之间达到最高。无论以植酸钙还是卵磷脂为有机磷源,相对于铵态氮处理,硝态氮处理能够使根际保持较高的磷酸单酯酶或磷酸双酯酶活性。有机磷的水解过程由磷酸酶活性和底物有效性两个因素控制,而植酸钙的水解受根际pH影响很大,在一定pH范围内,植酸钙的溶解度随根际pH值降低而升高,有效态磷浓度的增加,使得磷酸酶的底物有效性提高。在供应铵态氮时,根际pH值降低,玉米对植酸钙的利用效率高于硝态氮处理,尽管供硝态氮时磷酸单酯酶活性更高。同时,在供应铵态氮条件下,植株对植酸钙的利用率要显著高于卵磷脂,原因在于虽然磷酸双酯酶和磷酸单酯酶活性较低,由于植酸钙的溶解度较大,它的底物有效性更高。因此,植酸钙处理中植株的磷含量更高。相反,在供应硝态氮时,植酸钙溶解度减小而两种磷酸酶活性较高,卵磷脂处理中植株的磷含量更高。【结论】土壤中有机磷的水解过程由磷酸酶活性和有机磷底物有效性两个因素控制,酶活性与根际pH密切相关。本研究说明土壤有机磷的活化必须首先转化为溶解于水溶液中的状态,才能作为磷酸酶的底物被催化水解。我国长期施用化肥导致北方土壤大范围酸化,这种酸化无疑对土壤固有或随有机物料进入农田的有机磷的活化利用是具有重要贡献的,应该在北方土壤养分管理中应加以考虑。

[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 ( NO3- and NH4+) 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 the NH4+ form was supplied, rhizosphere pH was decreased from 5. 5 to 4. 0, while nitrogen was supplied in the NO3-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 NO3- treatment was higher than that in the NH4+ 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 the NH4+ treatment than that from in the NO3-treatment, although the phosphatase activity was not higher. In a similar way, when the NH4+ 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, when NO3- 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.