自生固氮菌活化土壤无机磷研究

Mobilization of inorganic phosphorus from soils by five azotobacters

以土壤为磷源,通过液体培养试验研究了5株自生固氮菌(Azotobacter sp.)对土壤无机磷的活化利用。结果表明,自生固氮菌能释放大量的氢离子,使液体培养基的pH大幅度降低,氢离子的浓度至少提高58倍以上。自生固氮菌分泌有机酸的种类与数量因菌株不同而异,这些有机酸包括甲酸、乙酸、草酸、丁二酸、柠檬酸、苹果酸和乳酸等,其中均能分泌草酸和苹果酸。在接种自生固氮菌的液体培养基中,全磷含量显著高于不接种的液体培养基,土壤无机磷总量则显著降低。由于土壤是培养基磷的唯一来源,故自生固氮菌促进了土壤无机磷的溶解释放。相关分析表明,培养基的pH值与土壤无机磷总量呈极显著正相关(r=0.959**,n=6),与液体培养基中的无机磷和全磷呈显著或极显著负相关(r=-0.850*;r=-0.918**,n=6),说明自生固氮菌分泌的氢离子可能是溶解土壤无机磷的原因之一。接种自生固氮菌显著降低土壤钙磷,土壤中的铁磷、铝磷和闭蓄态磷的降幅因菌株不同而异,其原因可能与有机酸分泌的数量和种类有关。

Phosphorus （P） is one of the most important essential elements for plant growth and function. However, both the concentration and the availability of P are very low in most soils. The chemical P supplemented through fertilization is rapidly adsorbed by soil minerals or transformed by soil microbial organisms resulting in a 10% --20% of P use efficiency. Studies have found that Azotobacter sp. , which can fix nitrogen from the atmosphere to contribute plant nitrogen nutrition, also has the capacity to mobilize soil P for plant use. As a result, the improvement of plant P use efficiency through some specific soil Azotobacter to mobilize soil P has attracted much attention around the whole world. However, less information is available to illustrate the mechanisms how Azotobacter sp. could directly mobilize P from the soil. Five strains of Azotobacter sp. , which isolated from a gray brown purple soil in Chongqing, southern China and coded as N 01 ,N 02,N 03 ,N 04 and N 05, respectively, were grown a liquid medium to study their capacity to mobilize soil phosphorus （P）. The medium contained 1L H20, 10 g mannitol, 0.2 g KC1, 0.2 g MgSO4 ~ 7H20, O. 2 g NaC1, 0.2 g CaSO4 ~ 7H20 and 5.0 g CaCO3. Compared to the non-Azotobacter control, concentrations of proton in the liquid media under all five Azotobacter treatments were increased by 58 times leading a significant pH decrease after 7 days of incubation. All Azotobacter strains exuded oxalic acid and malie acid, but varied their capacity to exude succinic acid, formic acid, acetic acid, citric acid and lactic acid. Total P in the liquid medium was significantly higher whilst inorganic P was significantly lower in the soil in the Azotobacter treatments than in the non-Azotobacter treatment, pH in the liquid medium positively correlated with soil totalinorganic P （r=0. 959 , n --6）, but negative correlated correlations with both inorganic P and total phosphorus in the liquid medium （ r = -0. 850 or -0. 918 , n = 6 ）. Meanwhile, soil Ca-P was significantly reduced by all Azotobacter treatments, while A1-P, Fe-P and occluded P were decreased depending on the Azotobacter strain used in this study. On one hand, our results suggest that the capacity of exuding organic acids and hence mobilizing soil P by Azotobacter may be species dependent. On the other hand, our results showed that the pH decrease in the liquid medium might be one of the most important mechanisms to mobilize soil Ca-P and/or Mg-P while the organic acids might contribute directly in the mobilization of soil P through the complexion and acidic solution. As a result, our results demonstrated an alternative pathway to enhance plant P nutrition through Azotobacter and could offer a potential effective practice to increase crop productivity by intercropping or growing cereal crops with legumes, in which the latter could not only provide nitrogen, but also P, to meet the growth requirements of both nitrogen and P for the cereal crops. Nevertheless, more research on the capacity of Azotobacter to mobilze soil P and associated nitrogen and P benefits to its host plants and neighbouring plants are further required.