菜豆根瘤菌对土壤钾的活化作用

以土壤为钾源,通过液体培养试验研究了8株菜豆根瘤菌对土壤钾的活化作用。结果表明,菜豆根瘤菌能释放大量的氢离子,使液体培养基的pH值大幅度降低,氢离子的浓度至少提高22倍以上。根瘤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,其中全部菌株均能分泌草酸和苹果酸,大部分菌株能分泌乙酸。在接种根瘤菌的液体培养基中,可溶性钾含量显著高于不接种的液体培养基,土壤矿物结构钾则显著降低。由于土壤是培养基钾的唯一来源,故根瘤菌可促进土壤无效钾的溶解。相关分析表明,土壤矿物结构钾与有机酸分泌总量呈极显著负相关(r=-0.878**,n=9),与培养液pH值呈极显著正相关(r=0.863**,n=9),说明根瘤菌分泌的有机酸和氢离子可能溶解土壤无效钾。考虑到根瘤菌草酸分泌量大,络合钙、镁、铁、铝的能力强,且与有机酸分泌总量呈极显著正相关(r=0.870**,n=9),推测草酸分泌在活化土壤无效钾的过程中起重要作用。此外,根瘤菌分泌的有机酸电离产生的氢离子仅占培养液氢离子的4.15%—27.56%,推测根瘤菌直接分泌的氢离子可能是造成培养液pH值降低的主要原因之一。

Mobilization of potassium from Soils by rhizobium phaseoli

Potassium (K) is one of the most important essential elements for plant growth and function. However, both the concentration and the availability of K are very low in most soils. The chemical K supplemented through fertilization is rapidly adsorbed by soil minerals resulting in a 20%-35% of K use efficiency. Studies have found that rhizobia, which have the capacity not only to fix nitrogen from the atmosphere to contribute plant nitrogen nutrition, but also to mobilize soil K for plant use. As a result, the improvement of plant K use efficiency through some specific rhizobia to mobilize soil K has attracted much attention around the whole world. However, less information is available to illustrate the mechanisms how rhizobia could directly mobilize K from the soil. Eight strains of rhizobium sp., which isolated from a gray brown purple soil in Chongqing, southern China and coded as R 01, R 02, R 03, R 04, R 05, R 06, R 07 and R 08, respectively, were grown a liquid medium to study their capacity to mobilize soil K. The medium contained 1L H₂O, 10 g mannitol, 1.0 g yeast powder, 0.5 g NaH₂PO₄, 0.2 g MgSO₄·7H₂O, 0.1 g NaCl, 0.1 g CaCl₂·6H₂O and 4.0 mL microelement solution (each 1 L contained 5.0 g H₃BO₃ and 5.0 g Na₂MoO₄ dH₂O). Compared to the non-rhizobium control, concentrations of proton in the liquid media under all eight rhizobium treatments were increased by 22 times at least leading a significant pH decrease after 7 days of incubation. All rhizobium strains exuded oxalic acid and malic acid and most of them exuded acetic acid, but varied their capacity to exude succinic acid, formic acid, citric acid and lactic acid. Soluble K in the liquid medium was significantly higher whilst mineral structure K was significantly lower in the soil in the rhizobium treatments than in the non-rhizobium control. Mineral structure K in soil negatively correlated with total organic acids (r=-0.878^{* *}, n=9), but positively correlated with pH in the liquid medium (r=0.863^*, n=9). So, rhizobia could mobilize soil K through the exudation of organic acids and protons. Oxalic acid has a strong capacity to complex calcium, magnesium, iron and aluminum and positively correlated with total organic acids (r=0. 870^{* *}, n=9), which suggests that oxalic acid exuded by rhizobia might contribute directly in the mobilization of soil K. In addition, protons ionized from organic acids were from 4.15% to 27.56% of the total protons in liquid medium, which showed that rhizobia could exude directly protons and led pH reduction in liquid medium. Our results demonstrated an alternative pathway to enhance plant K nutrition through proton and organic acid exudation 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 to meet the growth requirements of both nitrogen and K for the cereal crops. Nevertheless, more research on the capacity of rhizobia to mobilize soil K and to benefit to its host plants and neighbouring plants are further required.