Effect of Inoculating Bradyrhizobium on Phosphorus Use Efficiency and Nutrient Uptake of Soybean in Calcareous Soil, Central Rift Valley, Ethiopia

At a soil pH value of above 7.0, inorganic phosphorus (P) is highly susceptible to precipitation as insoluble form that is unavailable to plants. Hence, a field experiment was conducted at Metehara Sugar Estate under irrigation during the 2014/15 cropping season to evaluate the effect of inoculating Bradyrhizobium on P uptake and P use efficiency of soybean intercropped with sugarcane. The treatments consisted of three levels of inoculation (Legume fix, SB6B1 and uninoculated) and four rates of P (0, 10, 20 and 30 kg Pha-1). The experiment was laid out in a randomized complete block design (RCBD) in a factorial arrangement and replicated three times. Analysis of the data indicated that Bradyrhizobium inoculation significantly increased plant N concentration and P uptake compared to the uninoculated treatment. The effect of P rates and its interaction with inoculation was not significant on N concentration and P uptake, but significantly increased total P uptake at the application of 30 kg Pha-1. Phosphorus use efficiency indices were improved in response to inoculating the crop with Bradyrhizobium. The highest AE (13.6 kg kg-1), PRE (31.8%) and PUE (10.6 kg kg-1) were obtained by SB6B1 inoculation and the highest PE (117.2 kg kg-1) and APE (161.7 kg kg-1) were obtained by Legumefix inoculation all at 10 kg P ha-1except PE which recorded the highest at 30 kg P ha-1.Thus, it can be concluded that SB6B1 isolate can be used as the best inoculant followed by Legumefix isolate with 10 kg P ha-1of P fertilizer. However, strategies for increasing P use efficiency by adopting best management practices like co-inoculation of phosphate solubilizing microorganism or mycorrhiza with these Bradyrhizobium inoculants should be adopted to enhance P use efficiencies.当土壤pH值高于7.0时，无机磷（inorganic phosphorus）极易以不溶态形式沉淀，无法被植物吸收利用。为此，研究团队于2014/15种植季在灌溉条件下的梅泰哈拉糖业庄园开展田间试验，旨在评估慢生根瘤菌（Bradyrhizobium）接种对甘蔗间作大豆的磷吸收量及磷利用效率的影响。 试验设置3个接种处理组（Legume fix、SB6B1及未接种对照组）与4个磷肥施用量梯度（0、10、20、30 kg P ha⁻¹）。试验采用完全随机区组设计（randomized complete block design, RCBD）的因子排列方案，设置3次重复。 数据分析结果显示：与未接种对照组相比，慢生根瘤菌接种可显著提升植株氮浓度与磷吸收量；磷施用量及其与接种处理的交互作用对植株氮浓度和磷吸收量无显著影响，但当施磷量为30 kg P ha⁻¹时，总磷吸收量显著提升。接种慢生根瘤菌可改善各项磷利用效率相关指标：在10 kg P ha⁻¹的施磷水平下，SB6B1接种处理获得了最高的表观吸收效率（AE, 13.6 kg kg⁻¹）、磷回收率（PRE, 31.8%）与磷利用效率（PUE, 10.6 kg kg⁻¹）；而Legumefix接种处理则获得了最高的磷素效率（PE, 117.2 kg kg⁻¹）与表观磷效率（APE, 161.7 kg kg⁻¹），仅磷素效率（PE）在30 kg P ha⁻¹时达到峰值。 综上可得出结论：SB6B1菌株可作为最优接种剂，其次为Legumefix菌株，配套施用量为10 kg P ha⁻¹的磷肥即可取得较佳效果。不过，为进一步提升磷利用效率，应采用最佳管理策略，例如将上述慢生根瘤菌接种剂与溶磷微生物或菌根真菌联合接种。