For manipulating N2 fixation and improve the N benefits to agricultural legumes, there is a need to reduce the inhibitory effect of applied N on Rhizobia inoculant. The optimum level of inorganic N as starter fertilizer with inoculation of Rhizobia is essential to increase the production of legumes in smallholder farmers. However, the effect of inorganic N level which should be used by farmers with an inoculation to increase the production of soybean is not yet well known in soils of western Ethiopia. Therefore, the study conducted to determine the amount of starter N application which could enhance N2 fixation and soybean seed yield using Rhizobial inoculation and different nitrogen doses. Six levels of nitrogen fertilizer (0, 9, 18, 27, 36, and 54 kg N ha−1) and two levels of inoculation (inoculated (1) and un-inoculated (0)) were arranged using split-plot RCBD with three replications. Seed inoculation with Rhizobia was significantly superior over un-inoculated treatments. Both rhizobia inoculation and N rates applied alone increased nodulation and yield of soybean, however, their combination further increased nodulation, growth performance, and seed yield and yield traits of soybean. Although nodulation parameters showed inconsistent results across cropping seasons, N application improved nodulation and shoot dry weight and interacted positively with inoculation across locations. Increasing rates of N from zero up to 18 kg ha−1 increased nodule number and dry weight but further increase of N reduced nodulation. Application of N at 18 kg ha−1 increased nodule number and dry weight by 98.3 and 115.0%, respectively over un-fertilized control as well as by 61.0 and 58.0%, respectively over the greatest N rate (54 kg N ha−1). The highest seed yield of soybean was obtained when inoculation applied with N at 18 and 27 kg ha−1 in 2016/17 and 2017/18, respectively across locations. However, seed yield in the 2016/17 cropping season was much higher than 2017/18. In 2016/17 and 2017/18, inoculation together with N at 18 kg ha−1 increased grain yield by 75.0 and 76.8% compared with for the control plants, 50.7 and 31.6% compared with inoculation alone, respectively. In both cropping seasons, the un-inoculated plants had shown a trend that an increase in seed yield with increased N rate. Application of fertilizer N up to 18 kg ha−1 may be an optimum rate for effective nodulation and enhanced N2 fixation and thereby improved soybean yield. The present results demonstrate the potential of improving soybean nodulation, growth, and seed yield profitability using Rhizobia inoculant together with starter N at low N Nitisols soils of Western Ethiopia.

中文翻译：

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大豆对埃塞俄比亚西部 Assosa 和 Begi 地区 Nitisols 根瘤菌接种和起始氮肥的响应

为了操纵 N2 固定并提高农业豆类的 N 效益，需要降低施用 N 对根瘤菌接种剂的抑制作用。接种根瘤菌的无机氮作为起始肥料的最佳水平对于增加小农豆类的产量至关重要。然而，在埃塞俄比亚西部的土壤中，农民应该使用无机氮水平来增加大豆产量的影响尚不清楚。因此，本研究旨在确定可以使用根瘤菌接种和不同氮剂量提高固氮和大豆种子产量的起始施氮量。六级氮肥（0、9、18、27、36、和 54 kg N ha-1) 和两个接种水平（接种 (1) 和未接种 (0)）使用裂区 RCBD 安排，三个重复。用根瘤菌接种的种子明显优于未接种的处理。根瘤菌接种和单独施用氮肥都增加了大豆的结瘤和产量，然而，它们的组合进一步增加了大豆的结瘤、生长性能以及种子产量和产量性状。尽管结瘤参数在整个作物季节显示出不一致的结果，但施氮改善了结瘤和芽干重，并与不同地点的接种呈正相关。将 N 的比率从零增加到 18 kg ha-1 会增加根瘤数量和干重，但进一步增加 N 会减少根瘤形成。以 18 kg ha-1 施氮使根瘤数和干重分别比未施肥控制增加 98.3% 和 115.0%，在最大施氮量（54 kg N ha-1）上分别增加 61.0% 和 58.0% . 大豆在 2016/17 和 2017/18 年分别在不同地点接种 18 kg ha-1 和 27 kg ha-1 时获得了最高的种子产量。然而，2016/17 作物季节的种子产量远高于 2017/18 年度。在 2016/17 和 2017/18 年，与对照植物相比，接种 18 kg ha-1 的 N 使谷物产量分别增加了 75.0% 和 76.8%，与单独接种相比，分别增加了 50.7% 和 31.6%。在两个种植季节，未接种的植株均表现出随着施氮量增加而种子产量增加的趋势。施用高达 18 kg ha-1 的氮肥可能是有效结瘤和增强 N2 固定从而提高大豆产量的最佳比率。目前的结果表明，在埃塞俄比亚西部的低氮 Nitisols 土壤中，使用根瘤菌接种剂和起始剂 N 可以改善大豆的结瘤、生长和种子产量盈利能力。