Low soil fertility and soil acidity are among the major bottlenecks that limit agricultural productivity in the humid tropics. Soil management systems that enhance soil fertility and biological cycling of nutrients are crucial to sustain soil productivity. This study was, therefore, conducted to determine the effects of coffee‐husk biochar (0, 2.7, 5.4, and 16.2 g biochar kg−1 soil), rhizobium inoculation (with and without), and P fertilizer application (0 and 9 mg P kg−1 soil) on arbuscular mycorrhyzal fungi (AMF) root colonization, yield, P accumulation, and N2 fixation of soybean [Glycine max (L.) Merrill cv. Clark 63‐K] grown in a tropical Nitisol in Ethiopia. ANOVA showed that integrated application of biochar and P fertilizer significantly improved soil chemical properties, P accumulation, and seed yield. Compared to the seed yield of the control (without inoculation, P, and biochar), inoculation, together with 9 and 16.2 g biochar kg−1 soil gave more than two‐fold increment of seed yield and the highest total P accumulation (4.5 g plant−1). However, the highest AMF root colonization (80%) was obtained at 16.2 g biochar kg−1 soil without P and declined with application of 9 mg P kg−1 soil. The highest total N content (4.2 g plant−1) and N2 fixed (4.6 g plant−1) were obtained with inoculation, 9 mg P kg−1, and 16.2 g biochar kg−1 soil. However, the highest %N derived from the atmosphere (%Ndfa) (> 98%) did not significantly change between 5.4 and 16.2 g kg−1 soil biochar treatments at each level of inoculation and P addition. The improved soil chemical properties, seed yield, P accumulation and N2 fixation through combined use of biochar and P fertilizer suggest the importance of integrated use of biochar with P fertilizer to ensure that soybean crops are adequately supplied with P for nodulation and N2‐fixation in tropical acid soils for sustainable soybean production in the long term.
There is little understanding as to whether the addition of biochar requires less fertilizer to obtain the potential yield. Furthermore, the additional yield ascribed to the non-nutrient effects of biochar is ambiguously quantified. Therefore, this study is aimed to elucidate the influence of biochar application rate and production temperature on (i) marginal agronomic efficiency (AE LN), (ii) potential yield (Y opt), (iii) the amount of mineral fertilizer required to obtain the potential yield (F opt); and (iv) nutrient use efficiency. AE LN , Y opt and F opt were calculated after fitting the yield response at different levels of mineral fertilizer with a second-degree polynomial. Application of biochar reduced marginal agronomic efficiency, implying that the plant utilized the applied nutrient more efficiently without biochar at a low dose of mineral fertilizer. Biochar increased potential yield but required more mineral fertilizer to obtain the optimum yield. The non-nutrient associated effect of biochar reached to 39% and is mainly attributed to its liming effect. The effect of biochar on AE LN , Y opt , F opt , fertilizer use efficiency and soil pH were more pronounced at the higher application rate. Addition of biochar, however, increased soil Mehlich-P and carbon content, irrespective of production temperature and application rate. This study demonstrated that the shortterm effect of biochar application on fertilizer utilization should be examined with caution in low-input cropping systems because the biochar effects were dependent on fertilizer level, biochar application rate, production temperature and their interactions. Further manipulative experiments are recommended to identify the mechanisms that explain the non-nutrient effect of biochar on yield.
Although the addition of biochar has been shown to reduce the phosphorus (P) adsorption capacity of soil, quantitative evidence of this has mainly been provided by incubation experiments and it is therefore essential to conduct longterm field trials to draw general conclusions. It is largely unknown whether bone char has a greater effect than lignocellulosic biochar on P adsorption-desorption processes and crop yield. The aim of this study was to determine the long-term (8 years) effect of bone char and biochar on P adsorption-desorption and crop yield in low-input acidic soils. The results showed that bone char decreased the maximum P adsorption capacity (Q m ) by 10% and increased the desorption capacity (D s ) by 150% compared with the control (i.e. without a soil amendment). The desorption ratio was highest for the bone char treatment (10.3%) and three times more than the control. Plant-available P was seven times greater under bone char than the control. There was no variation in adsorption-desorption characteristics, desorption ratio and plant-P available content between bone char and lignocellulosic biochar treatments. The average yield increment following the application of bone char and biochar was 1.7 and 1.4 Mg ha −1 for maize and 1.8 and 1.9 Mg ha −1 for soya bean, respectively. Despite the low application rate (4 t ha −1 year −1 ), these findings demonstrated that the long-term application of bone char and biochar-based amendments enhanced P availability in low-input cropping systems, mainly by altering the P adsorption and desorption capacity of soils.
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