• Energy Research

Abstract Biogas digestates contain substantial amounts of nutrients and are widely used as fertilizer. However, long-term results of their agronomic effects are limited. A ten-year field experiment was evaluated regarding the phosphorus (P) fertilizer value of digestates in dependence of different crop species, paying special attention to turnover and translocation of P in the soil profile. The input substrate (inputS) and digested substrate (digestS) from a biogas plant were applied annually to at a rate of 30 m³ ha−1. For control, mineral nitrogen and potassium, but no P, were given in equal amounts as applied with the biogas substrates. The average annual P uptake of crops was increased by the inputS (31.5 kg ha−1) and digestS (28.5 kg ha−1) compared to the control (23.5 kg ha−1). Regarding the plant available P content (soluble in double-lactate) no differences between the inputS and digestS treatments were found in the topsoil. In the subsoil all P pools measured as well as the degree of P saturation were highest when P was added with the digestS. The activities of soil enzymes were only increased after the application of inputS in comparison to the control and were also affected by the crop species cultivated. Our results indicate that digested slurries have a comparable P fertilizer effect to that of undigested slurries, but they involve a higher risk of P losses due to higher P concentrations in the subsoil.

Agriculture is the backbone of the Ethiopian economy, and the agricultural sector is dominated by smallholder farming systems. The farming systems are facing constraints such as small land size, lack of resources, and increasing degradation of soil quality that hamper sustainable crop production and food security. The effects of climate change (e.g., frequent occurrence of extreme weather events) exacerbate these problems. Applying appropriate technologies like climate-smart agriculture (CSA) can help to resolve the constraints of smallholder farming systems. This paper provides a comprehensive overview regarding opportunities and challenges of traditional and newly developed CSA practices in Ethiopia, such as integrated soil fertility management, water harvesting, and agroforestry. These practices are commonly related to drought resilience, stability of crop yields, carbon sequestration, greenhouse gas mitigation, and higher household income. However, the adoption of the practices by smallholder farmers is often limited, mainly due to shortage of cropland, land tenure issues, lack of adequate knowledge about CSA, slow return on investments, and insufficient policy and implementation schemes. It is suggested that additional measures be developed and made available to help CSA practices become more prevalent in smallholder farming systems. The measures should include the utilization of degraded and marginal lands, improvement of the soil organic matter management, provision of capacity-building opportunities and financial support, as well as the development of specific policies for smallholder farming.

Proper fertilizer management and applications could effectively reduce global warming potential (GWP) and improving soil fertility under cereal production. However, the effect of soil fertilizer management practices on GWP and soil fertility is less understood in the agricultural soils of Ethiopia. The present study evaluated the effects of fertilizer application strategy on GWP, and soil fertility in a Nitisol. Both field and laboratory incubation experiments were conducted with the same treatments. Seven treatments (Cont: no input; 100 min: 100% mineral fertilizer, 80 min: 80% mineral fertilizer + 1.4 t ha−1 compost; 60 min: 60% mineral fertilizer + 2.8 t ha−1 compost; 50 min: 50% mineral fertilizer + 3.5 t ha−1 compost; 30 min: 30% mineral fertilizer + 4.9 t ha−1 compost, and 100 comp: 100% compost) with four replications were applied on maize crop (Zea mays L. Bako hybrid 661) for two consecutive growing seasons. The laboratory incubation experiment was also carried out with two moisture levels (40% and 75% of water-filled pore space) to simulate the seasonal rainfall pattern. GWP was calculated by summing up the quantified gas emissions of nitrous oxide (N2O), carbon dioxide (CO2), and methane (CH4). The laboratory result for GWP shown that soil fertilized with mineral fertilizer alone was significantly (p −1 of mineral fertilizer with biowaste compost (4.9 or 3.5 t ha−1) would be a suitable combination to mitigate the GWP and improve soil quality in smallholder farming systems, due to a slow release of N during decomposition into the soil compared to mineral fertilizer alone. However, to evaluate GWP under the field conditions, future investigations are recommended.

Abstract The application of biogas residues to agricultural fields is important for nutrient cycling. A 3 year field experiment was conducted to assess the phosphorus (P) fertilizer value of digestate from biogas production, taking into account soil microbial activity. The input substrate (inputS) and digested substrate (digestS) from a biogas plant using dairy slurry, maize silage and wheat corn, were applied at a rate of 30 m³ ha−1 annually. For control, mineral N and K, but no P, were applied in equal amounts with the biogas substrates. Maize was cultivated every year, and the biomass yield and P and N uptake were determined. Soil samples were collected on different sampling dates, and the P contents, pH, organic matter contents and enzyme activity were analyzed. The CO2 efflux was measured biweekly using a portable soil respiration chamber (EGM 4). After 3 years, the P and N uptake increased by 25% in the digestS treatment compared with that of the control but did not differ from that of the inputS treatment. The plant-available P contents were also higher in the inputS- and digestS-amended soil. The fertilizer application did not influence the organic matter content but did influence the enzyme activity in soil. Averaging of all the sampling dates in 2010 and 2011, the activities of dehydrogenase and alkaline phosphatase were 50% lower in the soils that were amended with digestS compared with inputS. However, the CO2 efflux from the soil surface was the same for the inputS and digestS treatments. Our results indicate that the anaerobic digestion of substrates does not affect the plant P uptake but the performance of soil microorganisms.

Citric acid was produced with free and k-carrageenan-entrapped cells of the yeastYarrowia lipolyticain single and repeated batch-shake-flask fermentations on glycerol-based media. Simultaneous solubilization of hydroxyapatite of animal bone origin (HABO) was tested in all experiments. The highest citric acid production by free yeast cells of 20.4 g/L and 18.7 g/L was reached after 96 h of fermentation in the absence and presence of 3 g/L HABO, respectively. The maximum values for the same parameter achieved by gel-entrapped cells in conditions of single batch and repeated-batch fermentation processes were 18.7 g/L and 28.1 g/L registered after 96 h and the 3d batch cycle, respectively. The highest citric acid productivity of 0.58 g L−1h−1was obtained with immobilized cells in repeated batch mode of fermentation when the added hydroxyapatite of 3 g/L was solubilized to 399 mg/L whereas the maximum efficiency of 89.0% was obtained with 1 g/L of HABO.

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