The need to find and maximize the use of alternative sources of nutrients for plants and soil environment have been on the forefront of research in sustainable agriculture. These alternatives have to be affordable, accessible, reproduceable, and efficient to compete with established inorganic fertilizers while at the same time reduce any potential negative impacts on the environment. We aimed to evaluate the effectiveness of digestate fertilization in an agricultural system over a period of three years. The digestate utilized in the study consisted of animal waste-based digestates, namely pig manure digestate, chicken manure digestate, and cow manure digestate, and were compared with synthetic nitrogen fertilizer. Every year, the digestate and the synthetic nitrogen fertilizer were split applied at the rate of 90 and 80 kg N ha−1. The soil chemical composition after three years of fertilization showed a slight decrease, significantly different nitrogen and carbon changes while phosphorus and potassium were significantly higher in the digestate treatments. The third year of digestate application showed higher grain yield than previous years and the yield from the digestate treatments were significantly different from the synthetic nitrogen fertilizer. The nitrogen use efficiency for the three years was in the range of 20–25 percent in the digestate treatments, with a strong correlation between the nitrogen use efficiency and the grain yield. There were varied results in the grain quality and straw quality in the digestate and synthetic nitrogen fertilizer with no clear trend observed. Our results showed a relatively high potential of animal waste digestates over the short to mid-term use with a positive result obtained in comparison to synthetic nitrogen fertilizer under favorable climatic conditions.

This study evaluates the environmental impact assessment of sustainable pig farm via management of nutrient and co-product flows in the farm. Manure management and biogas production are among the most promising pathways towards fully utilizing organic waste within a circular bioeconomy as the most environmentally friendly solution mitigating gaseous emissions and producing bioenergy and high-quality bio-fertilizers. The concept of farm management includes rearing pig, growing all the feeds needed, and managing the nutrients and co-product flows in the farm. A consequential life cycle assessment (LCA) was performed to examine three scenarios in which all the generated manure is used as fertilizer for barley cultivation and mineral fertilizer is used where necessary (SC1); produced surplus straw is used for thermal energy generation and maize is used for sale, substituting maize biomass in the market (SC2); and all co-products are circulated in a closed system (SC3). The functional unit (FU) was defined as a “farm with 1000 fattening pigs at farm gate”. The analysis showed that heat generation from wheat, barley and legumes straw has a significantly higher positive environmental impact than the use of these cereal straw for biogas production. The partial replacement of mineral fertilizers with digestate has positive environmental effects in terms of abiotic depletion, photochemical oxidation, terrestrial ecotoxicity, freshwater aquatic ecotoxicity, human toxicity, and marine aquatic ecotoxic aspects. The amount of digestate generated on a farm is not sufficient to completely eliminate the use of mineral fertilizers for plant fertilization. The generated pig manure (SC1) and digestate (SC2) is only enough for the fertilization of 8.3% of the total cultivated land of farm applying 22.9 t/ha rate.

The increasing quantities of organic residues are becoming one of the most important problems for climate change mitigation. Sustainable utilization technologies are required to minimize the effect of recycling on the environment. Nevertheless, treated residues should be part of the circular bioeconomy. One of the most promising processes is the biogas system, with the final products biogas and digestate, which contain valuable nutrients and are therefore suitable as agricultural fertilizers. However, there is lack of research data on the effectiveness of digestate on environmental factors including soil quality as well as crop productivity and quality. In this study, we compare the roles of different digestates (chicken manure digestate, cow manure digestate, and pig manure digestate) on spring wheat productivity, soil microbial activities, and greenhouse gas emissions in loam and sandy loam soil under controlled climate conditions. The liquid digestate applied was equivalent to 170 kg N ha−1 of total N presented. Overall, results showed that the two soil types responded differently to the addition of the digestates, and the benefits depended on soil characteristics as well as on the type of the digestate applied. There was a higher effect on soil microbial activity in sandy loam soil compared to that of loam soil. Chicken manure digestate had the highest value of dehydrogenase activity and soil microbial biomass C of 9.23 µg TPFg−1 h−1 and 175.6 µg g−1 across the two soil types. CO2 and N2O emissions were moderately higher in loam soil when compared to that of sandy loam soil. The highest CO2 peak emission at 0.0107 µg ha−1 h−1 occurred in pig manure digestate in the sandy loam soil, and regular peak patterns observed in loam soil fertilized with pig digestate manure. Chicken manure digestate had the highest peak emissions across both soil types at 0.007950 mg ha−1 h−1 and 0.5667 mg ha−1 h−1 in the loam and sandy loam soil, respectively. The biomass yield varied across the soil types irrespective of the digestate applied. The agricultural benefits of digestates in different receiving soil ecosystems supplying essential nutrients for crop productivity, coupled with its environmental benefits, makes it an encouraging prospect in temperate climate zones.

The race is on to achieve an important level of efficiency in the attainment of a circular economy in agriculture especially with the aim of sustainable nitrogen management. This cycle in the agricultural sector cuts across livestock farming, agriculture-induced waste generation, recycling and utilization, energy generation, crop production, ecosystem protection and environmental management through the mitigation of climate changes. In this work, we assess the process and functionalities of livestock waste generated from the piggery farm and their combinations with other by-products such as biochar and ash in comparison with mineral fertilization as sources of nitrogen applied in agricultural soil. The experiment was performed in a controlled environment with wheat ( Triticum aestivum L.) grown in a neutral and an acidic soil. Pig manure was used as the primary feedstock, fed and processed to biogas and nutrient-rich digestate by the anaerobic digestion process. The results revealed that the co-amendments of pig manure digestate with biochar and ash had complimentary positive effect on measured indices such as mobile potassium, phosphorus, biomass yield and nitrogen use efficiency. The mineral nitrogen fertilizer significantly induced carbon dioxide emissions from day 35 when compared to emissions from the organic amendments. In contrast, the organic amendments influenced nitrous oxide emissions from the onset till day 30 before flattening out. The individual combination of pig manure digestate with biochar and ash had a negative influence on enzymatic activity (dehydrogenase). Soil microbial biomass carbon was induced across all treatments in both soil types. Pig manure digestate + ash and pig manure digestate had 32.1 and 48.8% soil microbial biomass increase in neutral soil and acidic soil, respectively. Overall, the processing and application of single-use amendment or in combination with biochar and ash holds huge potential in the optimization of nitrogen and carbon efficiency towards sustainable soil management via improving soil quality, carbon sequestration and climate change.

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