The amount of biogas slurry residues has increased significantly worldwide due to people interest in utilizing renewable energy. Biogas residues are generated from anaerobic degradation of various types of urban and rural organic garbage and have been recommended as bio-fertilizers because of its valuable macro-nutrients and micro-nutrients and high content of ammonium.
However, application of biogas residues to agricultural lands may be accompanied by adverse environmental impacts, since they may contain organic pollutants and heavy metals. Therefore the effects of biogas residues on the environment and soil ecosystem and on crop production immediately need to be examined before their wider application. It is found from different laboratory experiments and field investigation that biogas residues increase crop production to the same degree or more than traditional compost and mineral fertilizer. It is generated from source-separated household garbage have a trend to give higher soil microbial activities and crop yield than others.
The biogas slurry residues have different physical and chemical properties due to the various organic substrates are used in the biogas plants and also to differences in process parameters such as retention time and operating temperature. The high concentration of NH4+ in the residues makes its fertilization value equal to that of the mineral fertilizers and traditional compost, which means that they are able to give the crop with needed nitrogen. However, in several cases the outcome of fertilizer may be delayed because organic residues contain less instant plant-available N than mineral fertilizers. It also indicates that a Nitrogen pool of organic Nitrogen will be developed in the soil and is able to deliver the plants in later years.
The use of biogas residues had no long-term or short-term harmful effects on soil microbial actions. However, some severe instant inhibitory effects on enzymes involved in denitrification and nitrification were observed on usage of residues. This primary effect is of concern allowing for soil health and gives an early informing of the presence of possible hazardous substances in the biogas residues. However, inhibition of denitrification and nitrification also lowers Nitrogen losses from the structure.
It is found from the dose-response test study that residues enhance ammonium absorption by soil microbes, which might decrease nitrogen availability for the short-term to the plant. It has the ability to alter soil bacterial community formation depending on soil group, with sandy soil being extra prone to alter. This change in bacterial community formation brought about by the biogas residues is varied to that originated by cattle slurry. However, generally, no changes or only small in microbial community formation could be seen after several years of biogas residue use. Soil resistance to alter in genetic structure is helpful, as reversion to an earlier functional state is potential, such as, if a farmer for different reasons wants to alter back to original fertilizer class after having investigated a 'new' form of fertilizer.
Biogas residues, like other mineral fertilizers containing nitrogen, are able to stimulate releases of N2O from agricultural lands. However, compared with the animal slurries it have high NH4+ concentration and less organic carbon and therefore induce N2O productions from the top soil in a unusual process. This means that the capability of biogas residues to supply denitrifying microbes with required carbon is controlled, which will limit the decrease of N2O to Nitrogen and start higher N2O releases from some kinds of soils.
Soil properties also influence the performance of the residues with regard to N2O emissions. As a result, the class of soil and organic residue should be considered as trying to lessen N2O emissions. For example, it could be suitable to keep away from fertilization of the organic soils with Carbon rich substance such as normal animal manure and as an alternative fertilize with the biogas residues. In addition, in clay soil the threat of N2O emissions could be lowered by applying a C-rich fertilizer at humidity contents higher than 50% of water-holding capacity.
On the other hand, it should be highlighted that it is still significant to do large-scale researches where diverse biogas residues are analyzed under field conditions. Moreover, fertilization with solid fraction of the filtered biogas residues may start higher N2O productions compared with the unseparated biogas residue, particularly when used to organic soils. Further researches are needed to recognize the cause why filtered biogas residues add to N2O fluxes. Such information and research can be used to conduct the development of the new fertilizers and devise improvement options.
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