Effects of Manure and Biogas Residues Application on N20 Emissions and Soil C Sequestration
Research output: Book/Report › Ph.D. thesis › Research
Biogas production from manure has become increasingly widespread worldwide in recent years due to the potential for renewable energy production and mitigation of greenhouse gas emissions. However, the sustainable development of this technology requires the appropriate utilization of an enormous volume of its residues, known as digestates, to minimize negative environmental impacts when they are applied on soils. The overall aim of this PhD project was to understand how different feedstock sources or input substrates and the operational conditions for biogas production, e.g. the temperature and hydraulic retention time (HRT) of the biogas digester, affect emission of nitrous oxide (N2O) and carbon (C) sequestration in the soils after the digestate is applied. This PhD thesis is based on the results of four laboratory studies undertaken between November 2013 and February 2017, primarily at the Department of Plant and Soil Sciences of the University of Copenhagen. Throughout the project, the spatial and temporal distribution of O2 in soils after surface applications of manure and biogas digestates was investigated using the O2 planar optode imaging technique. Furthermore, the relationship between soil O2 dynamics and the temporal N2O emission and its production pathways from the amended soils was examined (Chapters 4 & 5) through a combination of O2 optode imaging and N2O isotopomer analysis. The study also determined the potential C sequestration of seventeen digestates originating from three main feedstock groups which were produced from a mesophilic anaerobic co-digestion of pig slurry with agro-industrial waste (75:25% wet weight) at HRTs of 15, 20, 30 and 45 days, and thermophilic codigestions of food waste with garden waste (75:25 and 50:50) at HRTs of 10, 15, 20 and 30 days (Chapter 5). The results of the optode images of soil O2 indicated that soil O2 was depleted extensively within the upper 1.0-1.5 cm and 1.5-2.0 cm soil areas after surface application of manure and digestates respectively, with the most intensive development occurring during the initial 24 h. It was evident that O2 consumption was reduced for the soil amended with cattle slurry incorporated with nitrification inhibitor 3,4-dimethyl pyrazole phosphate (DMPP) compared to the cattle slurry treatment (Chapter 4). Similarly, the soil O2 depletion zones were much greater for digestate with a shorter HRT compared one with a longer HRT, e.g. the codigestion of pig slurry with agro-industrial wastes at 15 d vs. 30 d (Chapter 5). However, in both studies, soil O2 was seen to recover after 48 h from application, most probably due to the reduction in respiration activities in the soil amendments. The higher O2 consumption in the cattle slurry soil amendment resulted in significantly higher N2O emissions (ca. 60 %) relative to that of the treatment with DMPP (Chapter 4). In contrast, the larger anoxic areas for the digestates produced with a shorter HRT probably led to the reduction of N2O to N2, and hence reduced total N2O emissions in comparison with those of digestates produced with a longer HRT (Chapter 5). Furthermore, the isotopomers of N2O revealed that denitrification, including fungal and bacterial denitrification, were probably the main source of N2O production after manure application, especially under acidic grassland soil conditions (Chapter 4). During the first few days after the application of manure and digestates, it was the denitrification of the residual soil nitrate that was most likely responsible for the early peak in N2O emissions. Finally, it was demonstrated that the composition of feedstock for biogas production and the biogas digester HRT affected the potential of C sequestration after the application of digestates to soils. The highest stable C fraction in soil was attributed to the digestates of food waste and garden waste (50:50 % w/w) and the digestate resulting from the co-digestion of pig slurry with agro-industrial waste (PO, 75:25) at 45 d HRT. However, the difference was not significant between any of the digestates investigated with HRT of between 15 and 30 d. With respect to the C balance in the entire biogas production chain, the C loss due to soil respiration accounted for 6-17% of the initial C in feedstock, whereas the C-CO2 loss was as much as 56% for undigested pig slurry with no C end up in biogas production. The potential C sequestration of digestates ranged between 13% and 28% depending on the feedstock source and the HRT. Overall, both the HRT of biogas digesters and the chemical compositions of digestates such as cellulose, hemicellulose and lignin content can be used as dependent parameters to predict the potential C sequestration in soil. However, using the HRT or the lignin content as a single parameter can poorly predict the C sequestration. The best prediction lies on all lignocellulose components including cellulose, hemicellulose, ash, the ratio of cellulose/lignin, holocellulose/lignin and lignin/ash, which can improve the prediction variance up to 70%..
|Publisher||Department of Plant and Environmental Sciences, Faculty of Science, University of Copenhagen|
|Publication status||Published - 2017|