Methanogenic pathways and δ13 C values from swine manure with a cavity ring-down spectrometer: Ammonia cross-interference and carbon isotope labeling

Pig manure degradation and carbon emission: Measuring and modeling combined aerobic-anaerobic transformations

Greenhouse gas emissions from liquid livestock manure storage significantly contribute to global warming. Accurate farm-scale models are essential for predicting these emissions and evaluating manure management strategies, but they rely on multiple parameters describing carbon loss dynamics. Surface respiration may significantly influence carbon loss and methane emission, yet it is not explicitly included in current models. We conducted experiments to measure pig manure surface respiration rate and its effect on organic matter degradation and methane and carbon dioxide emissions. Manure was incubated for 283 days at 10°C or 20°C under aerobic or anaerobic conditions, while measuring methane and carbon dioxide emission. This was followed by anaerobic digestion at 38°C. Surface respiration reduced the organic matter content, and the effect was temperature dependent. Methane emission was not affected by surface respiration, suggesting that substrate availability was not rate-limiting for methanogenesis. Surface respiration rates were 18.1 ± 3.5 g CO2 m-2 day-1 at 10°C and 37.1 ± 13.1 g CO2 m-2 day-1 at 20°C (mean ± standard deviation) and were consistent with microsensor measurements of oxygen consumption in different manure surfaces. Based on these results, temperature- and surface area-dependent respiration was incorporated in the existing anaerobic biodegradation model (ABM). Simulations showed that surface respiration accounts for 29% of carbon losses in a typical pig house and 8% for outdoor storage. Developing and refining algorithms for diverse carbon transformations, such as surface respiration, is crucial for evaluating the potential for methane emission and identification of variables that control emissions at the farm scale.

Reducing greenhouse gas emissions from pig slurry by acidification with organic and inorganic acids

Methane (CH₄) emission from pig slurry is a large contributor to the climate footprint of livestock production. Acidification of excreta from livestock animals with sulfuric acid, reduce CH₄ emission and is practiced at many Danish farms. Possible interaction effects with other acidic agents or management practices (e.g. frequent slurry removal and residual slurry acidification) have not been fully investigated. Here we assessed the effect of pig slurry acidification with a range of organic and inorganic acids with respect to their CH₄ inhibitor potential in several batch experiments (BS). After careful selection of promising CH₄ inhibitors, three continuous headspace experiments (CHS) were carried out to simulate management of manure in pig houses. In BS experiments, more than <99% CH₄ reduction was observed with HNO₃ treatment to pH 5.5. Treatments with HNO₃, H₂SO₄, and H₃PO₄ reduced CH₄ production more than acetic acid and other organic acids when acidified to the same initial pH of 5.5. Synergistic effects were not observed when mixing inorganic and organic acids as otherwise proposed in the literature, which was attributed to the high amount of acetic acid in the slurry to start with. In the CHS experiments, HNO₃ treatment reduced CH₄ more than H₂SO₄, but increased nitrous oxide (N₂O) emission, particularly when the acidification target pH was above 6, suggesting considerable denitrification activity. Due to increased N₂O emission from HNO₃ treatments, HNO₃ reduced total CO₂-eq by 67%, whereas H₂SO₄ reduced CO₂-eq by 91.5% compared to untreated slurry. In experiments with daily slurry addition, weekly slurry removal, and residual acidification, HNO₃ and H₂SO₄ treatments reduced CO₂-eq by 27% and 48%, respectively (not significant). More cycles of residual acidification are recommended in future research. The study provides solid evidence that HNO₃ treatment is not suitable for reducing CO₂-eq and H₂SO₄ should be the preferred acidic agent for slurry acidification.

Characterisation of cellulose-degrading organisms in an anaerobic digester

The recalcitrant nature of lignocellulosic biomass hinders efficient exploitation of this fraction for energy production. A better understanding of the microorganisms able to convert plant-based feedstocks is needed to improve anaerobic digestion of lignocellulosic biomass. In this study, active thermophilic cellulose-degrading microorganisms were identified from a full-scale anaerobic digester fed with maize by using metagenome-resolved protein stable isotope probing (protein-SIP). ¹³C-cellulose was converted into ¹³C-methane with a ^13/12C isotope ratio of 0.127 after two days of incubation. Metagenomic analysis revealed 238 different genes coding for carbohydrate-active enzymes (CAZymes), six of which were directly associated with cellulose degradation. The protein-SIP analysis identified twenty heavily labelled peptides deriving from microorganisms actively assimilating labelled carbon from the degradation of ¹³C-cellulose, highlighting several members of the order Clostridiales. Corynebacterium was identified through CAZyme screening, amplicon analysis, and in the metagenome giving a strong identification of being a cellulose degrader.

Effects of combined tannic acid/fluoride on sulfur transformations and methanogenic pathways in swine manure

Livestock manure emits reduced sulfur compounds and methane, which affect nature and the climate. These gases are efficiently mitigated by addition of a tannic acid-sodium fluoride combination inhibitor (TA-NaF), and to some extent by acidification. In this paper, TA-NaF treatment was performed on swine manure to study the treatment influence on methanogenic pathways and sulfur transformation pathways in various laboratory experiments. Stable carbon isotope labeling revealed that both untreated and TA-NaF treated swine manures were dominated by hydrogenotrophic methanogenesis. However, in supplementary experiments in wastewater sludge, TA-NaF clearly inhibited acetoclastic methanogenesis, whereas acidification inhibited hydrogenotrophic methanogenesis. In swine manure, TA-NaF inhibited s-amino acid catabolism to a larger extent than sulfate reduction. Conversely, acidification reduced sulfate reduction activity more than s-amino acid degradation. TA-NaF treatment had no significant effect on methanogenic community structure, which was surprising considering clear effects on isotope ratios of methane and carbon dioxide. Halophile sulfate reducers adapted well to TA-NaF treatment, but the community change also depended on temperature. The combined experimental work resulted in a proposed inhibition scheme for sulfur transformations and methanogenic pathways as affected by TA-NaF and acidification in swine manure and in other inocula.

A mechanistic model of methane emission from animal slurry with a focus on microbial groups

Liquid manure (slurry) from livestock releases methane (CH4) that contributes significantly to global warming. Existing models for slurry CH4 production-used for mitigation and inventories-include effects of organic matter loading, temperature, and retention time but cannot predict important effects of management, or adequately capture essential temperature-driven dynamics. Here we present a new model that includes multiple methanogenic groups whose relative abundance shifts in response to changes in temperature or other environmental conditions. By default, the temperature responses of five groups correspond to those of four methanogenic species and one uncultured methanogen, although any number of groups could be defined. We argue that this simple mechanistic approach is able to describe both short- and long-term responses to temperature where other existing approaches fall short. The model is available in the open-source R package ABM (https://github.com/sashahafner/ABM) as a single flexible function that can include effects of slurry management (e.g., removal frequency and treatment methods) and changes in environmental conditions over time. Model simulations suggest that the reduction of CH4 emission by frequent emptying of slurry pits is due to washout of active methanogens. Application of the model to represent a full-scale slurry storage tank showed it can reproduce important trends, including a delayed response to temperature changes. However, the magnitude of predicted emission is uncertain, primarily as a result of sensitivity to the hydrolysis rate constant, due to a wide range in reported values. Results indicated that with additional work-particularly on the magnitude of hydrolysis rate-the model could be a tool for estimation of CH4 emissions for inventories.