nitrification – Livestock and Poultry Environmental Learning Community

Purpose

This study seeks to quantify the impact of swine slurry nitrification on biogas productivity. Ammonia (NH3) is produced during anaerobic digestion of manure and emitted during storage. Ammonia emissions have adverse impacts on swine health and growth, caretaker health, and local air and water quality. Ammonia is also known to inhibit methanogenic activity during anaerobic digestion, reducing methane potential. Thus, reducing ammoniacal nitrogen in digester feedstock can improve digester performance. A novel approach to nitrogen management, developed by a commercial partner, is nitrifying flush water that feeds into the digester. This technology leverages nitrification to suppress NH3 volatilization through using low-pH, highly nitrified substrate to flush the barns. This alternative reduces in-barn NH3 concentration surge during flushing events. In addition, equilibrium between nitrified (oxidized) flush liquid and reduced urine-feces will reduce ammoniacal nitrogen levels in the feed entering the digester. A barn-scale system (17,000 gallons per day capacity) is currently under testing on a NC swine farm that has an anaerobic digester as part of the waste management system (Figure 1). Understanding the impacts of this treatment on anaerobic digestion under controlled conditions under different organic loading rates is needed. This study aimed to quantify impacts of flush water nitrification on biomethane yield (BMY) in swine manure under two different organic loading rates (OLRs).

What Did We Do?

Figure 1: Commercial swine farm used for sample collection.

Three different substrates were collected for this study. Substrates were sourced from the same farm every 2 to 3 weeks (Figure 2). Swine slurry was processed through settling > decanting > maceration > screening to create liquid (<1% solids) and solid (>5% solids) fractions needed to formulate desired OLRs. Two OLRs were tested in this study, 1 g VS/L-d (low, L) and 2 g VS/L-d (high, H). For each OLR, two substrate formulations were tested: nitrified (treatment, T) and baseline (control, C). Therefore, four combinations of substrate and OLR were evaluated in this study and were abbreviated as: CH, CL, TH, and TL.

Figure 2: Nitrification System installed onsite

Eight mesophilic reactors at 95°F (35 °C), each with a two-liter active volume, were used to study the impacts of OLR and substrate type, with two replicates per OLR-substrate combination, represented by 1 or 2, respectively. Reactors were fed once daily, 6 days per week, unless otherwise noted. Influent and digestate total solids (TS), volatile solids (VS), chemical oxygen demand (COD), pH, alkalinity, and nitrogen forms were analyzed during the study. Biogas composition (% carbon dioxide (CO2), methane (CH4), and nitrogen gas (N2)), specific CH4 productivity (mL/g VS-fed), and volatile solids and COD reduction (%) were compared across treatments.

What Have We Learned?

Overall, comparable BMY values were observed across reactors with mean reactor productivity ranging from 275 to 354 mLCH4/g VS-fed. Average BMY for the reactors represented around 61% of typical values of ultimate biomethane potential (BMP) for swine manure reported in the literature, i.e., 450 to 550 mLCH4/gVS. Increasing OLR from 1 to 2 gVS/L-d resulted in a 14% decrease in BMY. The nitrogen treatment effect appears to be minimal and only limited to low OLR treatments. The percentage deviation of biomethane productivity between C and T reactors was less than 1%.

Similar to CH4, concentrations of CO2 were impacted more by OLR than the nitrogen treatment implemented. For low OLR reactors, Average CO2 concentrations in the biogas were for treatment reactors. Increasing the OLR showed an increase in CO2 concentration in the biogas, with control and treatment reactors containing approximately , respectively.

Figure 3 – Cumulative biomethane yields (20 hr) by reactor ID. The ID of each reactor denotes the combination of substrate, OLR, and replicate. Control (C) and nitrified (T) substrates were fed to corresponding reactors according to 2 OLRs, 1 g VS/L-d (L) and 2 g VS/L-d (H).

Future Plans

We plan to continue our data analysis to quantify reduction in VS and COD. Similarly, digestate characterization to quantify alkalinity and volatile fatty acids (VFAs) in the feedstock and digestates is ongoing. Two-way analysis of variance (ANOVA) will be conducted to assess treatment impacts on specific methane yield, VS and COD reduction. Denitrification occurring within reactors was further investigated via GC-TCD headspace analysis. We plan to closely analyze denitrification dynamics to capture the effect of treatment on nitrogen forms and organic matter in the substrates.

Acknowledgements

This work was funded by Pancopia, Inc. as part of a Department of Energy, Small Business Innovation Research program grant (DOE SBIR, Grant No. DE-SC0020833). Authors would like to acknowledge Smithfield Foods for access and support sampling. and undergraduate student researchers: Brian Ngo, Nick Bell, Kiarra Condon, Himanth Mandapati, and Jackson Boney for assistance and support conducting this study.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2025. Title of presentation. Waste to Worth. Boise, ID. April 7-11, 2025. URL of this page. Accessed on: today’s date.

[Abstract] Subsurface manure application is theoretically susceptible to greater denitrification losses and nitrous oxide (N2O) emissions compared to surface application methods – primarily attributed to manure being placed in a more anaerobic environment. A review of field studies suggest N2O emissions typically range from 0.1% to 3% of total applied N from subsurface application methods, but there is considerable variation in emissions depending on pre- and post-application soil moisture conditions, readily-available carbon content in manure compared to background levels in soil, localized nitrogen form and oxygen concentration at the application site, and application depth. This paper will summarize peer-reviewed literature of field studies that quantify N2O emissions subsequent to subsurface manure application and identify the most prominent determining factors cited by authors.

Why Study Nitrous Oxide Emissions of Manure?

Ammonia abatement efficiencies of up to 90 percent have been documented with subsurface application and incorporation of animal manures compared to conventional surface application methods. While reducing ammonia emissions has positive implications for air and water quality, a portion of the nitrogen conserved may come at the expense of increased nitrous oxide emissions produced during denitrification and nitrification processes in the soil. As a greenhouse gas 300 times more potent than carbon dioxide at trapping heat, nitrous oxide has been linked to anthropogenic climate change and depletion of stratospheric ozone. Release of nitrous oxide from agriculturally-productive soils into the atmosphere also represents a loss of crop nutrients. Understanding the circumstances and manageable factors that contribute to nitrous oxide formation in soils subsequent to manure application is important for retaining crop nutrients and preventing greenhouse gas emissions.

What did we do?

A literature review was performed to investigate the factors that contribute to nitrous oxide emissions following subsurface application of animal manure to both grassland and arable land, compare results from different application techniques, and examine the conditions and circumstances that lead to nitrous oxide emissions.

What have we learned?

Several studies demonstrate significant increases in nitrous oxide emissions (from 0.1 to 3 percent) attributable to factors including increasing soil moisture content, high concentrations of readily-available carbon in manure substrate, increased nitrate concentration in soil, shallow application depth, high soil temperature, and ambient conditions during and immediately following application (table 1). Other studies show no difference in nitrous oxide emissions as compared to surface application methods. Reasons that subsurface application techniques will not necessarily result in greater nitrous oxide emissions were: 1) the length of the diffusion path from the site of denitrification to the soil surface may lead to a greater portion of denitrified nitrogen being emitted as nitrogen gas; 2) the soil moisture conditions and aeration level at the time of application may not be suitable for increased nitrous oxide production; 3) prior to manur e application, soils may already contain readily-metabolizable carbon and mineral nitrogen, thus any increase in nitrous oxide emission following application may not have a significant impact; and 4) weather events subsequent to manure application may effect soil moisture content and water-filled-pore-space, thereby affecting nitrous oxide emissions. Several studies document nitrous oxide emissions due to subsurface application methods (including manure incorporation and shallow injection) but research comparing nitrous oxide emissions from different subsurface application techniques and application depth is limited. Lack or absence of data in literature about manure chemistry, nitrogen application rates, application technique or method, as well as soil and atmospheric conditions during and after application made it more difficult to draw specific conclusions on factors affecting nitrous oxide emissions from subsurface-applied manure.

Further research is needed to determine the environmental and economic tradeoffs of implementing subsurface manure application methods for abatement of NH₃ considering different future greenhouse gas emissions and market scenarios. Recent work suggests a link between denitrifier community density, organic C, and N₂O emissions. Characterization of these biological mechanisms and identification of genetic markers for key enzymes should continue, particularly with respect to various subsurface manure application techniques, different manure types and N application rates, soil types, environmental conditions, and soil chemistry. Subsurface application depth plays an important role in determining the proportion of N₂O to N₂ emitted during denitrification; however, the number of field studies that examine the impact of application depth is limited. More research is needed to determine optimal manure application depth as influenced by soil type, soil chemistry, timing of application, and vegetative cover. Finally, future research on subsurface manure application will allow existing and future prediction models to improve estimation of annual N₂O emissions at landscape scale and airshed levels. Refinement of greenhouse gas inventories, including N₂O emissions from agricultural production systems, will assist agriculture producers, scientists, and policy makers in making informed decisions on greenhouse gas emission mitigation.

research articles reporting factors of Nitrous Oxide

Future Plans

Future agricultural greenhouse gas regulations and/or carbon market incentives have potential implications for agricultural producers, including the method and timing of manure application. Controlled, replicated, and well-documented research on subsurface manure application and subsequent nitrous oxide release is critical for estimating the costs and benefits of different manure application techniques.

Authors

David W. Smith, Extension Program Specialist, Texas A&M AgriLife Extension DWSmith@ag.tamu.edu

Dr. Saqib Mukhtar, Professor and Associate Department Head for Extension, Texas A&M AgriLife Extension

Additional information

The publication ‘Estimation and Attribution of Nitrous Oxide Emissions Following Subsurface Application of Animal Manure: A Review’ has been accepted for publication in Transactions of the ASABE.

Acknowledgements

Funding for this effort provided by USDA-NIFA grant No. 2011-67003-30206.

The authors are solely responsible for the content of these proceedings. The technical information does not necessarily reflect the official position of the sponsoring agencies or institutions represented by planning committee members, and inclusion and distribution herein does not constitute an endorsement of views expressed by the same. Printed materials included herein are not refereed publications. Citations should appear as follows. EXAMPLE: Authors. 2015. Title of presentation. Waste to Worth: Spreading Science and Solutions. Seattle, WA. March 31-April 3, 2015. URL of this page. Accessed on: today’s date.