Felipe Montes

Ammonia emission model for whole farm evaluation of dairy production systems.

Ammonia (NH) emissions vary considerably among farms as influenced by climate and management. Because emission measurement is difficult and expensive, process-based models provide an alternative for estimating whole farm emissions. A model that simulates the processes of NH formation, speciation, aqueous-gas partitioning, and mass transfer was developed and incorporated in a whole farm simulation model (the Integrated Farm System Model). Farm sources included manure on the floor of the housing facility, manure in storage (if used), field-applied manure, and deposits on pasture (if grazing is used). In a comprehensive evaluation of the model, simulated daily, seasonal, and annual emissions compared well with data measured over 2 yr for five free stall barns and two manure storages on dairy farms in the eastern United States. In a further comparison with published data, simulated and measured barn emissions were similar over differing barn designs, protein feeding levels, and seasons of the year. Simulated emissions from manure storage were also highly correlated with published emission data across locations, seasons, and different storage covers. For field applied manure, the range in simulated annual emissions normally bounded reported mean values for different manure dry matter contents and application methods. Emissions from pastures measured in northern Europe across seasons and fertilization levels were also represented well by the model. After this evaluation, simulations of a representative dairy farm in Pennsylvania illustrated the effects of animal housing and manure management on whole farm emissions and their interactions with greenhouse gas emissions, nitrate leaching, production costs, and farm profitability.

Landscape control of nitrous oxide emissions during the transition from conservation reserve program to perennial grasses for bioenergy

Future liquid fuel demand from renewable sources may, in part, be met by converting the seasonally wet portions of the landscape currently managed for soil and water conservation to perennial energy crops. However, this shift may increase nitrous oxide (N2O) emissions, thus limiting the carbon (C) benefits of energy crops. Particularly high emissions may occur during the transition period when the soil is disturbed, plants are establishing, and nitrate and water accumulation may favor emissions. We measured N2O emissions and associated environmental drivers during the transition of perennial grassland in a Conservation Reserve Program (CRP) to switchgrass (Panicum virgatum L.) and Miscanthus x giganteus in the bottom 3‐ha of a watershed in the Ridge and Valley ecoregion of the northeastern United States. Replicated treatments of CRP (unconverted), unfertilized switchgrass (switchgrass), nitrogen (N) fertilized switchgrass (switchgrass‐N), and Miscanthus were randomized in four blocks. Each plot was divided into shoulder, backslope, and footslope positions based on the slope and moisture gradient. Soil N2O flux, soil moisture, and soil mineral nitrogen availability were monitored during the growing season of 2013, the year after the land conversion. Growing season N2O flux showed a significant vegetation‐by‐landscape position interaction (P < 0.009). Switchgrass‐N and Miscanthus treatments had 3 and 6‐times higher cumulative flux respectively than the CRP in the footslope, but at other landscape positions fluxes were similar among land uses. A peak N2O emission event, contributing 26% of the cumulative flux, occurred after a 10.8‐cm of rain during early June. Prolonged subsoil saturation coinciding with high mineral N concentration fueled N2O emission hot spots in the footslopes under energy crops. Our results suggest that mitigating N2O emissions during the transition of CRP to energy crops would mostly require a site‐specific management of the footslopes.

Measuring Emissions of Volatile Organic Compounds from Silage

Volatile organic compounds (VOCs), a necessary reactant for photochemical smog formation, are emitted from numerous sources. Limited available data suggest that dairy farms emit VOCs with cattle feed, primarily silage, being the primary source. Process-based models of VOC transfer within and from silage during storage and feeding are presented. These models are based upon well-established theory for mass transport processes in porous media with parameters determined from silage properties using relationships developed for soils. Preliminary results indicate that VOC emission by advective flow of silage gas is generally insignificant compared to emission by surface convection and diffusion from within silage. VOC emissions are dependent upon silage properties, temperature, wind speed, and exposure duration, which have implications for measuring, predicting, and controlling VOC emissions from silage. Emissions appear to be co-limited by convection and diffusion; therefore, the EPA-style emission isolation flux chamber design previously used to measure VOC emissions from silage is not suitable for this task.

Process Modeling of Ammonia Volatilization from Ammonium Solution and Manure Surfaces

Ammonia emissions occur from manure surfaces on the barn floor, during storage, and following field application. Based upon theoretical principles and associated published information on ammonia emission, relationships were refined for modeling the dissociation constant, Henry’s law constant and mass transfer coefficient to better predict ammonia loss from manure surfaces. Expressions were obtained that relate these coefficients to the temperature, pH and ionic strength of the material, and the air velocity over the material. These expressions were tested by comparing predicted ammonia emission rates against values measured in controlled laboratory experiments for buffered ammonium-water solutions and dairy cattle manure. Experimental results compared well to values predicted using these theoretical expressions derived from ammonia volatilization literature. This process-based model provides a basis for developing predictive tools that better quantify management effects on ammonia emissions from farms and thus assist in the development and evaluation of strategies for reducing emissions.

A Farm-Level Model of VOC Emission from Silage

Recent measurements suggest that dairy farms can be a significant emission source of volatile organic compounds (VOCs). However, accurate estimates of farm-level emissions currently do not exist. We developed a preliminary process-based model to estimate VOC emissions from silage on farms and to assess the effectiveness of management changes on reducing emissions. Using ethanol as a representative VOC, we evaluated the effects of environmental conditions (temperature and air velocity) and management practices on emission. Model predictions suggest that VOC emission is sensitive to environmental conditions, with the greatest emission occurring under hot and windy conditions. Predictions indicate that changes in silage management can substantially reduce VOC emission, but that changes in individual sources will not lead to significant reductions on their own. Combined changes in storage and feeding practices can lead to substantial emission reductions, according to model predictions. Preliminary predictions of ethanol emission for typical conditions are substantially greater than previous estimates of VOC emission from silage. Additional measurements are needed, however, to complete the model for all important VOCs and to fully verify farm-level predictions. When complete, this model will provide a useful tool for evaluating strategies for reducing VOC emissions from silage.

Effects of carbon dioxide emission, kinetically-limited reactions, and diffusive transport on ammonia emission from manure

Volatilization of ammonia from animal manure causes significant loss of fixed N from livestock operations. Ammonia emission from manure is the culmination of biological, chemical, and physical processes. In this work we present a speciation and transport model for simultaneous CO2 and NH3 emission from manure. Our model was implemented using the geochemical software PHREEQC in order to assess the importance of CO2 volatilization, equilibrium speciation, kinetically-limited reactions, and aqueous diffusion on NH3 emission dynamics from thin layers (1-10 mm) of dairy cattle manure. Preliminary predictions show that emission of CO2 leads to a rapid increase in manure pH, which causes a substantial increase in NH3 flux. Kinetic limitations on conversion of carbonic acid to dissolved CO2 was predicted to significantly influence NH3 emission by limiting the effect of CO2 volatilization on solution pH, therefore these reactions should be considered in the development of chemical models of NH3 emission. For the limited number of conditions studied in this work, diffusion of solution species through the aqueous phase had only small effects on NH3 emission from 1 and 5 mm layers. However, aqueous-phase diffusion can be limiting for thicker layers or for conditions that cause higher convective mass transfer from the surface. Additional work is needed to assess the importance of the processes studied here for different conditions.

Dissociation and Mass Transfer Coefficients for Ammonia Volatilization Models

Laboratory trials were conducted to measure the dissociation and mass transfer coefficients for ammonia volatilization from buffered ammonium-water solutions and dairy cattle manure. Effects of ionic strength, ammoniacal N concentration, temperature and pH of the media, and air velocity over the media were evaluated. Current models were found to adequately predict ammonia emissions; even though, important differences were found in measured and model-predicted dissociation between ammonium solution and dairy manure. The results imply that model refinements can be made to improve accuracy in predicting emissions from livestock manure.