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Why Study Composting Separated Manure Solids?
This research is evaluating management options for conventional hog producing facilities in regions of Manitoba that will have insufficient land base for sustainably applying raw slurry manure when manure application regulations switch from nitrogen based to phosphorus based rates. Producers are being encouraged to use solid-liquid separation, such as centrifugation, to remove the phosphorus rich solid fraction so that it can be transported and applied further away where there is a phosphorus deficiency. However, the resulting separated hog solids (SHS) product is still odorous and prone to nitrogen losses through ammonia volatilization. Therefore, it has been suggested that composting the SHS before it is applied is a beneficial management practice that would allow producers to capitalize on agricultural and environmental benefits such as reduced odours, stabilization of minerals, application of a homogeneous product, and acts as a multi-beneficial soil conditioner. However, the low starting C:N of 15:1 and small particle size of SHS make it a unique and challenging product to compost in windrows, a common form of large production on-farm composting. The SHS must be combined with a bulking agent that allows adequate nutrient balance for decomposition as well as a porous structure. Therefore, this project is comparing wood shavings (WS) and wheat straw (WHT) as bulking agents to evaluate which is the better management practice based upon minimizing greenhouse gas emissions and additional nitrogen gas losses as well as overall quality of the mature compost.
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What Did We Do?
Starting in October 17, 2012 we created two windrows containing SHS, one with wood shavings as a bulking agent and one with wheat straw. The materials were mixed in a feed mixer to produce a homogeneous mixture with the initial starting parameters shown in Table 1. The windrows were turned once a week for the first four weeks with a Backus windrow turner.
Gas emissions were measured with the use of the highly innovative technology of the LI-8100a automated chamber system (LICOR BioSciences) and Fourier Transform Infrared spectroscopy (FTIR) multi-gas analyzer (Gasmet DX4015). By combining these two instruments it has the advantage of nearly continuous unattended data collection and simultaneous measurement of greenhouse gases (carbon dioxide, methane, nitrous oxide) and additional nitrogen gases (ammonia, nitrous dioxide, and nitrogen monoxide). There were four automated chambers on each windrow; a flux measurement was taken every half hour, alternating between the two windrows. Flux emissions were calculated using linear regression analysis.
| Table 1. Initial starting parameters for the two windrows | ||||
|
Initial In-process Compost |
Starting C:N |
Starting Moisture % |
Starting Bulk Density (kg/m3) |
Starting pH |
|
WHT + SHS |
32.5 |
63.70 |
170.5 |
6.86 |
|
WS + SHS |
35.5 |
60.45 |
350 |
6.5 |
The temperature, % oxygen, and moisture content of the windrows were recorded to identify when the compost needed to be turned and to track the composting process and relate it to the gases emitted.
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What Have We Learned?
In September 2011 we conducted a trial that used straw as a bulking agent but found the contact between the separated hog solids and straw was poor due to the difference in particle size allowing for large pore spaces and the waxy texture of straw. The porous structure made it difficult to maintain moisture in the compost windrow and when water was added some of the separated hog solids actually “washed off”. In the winter, the windrow wasn’t big enough or it was too porous that it did not insulate well so self heating stopped and the pile froze in January. These problems slowed the decomposition process and resulted in compost with straw pieces still visible.
For this trial we decided to try using wood shavings as an alternative bulking agent, because wood shavings have a smaller particle size which we predicted would result in better contact with the separated hog solids and a less porous structure allowing better insulation against the weather (water loss in the summer, heat loss in the winter). Additionally, it is expected that wood shavings are also beneficial in reducing ammonia losses.
However, during this trial we experienced much wetter and cooler conditions compared to the year before, so we did not have to add water to the windrows. This was beneficial for the windrow with straw because the moisture content did not decline resulting in a steady rate of decomposition during the first month of composting noted by continuous CO2 emissions. Eventually the moisture content became too high creating anaerobic conditions and the production of CH4 after the second and fourth turnings. NO2 emissions were also detected during the same time as CH4, indicating some aerobic respiration occurring. After CO2 emissions reduced there was a small amount of N2O and NO measured.
The windrow with wood shavings took a little longer to start producing CO2 because it became anaerobic from the start. CH4 was produced much early and at higher emission rates compared to the windrow with the straw as a bulking agent. N2O, NO, and NO2 were emitted at the same time as CH4, indicating there were anaerobic and aerobic pockets throughout the windrow. N2O emissions continued after CO2 emissions declined.
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After the windrows had been in the active stage of composting for three months, the temperature within the windrows gradually declined and both windrows froze up in early January.
We are currently in the process of calculating the ammonia flux determinations. Due to the nature of ammonia it is prone to absorbtion reactions on the surface of the LI-8100a and FTIR systems’ tubing. The surface reactions cause a time delay for the FTIR to analyze the concentration compared to the other gases. Thus, this gas requires a different time interval to calculate the flux.
Future Plans
A common problem with using chamber measurements on compost windrows is underestimation of gas emissions from chambers placed on the top of the windrow when high winds blow through the windrow horizontally, reducing the “chimney effect”. Having the ability to collect gas emission data at such a high frequency using the LI-8100a automated chamber and FTIR system allows us to identify when gases emissions may be underestimated due to wind. The next step is to determine if we can correlate the wind speed and direction with under estimation of gas losses.
Authors
Jolene Rutter, MSc. Candidate, University of Manitoba, Jolene_rutter@hotmail.com
Mario Tenuta, Canada Research Chair in Applied Soil Ecology, University of Manitoba
Matt Gervais, Soil Ecology Field Technician, University of Manitoba
Acknowledgements
Western Economic Diversification Canada, Manitoba Pork Council, Manitoba Horticultural Productivity Enhancement Centre, Manitoba Rural Adaptation Council, NSERC, National Center for Livestock and the Environment, University of Manitoba Soil Ecology Laboratory, Glenlea Research Farm, Prairie Agricultural Machinery Institute, Compo-stages, Puratone
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