Composting offers the potential to significantly reduce problems associated with manure management including odors, pathogens, ground water pollution, and utilization costs. Two variables that directly affect on-farm composting costs are windrow size and windrow turning frequency. However the size of a windrow is limited by the depth of penetration of oxygen and high temperatures as well as available equipment. In this study three full scale compost sets were set-up at the Ohio Agricultural Research and Developing Center (OARDC) compost pad to evaluate the effects of turning frequency, pile size and seasonal variability on physical (temperature, oxygen, bulk density, moisture and weigh loss), chemical (volatile solid loss, pH, Carbon and Nitrogen concentrations) and biological (plant growth bioassays and microbial community structure) parameters during dairy manure/sawdust composting (DM+S). Based on these data the operational costs for producing and transporting compost were estimated and compared to those for liquid manure and fertilizer.
The three treatments consisted of a set of windrows (A) which were turned using a self propelled and tractor drawn windrow turner every three days for a total of 32 turns during 16 weeks, a second set (B) that was turned once every ten days and a third set (C) consisting of much larger piles turned that was
also turned every ten days with a loader. All three sets were composted in both winter and summer for 120 days.
The hypotheses of the study was that; turning frequency, pile size and season do not significantly affect compost process parameters or the final chemical, physical or biological properties of cured composts.
Results showed that neither physical chemical nor biological properties of the final cured composts were significantly affected by turning frequency, season or pile size (p > 0.05). During composting, he the surface area, oxygen concentrations and Total nitrogen losses were significantly affected by pile size (p < 0.05). Turning frequency affected (p < 0.05) mass losses, bulk density and total nitrogen losses. The seasonal effects on composting during the process were primarily related to moisture (p < 0.05), mostly due to ambient temperatures which affects water holding capacity of air. Despite these process differences, the final cured composts from all treatments and seasons had similar properties (p > 0.5).
Plant growth bioassays showed a high emergence percentage (> 80%). The fertilized cucumber plants grown in composts from the various treatments in summer had higher shoot dry weights than peat controls (≥ 100%) except for day 30 in pile C (89%). The unfertilized cucumbers plants showed an increase of shoot dry weight at the end of the composting process (day 120) except for windrow A in summer. However the bioassay tests were inconclusive.
Microbial Community analysis, based on Terminal Restriction Fragment Length Polymorphisms (T-RFLP), showed that management differences (turning frequency, pile size and season) did not significantly affect (p > 0.05) microbial community structure. Clustering, pairwise comparison, principal component analysis (PCA) and Kruskal Wallis tests were used to determine the similarities and differences between microbial communities in the different treatments. In each treatment a different subset of TRFs were present suggesting that different classes of organisms predominate during different stages of composting.. However, one terminal restriction fragment H371 contributed significantly (p < 0.1) to the observed variation as a function of compost age.
The Restriction Fragment (TRF) sizes obtained in the different treatments were compared to fragment sizes predicted by in silico amplification and digestion (RDP v.9.0) to characterize the microbial community in the composts. TRFs fragments sizes were also compared to a clone library of 263 sequences from composted dairy manure. Representative TRFs (61, 93, 99, 159, 167, 205, 215, 227, 365, 373, 437 and 481) in the compost samples were consistent with the predicted TRFs of Proteobacteria, Firmicutes, Bacteroidetes and Actinobacteria.
The main factor affecting total compost production operational cost was the cost of the bulking agent. Operational costs for frequently turned windrow were higher ($109/Mg) compared to the infrequently turned windrow ($95/Mg), and the infrequently turned piles ($93/Mg). These differences are due to the time that is needed to turn and the equipment fuel costs. Thus, infrequent turning (every ten days) with larger windrow sizes reduced the operating costs associated with unseparated dairy manure composting compared to more frequently turning windrows. It is recommended for the farmers to use a turning frequency of ten days and piles with a surface to volume ratio of 0.9-1.2 m2/m3 to minimize operational costs. If composting is performed in temperate climates there is a need to consider the moisture content at the beginning of the process to prevent moisture irregularities during the process.