Assessing how the ratio of wood chips to food waste in compost affects rates of microbial processing

Background/Question/Methods

Food waste can be utilized to produce compost for degraded urban soils and presents an opportunity to scale up urban agriculture. Wood chips are often added to compost material to create the oxygenated conditions microbes require to break down compost material. Because wood chip availability is often limited in urban areas due to competing uses such as biomass incineration, research is needed to optimize wood chip use in composting. Here, we assessed how the ratio of wood chips in compost affects rates of microbial processing, nutrient leaching, and compost quality. We constructed ten compost piles (1 m³) consisting of barley mash and wood chips. Each pile contained different ratios (by volume) of wood chips to barley mash  (0:100, 10:90, 20:80, 30:70, 40:60, 50:50, 60:40, 70:30, 80:20, and 90:10 by volume).  Weekly measurements of temperature, moisture content, internal CO₂ levels, and internal O₂ levels were acquired for twelve weeks to assess rates of microbial activity (microbes consume O₂ and emit CO₂). In addition, leachate samples were collected four times during the experiment. These samples were analyzed to determine the amount of phosphorus runoff.

Results/Conclusions

During the first 4 weeks of the experiment, compost containing 30%, 40%, 50%, and 60% wood chips maintained optimal internal temperature above 55° C (required to kill food-borne pathogens) and maintained optimal moisture content between 40% and 60% (necessary for microbes to thrive). In addition, these treatments had internal CO₂ levels above 10% and internal O₂ levels below 5%, indicating significant microbial activity (compared to the other treatments). After the initial 4 weeks of the experiment, decreasing microbial activity occurred in compost containing 40%, 50%, and 60% wood chips. However, compost containing 30% wood chips was able to maintain microbial activity throughout the 12-week experiment. Additionally, it was found that phosphorus concentration in leachate samples generally increased in treatments with fewer wood chips (less than 50%) indicating the importance of woodchips in minimizing nutrient runoff and pollution. Currently, organic gardeners generally recommend 50% wood chips for compost. Our results suggest that fewer wood chips could produce similar composting conditions, although less wood chip use could increase the environmental impact of compost production. These results will help determine the extent to which wood chips should be made available for composting rather than other uses such as biomass incineration.