Physico-chemical assessment of on-farm bioconversion of organic waste in dairy farms in context to sustainability and circular bioeconomy

On a milk-producing dairy farm, milk production is correlated with manure production and number of cattle, and manure is widely used as soil fertilizer. However, excessive dairy manure production is linked with greenhouse gas emissions and water quality issues. On-farm planning of manure storage and application to enhance soil nutrients are essential in a circular economy to reduce environmental impact, where manure is not landfilled and incinerated. Instead, it creates a nutrient resource for crops, and soil. Dairy manure, which is rich in nutrients, is valuable fertilizer that contains many nutrients such as nitrogen (N), organic matter (OM), phosphorous (P), and Potassium (K) and micronutrients. In this work, a pilot field research was conducted between 2016 and 2018 in various parts of California, USA (San Joaquin Valley, Sacramento Valley, Shasta Cascade, and the North Coast of California) to assess physio-chemical characteristics of solid fractions of dairy manure among various dairy farms. A total of 156 samples were collected from the gut (n = 107) and toe (n = 49) of the manure piles across California for determining total solid (TS), volatile solids (VS), temperature, moisture content, and carbon nitrogen ratio (C:N). Here, using the observations of field study and analysis, we show that C:N, OM, and MC of solid fractions of dairy manure vary significantly among dairy farms. Average C:N ratio of manure (26.32) among various regions was close to an ideal C:N value of 24:1 for soil microbes to stimulate nutrient release to crops. Manure pH varied between 7.0 and 8.0, which was close to an optimal pH rages for common crops (6.0-8.0). Moreover, considering less cost and surplus availability, manure will likely to continue providing a cost-effective organic fertilizer resource compared to commercial chemical fertilizers.