Where does the nitrogen used in biosolids primarily originate?

The nitrogen used in biosolids primarily originates from the atmosphere. This is because atmospheric nitrogen is converted into a usable form through various biological processes, such as nitrogen fixation, and is then incorporated into organic matter and eventually, into the biosolids produced during wastewater treatment. These biosolids are rich in nutrients, including nitrogen, which is essential for plant growth.

While other sources of nitrogen, such as soil, water, and crop residues, certainly contribute to the nitrogen cycle, they don't represent the primary source of nitrogen for biosolids. Soil contains organic and inorganic nitrogen, but this nitrogen is recycled from other processes, including the breakdown of plant and animal materials. Water can contain nitrogen from runoff or leaching, but it is not the primary source. Crop residues do contribute to soil nitrogen levels when they decompose, but again, they derive from the atmospheric nitrogen that plants assimilated during their growth. Thus, the correct identification of atmospheric nitrogen emphasizes its role as the fundamental source from which biosolid nitrogen is ultimately derived.