Nitrogen release and plant available nitrogen of composted and un‐composted biosolids

Abstract

The nitrogen (N) release from composted and un‐composted biosolids and plant available N (PAN) of the biosolids were quantified to evaluate if composting can contribute to stabilize biosolids N and reduce the nitrate (NO3-) leaching potential in biosolids‐amended soil. Biosolids were composted at >55°C for 21 days after mixing the biosolids with yard waste at 1:1 (w/w) ratio. In the N release study, we installed field lysimeters filled with soil (sand and clay) amended with composted and un‐composted biosolids at two rates (30 and 150 dry Mg/ha) and measured the inorganic N in leachate after each rainfall and soil inorganic N monthly. The N released from composted biosolids during the two‐year study period were lower (6% of organic N added for clay and 11% for sandy loam soil) as compared to un‐composted biosolids (14% of organic N added for clay and 21% for sandy soils). Composted biosolids showed a lower N release rate constant k value of 0.0014 and 0.0027 month−1 for clay and sandy soil, respectively, compared to corresponding values of 0.0035 and 0.0068 month−1 for un‐composted biosolids. We used greenhouse bioassay with corn (Zea mays), ryegrass (Lolium perenne), and Miscanthus (Miscanthus giganteus) as test plants grown for six months with reference to N chemical fertilizer ranging from 0, 75, 150 to 300 kg N/ha to evaluate the PAN of the biosolids. Based on our study, plant growth was not affected by using either composted or un‐composted biosolids but the PAN was lower in composted biosolids (4.0%–5.9%) than un‐composted biosolids (11.4%–13.6%). Composting results in higher N‐retention efficiency in biosolids and composted biosolids are a valuable source of N to support the plant growth with lower N released to the environment. Thus, the potential of N leaching would still be low in the situations where a high rate of biosolids needs to be applied for land reclamation or landscaping soil reconstruction. Practitioner points Composting enhances N‐retention efficiency in biosolids and composted biosolids are a valuable source of N to support the plant growth with lower N released to the environment. Potential of N leaching would still be low in the situations where a high rate of biosolids needs to be applied for land reclamation or landscaping soil reconstruction. N released from composted and un‐composted biosolids can be adequately described by first‐order kinetic model.