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 ( ${\text{NO}}_3^{-}$ ) 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.

Keywords: bioassay; lysimeters; mineralization kinetics; nitrates leaching; nitrogen mineralization; plant available nitrogen.