Valorization of nitrogen-rich melamine as a nitrogen source in the production of maize (Zea mays L.)

Maize is important for the biofuel and bio-based products. Building a clean maize production system could minimize the environment pollution from the N loss from the sloping land and achieve the industrial sustainable development. As a nitrogen-rich source, melamine could be used as a slow-release nitrogen (N) fertilizer, which provides a route for the recycling of melamine waste and the combatting of N loss from fertilizers. Model N release of melamine was compared with urea, sulfur(S)-coated urea via N loss in leaching column under inter-mittent leaching and under the ploughing fertilizer in the farmland soil. Only 7.12 % of the total nitrogen (TN) was lost from the 21 irrigation events, while the loss of TN from urea reached 77.28 %. TN from urea would be release totally in soil within 14 days in the ploughing fertilizer experiment, but 62 % of melamine was remained after 144 days. In order to evaluate the efficacy of melamine as an N fertilizer, plot experiments with an N rate of 180 kg ha-1 were carried out using pure melamine, urea and S-coated urea as well as mixed melamine/urea at 2:8, 1:1 and 7:3 by the N ratio. Melamine fertilization alone was unable to satisfy the N demand of maize, but the melamine/urea mixture (1:1) was comparable to the commercial S-coated urea in both years in terms of the grain yield (2217 kg ha-1 in 2019, 2368 kg ha-1 in 2020), total aboveground dry matter (5076 kg ha-1 in 2019, 4815 kg ha-1 in 2020), aboveground N uptake (36 kg ha-1 in 2019, 48 kg ha-1 in 2020), partial factor productivity of nitrogen (12.32 kg kg-1 in 2019 and 13.16 kg kg-1 in 2020) and net return (1474 USD in 2019, 1606 USD in 2020). Melamine accumulation in the maize grain measured from ELISA (enzyme-linked immunosorbent assay) ranged from 0.24 to 1.8 mg kg-1. The valorization of the melamine waste into N fertilizer by blending with urea was a possible way of reducing melamine pollution, alleviating N loss in hilly area and building a recyclable agriculture system.