Enhancing Spinach Growth and Soil Edaphic Factors Using Aquatic Weed–Derived Biochar

<i>Phragmites australis</i> (Cav.) Trin. ex Steud and <i>Lemna minor</i> L. are non-native aquatic weeds. They disturb the habitat dynamics by competing with native flora for water and nutrition, deplete oxygen in water bodies, destroy water quality, and create habitat for mosquitoes and other parasites. Valorization of this huge biomass into biochar is a sustainable approach to address both environmental and agricultural challenges. It not only mitigates the climate issues by proper management but also sequesters carbon and improves soil quality. The current study manifests the utility of <i>Phragmites australis</i> and <i>Lemna minor</i> biomass as a feedstock for pyrolysis, at 400°C, 500°C, and 600°C, to design <i>Lemna minor</i> biochar (LMBC400, LMBC500, and LMBC600) and <i>Phragmites australis biochar</i> (PABC400, PABC500, and PABC600). These biochars were added as a soil conditioner to estimate the productivity of test crops. The results demonstrate that ash content, pH, electrical conductivity (ECe), and fixed carbon are directly proportional to pyrolysis temperature, whereas oxygen, hydrogen, nitrogen, volatile contents, and bulk density (BD) are inversely proportional to pyrolysis temperature. Scanning electron microscopy (SEM) and Fourier transform infrared (FTIR) spectroscopy analyses proved that high pyrolysis temperature increases the porosity and phenolic compounds in biochar, which improves the surface quality. The percentage of nutrients such as Mg, Fe, N, Ca, N, P, K, and Zn increases by elevating pyrolysis temperature due to unlocking and release under the influence of heat. Soil quality parameters (viz. pH, BD, ECe, water holding capacity [WHC], total dissolved solids [TDS], and soil organic matter [SOC]) improved in favor of plant growth conditions, so the yield of test crops increased. So, LMBC600 and PABC600 had great potential to improve soil and productivity. It is a promising approach to manage this colossal volume of <i>P. australis</i> and <i>L. minor</i> through green technology by recycling this biowaste into a worthy product biochar, which is an alternative to chemical fertilizer in agronomical practices, which leave eco-toxic footprints and play havoc to the environment.