Reclamation of phosphorus (P) from wastewater using metal-engineered biochar and its reuses as phosphate fertilizers could be an eco-friendly and effective technology for the sustainability of environmental and agricultural systems. Herein, we studied the P removal capacity of filter cake-engineered biochar loaded with Al and Fe and examined their available P release capability into the sand sample. The Al-biochar (63.7 mg/g) had a higher P retention capacity than the Fe-biochar (33.2 mg/g). Over 45 days of the incubation, Al-and Fe-biochar incorporation steadily or increasingly released adequate concentrations of available P to the sand sample. Furthermore, a considerable P fraction (99.4-99.8% of the total P) remained in all metal-biochar samples, suggesting that the metal biochar could be a long-term source of the available P pool. This study highlighted the potential use of Al-and Fe-biochar for providing available P availability in sandy soils.

Silicate minerals constitute the main components in silicon (Si)-rich biomass, affecting the phosphorus (P) adsorption and release competencies of mineral-engineered biochar; however, the mechanisms underlying their differences remain largely unresolved. To examine these interactions, we investigated the mineralogical compositions and quantified the P-adsorption capacities of Al-, Fe-, Mn-, Zn-, and Mg-engineered biochars from Si-rich rice husk material. The potential uses of P-laden mineral-engineered biochar for P fertilizers were assessed using citric acid extraction. The results from X-ray diffraction, scanning electron microscopy, and Fourier transform infrared spectrometry revealed that mixed metal (oxyhydr)oxides and metal-silicate compounds precipitated in the biochar structure and acted as the main P adsorbents. Micro-crystalline silicates derived from the biomass-induced metal-silicate precipitates in all engineered biochars, which effectively retained the aqueous P with varying excellent capacities (25.6–46.5 mg/g) but relatively slow kinetics (48 h). The suitability of the Zn-, Mg-, Mn-, and Fe-biochars as P-recycled fertilizers was confirmed by the high amounts of citric acid extractable P (19–69% of the total P). Varying amounts of Zn, Mg, and Mn (34–47% of the total host metals) were also released from the engineered biochar through ligand-promoted dissolution. Our data shed light on the novel potential utilization of Mn-, Mg- and Zn-biochars from Si-rich biomass for P retrieval and their use for P, Mg, and micronutrient (Mn and Zn) fertilizers. Regarding the P removal capacity, the mineral-engineered biochar needed a longer adsorption period than conventional metal-engineered biochar. Graphical Abstract

Dwindling carbon (C) and nitrogen (N) levels in paddy soils decreases rice production and threaten human food security globally. The efficient maintenance of C and N fluxes in soil-rice systems is a crucial prerequisite for agricultural and environmental sustainability. Herein, we examined the C and N fluxes from 63 rainfed rice paddy fields under conventional farming (CF) and organic farming (OF) systems in Thailand. The C and N fluxes were measured based on a detailed analysis of relevant influxes (fertilizer, manure, and biomass addition) and effluxes (biomass harvest and greenhouse gas emission). The results demonstrated that the harvested grain and straw contributed to the most abundant C and N effluxes for both farming systems. The CH4 effluxes were moderate, whereas the N2O effluxes were meager relative to their total effluxes. Stubble incorporation and animal manure addition to soil were the most extensive C influxes. However, the primary N influxes were stubble incorporation and animal manure addition for the OF system, and chemical-N fertilizers for the CF system. Net C depletions were observed in both the CF and OF systems. However, net N was depleted and accumulated in the CF and OF systems, respectively. Straw incorporation to soils could restore the net C accumulations for the CF and OF systems and elevate the net N accumulation for both systems. This study highlighted that complete straw removal has exacerbated the C and N stock in soil-rice systems, inducing insecurity for the environment and the agricultural systems. Effective straw management is a simple approach for sustaining paddy rice production.