Effects of layered double hydroxide functionalized biochars and hydrochars on dry matter and N, P and K concentrations of corn

Phosphorus (P) is an essential element for living organisms. Discharging P from various sources, such as industrial wastewater and agricultural waters, into surface water causes eutrophication and undermines the balance of aquatic ecosystems and imposes many costs due to water quality degradation. In addition, mineral resources of P-fertilizers in the world are unrecoverable and are coming to an end. Therefore, it is very important to develop adsorbents to remove P from contaminated water and then be used as P-fertilizer for surmounting the eutrophication and P-fertilizer exhausting challenges. In the last few years, biochar and hydrochar have been considered as low-cost porous eco-friendly adsorbents with a high surface area and easy to produce and use. Biochar and hydrochar are carbonaceous solids that are produced from the carbonization of biomasses and could be used as adsorbents and soil amendments. However, because of their high negative charge and very low ability to absorb anions, especially phosphate, they cannot be used as phosphate adsorbents. In recent years, several methods have been introduced to change the surface of biochar and hydrochar to increase their anion adsorption capacity. In this respect, the successful results of the production and the use of engineered biochars, such as layered double hydroxides (LDHs) functionalized biochar (LDH-biochar) and LDH-hydrochar composites have been provided. Layered double hydroxides (LDHs) are brucite-like compounds with a large specific surface area, high positive charge, and exchangeable interlayer anions. LDHs functionalized biochar and hydrochar composites are environmentally friendly adsorbents for the removal of phosphate from aqueous solutions. Also, P-loaded LDH-biochar and LDH-hydrochar composites have the potential application as a P-fertilizer. These composites may increase soil available-P through the slow release of P and can improve soil properties and fertility due to the presence of the biochar and hydrochar in their structure. So, the P-loaded LDH-biochar and LDH-hydrochar may affect the availability of soil nutrients and plant growth. Nitrogen (N), P, and potassium (K) are the macronutrients that have a direct and great influence on plants growth. Therefore, the aims of this study were: (I) producing LDH-biochar and LDH- hydrochar composites and loading them with phosphate. (II) Investigating the effects of the biochar, hydrochar, LDH, LDH-biochar, LDH-hydrochar, the P-loaded LDH-biochar (LDH-biochar-P), and LDH-hydrochar (LDH-hydrochar-P) on dry matter and concentrations of P, N, and K in corn shoot and root. Biochar was produced from applewood feedstock through slow pyrolysis at 600 ºC for 1 h under Argon flow conditions. Hydrochar was produced through hydrothermal carbonization of the applewood feedstock at 180 ºC and 11 bars pressure for 12 h. Then by precipitation of LDH particles on the biochar and hydrochar surfaces, LDH-biochar and LDH-hydrochar composites were prepared. The LDH particles were synthesized via a combined fast co-precipitation and hydrothermal treatment route. Each gram of LDH-biochar and LDH-hydrochar composites was loaded with 51 and 47 mg P, respectively. Then using a factorial experiment on the basis of completely randomized design with three replications, the effects of biochar, hydrochar, LDH, LDH-biochar, LDH-hydrochar, LDH-biochar-P, and LDH-hydrochar-P were studied in presence and absence of monocalcium phosphate fertilizer on corn dry matter and concentrations of N, P, and K in corn shoot and concentrations of P and K in corn root. The results showed that the biochar had a higher yield and ash percentage, pH and electrical conductivity (EC) as compared with the hydrochar. The concentrations of all studied nutrients in the biochar, except for N, were greater than those of hydrochar and biomass. The P, K, Na, Fe, Mn, and Zn concentrations in biochar and hydrochar were significantly greater than the initial biomass. The application of P-fertilizer increased root and shoot dry matters in all treatments, except for LDH-biochar-P and LDH-hydrochar-P treatments. Biochar and hydrochar had no significant effects on root and shoot dry matter in non-P-fertilized treatments and had no significant effects on P and K concentrations of corn root and shoot. However, biochar and hydrochar increased shoot dry matter in P-fertilized treatments. The highest root and shoot dry matters, P concentrations of root and shoot, and N concentration of shoot were obtained in the presence of the LDH-biochar-P and LDH-hydrochar-P, and the lowest root and shoot dry matters of corn were observed in the presence of the LDH. Application of P-fertilizer increased P concentrations of corn root and shoot in the presence of the LDH-biochar and LDH-hydrochar but decreased the K concentration of root in biochar, LDH-biochar and no amendment treatments and had no significant effects on N and K concentrations in the shoot. The application of P-fertilizer decreased P translocation factor in presence of the LDH-biochar and LDH-hydrochar and had no significant effect on P translocation factor in all other treatments. Using P-fertilizer had no significant effect on K translocation factor in all treatments. Biochar, hydrochar, LDH, LDH-biochar, and LDH-hydrochar had no significant effects on P and K translocation factors. The translocation factor of P was greater than 1 in all treatments, except for the LDH-biochar-P and LDH-hydrochar-P treatments. Also, the translocation factor of K was greater than that of P in all treatments. Due to the structural similarities between biochar and hydrochar, LDH-biochar and LDH-hydrochar, and LDH-biochar-P and LDH-hydrochar-P, the root and shoot dry matter and concentrations of the studied elements in corn root and shoot were not significantly different between the biochar and hydrochar, LDH-biochar and LDH-hydrochar, and LDH-biochar-P and LDH-hydrochar-P treatments, respectively. P-fertilizer had synergistic relationships with biochar, hydrochar, LDH-biochar, and LDH-hydrochar but antagonistic relationships with LDH, LDH-biochar-P, and LDH-hydrochar-P composites in terms of dry matter and P concentrations in corn root and shoot. So, applications of the biochar, hydrochar, LDH-biochar, and LDH-hydrochar accompanied by P-fertilizer and the use of LDH-biochar-P and LDH-hydrochar-P without the application of P-fertilizer can be proposed for corn cultivation under similar conditions.