Biochar-based fertilizers as technology to improve nutrient use efficiency in tropical soils

Abstract

Highly weathered soils under natural conditions have low macro and micronutrient availability and very low efficiency for most soluble fertilizers used to overcome this issue. Therefore, we sought to develop new fertilizers technologies to increase the nutrient use efficiency, such as phosphorus (P), copper (Cu), and zinc (Zn) in tropical soils. Two studies were carried out, the first with P and the second with micronutrients (Cu and Zn). It is known the incorporation of phosphorus (P) into an organic matrix may be an effective strategy to increase plant P use efficiency in high P-fixing soils. Therefore, the objective of the first study was to evaluate the effect of biochar-based fertilizers (BBFs), produced from poultry litter (PLB) and coffee husk (CHB) enriched with phosphoric acid and magnesium oxide, in combination with triple superphosphate (TSP) on plant growth and soil P transformations. Treatments were prepared as: TSP, CHB, PLB, CHB + TSP [1:1], CHB + TSP [3:1], PLB + TSP [1:1] and PLB + TSP [3:1]; with numbers in brackets representing the proportion of BBF and TSP on a weight basis. Cultivations were: Mombasa grass, maize, and common bean interspersed with fallow periods. After cultivations, a sequential extraction procedure was employed to determine P distribution among different P pools. A kinetic study was performed and revealed that TSP released approximately 90% of total P, and BBFs less than 10% in the first hour. BBF alone or in combination with TSP presented higher or similar biomass yields, relative agronomic effectiveness, and P uptake when compared with TSP. As for the soil, BBFs increased non-labile P fractions, which can be due to pyrophosphate formed during pyrolysis. According to these results, BBFs could totally or partially replace conventional soluble P fertilizers without compromising crop yield either in the short and long-term. In the second study the effect of poultry litter biochar-graphene oxide composite (PLB-GO) as a novel adsorbent for copper (Cu) and zinc (Zn) was evaluated, as well as fertilizing effects on plant growth, nutrient use efficiency and soil fertility. In order to do so, poultry litter biochar (PLB) and PLB-GO were produced, characterized and evaluated by isotherm adsorption and kinetics to evaluate their Cu and Zn sorption and desorption properties. Cu and Zn loaded to PLB and PLB-GO as biochar-based fertilizers (BBF) were researched. PLB-GO showed higher adsorption capacity for Cu (16.2%) and Zn (17.7%) than pristine PLB and in both cases < 0.5% of the sorbed metal content was released in water. Plant effects on growth, nutrient uptake and nutrient use efficiency were, in general, as shown in the following order both for Cu and Zn: PLB-GO ≥ PLB ≥ Sulfates. PLB increased Cu availability while PLB-GO increased Zn availability after cultivation, even after increasing nutrient uptake and being little soluble in water. Addition of small amounts (≤ 0.5%) of graphene oxide in biochar has potential to increase its properties to retain micronutrients and enhance fertilizer use effectiveness in highly weathered soils. In general, both biochar-based fertilizers studied showed promising results when applied to highly weathered soils with high fixation capacity. The use of these materials increased the nutrient use efficiency by plants, showing that they have the potential for use in agriculture.