Effects of Modified Almond Shell Biochar with Acid on the Amount and Rate of Phosphorus Release in Calcareous Sandy Soils

Background and Objectives: Phosphorus (P) is a vital macronutrient for plant development and productivity, playing a crucial role in key physiological and biochemical processes, including photosynthesis, energy transfer via ATP, nucleic acid synthesis, root growth, and cell division. Despite its importance, the availability of phosphorus in soils—particularly in calcareous soils—is significantly limited due to its tendency to react with calcium and other soil minerals, leading to the formation of insoluble phosphate compounds. These reactions immobilize added phosphorus fertilizers, making them inaccessible to plants. In calcareous soils, where the pH is high and calcium carbonate is abundant, phosphorus readily precipitates as calcium phosphates. Consequently, only a small fraction of applied phosphorus remains in a form that plants can absorb. This inefficiency not only reduces crop yield potential but also increases the economic and environmental costs associated with excessive fertilizer use. To address this issue, researchers have increasingly focused on sustainable soil amendment strategies that can enhance phosphorus availability. Among these, biochar—produced through pyrolysis of organic materials under limited oxygen conditions—has emerged as a promising candidate. Biochar possesses a high specific surface area, stable carbon structures, significant cation exchange capacity, and the potential for surface functionalization. These properties make it suitable for modifying nutrient dynamics in soil. However, most biochars, especially those derived from woody feedstocks, tend to have alkaline pH values, which can further exacerbate phosphorus immobilization in alkaline soils. Therefore, modifying biochar through acid treatment has been proposed as an effective way to improve its properties for nutrient retention and release, particularly phosphorus. Acid treatment can lower the pH, introduce more functional groups, and remove base cations, potentially enhancing the biochar’s capacity to retain and release phosphorus in a plant-available form.
Materials and Methods: This study was conducted to evaluate the effect of acid-modified almond shell biochar on the release and availability of phosphorus in two calcareous soils with contrasting textures—sandy and clay. A factorial experiment with a completely randomized design was carried out, incorporating three main factors: (1) biochar amendment (no amendment/control, 1% w/w unmodified almond shell biochar, and 1% w/w acid-modified almond shell biochar), (2) soil texture (sandy and clay), and (3) incubation period (30 and 60 days), each with three replications. Biochar was produced by pyrolyzing almond shell wood at a temperature of 400°C. For acid modification, 200 grams of the produced biochar were saturated with distilled water, followed by the addition of 11.3 mL of concentrated phosphoric acid (H₃PO₄) and 34.9 mL of concentrated nitric acid (HNO₃). The mixture was stirred thoroughly with a glass rod and then oven-dried at 60°C. These treatments aimed to alter the surface characteristics of the biochar, particularly its pH and electrical conductivity (EC), as well as to introduce oxygen-containing functional groups that could facilitate phosphorus release. The treated soils were incubated with the respective biochar amendments at 70% field capacity to simulate realistic soil moisture conditions. Phosphorus release was monitored through sequential extraction using sodium bicarbonate solution (NaHCO₃) at multiple time intervals: 2, 4, 8, 24, 48, 72, 96, 240, 336, 456, 600, and 768 hours. The extracted phosphorus was quantified to determine the release pattern over time. To model the kinetics of phosphorus release, four commonly used equations were fitted to the data: first-order kinetics, power function, simple Elovich, and parabolic diffusion. These models help in understanding the mechanisms and rates of phosphorus release under various treatments and conditions. The determination of the most suitable kinetic equation was generally based on the coefficient of determination (R²) and the standard error (SE) associated with each equation. Equations exhibiting higher R² values and lower SE were considered the most appropriate for describing the elemental release data.
Results: The results indicated that the acid-modified biochar significantly enhanced phosphorus release in both soil types compared to the control (p < 0.05), while the unmodified almond shell biochar did not lead to a significant increase (P > 0.05). The highest cumulative phosphorus release, 144.6 mg/kg, was recorded in the clay soil treated with acid-modified biochar after 30 days of incubation. In general, phosphorus release was greater in clay soil than in sandy soil, which may be attributed to the higher cation exchange capacity and greater organic matter content of the clay soil, both of which enhance phosphorus retention and release dynamics. The acid-modified biochar exhibited a markedly lower pH (3.0) compared to the unmodified biochar (7.1), and its electrical conductivity was also significantly reduced (0.0026 vs. 0.46 dS/m). This reduction in EC suggests the successful removal of soluble base cations during the acid washing process, a change that likely contributed to improved phosphorus desorption and availability in calcareous conditions. Among the kinetic models applied, the power function and simple Elovich equations provided the best fit for the experimental data, based on their high R² and low SE. These models indicated that diffusion was the dominant mechanism controlling phosphorus release from the soil-biochar system, especially under the influence of acid-modified biochar. The model parameters—particularly the intercepts and slopes—provided further insights into the dynamics of phosphorus availability. A higher intercept signifies a greater initial release of phosphorus, while a lower slope is indicative of faster release rates. The intercepts of the best-fit models were highest for acid-modified biochar in both soil types, suggesting that these treatments facilitated the most rapid initial release of phosphorus. Conversely, the slopes were lowest in acid-biochar treatments, reaffirming the faster rate of release. Over time, as incubation progressed to 60 days, the intercepts generally decreased while the slopes increased, reflecting the natural decline in phosphorus release as the soil-biochar system approached equilibrium. Notably, the acid-modified biochar maintained a higher phosphorus release rate over time compared to the other treatments, underscoring its sustained effectiveness.
Conclusion: The findings of this study demonstrate that acid treatment of almond shell biochar using phosphoric and nitric acids can substantially enhance its capacity to release phosphorus in calcareous soils. Compared to unmodified biochar, the acid-modified variant showed superior performance in increasing phosphorus availability, particularly in clay soils with higher nutrient retention capacity. This suggests that acid-modified biochars can be effectively used as soil amendments in alkaline and calcareous agricultural systems to improve phosphorus use efficiency and reduce reliance on synthetic fertilizers. Furthermore, kinetic modeling provided valuable information on the temporal dynamics of phosphorus release, offering a scientific basis for designing more efficient phosphorus fertilization strategies.