Citric Acid Acidification of Wheat Straw Derived Biochar for Overcoming Phosphorus Deficiency in Soil

Phosphorus (P) is an irreplaceable element that has no substitute to sustain life on Earth [1]. Most importantly, it is obtained from limited phosphate rock resources [2]. In the soil, the available phosphorus of plants is relatively low because most of the phosphorus is usually combined with Calcium (Ca) and Magnesium (Mg) in calcareous soils and combined with Iron (Fe) and Aluminum (Al) in acid soils, Resulting in a decrease in the availability of phosphorus in this plant [3]. The fixed phosphorus in the soil is prone to runoff loss, leading to eutrophication of freshwater bodies [4].

Therefore, under acidic and alkaline soil conditions, a large amount of phosphate fertilizer (>90%) becomes unavailable, and the effectiveness of phosphorus in farmland systems becomes the primary limiting factor limiting phosphate fertilizer [5]. Therefore, exploring advanced phosphorus recycling technologies and management strategies is inevitable. These technologies and management strategies can provide phosphorus in the form available to plants and reduce the loss of available phosphorus to meet the increasing demand for phosphorus and food [6]. One option to promote phosphorus management in alkaline soils is to use biochar, a carbon-rich solid product at high temperatures, by exposing organic waste (such as wood chips, crop residues, or fertilizers) under anaerobic conditions [7]. Applying biochar to the soil can replace or partially reduce the use of inorganic phosphate fertilizers and may be one of the lowest costs, most efficient and most sustainable methods to save P resources and prevent P pollution [8].

In a recent Meta study with 108 pairwise comparisons of peer-reviewed studies on biochar effects on P availability in varying soils suggested that: P availability was increased by a factor of 5.1 and 2.4 is acidic (pH<6.5) and neutral (pH 6.5-7.5) soils, respectively and no significant effect was observed in alkaline soils [6]. Thus, the biochar amendment effects on P availability are inconsistent and regulated by changes in soil chemical properties, specifically soil pH. Recently, biochar feedstock/biochar modifications have been proposed to produce biochar with superior characteristics and improve its agronomic benefits [9,10]. However, chemical modifications using Citric Acid (CA) are not well studied. Therefore, improving our understanding of the potential effects of citric acid-modified biochar on P processes in soil and plant bioavailability in agricultural systems is mandatory.

The current study aimed to evaluate how biochar produced from wheat straw, and their chemical modification, treating with 0.01 N-Citric acid, and P application (control No P application or yes, P at 250mgkg^-1 soil (in the form of KH₂PO₄.3H₂O solution) affect P availability under alkaline soil condition.

Biochar production and properties

Figure 1:Scheme for the preparation of CA-modified biochar.

Wheat straw was crushed into powder and dried to constant weight at 65°C in a thermo-ventilated oven and then pyrolyzed in a muffle furnace at atmospheric pressure by applying 400°C for 1h30 under limited oxygen condition in a 250-c. The detailed preparation process of AWSB described above is shown in Figure 1. The resulting biochar materials were grounded and sieved through a 250μm square-mesh sieve prior to analysis. pH and Electrical Conductivity (EC) were measured in distilled water at 1:10 biochar to water mass ratio after shaking for 30 min according to ASTM D1762-84 2007 (Standard test method for chemical analysis of wood charcoal [11]. Biochar Organic Carbon Content (OC) was determined by the Walkley-Black method. For the determination of total P in biochars, the samples were digested in sulfuric acid. Later, the P concentration was measured following the method of [12] by using a using spectrophotometer at 882nm.

Incubation experiment

Table 1:Treatment details used for the experimentation.

The top 0-30cm composite soil samples were collected from a deprived area. The soil was sandy, neutral, calcareous in nature (CaCO3~15%) and poor in organic matter as well as in available P. The incubation experiments included ten treatments, and each treatment was carried out with 100g portions of the air-dried soil packed uniformly in plastic cups. In eight treatments, each soil was amended with one of biochar’s produced: acidified or no-acidified biochar at rates of 4%, and 8 % (w/w) of initial soil in dry weight, with or without addition of KH₂PO₄ at a P rate of 250mgkg^-1 soil. Two controls without biochar input, one with the unfertilized soil and the other with only KH₂PO₄, were included. The different treatments used were as follows (Table 1). In the experimentation, two replicates of each treatment were prepared, randomly placed and incubated in the laboratory at ambient temperature 25± 2°C and 80% soil moisture (v/w) for 9, 18, 27, 36, 45 and 54 days. At the end of each incubation period, samples (~20g) were removed from each plastic cup and analyzed for available P (Olsen-P).

Data analysis

Average and stranded deviation were calculated. Statistical analysis was performed using Stat-soft Statistica Software, 10^th Edition (Stat-soft, Tulsa OK). In addition, Duncan’s multiple range test, which combines means of similar values into ordered homogenous groups, was applied.

The wheat straw that was charred at 400 °C yielded an alkaline biochar (pH= 8.9), rich in OC (55.4 %), moderately low in available P (9mgkg-1) and total P (68.64mgkg^-1). However, a higher EC value was found (26.12dSm-1), indicating the existence of water-soluble salts. In this experiment, the CA modification/ amelioration of biochar focused on enhancing P availability and its half-life in low-P soils. It is clearly demonstrated that application of 4-8% biochar significantly increased P availability in studied soil. All biochar treatments individually or in combination with or without phosphorus input yielded a significant increase in the available P in studied sandy soil. When compared with the control, content of available P in soil samples increased significantly (p<0.001) after amendment with WSB compared with the controls without biochar amendment (Table 2).

Incubation experiment

Table 2:Evolution of soil available P of different treatments during the incubation time.

Values represent mean of two replicates ± SD. Values followed by different letters are significantly differences among each other as determined by the DMR test (P<0.05). ***Stands significant at P<0.001.

The addition of biochar alone without P amendment increases the availability of P in soil. Our results are similar to previous studies which indicated that the addition of biochar increases the availability of P even without adding P [13]. In addition, the amount of soil available P in AWSB1 and AWSB2 amended treatments is higher compared to the control (without biochar input) and even compared with WSB treatments (Figure 2). This could probably be attributed to the acidification of soil/biochar by citric acid which can compete strongly with P for adsorption sites by blocking phosphate sorption sites on soil thereby reducing phosphate adsorption [14]. Organic acids have been reported to strongly enhance the mobilization of P from soils [15,16]. The low pH of the soil plays a vital role in the dissolution of immobilized soil P [17]. Addition of organic acids to the soil caused an immediate pH drop and the extent of pH decrease depended on both soil type and organic compound [18,14]. According to Hopkins et al. [19], the decrease of soil pH (6.5) will increase the mobility of fixed soil P. In the case of low soil pH, the activity of H+ will decompose the bond of calcium and phosphorus in the calcareous soil. Breaking of the bond between calcium and phosphorus leads to the dissolution and availability of phosphorus in the soil [20]. In addition, P is also part of the biochar structure [21]. As observed in the AWSB of this study, lowering the pH of biochar also increases the release of phosphorus from the biochar into the soil solution [22]. Biochar improves P availability in calcareous soils mainly due to changes in soil microbial community [23,24]. Indirect application of biochar can also promote the secretion of phosphomonoesterase by soil microorganisms, thereby enhancing the mineralization of soil (Paz-Ferreiro et al., 2012). According to Laird [25], the application of biochar can increase the Cation Exchange Capacity (CEC) of the soil by 20%. The CEC is an indirect measure that can increase the retention rate of water and nutrients by reducing its leaching loss. The modification of various bio-sorbents (such as biochar) by citric acid may change its quality by introducing additional carboxyl groups on its surface Wang et al., 2014 & [26], thereby increasing the availability of soil and nutrients availability [10].

Biochar production and properties

Figure 2:Effect of wheat straw biochar acidification (yes/no) and phosphorous application (no application or 250mgPkg-1 soil) on P availability under alkaline condition. Bars represent means ± S.E and values followed by different letters are significantly differences among each other as determined by the Duncan’s test (P<0.05).

Results of the incubation experiment showed that the acidification of biochar exerted a positive effect by enhancing the availability of soil P. A greater Olsen’s P availability was obtained with acidified bio-chars instead of non-acidified ones. The capacity for a reduction in P adsorption and increase in P availability appears to follow the order of AWSB treatments > WSB treatments > control treatments. This indicates that treating biochar with an acidifying agent such as citric acid is effective enough to reduce the rate of orthophosphate conversion to unavailable form. However, these findings need to be further confirmed by field experiments over the long term, considering the possible adverse effects of biochar application at high rates.

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