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Article

Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications

by
Meng Wang
1,2,3,
Jim J. Wang
2,*,
Jong-Hwan Park
2,3,
Jian Wang
4,
Xudong Wang
5,
Zuoping Zhao
1,6,
Fengmin Song
1,6 and
Bo Tang
1,6
1
College of Chemical and Environment Science, Shaanxi University of Technology, Hanzhong 723001, China
2
School of Plant, Environmental and Soil Science, Louisiana State University, 104 Sturgis Hall, Baton Rouge, LA 70803, USA
3
Department of Life Resources Industry, Dong-A University, Busan 49315, Korea
4
School of Humanities, Shaanxi University of Technology, Hanzhong 723001, China
5
College of Natural Resources and Environment, Northwest A&F University, Xianyang 712100, China
6
Qinba Mountains of Bio-Resource Collaborative Innovation Center of Southern Shaanxi Province, Hanzhong 723001, China
*
Author to whom correspondence should be addressed.
Agronomy 2022, 12(8), 1923; https://doi.org/10.3390/agronomy12081923
Submission received: 28 June 2022 / Revised: 24 July 2022 / Accepted: 12 August 2022 / Published: 15 August 2022
(This article belongs to the Topic Advances in Sustainable Agri-Food Systems: Insights into Production, Processing, and Consumption Perspectives)
Browse Figures
Review Reports Versions Notes

Abstract

:
Alkali-enhanced biochars, as an environment-friendly material, combine the advantages of biomass nutrients and carbon fixation. In this study, rice-residue-derived biochars were evaluated for P and C solubility and their amendment upon plant P uptake. Biochars from rice straw (RS) and husk (RH), including raw biochar without alkaline pretreatment (0B), alkali-enhanced biochars with KOH (5KB, 5 g KOH per 100 g feedstock;10KB, 10 g KOH per 100 g feedstock), K2CO3 (5K2B, 5 g K2CO3 per 100 g feedstock; 10K2B, 10 g K2CO3 per 100 g feedstock), and CaO (5CB, 5 g CaO per 100 g feedstock; 10CB,10 g CaO per 100 g feedstock) were prepared at 350 °C~550 °C pyrolysis conditions. Alkali-enhanced biochars on soil water soluble P(WSP) and C(WSC) levels were assessed through a soil-biochar incubation experiment. The effect of alkali-enhanced biochar on rice P uptake was evaluated in a greenhouse pot study. The WSP content in KOH- and K2CO3-enhanced biochars produced at 550 °C was significantly increased by up to 144% compared with that produced by the corresponding biochars at 350 °C, while the WSC content in all alkali-enhanced biochars (except for RS-5CB) prepared at 550 °C significantly decreased by up to 6426% compared with that produced by the corresponding biochars at 350 °C. The application of 3% 10KB and 10K2B rice straw biochars (produced at 550 °C) significantly elevated the WSP content in soils. Rice grown in the RH-10K2B-550 treated soil significantly increased the grain P uptake by 15% and 8% compared with RH-0B-350 and RH-10K2B-350, respectively. The water soluble P of the KOH- and K2CO3-enhanced biochars increased with increasing the pyrolysis temperature. RS-10KB and RS-10K2B increased the soil WSP and WSC content compared with the unenhanced biochar (RS-0B), and showed a clear positive effect on increasing the rice P uptake. Overall, KOH- and K2CO3-enhanced biochars pyrolyzed at 550 °C as Si sources could also serve as a potential P pool with multi-functions in C sequestration and K nutrition.

1. Introduction

Phosphorus is an essential element for plant growth, but 80% of applied P fertilizers transfer to an unavailable form for plant utilization because of precipitation, sorption, and microbial immobilization [1]. Its deficiency in soil is one of the main factors restricting 50% of world crop production [2,3]. Thus, repeated P fertilization is required to maintain an appropriate soil available P level to meet the needs of plant uptake. Phosphorus fertilizers are mainly produced from phosphate rocks, which are non-renewable and could be depleted in the next 75~400 years [4,5,6]. Thus, the current situation of global food production depending on single P resources needs to be resolved urgently [7]. In addition, repeated P fertilization and enrichment in topsoil could increase the potential risk of water eutrophication [8].
Recovering P from various P-rich sources such as biosolids from animal and municipal waste for use as fertilizers has been extensively studied [9,10]. Recently, studies have also confirmed that biochars could gradually release P and increase soil P availability; therefore, biochar could serve as a phosphorus pool for soils [11,12,13]. The pyrolysis process of converting biomass waste materials has remained most of the inorganic phosphorus in biochar with a relatively higher P availability, and could provide a durable source of P to soil compared with the crop residues [14,15]. On the other hand, the pyrolysis temperature was found to significantly affect phosphorus transformation, regardless of biomass materials [16]. As the pyrolysis temperature increased from 250 to 650 °C, the total P and Olsen P biochar contents reached a maximum at 650 °C for both canola stalk and rice straw biochar [17]. Besides conventional pyrolysis, alkali-enhanced biochar has also been developed for use as an alternative Si source, which has been shown to have a positive effect as an environment-friendly disease control for perennial ryegrass and/or turfgrass [18,19]. However, the effect of pyrolysis temperature on alkali-enhanced biochar as a phosphorus reservoir has not yet been evaluated.
Moreover, dissolved organic carbon (DOC) in biochars play an important role in stimulating microbial activity and improving microbial abundance [20]. Biochar water extractable organic C (WSC) is affected by pyrolysis conditions, feedstock types, and the chemical pretreatments of biomass such as H3PO4 or KOH [21]. Rostad et al. [22] noted that the DOC content in pine and switchgrass biochars increased as the pyrolysis temperature decreased from 900 to 250 °C. The biochar produced at a low pyrolysis temperature showed a higher labile organic C level, and due to the competitive reaction between phosphorus and the low molecular weight organic acid and anions, the sorption of phosphorus by soil could be reduced [23]. As biochar is increasingly used as a multi-functional source, the effect of pyrolysis temperature along with alkali treatments on C solubility in bio-based biochar needs to be further investigated.
Currently, there is no study that has evaluated the potential of pyrolysis temperature on increasing nutrient availability in alkali-enhanced biochar as a multi-functional bio-based fertilizer. However, as alkaline solutions are known to increase the solubility of phosphorous and organic C [24,25], we hypothesize that the alkali regent retreatment of biomass to produce Si-rich biochar could also influence the release of P and C from the resulting biochar, which would subsequently affect the dynamics of these elements in the soil. Therefore, the primary objective of this study was to investigate the impact of pyrolysis temperature on P and C solubility of alkali-enhanced biochars, as well as their application effects on the behavior of P and C in soils.

2. Materials and Methods

2.1. Alkali-Enhanced Biochar Preparation

Rice straw (RS) and husk (RH) collected from Louisiana State University AgCenter Research Stations (Crowley, St. Gabriel, LA, USA, and Baton Rouge, LA, USA, respectively) were used for the biochar preparation. The collected feedstock samples were washed using deionized water to remove the dust and were dried at 60 °C for 24 h. The dried rice straw and husk biomass samples were ground using a high-speed rotary cutting mill and passed through a 1 mm sieve. For making the biochar, rice straw or husk were mixed with three alkali chemicals (KOH, K2CO3, or CaO), respectively, at an alkali/biomass ratios (on weight basis) of 0:100, 5:100, and 10:100 in a porcelain crucible, followed by adding 100 mL of ultra-pure water (based on 50 g of biomass), which were allowed to dissolve with an alkali reagent for 90 min. The crucibles were then placed in a muffle furnace under N2 flow at 400 mL min−1 for 30 min to remove the air from the system. The mixtures were dried with a muffle furnace temperature set at 180 °C for 30 min, followed pyrolysis at 350, 450, and 550 °C, respectively, for 60 min under an N2 flow rate of 200 mL min−1 [18]. The produced biochar samples were cooled and ground to pass through a 1 mm sieve before characterization. For convenience of discussion, these resulting biochars are referred as 0B (no alkali pretreatment), 5KB, and 10KB (pretreatment with KOH); 5K2B and 10K2B (pretreatment with K2CO3); and 5CB and 10CB (pretreatment with CaO), respectively.

2.2. Biochar Characterization

The biochar sample pH was determined based on a 1:100 biochar to deionized water ratio. The ash content was determined using a furnace at 550 °C for 5 h. The C and N contents were determined using a C/N elemental analyzer (Elementar Analysen systeme GmbH, Langenselbold, Germany). The total P, K, Ca, Mg, Cu, and Fe were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, SPECTRO Plasma 3200, Kleve, Germany) after the samples were digested using 68–70% nitric acid and 30% H2O2 (Huang and Schulte 1985).

2.3. Biochar and Soil Water Soluble P and C

The biochar and deionized water were mixed at a solid to liquid ratio of 1:20, and were shaken at 120 rpm for 3 h. The samples were filtered through a 0.45 μm filter membrane. Water soluble C (WSC) was determined using a TOC analyzer (TOC-VCSH, Shimadzu, Torrance, CA, USA), and the water soluble P (WSP) was analyzed by a colorimetric method at 880 nm with a SPECTRONIC 501 spectrophotometer (Thermo Scientific, Wilmington, DE, USA).
The effects of the alkali-enhanced biochars on the soil WSP and WSC levels were assessed through a soil-biochar incubation experiment. The 550 °C-prepared rice straw biochar samples of RS-0B, RS-5KB, RS-10KB, RS-5K2B, RS-10K2B, RS-5CB, and RS-10CB, along with RS, were added to two different acid soils. Thirty grams of Briley silt loam (pH 5.8, loamy, siliceous, semi active, thermic Arenic Paleudults) and Commerce silt loam (pH 5.2, fine-silty, mixed, super active, thermic Fluvaquentic Endoaquepts) were complexed with biochar samples at a 0, 1, and 3% application rate, respectively. Incubation was maintained at a 70% field water-holding capacity for 32 days. The experiments were replicated twice. The incubated samples were extracted by deionized water at a 1:10 soil to solution ratio for 1 h shaking, and then filtered through quantitative Whatman filter paper. The filtrates were measured using a TOC analyzer (TOC-VCSH, Shimadzu, Columbia, SC, USA) for the soil WSC content and a SPECTRONIC 501 spectrophotometer (Thermo Scientific, Wilmington, DE, USA) at 880 nm for the soil WSP.

2.4. Rice Greenhouse Study

The rice (Jupiter) potting study was conducted in a greenhouse at Louisiana State University, USA. The soil used in the potting study was Crowley silt loam (pH 7.6), collected from the Louisiana Agricultural Center Rice Research Station, Rayne, Louisiana, USA, which contained Mehlich-III extractable P and K of 10.42 and 68.97 mg kg−1, respectively. Based on a preliminary evaluation of the nutrient levels, eight types of biochar sources based on combinations of two feedstocks (RS and RH), two pyrolysis temperatures (350 °C and 550 °C), and two levels of alkali pretreatments (0B and 10K2B) were used. They were designated as four alkali-enhanced RS biochars (RS-0B-350, RS-10K2B-350, RS-0B-550, and RS-10K2B-550) and four alkali-enhanced RH biochars (RH-0B-350, RH-10K2B-350, RH-0B-550, and RH-10K2B-550), along with the control (CK), RS, and RH. The potting study was carried out using a completely randomized block design. Each source of these materials was applied to 1.5 kg soil at 0.22% (equivalent to 5 t ha−1) with three replicates. All of the pots were also treated with N 0.0357 g kg−1 (80 kg N ha−1) as urea, P2O5 0.0302 g kg−1 (68 kg P2O5 ha−1) as superphosphate, and K2O 0.0302 g kg−1 (68 kg K2O ha−1) as potash at the beginning of the trial. Each pot contained two rice seeds and was placed in a greenhouse on 12 May 2017. A second N application of 0.025 g kg−1 (56 kg ha−1) was conducted on 3 June for each pot. All of the pots were flooded on 7 June and harvested on 21 September. The rice grain, stem, and leaves were washed with deionized water and dried at 60 °C for 48 h. The total P and K were measured by inductively coupled plasma atomic emission spectrometry (ICP-AES, SPECTRO Plasma 3200, Germany) after the samples were digested using HNO3-H2O2 (Huang and Schulte 1985).

2.5. Statistical Analysis

All of the statistical analyses were performed using the Statistical Analysis Software, version 9.0 (SAS Institute, Cary, NC, USA). The pyrolysis temperature and alkali level treatment effects on biochar and soil soluble P and C were analyzed using one-way ANOVA based on the GLIMMIX procedure. When an ANOVA test was significant, individual treatment level effects were assessed by Duncan’s multiple range test at a p < 0.05 level.

3. Results and Discussion

3.1. Characteristic of Alkali-Enhanced Biochars at Different Preparation Temperatures

The basic physical and chemical properties of alkali-enhanced biochars could be found elsewhere [26]. Additional nutrient contents are shown in Tables S1 and S2. In general, the biochar yield decreased with increasing the pyrolysis temperature and increased with increasing the alkali application amount. The biochar pH and ash content also increased with increasing the temperature and alkali application. The increasing biochar pH was likely caused by the conversion of alkaline cations such as Ca, Mg, K, and Na to oxides, hydroxides, and carbonates during the pyrolysis process [27,28,29]. In addition, the removal of acidic functional groups from feedstock biomass resulted in the biochar being more basic as the pyrolysis temperature was increased [30]. Increasing the pyrolysis temperature concentrated the total nutrient contents due to the losses of easily decomposable substances and volatile compounds [31], and the addition of KOH, K2CO3, and CaO also changed the biochar elemental compositions by increasing the pyrolysis temperature [26]. As the alkali ratio increased, the biochar C and N content decreased but the total K contents increased in the KOH- or K2CO3-pretreated biochars, which could function as K sources in the soil biochar amendment. The CaO pre-treatment also increased the total Ca content of the prepared biochars.

3.2. Water Soluble P Content of Alkali-Enhanced Biochar at Different Preparation Temperatures

As shown from Figure 1A, after increasing the pyrolysis temperature, the WSP content of the KOH- and K2CO3-enhanced rice straw biochars increased and reached a maximum value at 550 °C. The WSP content of RS-10K2B-550 was 40.8 and 19.6% higher than the 350 and 450 °C-prepared biochar samples, respectively. In Figure 1B, the changing trend of the rice husk biochar WSP was similar to that of the rice straw biochar. The WSP content of RH-5K2B did not fluctuate significantly with increasing the temperature, and remained at 620 mg kg−1, which was 310~324% higher than for the RH feedstock. The generally higher WSP contents of the 5KB, 10KB, and 10K2B rice straw and husk biochars that increased with increasing the pyrolysis temperature were similar to those observed for the higher available P content of rice husk biochars prepared from 300 to 700 °C without any alkali pretreatment of the biomass [32]. The disproportionate volatilization of carbon, which leads to cleavage of organic phosphorus bonds, was found to be the main cause for the increased soluble P in biochar with the increasing pyrolysis temperature [33]. Our results suggest alkali pretreatment with KOH and K2CO3 further enhanced the available P in the produced biochars compared with the unenhanced pristine biochars.
While increasing the pyrolysis temperature generally concentrated the total P in the biochar from 350 to 550 °C (Table S1), likely due to the lower volatility of P in comparison with C, N, O, and S [34], the loss in total P content could also occur above 800 °C due to the volatilization of P-containing compounds near 760 °C. On the other hand, other studies have shown that plant-available P in biochar declined when increasing the temperature above 700 °C [35,36,37] and attributed the P unavailability to the crystallization of ortho-P with insoluble mangnesic, ferric, and calcic phosphates during the pyrolysis process [35,38]. However, the P content of CaO-enhanced rice straw biochar decreased significantly with increasing the temperature and alkali application ratio (Figure 1). The WSP content of CaO-enhanced bichar was lower than that of the unenhanced biochar (0B) and other alkali-enhanced biochar. This was likely due to the addition of a large amount of Ca2+ that could react with phosphorus in the biochar to form poorly soluble phosphate precipitation [39].

3.3. Water Soluble C Content of Alkali-Enhanced Biochar at Different Preparation Temperatures

The WSC content of all biochar treatments decreased as the pyrolysis temperature increased, except for RS-5CB and RH-10CB (Figure 2), indicating that the solubility of carbon in the biochar was generally reduced and the structure became more stable. This result was consistent with Xiao et al. [40] and Lin et al. [21]. Organic carbon in the biochar was lost due to phase conversion from solid to liquid, and then gaseous, during pyrolysis [41].
Under the same feedstock and pyrolysis temperature, the WSC content increased with increasing the addition of KOH and K2CO3, at 350 and 450 °C, as opposed to the unenhanced biochars (0B). Previously, the addition of KOH or K2CO3 was found to accelerate the hydrolysis reaction of the ester groups formed by cross linking between the lignin and cellulose on biochar surfaces, which led to the enhancement of the dissolution of humics during the pyrolysis process [21,42]. Interestingly, the KOH- and K2CO3-enhanced biochars had comparable WSC contents to the 0B biochar samples at 550 °C, indicating the relatively greater stability of biochar C at this pyrolysis temperature. As biochar C is known to be dominated with a more stable aromatic structure at 550 °C [43], this result suggests that these alkali pretreatments (KOH and K2CO3) of biomass had less of an impact on biochar C stability when the 550 °C pyrolysis temperature was used. However, CaO amendment and pyrolysis did not have a consistent effect on the WSC content of CaO-enhanced rice straw and husk biochars.

3.4. Effect of Alkali-Enhanced Biochar Amendment on Water-Soluble P in Acid Soil

The 550 °C-prepared KOH- and K2CO3-enhanced biochars were evaluated in soil amendment treatments as they showed higher WSP and lower WSC levels. The application effects of the alkali-enhanced biochars on the soil WSP varied with soil, with the WSP content of Briley silt loam soil being generally higher than that of Commerce silt loam soil (Figure 3). For Commerce silt loam soil, the WSP contents of RS-10KB and RS-10K2B were 100.6~264.1% and 348.6~1034.2% higher than FS and 0B at 1% and 3% application rates, respectively, whereas the WSP contents of RS-5CB and RS-10CB were similar to those of FS and 0B. For the Briley silt loam soil, the WSP contents of RS-10KB and RS-10K2B were significantly higher than for the other treatments at 1 and/or 3% application rates. Specifically, the soil WSP content of RS-10KB was 12.7~124.1% higher than that of FS and 0B at 1 and 3% application rates. However, the soil WSP content of CaO-enhanced biochars was significantly lower than FS and 0B.
The soil application of biochars made from P-enriched feedstocks was previously reported to increase the plant-available P content in soil [44,45]. Biochar amendment in acid soil could cause the dissolution of phosphates bound with Fe3+ and Al3+ in the soil, as well as phosphate bound with Ca2+ and Mg2+ in the biochar itself, which led to the overall release of plant-available P [3]. In addition, the application of Na2SiO3 was found to stimulate the secretion of organic acids in plant roots, which enhanced the activity of root transporter protein and ultimately led to an increased P content in wheat leaves and stems [46]. Thus, alkali-enhanced biochars could have both positive Si and biochar effects on increasing the plant-available P in soil.
As shown from Table 1 and Table 2, in two acid soils, the soil-available Si and WSP was significantly and positively correlated. While biochar amendment increased soil pH [18] and promoted the release of iron and aluminum-bound phosphorus P in soil [47,48], similar chemical properties of Si and P as H3SiO4- and H2PO4-, respectively, would compete for adsorption onto the soil colloid surfaces. As the applied Si occupied the adsorption sites of iron/aluminum oxides or clay mineral surfaces in soil, it inhibited the adsorption and/or fixation of P in soil, thereby increasing the overall P desorption rate [49]. On the other hand, as expected, the WSP content of Briley silt loam soil treated with CaO-enhanced RS biochar was significantly lower than that of RS feesdstock and pristine biochar (0B), due to the formation of poorly soluble calcium phosphate precipitates [39]. This result was consistent with the decreased WSP content of CaO-enhanced biochar (Figure 1).

3.5. Effect of Alkali-Enhanced Biochar Amendment on Water-Soluble C in Acid Soil

Figure 4 shows that the soil WSC content of 10KB treatment was 43.7~172.9% higher than 0B at both application rates in Commerce silt loam soil. In addition, 10KB and 10K2B significantly increased the soil WSC content compared with other treatments at a 3% application rate in Briley silt loam soil. Specifically, the soil WSC content of 10KB treatment increased by 48.0~153.3% compared with FS, 0B, and 5KB at a 3% application rate, and 10K2B increased by 28.8~82.8% compared with FS, 0B, and 5K2B in the Briley silt loam soil. For CaO-enhanced biochar, the soil WSC content of biochar treatments (0B, 5CB, and 10CB) was significantly lower than that of FS feedstock treatment (Figure 4).
Soil WSC is a small portion of soil organic carbon, but plays a pivotal role in many soil microbial activities [50,51,52]. As shown in Table 1 and Table 2, a significant positive correlation existed between the soil available Si content and WSC content after feedstock and alkali-enhanced biochar amendments in Commerce and Briley silt loam soil. An increased soil WSC content could promote microbial growth and increase soil CO2 release rate [53,54]. On the other hand, biochar amendment could increase soil pH [28,29], and a 3% application rate was more effective for soil pH improvement than 1%. The increase in pH may lead to the deprotonation of weakly acidic functional groups of water-soluble C compounds, thus increasing the hydrophilicity and charge density of active organic carbon, ultimately improving the solubility of soil WSC [55]. This likely explained the results that WSC levels measured in soils treated with alkali-enhanced biochars were higher than those with unenhanced biochar (0B) observed in this study (Figure 3 and Figure 4). As WSC is closely related to soil labile carbon [56], its increase through application alkali-enhanced biochars could indicate an improvement in soil health, which is important for sustainable crop production [57]. In contrast, in the CaO-biochar amendment treatments, the soil WSC content was lower than the corresponding KOH- and K2CO3-enhanced biochar. This was probably because CaO-enhanced biochar increased the soil Ca2+, which could complex WSC compounds, resulting in a decrease in soil WSC content.

3.6. Effect of Alkali-Enhanced Biochar Application on Plant P and K Uptake

The results of rice potting study for assessing actual plant P uptake under different feedstocks and biochars are shown in Figure 5. The straw and grain P contents of the rice growing in pots treated with RS and RH-derived biochars were all significantly higher than that of the control (CK). The grain P uptake of RS-derived biochar amendments was slightly greater than those of RH-derived biochars, whereas the straw P uptake showed the opposite trend. Specifically, RH-0B-550 and RH-10K2B-550 treatments yielded 3.13 and 3.17 g kg−1 P contents in the rice grain, which were significantly higher than the RH-0B-350 and RH-10K2B-350 treatments by 7.9% and 13.4%, as well as higher than the control (CK) and RH feedstock amendment by 19~21%, respectively. To the best of our knowledge, this study is the first to report that alkali (K2CO3)-enhanced biochar significantly elevated rice straw and grain P levels, emphasizing the positive effect of enhanced Si-source biochar on P uptake. Other studies have also shown that pristine biochars could increase P uptake in lotus tissue [58], wheat shoots [59], and ryegrass shoots [60]. However, it was clear that K2CO3-enhanced RS and RH biochars at an equivalent of 5 t ha−1 significantly elevated rice straw and grain P compared with the control, besides improving Si uptake [26].
Rice straw and grain K uptake with different biochar and feedstock treatments are shown in Figure 6. Straw K content of RS and RH feedstock amendments was similar to that of control (CK), while biochar treatments were all significantly higher than CK. Specifically, RS-10K2B-350 and RS-10K2B-550 treatments yielded 28~39% and 31~43% greater K uptake in rice straw than the amendments with RS-0B-350 and RS-0B-550, respectively. The RH-10K2B-350 and RH-10K2B-550 treatments also had 18~45% higher straw K uptake than that of CK, RH, RH-0B-350 and RH-0B-550. On the other hand, RS-10K2B-350 and RS-10K2B-550 treatments yielded significantly greater K uptake in rice grain by 19~20% than that of CK and RS. Although K uptake by both straw and grain in the potting study were similar among amendments of K2CO3-enhanced biochars and greater than unenhanced 0B biochars, regardless of pyrolysis temperature variations, there was a significant difference between rice straw and grain K uptake with K content being much higher in straw than in grain (Figure 6). This was likely due to K located in stem and blade other than grain [61]. As stalk or stem K concentration is positively correlated to stalk bending resistance [61,62], the application of K2CO3-enhanced biochars could increase plant resistance to lodging besides the benefits of Si in preventing crop disease.

4. Conclusions

Pyrolysis temperature and alkali pre-treatment showed a great influence on biochar P and C solubility. Water-soluble P of KOH- and K2CO3-enhanced biochars increased with increasing the pyrolysis temperature, whereas their WSC decreased. The CaO-enhanced biochar showed an inconsistent effect. Soil amendment of KOH-enhanced biochars increased the soil WSP content compared with the unenhanced biochar (0B). The soil WSP and WSC contents of RS-10KB were 101~492% and 44~173% higher than RS-0B at 1% and 3% application rates in Commerce silt loam soil. On the other hand, RS-10KB and RS-10K2B significantly increased the soil WSC contents compared with the unenhanced biochar (RS-0B). Potting study also demonstrated that K2CO3-enhanced biochar prepared at both 350 and 550 °C could significantly increase rice straw and grain P uptake. Based on the P release from the water extraction of these biochar sources, it is estimated that an application rate of 1 metric ton per hectare of RS-10KB and RH-10K2B produced by 550 °C pyrolysis temperature has the potential to input 0.032 and 0.303 kg (or 4 and 35%) more WSP over RS-0B into soil, while the same rate of amendment with RH-10KB and RH-K2B at 550 °C could have 0.311 and 0.343 kg (or 115 and 126%) more WSP than RH-0B, respectively. The actual extent of the WSP increase requires the calibration for the specific soil type and alkali-biochar application rate. Overall, KOH- and K2CO3-enhanced biochars, particularly 10KB and 10K2B, showed a clear positive effect on increasing rice P uptake, in addition to benefits as a source of Si and K for plant growth.

Supplementary Materials

The following supporting information can be downloaded at: https://www.mdpi.com/article/10.3390/agronomy12081923/s1. Table S1: Chemical and physical properties of alkali-enhanced rice straw biochars prepared at different pyrolysis temperatures. Table S2: Chemical and physical properties of alkali-enhanced rice husk biochars prepared at different pyrolysis temperatures.

Author Contributions

Conceptualization, M.W. and J.J.W.; methodology, J.W. and J.-H.P.; software, M.W. and X.W.; validation, J.J.W., and X.W.; formal analysis, Z.Z. and J.J.W.; investigation, Z.Z. and F.S.; resources, J.J.W.; data curation, J.W. and B.T.; writing—original draft preparation, M.W.; writing—review and editing, J.J.W. and J.-H.P.; supervision, J.J.W.; project administration, J.J.W.; funding acquisition, M.W. and J.J.W. All authors have read and agreed to the published version of the manuscript.

Funding

This work was, in part, supported by the Louisiana Board of Regents Support Fund #LEQSF(2017-18)-RD-D02 and #LEQSF(2019-20)-RD-D-01; the USDA National Institute of Food and Agriculture Hatch Project #1013888; the Shaanxi University of Technology Talent Startup Program (SLGRC19), Hanzhong, Shaanxi, China; and the Scientific Research Foundation of Education Department of Shaanxi province (20JY008).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

The data presented in this study are available upon request from the corresponding author. The data are not publicly available due to patent application.

Conflicts of Interest

The authors have no conflicts of interest to declare that are relevant to the content of this article.

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Figure 1. Water soluble P content of alkali-enhanced (A) rice straw and (B) husk biochar prepared at different pyrolysis temperatures. Note: In the figure, subgraph (A1–A3,B1–B3) indicates KOH, K2CO3, and CaO of alkali treatments of rice straw (RS) and rice husk (RH), respectively; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Here, 350, 450, and 550 °C indicate the pyrolysis temperatures. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 1. Water soluble P content of alkali-enhanced (A) rice straw and (B) husk biochar prepared at different pyrolysis temperatures. Note: In the figure, subgraph (A1–A3,B1–B3) indicates KOH, K2CO3, and CaO of alkali treatments of rice straw (RS) and rice husk (RH), respectively; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Here, 350, 450, and 550 °C indicate the pyrolysis temperatures. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Figure 2. Water soluble C content of alkali-enhanced (A) rice straw and (B) husk biochar prepared at different pyrolysis temperatures. Note: In the figure, subgraph (A1–A3,B1–B3) indicates KOH, K2CO3, and CaO of alkali treatments of rice straw (RS) and rice husk (RH), respectively; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Here, 350, 450, and 550 °C indicate the pyrolysis temperatures. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 2. Water soluble C content of alkali-enhanced (A) rice straw and (B) husk biochar prepared at different pyrolysis temperatures. Note: In the figure, subgraph (A1–A3,B1–B3) indicates KOH, K2CO3, and CaO of alkali treatments of rice straw (RS) and rice husk (RH), respectively; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Here, 350, 450, and 550 °C indicate the pyrolysis temperatures. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Figure 3. Water soluble P content of Si source amendments at applications of 1 and 3% in Commerce and Briley silt loam soil. Note: In the figure, subgraph (A–C) indicates KOH, K2CO3, and CaO of the alkali treatments, respectively. FS, rice straw feedstock; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 3. Water soluble P content of Si source amendments at applications of 1 and 3% in Commerce and Briley silt loam soil. Note: In the figure, subgraph (A–C) indicates KOH, K2CO3, and CaO of the alkali treatments, respectively. FS, rice straw feedstock; 0B, 5KB/K2B/CB, and 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Figure 4. Water soluble C content of Si source amendments at application rates of 1 and 3% in Commerce and Briley silt loam soil. Note: In the figure, subgraph (A–C) indicates KOH, K2CO3, and CaO of alkali treatments, respectively. FS, rice straw feedstock; 0B, 5KB/K2B/CB, 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 4. Water soluble C content of Si source amendments at application rates of 1 and 3% in Commerce and Briley silt loam soil. Note: In the figure, subgraph (A–C) indicates KOH, K2CO3, and CaO of alkali treatments, respectively. FS, rice straw feedstock; 0B, 5KB/K2B/CB, 10KB/K2B/CB indicate the biochars prepared at the proportion of KOH/K2CO3/CaO to feedstock of 0:100, 5:100, and 10:100. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Figure 5. Rice P uptake with alkali-enhanced (A) rice straw and (B) husk biochar amendment. Note: CK, non-amendment; RS, rice straw; RH, rice husk; RS/RH-0B-350 and RS/RH-10K2B-350 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 350 °C, respectively; RS/RH-0B-550 and RS/RH-10K2B-550 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 550 °C, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 5. Rice P uptake with alkali-enhanced (A) rice straw and (B) husk biochar amendment. Note: CK, non-amendment; RS, rice straw; RH, rice husk; RS/RH-0B-350 and RS/RH-10K2B-350 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 350 °C, respectively; RS/RH-0B-550 and RS/RH-10K2B-550 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 550 °C, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Figure 6. Rice K uptake with alkali-enhanced (A) rice straw and (B) husk biochar amendment. Note: CK, non-amendment; RS, rice straw; RH, rice husk; RS/RH-0B-350 and RS/RH-10K2B-350 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 350 °C, respectively; RS/RH-0B-550 and RS/RH-10K2B-550 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 550 °C, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
Figure 6. Rice K uptake with alkali-enhanced (A) rice straw and (B) husk biochar amendment. Note: CK, non-amendment; RS, rice straw; RH, rice husk; RS/RH-0B-350 and RS/RH-10K2B-350 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 350 °C, respectively; RS/RH-0B-550 and RS/RH-10K2B-550 are rice straw/husk biochars prepared with the proportion of K2CO3 to feedstock at 0 and 10 at 550 °C, respectively. Different letters indicate significant differences within treatment (p < 0.05) based on Duncan’s multiple range test.
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Table
Table 1. Correlation analysis between soil available Si, water soluble P, and water soluble C after Si source amendments (1 and 3% application rate).
Table 1. Correlation analysis between soil available Si, water soluble P, and water soluble C after Si source amendments (1 and 3% application rate).
Soil TypeIndicatorsCorrelation Coefficient
Si(X)
Commerce silt loam soilP(Y)Y = 0.1382X − 4.7082
R2 = 0.6284 **
C(Y)Y = 1.5712X + 170.1
R2 = 0.3399 **
Briley silt loam soilP(Y)
C(Y)		Y = 0.5277X + 12.039
R2 = 0.4487 **
Y = 4.7794X + 243.76
R2 = 0.4049 **
Note: ** p < 0.01; Si indicates soil available Si; P indicate soil water soluble P; C indicates soil water soluble C.
Table
Table 2. Correlation analysis between soil available Si, water soluble P, and water soluble C in both Commerce and Briley silt loam soil after Si source amendments (1and 3% application rate).
Table 2. Correlation analysis between soil available Si, water soluble P, and water soluble C in both Commerce and Briley silt loam soil after Si source amendments (1and 3% application rate).
IndicatorsCorrelation Coefficient
Si(X)
P(Y)Y = 0.3022X + 5.4358
R2 = 0.1576 **
C(Y)Y = 2.9806X + 216.59
R2 = 0.201 **
Note: ** p < 0.01; Si indicates soil available Si; P indicate soil water soluble P; C indicates soil water soluble C.
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Wang, M.; Wang, J.J.; Park, J.-H.; Wang, J.; Wang, X.; Zhao, Z.; Song, F.; Tang, B. Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications. Agronomy 2022, 12, 1923. https://doi.org/10.3390/agronomy12081923

AMA Style

Wang M, Wang JJ, Park J-H, Wang J, Wang X, Zhao Z, Song F, Tang B. Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications. Agronomy. 2022; 12(8):1923. https://doi.org/10.3390/agronomy12081923

Chicago/Turabian Style

Wang, Meng, Jim J. Wang, Jong-Hwan Park, Jian Wang, Xudong Wang, Zuoping Zhao, Fengmin Song, and Bo Tang. 2022. "Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications" Agronomy 12, no. 8: 1923. https://doi.org/10.3390/agronomy12081923

APA Style

Wang, M., Wang, J. J., Park, J. -H., Wang, J., Wang, X., Zhao, Z., Song, F., & Tang, B. (2022). Pyrolysis Temperature Affects Dissolved Phosphorus and Carbon Levels in Alkali-Enhanced Biochar and Its Soil Applications. Agronomy, 12(8), 1923. https://doi.org/10.3390/agronomy12081923

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