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Journal of Arid Land
Journal of Arid Land  2025, Vol. 17 Issue (8): 1103-1117    DOI: 10.1007/s40333-025-0025-7    
Research article     
Effects of acidified municipal waste and coffee ground biochars, and sodium bentonite on soil potassium equilibration and release
Mahdi NAJAFI-GHIRI1,*(), Hamid Reza BOOSTANI1, Niloofar SADRI2
1Department of Soil Science, College of Agriculture and Natural Resources of Darab, Shiraz University, Darab 71956-15735, Iran
2Department of Soil Science, College of Agriculture, Shiraz University, Shiraz 71956-15735, Iran
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Abstract  

In addition to sequestering carbon in soil, biochars can also play a role in changing the potassium equilibration and dynamics of the soil. Nowadays, acidification of biochar is commonly used to improve its properties, which can impact the potassium content in the soil. Simultaneous application of acidified biochar and sodium bentonite can complicate this effect. In the present study, the effects of adding two types of biochars prepared from municipal waste and used coffee grounds and their acidified types, along with sodium bentonite at three levels (0.00%, 1.00%, and 2.00%), on soil physical-chemical properties (pH, salinity, cation exchange capacity, concentration of soluble cations and their ratio, and sodium adsorption ratio) and the release of potassium from a calcareous soil were investigated. The results showed that the addition of coffee ground biochar increased the concentration of soluble potassium and decreased the ratio of calcium to potassium, while the acidified coffee ground biochar decreased the amount of soluble potassium and increased the ratio of calcium to potassium. Alkaline and acidified municipal waste biochars had no effect on soluble potassium and soluble cations ratio. Application of bentonite increased the amount of soluble calcium and sodium and the ratio of calcium to potassium. Addition of bentonite also increased the amount of exchangeable potassium and exchangeable sodium percentage, but use of different biochars reduced negative effect of bentonite. Use of bentonite also caused an increase in the exchangeable potassium and a decrease in the non-exchangeable potassium contents. Alkaline and acidified coffee ground biochars increased the amount of exchangeable, non-exchangeable, and total potassium, but this effect was greater by alkaline biochar. Application of municipal waste biochar did not affect the amount of exchangeable potassium but increased the amount of non-exchangeable and total potassium, with no significant difference observed between alkaline and acidified biochars. Potassium saturation percentage was not affected by bentonite, but coffee ground biochar increased its amount and municipal waste biochar had no effect on it. Acidified and alkaline coffee ground biochars were able to release more potassium from the soil (475 and 71 mg/kg, respectively), while alkaline municipal waste biochar did not affect it and acidified municipal waste biochar reduced it by 113 mg/kg. In general, it can be concluded that alkaline biochars in calcareous soils can improve potassium fertility by reduction of the ratio of calcium to potassium and increasing its various forms, while acidified biochars and bentonite may aggravate potassium deficiency in these soils. Considering the lack of significant change in the pH of calcareous soils with the use of different biochars, it is suggested to use alkaline biochars, which can improve the potassium status of the soil while reducing the costs associated with biochar modification.

Key words： calcium to potassium ratio      potassium forms      sodium adsorption ratio      calcareous soils      potassium fixation     
Received: 19 April 2025      Published: 31 August 2025
Corresponding Authors: *Mahdi NAJAFI-GHIRI (E-mail: mnajafighiri@shirazu.ac.ir)
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Mahdi NAJAFI-GHIRI
Hamid Reza BOOSTANI
Niloofar SADRI
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Mahdi NAJAFI-GHIRI, Hamid Reza BOOSTANI, Niloofar SADRI. Effects of acidified municipal waste and coffee ground biochars, and sodium bentonite on soil potassium equilibration and release. Journal of Arid Land, 2025, 17(8): 1103-1117.

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http://jal.xjegi.com/10.1007/s40333-025-0025-7     OR     http://jal.xjegi.com/Y2025/V17/I8/1103

Property Soil Municipal waste biochar Acidified municipal waste biochar Coffee ground biochar Acidified coffee ground biochar Bentonite
pH 7.85 8.50 5.64 8.50 3.62 8.20
EC (dS/m) 0.17 0.42 0.24 0.48 0.40 1.00
CEC (cmol/kg) 12.3 10.3 22.9 10.9 12.7 76.0
Sand (%) 45.00 - - - - -
Silt (%) 40.00 - - - - -
Clay (%) 15.00 - - - - -
CCE (%) 44.00 - - - - -
Carbon (%) 1.00 10.60 6.20 77.00 74.00 -
Hydrogen (%) - 0.30 0.80 4.30 4.50 -
Nitrogen (%) - 0.60 0.70 4.40 4.80 -
Potassium (mg/kg) 629 1560 1480 15,700 2160 -
Table 1 Physical-chemical properties of the used soil, biochar, and bentonite
Source of
variation		 df pH EC CEC Soluble
Ca		 Soluble Mg Soluble
Na		 Ca/K ratio K/Na ratio SAR Exch. Na ESP
Factor A 2 0.014** 0.007** 15.800** 2036.00** 28.1ns 2251.00** 0.58** 80.10** 0.401** 27,042** 18.00**
Factor B 4 0.019** 0.012** 0.064* 65.30** 549.0** 6.17** 0.22** 11.10** 0.003** 508** 0.44**
A×B 8 0.004** 0.001** 0.002ns 5.33* 397.0** 3.74* 0.01** 2.19** 0.001** 306** 0.26**
Table 2 Analysis of variance (ANOVA) for the effects of bentonite (A), biochar (B), and their interaction on soil properties, soluble cations, exchangeable Na, and ESP
Amendment Bentonite pH EC
(dS/m)		 Soluble cation (mg/L) Ca/K ratio K/Na ratio SAR
Ca K Na Mg
Control B0 7.85a-d 0.17h 19.4gh 13.9de 11.3d 83cd 1.40e 1.2b 0.17d
B1 7.86a-c 0.26c-e 36.2e 12.9ef 29.0c 89cd 2.81c 0.4de 0.42bc
B2 7.93a 0.27b-e 49.6bc 11.7g 39.0b 84cd 4.25b 0.3f 0.55a
Municipal waste
biochar		 B0 7.75de 0.31ab 23.0g 14.9d 12.2d 62f 1.55e 1.2b 0.21d
B1 7.85a-d 0.33a 37.3de 14.0de 27.8c 94bc 2.67c 0.5d 0.39c
B2 7.90ab 0.31ab 53.2ab 13.2e 43.8a 111a 4.03b 0.3f 0.55a
Coffee ground
biochar		 B0 7.77c-e 0.25de 21.4gh 26.1a 11.5d 79de 0.82f 2.3a 0.18d
B1 7.76c-e 0.29a-d 38.3de 23.6b 28.5c 63f 1.62e 0.8c 0.46b
B2 7.74e 0.30a-d 50.2a-c 22.1c 40.3ab 68ef 2.28d 0.5d 0.61a
Acidified municipal
waste biochar		 B0 7.79c-e 0.18gh 17.5h 13.8de 10.5d 101ac 1.27e 1.3b 0.15d
B1 7.91ab 0.22e-g 35.5e 12.9ef 26.5c 93b-d 2.75c 0.5d 0.37c
B2 7.95a 0.25de 46.7c 12.0fg 39.4b 87cd 3.89b 0.3f 0.55a
Acidified coffee ground biochar B0 7.79c-e 0.20f-h 30.0f 10.3h 11.0d 92b-d 2.91c 0.9c 0.16d
B1 7.77c-e 0.20f-h 41.8d 9.9h 28.7c 85cd 4.22b 0.3e 0.41bc
B2 7.81b-e 0.24ef 54.4a 9.8h 43.7a 88cd 5.55a 0.2f 0.60a
Table 3 Soil characteristics and soluble Ca, K, Na, and Mg status as influenced by interaction between bentonite and biochar treatments
Fig. 1 Effects of bentonite and biochar on cation exchange capacity (CEC). B0, B1, and B2 indicate 0.00%, 1.00%, and 2.00% bentonite, respectively. MWB, municipal waste biochar; CGB, coffee ground biochar; AMWB, acidified municipal waste biochar; ACGB, acidified coffee ground biochar. Different lowercase letters within the same treatment indicate significant difference at P<0.05 level. Bars are standard errors. The abbreviations are the same in the following figures and tables.
Fig. 2 Effects of bentonite and biochar on exchangeable Na (a) and exchangeable sodium percentage (ESP). Different lowercase letters within the same treatment indicate significant difference at P<0.05 level. Bars are standard errors.
Source of
variation		 df K form Percentage of K form KSP K
desorption		 K
release
Soluble Exch. Non-exch. Nitric acid Soluble Exch. Non-exch.
Factor A 2 272.0** 2915.0** 2644.0** 185.0ns 3.67** 61.1** 34.8** 3.67** 6468ns 1.79*
Factor B 4 4322.0** 57,784.0** 37,102.0** 222,523.0** 18.80** 108.9** 165.5** 18.80** 301,796** 34.80**
A×B 8 21.4** 30.9ns 44.1ns 81.5ns 0.19* 0.8* 0.5ns 0.19* 4415ns 3.18**
Table 4 ANOVA for the effects of bentonite (A), biochar (B), and their interaction on potassium (K) fractionations and release
Fig. 3 Effect of bentonite (a) and biochar (b) on concentrations of different K forms. Different lowercase letters with the same K forms indicate significant difference at P<0.05 level among different bentonite or biochar treatments. Bars are standard errors. Exch., exchangeable.
Fig. 4 Effects of bentonite and biochar on percentages of soluble K (a), exchangeable K (b), and non-exchangeable K (c). Different lowercase letters indicate significant difference at P<0.05 level among different bentonite and biochar treatments. Bars are standard errors.
Fig. 5 Effects of bentonite and biochar treatments on K saturation percentage (KSP). Different lowercase letters indicate significant difference at P<0.05 level among different bentonite and biochar treatments. Bars are standard errors.
Fig. 6 K release as influenced by biochar treatments. Different lowercase letters indicate significant difference at P<0.05 level among different biochar treatments. Bars are standard errors.
Bentonite Control Biochar treatment
MWB CGB AMWB ACGB
(mg/kg)
B0 49.1bc 49.5bc 51.8ab 48.4cd 48.2cd
B1 48.0cd 49.6bc 53.2a 44.7e 47.6cd
B2 47.7cd 47.3cde 53.5a 45.8e 49.2bc
Table 5 Effects of bentonite and biochar treatments on cumulative K release
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