Contribution of modified P-enriched biochar on pH buffering capacity of acidic soil

Beatrice Arwenyo; Jac J Varco; Andrew Dygert; Sydney Brown; Charles U Pittman; Todd Mlsna

doi:10.1016/j.jenvman.2023.117863

Contribution of modified P-enriched biochar on pH buffering capacity of acidic soil

J Environ Manage. 2023 Aug 1:339:117863. doi: 10.1016/j.jenvman.2023.117863. Epub 2023 Apr 18.

Authors

Beatrice Arwenyo¹, Jac J Varco², Andrew Dygert², Sydney Brown³, Charles U Pittman³, Todd Mlsna⁴

Affiliations

¹ Department of Chemistry, Mississippi State University, Mississippi State, MS, 39762, USA; Department of Chemistry, Gulu University, P O Box 166, Gulu, Uganda.
² Department of Plant and Soil Sciences, Mississippi State University, Mississippi State, MS, 39762, USA.
³ Department of Chemistry, Mississippi State University, Mississippi State, MS, 39762, USA.
⁴ Department of Chemistry, Mississippi State University, Mississippi State, MS, 39762, USA. Electronic address: tmlsna@chemistry.msstate.edu.

PMID: 37080104
DOI: 10.1016/j.jenvman.2023.117863

Abstract

Biochar can directly hold cations in soil because of the negative charge that exists on its surfaces. Besides, improving soil cation exchange capacity, the negative charges on biochar surfaces can buffer acid soil by protonation and deprotonation mechanisms. Moreover, biochar ameliorates soil acidity due to the presence of oxides, carbonates, and hydroxides of its basic cations (Ca, Na, K, and Mg). Both biochar surface functional group and basic cation concentrations can be altered by modification with chemical agents which can affect its soil pH buffering capacity. However, the impact of modified biochar application on soil pH buffering capacity is still scanty. This study investigated the pH buffering capacity of acidic soil amended with three P-enriched modified Douglas fir biochars and compared this buffering capacity to amendment with untreated Douglas fir biochar. These three P-enriched biochars, were prepared by treating Douglas fir biochar (DFB), respectively, with: 1) anhydrous calcium chloride (CaCl₂) and potassium phosphate monobasic (KH₂PO₄), 2) calcium carbonate (CaCO₃) and diammonium phosphate {(NH₄)₂HPO₄} and 3) an aqueous solution of magnesium sulfate (MgSO₄), potassium hydroxide (KOH) and potassium phosphate monobasic (KH₂PO₄). The three P-enriched biochars were designated as CCPP, CAPP and MSPP, respectively. The soil pH buffering abilities were largely dependent on the added biochar's alkalinity and ash contents. The residual soil CEC was highly correlated (r ≥ 0.9), with the soil buffering capacity. Both alkalinity and pH buffering capacity improved following the order CCAP > CCPP > MSPP > DFB, while residual soil CEC followed the order CAPP > MSPP > CCPP > DFB. The pH buffering capacity of the soil after amendments with 10% CAPP, CCPP MSPP and BFB rose by 84.8, 58.3, 3.0 and 2.5%, respectively. Whereas MSPP had higher concentrations of K⁺ and Mg²⁺, greater concentrations of Ca²⁺ were present in CCAP and CCPP than MSPP. So, Ca²⁺ concentrations in biochar exerts a greater influence on alkalinity and buffering capacity than Mg²⁺ and K⁺ because of 1) its smaller effective hydration radius and larger charge density. 2) calcium hydroxide has a greater water solubility than magnesium hydroxide providing more available base. Since pH buffering capacity depends on cation exchange sites, soil additives containing Ca²⁺ are prone to create greater impacts than Mg²⁺ and K⁺ additives.

Keywords: Alkalinity; Cation exchange capacity; Effective hydration radius; Soluble base cations.

MeSH terms

Cations
Charcoal / chemistry
Hydrogen-Ion Concentration
Soil Pollutants* / chemistry
Soil* / chemistry

Substances

biochar
potassium phosphate
Soil
Charcoal
Cations
Soil Pollutants