Effects of soil type and biochar rate on sweet pepper productivity and microbial dynamics in regenerative organic soils

Abstract

Biochar is widely used to improve crop productivity and soil health; however, its adoption remains limited due to insufficient quantification of benefits and unclear mechanisms, particularly within regenerative organic systems. We conducted a pot experiment applying biochar at 0 (control), 11.2, and 22.4 Mg ha⁻¹ to sandy loam and loam soils planted with sweet pepper ( Capsicum annuum L.) and measured plant performance, microbial biomass, and soil organic carbon (SOC). Yield was higher in sandy loam than loam (282 vs. 146 g plant⁻¹). Biochar increased loam yield from < 100 g plant⁻¹ (control) to ~ 240 g plant⁻¹ at 22.4 Mg ha⁻¹, while sandy loam yield plateaued beyond 11.2 Mg ha⁻¹. Microbial biomass was greater in sandy loam (6845 ng g⁻¹) than loam (3737 ng g⁻¹) and declined ~ 17% at the highest rate, from 5628 to 4682 ng g⁻¹. Arbuscular mycorrhizal fungi colonization peaked at the moderate rate and declined at the highest rate. Although biochar increased SOC, SOC related weakly to microbial biomass in sandy loam and a significant negative relationship in loam (declining from ~ 4200 to 2800 ng g⁻¹ as SOC rose from 1.8 to 5.0%), indicating that biochar-derived carbon was not readily available to soil microorganisms. Fruit nutrient concentrations were unchanged across treatments. These results show biochar effects are soil- and rate-dependent: moderate application (11.2 Mg ha⁻¹) improved productivity without suppressing microbial biomass, whereas the highest rate (22.4 Mg ha⁻¹) further improved loam yield but reduced microbial biomass and decoupled SOC accumulation from microbial activity, without improving fruit nutritional quality.