Evaluation of the long‐term effect of biochar on properties of temperate agricultural soil at pre‐industrial charcoal kiln sites in Wallonia, Belgium

Summary

This field study exploited a natural long-term experiment at pre-industrial charcoal kiln sites in Belgium to evaluate the persistence and soil effects of biochar over 150+ years. Charcoal residues significantly elevated soil organic C:N and C:P ratios, increased cation exchange capacity (particularly for charcoal-C), and enhanced exchangeable Ca²⁺ and Mg²⁺ contents, whilst having minimal effect on available phosphorus but forming strong complexes with copper. The findings highlight that biochar is highly persistent in soil and that long-term implications of biochar amendment warrant careful consideration in agricultural management.

UK applicability

The study's temperate climate and soil types are broadly analogous to southern and central UK conditions, making the findings relevant to UK agricultural policy considering biochar as a soil amendment. However, UK soils may have different baseline properties, and the absence of lime application (common in Belgium) in some UK systems could alter the pH dynamics and thus the relative effects of biochar.

Key measures

Charcoal-C content (differential scanning calorimetry, 1.8–33.1 g kg⁻¹); organic C:N and C:P ratios; soil organic carbon; total N; nitrates; pH; cation exchange capacity (CEC; 414 cmol_c kg⁻¹ for charcoal-C versus 213 cmol_c kg⁻¹ for uncharred SOC); exchangeable cations (K⁺, Ca²⁺, Mg²⁺); available, inorganic and total phosphorus; copper complexation

Outcomes reported

The study characterised soil properties at 17 pre-industrial charcoal kiln sites in Wallonia where biochar residues had accumulated over 150+ years, comparing them with adjacent reference soils. Measurements included charcoal-C content, organic matter, nutrient status (N, P, K, Ca, Mg), cation exchange capacity, pH, and trace metal complexation.

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