Three-year field trials with seven biochars reveal minor changes in soil chemical properties but no impact on crop yield

Biochar is frequently described as a “win-win” solution due to its potential to mitigate climate change while improving agricultural production. Intensive investigation has revealed that biochar can deliver agronomic benefits such as increases in soil water retention, fertility, and yield. However, benefits may be greatest in marginal soils such as those with low fertility, coarse-textured, contaminated, highly acidic, or receiving limited fertilizer and irrigation. While the agronomic benefits of biochar may be more limited in fertile soils, there is nevertheless a strong interest in applying it to prime soils for purposes other than crop production, such as climate change mitigation. The objectives of this study were to explore the impacts of biochar on processing tomato production in two fertile soils in a Mediterranean climate, and to evaluate the fertilizer rate, soil texture, and biochar production parameters and application rate which may optimize potential benefits and minimize negative consequences. Seven biochars, produced from different feedstocks at different temperatures, were amended to a silt loam and sandy loam in three-year field trials, at two application rates, in conjunction with two synthetic fertilizer rates. No biochar had a significant effect on processing tomato yield, crop ripening, or plant nitrogen (N) uptake at any rate in any year in any location. Minor increases in soil pH, EC, and mineral N concentration were observed in years 2 and 3, though effects were inconsistent across biochars, locations, and years. The findings suggest that biochar may confer limited agronomic benefits in fertile soils in Mediterranean climates, though its application resulted in minimal observed negative consequences. In similar contexts with comparable materials, biochar application for climate change mitigation may be pursued with minimal tradeoffs for agricultural production. • Biochars did not increase crop yield in two fertile soils in 3-year field trials. • Minor and inconsistent increases in pH, EC, and soil mineral N were observed. • Biochar may be better suited to deliver agronomic benefits in marginal soils. • No negative effects of biochar amendments were observed. • Climate change mitigation can be pursued in such contexts with minimal consequences.