Biochar application to soil alleviates nutrient limitation, drought stress, and soil hardness, while improving aboveground productivity in a Mediterranean sown pasture

Background and aims Acidic-coarse granitic-soils dominate North and Central Portugal, their productivity being limited by nutrient limitations and Al/Mn toxicity, low water retention causing drought stress, and soil hardness. Al ³⁺ /Mn ²⁺ induce P-immobilization and antagonize Ca ²⁺ /Mg ²⁺ uptake, high sand content limits water retention, and hard-setting creates a cement-like structure that limits root growth. We hypothesized that biochar incorporation would alleviate these three limitations and improve pasture productivity and quality. Methods 10 plots with woody biochar (4% w/w) and 10 controls in randomized block design field experiment. Results Contrary to common findings, biochar alleviated nutrient limitation and improved soil nutrient status not by increasing available nutrients (except K ⁺ ), rather by decreasing antagonist soil metal concentrations, their antagonism on Ca ²⁺ /Mg ²⁺ uptake, and their ratios against available P. Decrease in Al ³⁺ occurred due to liming effect, whereas that of Mn ²⁺ and Fe ²⁺ /Fe ³⁺ is not fully clear. Biochar increased available K, total N, and SOM, while increasing K/decreasing Mn grass uptake, increasing forb N and S uptake, but decreasing legume Mg and Zn tissue content. SMC was increased in the root zone (31%) but decreased in deep layers of biochar treatment, whereas extreme drought stress was completely eliminated during critical flowering period. Biochar reduced soil hardness (29-68%) in deep layers but not in top-soil, and increased aboveground productivity from 6 to 9 t ha ⁻¹ , increasing grass and legume, but not forb biomass, and increasing protein yield (52%). Conclusion Biochar offers a multifunctional solution for acidic-coarse drought-affected granitic-soils, reducing needs for costly separate measures (dolomitic limestones/mechanical treatments/irrigation) while substantially increasing fodder quantity and quality.