Softwood biochar improves soil (bio)chemistry, microbial community structure and function, and Sulla coronaria growth in mediterranean acidic soils

This study evaluated the potential of a softwood biochar (B, added to soil at 5% w/w) to improve the chemical and biological properties of an acidic soil (pH=5.2) of a typical Mediterranean subhumid dryland silvopastoral ecosystem. Our hypothesis was that biochar can be a complementary or alternative material to lime to buffer soil acidity, thereby enabling the establishment and growth of acid-sensitive but drought-tolerant forage legumes in Mediterranean grazed grasslands. The experiment also included treatments with lime (L, added at 0.2% w/w), a combination of B+L (added at 4.8% + 0.2% w/w respectively) and an untreated control soil (Ctr). Soil pH increased by 1.0 unit after biochar addition and significant increases were also recorded for cation exchange capacity, dissolved organic carbon, and available phosphorus. The urease, β-glucosidase and phosphatase activities increased in B-treated soil by ∼2.20, 1.03 and 1.06-fold respectively vs Ctr, while the dehydrogenase activity decreased by ∼30%. Biochar (alone and with lime) increased soil microbial biomass and basal respiration (on average by ∼2.75 and 1.6-fold vs Ctr, respectively) and had a significant impact on soil culturable microorganisms and community structure as assessed with Biolog Ecoplates. Sequencing of the partial 16S rRNA gene showed that all amendments augmented the bacterial α-diversity, with biochar increasing the relative abundance of taxa including several potential plant growth-promoting bacteria. The results from a pot experiment and a field trial showed that biochar, alone and with lime respectively, promoted significant increases in the growth and forage production of Sulla coronaria, an acid-sensitive, drought resistant perennial forage legume. Over two growing seasons, the forage production in the field trial was + 50% higher for B+L (4700 kg ha−1) vs Ctr (3100 kg ha−1) highlighting the suitability of biochar, in combination with lime or as possible alternative, to improve soil health and productivity of acidic dryland environments.