The microbial mobilisation of sulphur and phosphorus in biochar amended soils

Biochar amended soils have demonstrated various agronomic benefits, principally a plant growth promotion effect. The char’s impact, however, on macronutrient bio-availability and the microorganisms involved in their transformation remain largely elusive. This is particularly the case with sulphur (S) and phosphorus (P). Plants rely on microorganisms to mineralize organic and inorganically bonded S and P which the plant can then readily utilise. The aim of this study was to investigate the role of S and P mobilizing bacteria in the observed plant growth promotion effect associated with biochar amendment. The bacterial genera associated with S and P mobilisation in Irish soils were isolated from the rhizosphere of Zea mays. This study revealed a broad metabolic plasticity in agriculturally important genera such as Pseudomonas, Burkholderia and Rhizobium. Replicate pot experiments of Lolium perenne, Hordeum vulgare and Lycopersicon esculentum were established on S and P limited soil. Pot soils were treated with 1 or 2% biochar (w/w) (Miscanthus giganteus) or were kept biochar-free as a control. After growth of approximately 3 months the plant shoots were harvested. Both biochar amendments resulted in a significant growth promotion effect (P<0.01) with all plants tested. Rhizospheric soil was sampled and bacteria capable of mobilising a) S from aromatic sulphonates, b) P from phosphate-esters, c) P from phosphonates, and d) P from tri-calcium-phosphates as sole source of S or P, respectively were quantified. A significant increase in the abundance of all S and P mobilising bacteria (P<0.05) was associated with both biochar treatments compared to the control for all plants, suggesting increased plant growth through enhanced nutrient mobilisation. Denaturing Gradient Gel Electrophoresis of the 16S bacterial community profiles revealed a clear separation between both biochar treatments and the control. Clone library analysis of the community asfA and phnJ genes of the rhizosphere of Lolium perenne revealed broad sequence diversity in bacterial desulphonating and phosphonate mineralizing capabilities. XANES and 31P NMR analysis demonstrated that S and P were retained in biochar during the pyrolysis process, indicating the char may act as a source of these essential macronutrients. These findings indicate that biochar amendment supports enhanced microbially-mediated nutrient mobilisation of S and P resulting in enhanced plant growth.