Effects of sugar cane bagasse biochar and spent mushroom compost on phosphorus fractionation in calcareous soils

In the present study we investigated the effects of using sugar cane bagasse biochar and spent mushroom compost (SMC) on different fractions of phosphorus and plant availability in three calcareous soils with a loam, clay loam and sandy loam texture. The incubation experiment was performed using a completely randomised design, with five treatments (B1 and B2 (15 and 30 t biochar ha–1 respectively), SMC1 and SMC2 (20 and 40 t SMC ha–1 respectively) and C (control)) and three incubation periods (14, 60 and 120 days) over three replicates. The different P fractions evaluated in the soil were Olsen P, dicalcium phosphate (Ca2-P), octacalcium phosphate (Ca8-P), aluminium phosphate (Al-P), iron phosphate, occluded phosphate and apatite. Application of SMC at both levels increased Olsen P, whereas biochar application was less effective. SMC2 increased Olsen P by 473%, 227% and 89% in clay loam, loam and sandy loam soils respectively. In addition, for all soils and all incubation times, SMC1 and SMC2 significantly increased Ca2-P compared with C, which had an increasing trend with time, but biochar only increased Ca2-P significantly in sandy loam soil. SMC2 also increased Ca8-P and Al-P at 120 days. In conclusion, application of SMC can enhance plant-available P and affect P fractions and distribution, with the degree of the increase being soil specific. In contrast, the effects of biochar on P availability, fractions and distribution need more time to become apparent.