Properties of a sandy clay loam Haplic Ferralsol and soybean grain yield in a five-year field trial as affected by biochar amendment

Biochar may increase soil fertility and crop yield by enhancing soil organic carbon (SOC), nutrient availability and water retention. However, the mechanisms through which biochar effects crop yield at field scale and the time span over which biochar can affect soil properties and crop yield are still little known. The aim of this study was to evaluate the effect of wood biochar as soil amendment, combined with the application of synthetic fertilizer in a zero-tillage system, on soil physical-chemical properties and soybean grain yield in a 5-year field trial settled on a sandy clay loam Haplic Ferralsol of the Brazilian savannah. Immediately after biochar application (year 0), soil pH and plant available water (PAW) increased and Al3+ and bulk density (BD) decreased linearly with biochar application rate. Later, in year 5, biochar had no effect for PAW and BD increased with biochar rates. The SOC was not affected until the third year after application, but increased linearly with biochar rates at 4 and 5 years after its application. Macro-porosity (MAC) was affected in a quadratic manner reaching a maximum at 8 Mg ha(-1) biochar rate in years 0 and 2, however in year 5 it decreased with increasing biochar rates. Soybean grain yield increased with biochar rates until the third year after its application. There was almost no interaction effect of biochar and synthetic fertilizer; except for potassium (K+) availability and grain yield in year 3. Possible control mechanisms of biochar effect on soybean grain yield through its effect on soil fertility and physical-hydraulic properties are discussed. The effect of biochar on soybean yield depended on multiple independent factors that included soil and environmental variables. Because of this its effect on yield was variable through the years, which makes it rather unpredictable. Biochar, however, may have an important contribution to sustainable production through enhancing the effect of conservation soil management, like the use of green manure or mulching, on SOC accumulation.