Hardwood biochar and manure co-application to a calcareous soil

Biochar may improve nutrient retention when applied to soils, so co-applying biochar with manure may be synergistically beneficial to soils. In a laboratory incubation study, dairy manure (2% by weight) and a hardwood-based, fast pyrolysis biochar was applied (0, 1, 2, and 10% by weight) to a calcareous soil. Destructive sampling occurred at 1, 2, 3, 4, 5, 6 and 12 months, and monitored for changes in soil chemistry, water content, microbial respiration, bacterial populations, and microbial community structure. Increasing biochar application rate improved the soil water content, which may be beneficial in limited irrigation or rainfall areas. Biochar application increased soil organic carbon content and plant-available iron and manganese, while a synergistic biochar-manure effect increased plant-available zinc. Compared to the other rates, the 10% biochar application lowered concentrations of nitrate-nitrogen; effects appeared masked at lower biochar rates due to manure application. Over time, soil nitrate-nitrogen increased likely due to manure N mineralization, yet the 10% biochar rate limited excessive soil nitrate-nitrogen accumulation as compared to other treatments. In the presence of manure, the 10% biochar application caused subtle microbial community structure shifts by increasing the relative amounts of two fatty acids associated with Gram-negative bacteria and decreasing Gram-positive bacterial fatty acids, each by ~1%. The 10% biochar application rate, co-applied with 2% manure, appeared to prevent excess mineralization; co-application may lead to more efficient N use without having a large effect on the soil microbial community