I purchased a sack of biochar a while back from what I would consider the most reputable source in my state. I took one section of a bed and incorporated biochar into the soil, the other section received no biochar. I applied compost and azomite to both sections. Kynon oats were then planted in both sections.
It looks as though the biochar has locked up some nutrient as the oats which received no biochar performed much better. I used biochar on some different crops but didn't see this type of pronounced affect. It's also possible there was gopher damage to the one end where the biochar was applied. If the biochar did cause this though, then it might not be a great soil amendment for short term results. At the very least it would be good to know what nutrient was being locked up and apply that along with the biochar.