Officers initially detained the owner of the restaurant for allegedly serving unsanitary food, but the dead businessman's family refused to believe it was food poisoning and pressed police to make further checks.
Local official Huang Guang has now been arrested on suspicion of poisoning the dish with a toxic herb called Gelsemium elegans. Police say the pair had fallen out over money.
Mr Long attended the lunch to discuss plans to develop a piece of woodland, according to state newspaper Nanfang Daily.
One of the men present said the meal tasted "more bitter" than usual.
Businessman Mr Long reported feeling sick and dizzy before going into fatal cardiac arrest.
His family has posted a 100,000 yuan (£10,100) reward for further information about his death.