Or maybe "if you use redwood for the bed and something easier to decompose in the center, the redwood won't be much touched while the center transforms."
Sally Brown, a research assistant professor of soils at the University of Washington, knows her way around both food and metals. Starting out as a chef and then a food broker between farmers and restaurants, she became fascinated with soils and went on to earn a PhD in agronomy. Brown’s current research includes identifying the mechanisms by which organic residuals reduce the availability of soil metals to plants. She has some hard-earned opinions.
Brown says that if you already have the older, arsenic-treated wood in your garden, don’t panic. Plants will not take up arsenic unless the soils are deficient in phosphorus. That is not a problem for gardeners who use compost generously. As for the new copper-based wood treatments, Brown believes the actual risk is minimal. First of all, if plants take up too much copper, they will die before a gardener can eat them. In addition, if homegrown vegetables make up a small percentage of the diet, exposure to any metal taken up is insignificant. Do not use copper near ponds and streams because it is toxic to aquatic life.