We asked how organic matter pools and microbial communities vary across polygon types and soil layers in Arctic lowland tundra. Low-center polygons had lower microbial abundance, enzyme potential, and organic matter bioavailability. Organic topsoils were microbial hotspots, but thaw may also rapidly mobilize carbon from upper permafrost. Patterns consistently followed gradients in organic matter and redox conditions, offering a practical framework for scaling landscape-scale soil carbon changes.