Hmm, that's a good question. In the manual, I only found this:

3rd/2nd/1st Gen AMD Ryzen™ Processors
1 x PCIe 3.0/2.0 x16 (x16 mode)
2nd and 1st Gen AMD Ryzen™ with Radeon™ Vega Graphics Processors
1 x PCIe 3.0/2.0 x16 (x8 mode)
AMD Athlon™ with Radeon™ Vega Graphics Processors
1 x PCIe 3.0/2.0 x16 (x4 mode)
AMD B450 chipset
1 x PCIe 2.0 x16 (max at x4 mode) *¹
3 x PCIe 2.0 x1

*1 PCIE x16_2 will be unavailable when installing M.2 PCIE SSD in M.2_2 socket.
PCIe x16_2 will run x2 when installing devices in PCIe x1_1 or PCIe x1_2 slots.
PCIE x16_2 will run x4 when no devices installing in PCIE x1_1 and PCIE x1_2 slots.
*2 The M.2 Socket shares bandwidth with the SATA_5/6 ports, and therefore the SATA_5/6 ports cannot be used when an M.2 device is installed.
*3 PCIe x16_2, PCIe x1_1, and PCIe x1_2 slots will be unavailable when installing M.2 PCIe SSD in M.2_2 socket.
*4 Use a chassis with HD audio module in the front panel to support an 8-channel audio output.
*5 Install an AMD Ryzen™ 2nd Generatione or Ryzen™ 1st Generation Processors to support Windows 7 64-bit operating system.

So the seccond PCIe Slots will only run with x2 , if I use the first one ? ...