Forest soils host bacteria that pull methane (a powerful greenhouse gas) from the air before it warms the planet. Over recent decades this natural filter has weakened sharply, and the standard explanation blames rising rainfall. Drawing on 27 years of field measurements, we show that rainfall cannot explain the loss. The timing and pattern trace the cause instead to the bacteria and the soil they live in. Such a breakdown may not heal on its own, leaving more methane in the air.