Separating natural from human enhanced methane emissions in headwater streams

Headwater streams are natural sources of methane but are suffering severe anthropogenic disturbance, particularly land use change and climate warming. The widespread intensification of agriculture since the 1940s has increased the export of fine sediments from land to streams, but systematic assessment of their effects on stream methane is lacking. Here we show that excess fine sediment delivery is widespread in UK streams ( n = 236) and, set against a pre-1940s baseline, has markedly increased streambed organic matter (23 to 100 g m −2 ), amplified streambed methane production and ultimately tripled methane emissions (0.2 to 0.7 mmol CH 4 m −2 d −1 , n = 29). While streambed methane production responds strongly to organic matter, we estimate the effect of the approximate 0.7 °C of warming since the 1940s to be comparatively modest. By separating natural from human enhanced methane emissions we highlight how catchment management targeting the delivery of excess fine sediment could mitigate stream methane emissions by some 70%. The effects of fertiliser from intensive agriculture are well recognised, but not so well for fine-sediment. Here we show how widespread ingress of agriculturally derived fine-sediment since the 1940s markedly amplifies methane emissions from streams.