Phosphorus availability as a primary constraint on methane emission from a freshwater wetland

Human activities have increased phosphorus (P) loading in wetland ecosystems worldwide. However, little is known about the effect of P enrichment on CH4 emissions from these ecosystems. In this study, a 4-year P addition experiment was conducted to examine the effects of increased P availability on CH4 emission in a Deyeuxia angustifolia-dominated freshwater marsh in the Sanjiang Plain in northeastern China. Phosphorus was added at four rates (0, 1.2, 4.8 and 9.6 g P m−2 year−1). We investigated CH4 emission during the growing season (early May to late September) using opaque chamber and gas chromatography method. Our results indicated that the effect of P enrichment on CH4 emission was time-dependent. Increased P availability did not affect CH4 emission in 2007 and 2008, but decreased in 2009 and 2010. Notably, four years of P addition decreased cumulative CH4 emission during the growing season in the freshwater marsh, and the effect did not change with fertilization rates. From 2007 to 2010, P additions of 1.2, 4.8 and 9.6 g P m−2 year−1 caused a decline in growing-season CH4 emissions by averages of 23%, 38% and 26%, respectively. Our results suggest that long-term P enrichment driven by agricultural activities would reduce CH4 emission from temperate freshwater wetlands. This study also highlights the impact of experimental duration on accurate assessments of the effect of P addition on wetland CH4 budget.