Land Use, Stream Order, and Temperature Dependences of Methane Emissions across Subtropical Rivers in Southeast China

Rivers emit significant greenhouse gases due to global warming and human activities. The effects of land use and stream orders on methane (CH4) emissions remain poorly understood. Here, we investigated the patterns and magnitudes of CH4 emissions in subtropical rivers spanning land uses and stream orders in Southeast China. CH4 concentrations and fluxes ranged from 129 to 2131 nmol L-1 and from -0.56 to 140 μg m-2 h-1, respectively. Rivers in urban areas showed the highest CH4 concentrations and fluxes, which increased by 55% and 71%, respectively, compared to those in cropland and forest areas. Water variables explained 34% of CH4 variability, while land use and stream order explained 17.6% by mediating water variables. CH4 flux was positively correlated with N2O and CO2 fluxes, suggesting that NO3- and carbon respiration facilitated CH4 emissions. In addition, CH4 flux was temperature-dependent and was conservatively projected to increase by 54 ± 7.6% and 109 ± 9.2% under 2 and 4 °C warming scenarios, respectively. Therefore, land use, stream order, and water variables have pressured significant variability of CH4 emissions. This variability can be further enhanced in the context of climate warming and human activities, yet it is modified by river hydraulics and morphological characteristics.