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.