Topography-dependent mechanisms underlying the impacts of nitrogen addition on soil methane uptake in a subtropical karst forest

Abstract

The influences of atmospheric nitrogen (N) deposition and topographical position on soil methane (CH4) uptake have been intensively investigated; however, it remains unclear whether topographic position regulates the responses of soil CH4 uptake to N deposition. Here, we examined the effects of N addition on soil CH4 uptake in the valley and on the slope of a subtropical karst forest. Control (0 kg N ha−1 yr−1), moderate N addition (50 kg N ha−1 yr−1), and high N addition (100 kg N ha−1 yr−1) treatments were included at each topographic position. CH4 fluxes were determined from 2017 to 2019 along with potential methane production rate (PMPR), potential methane oxidation rate (PMOR), mcrA and pmoA gene abundances, and plant and soil properties. The averaged annual CH4 uptake under the control was 0.67 ± 0.07 kg CH4 ha−1 yr−1 in the valley and 0.99 ± 0.32 kg CH4 ha−1 yr−1 on the slope across the three years. High N addition consistently increased CH4 uptake in the valley in 2017 and 2019 and on the slope in 2017 and 2018. The annual CH4 uptake was stimulated by high N addition by 81.27 % in the valley owing to inhibited PMPR caused by increased NO3−, and to stimulated PMOR caused by increased NO3− and soil organic carbon. On the slope, annual CH4 uptake was enhanced by high N addition by 75.83 % attributed to inhibited PMPR caused by increased fine root biomass and to stimulated PMOR induced by increased soil phosphorus availability. Our study highlights the importance of topography in regulating soil CH4 uptake under elevated N addition.