Low-Level Nitrogen Addition Promotes Net Methane Uptake in a Boreal Forest across the Great Xing' an Mountain Region, China

Chronic nitrogen (N) deposition may alter the amount and direction of methane (CH4) uptake in boreal forest soils, but the critical level of N deposition eliciting the alteration of soil CH4 uptake remains unknown. In a cold-temperate coniferous forest in the Great Xing' an Mountain of northeastern China, NH4NO3 fertilizer was added at four rates: control (0 kg N ha(-1) year(-1)), low N (10 kg N ha(-1) year(-1)), medium N (20 kg N ha' year(-1)), and high N (40 kg N ha(-1) year(-1)). Soil atmospheric CH4 fluxes as well as soil temperature, soil moisture, mineral N, dissolved organic carbon (DOC), and pH values were determined to explore the main factors controlling soil CH4 uptake fluxes under different N addition levels. Our results showed that N addition did not alter soil temperature, soil moisture, and soil NH4+-N concentrations. Low N rather than medium and high N significantly increased soil DOC and NO3--N concentrations in mineral horizon as well as soil CH4 uptake fluxes. In addition, soil acidification occurred in the fertilized forest soils, because soil pH declined by 0.4 unit. CH4 uptake in the cold-temperate forest soil was dominated by soil moisture followed by soil DOC and soil mineral N. Low N seemed to stimulate soil CH4 uptake through increasing the contents of soil DOC and NO3--N. These results suggest that low-level N addition (< 20 kg N ha-1 year(-1)) can promote CH4 uptake in the cold-temperate coniferous forest soil, which is also conducive to carbon sequestration of the boreal forests