Humid Subtropical Forests Constitute a Net Methane Source: A Catchment‐Scale Study. Issue 9 (11th September 2019)

• Record Type: Journal Article
• Authors: Yu, Longfei; Zhu, Jing; Zhang, Xiaoshan; Wang, Zhangwei; Dörsch, Peter; Mulder, Jan
• Is Part Of: Journal of geophysical research. Volume 124:Issue 9(2019)
• Page Start: 2927
• Page End: 2942
• Publication Date: 2019-09-11
• Subjects: CH4 emission -- CH4 uptake -- hydrological continuum -- methane budget -- groundwater discharge zone -- subtropical forest catchment; Geobiology -- Periodicals; Biogeochemistry -- Periodicals; Biotic communities -- Periodicals; Geophysics -- Periodicals; 577.14
• DOI: 10.1029/2019JG005210
• Languages: English
• ISSNs: 2169-8953

Abstract

Abstract: Commonly, well‐drained forest soils are net methane (CH4 ) sinks, whereas poorly drained soils in groundwater discharge zones could contribute significant CH4 emissions. Climate change is projected to bring more summer precipitation extremes to subtropics, which may affect forest CH4 balance. Chinese forests often have a hilly topography with well‐drained hillslopes and pronounced groundwater discharge zones. Although different landscape elements are likely to have different source and sink functions for CH4, the climatic influence on CH4 budget at the catchment scale remains poorly studied. Here, we measured CH4 fluxes over three years along a topographic gradient in a subtropical forested catchment in SW China. CH4 fluxes exhibited a clear spatial pattern indicating moderate CH4 uptake or emission from the well‐drained soils on the hillslope (−5.8 to +1.0 kg CH4 ‐C·ha −1 ·year −1 ) and significant CH4 emission from the wetter soils in the groundwater discharge zone (94.3 to 479.5 kg CH4 ‐C·ha −1 ·year −1 ). Despite its small area contribution to the catchment (0.6%), the groundwater discharge zone emitted substantial amounts of CH4 in monsoonal summers, affecting the whole‐catchment budget. Hillslope soils showed weaker CH4 uptake or even turned into a small CH4 source under wet climatic conditions. Estimates of catchment‐scale CH4 budgets indicate that subtropical forest catchments can tip from a net CH4 sink in drier years to a net CH4 source in wetter years, highlighting the importance of interannual climate variabilities. Our findings suggest that subtropical forests under projected climate change could contribute a net CH4 source with consequences for the regional CH4 balance. Plain Language Summary: Forest soils are commonly believed to uptake methane from the atmosphere, which is an important process to counteract the growth of atmospheric methane level. However, sparsely distributed wet soils in forests could contribute transient but large methane emissions to atmosphere. Under climate change, methane emission from wet soils may significantly change the role of forest ecosystems in regional methane budgets. Therefore, for three years, we measured soil‐atmosphere exchange of methane in upper drier and lower wetter soils at a subtropical forest in Southwest China. Under the influence of Eastern Asian monsoon, we observed strong methane emissions from the wetter soils of the forest during summers, despite that these areas only account for a small part of the forest. In a wet year, methane uptake in drier soils decreased and methane emission in wetter soils increased. Taking into account the contributions of both wetter and drier soils, estimates of annual methane budgets suggest that the studied subtropical forest contributes a net emission of methane to the atmosphere in a wet year. This work highlights an overlooked source of methane for the subtropical region and provides insights into the future role of forest ecosystem in greenhouse gas budgets under climate change. Key Points: Soils in the groundwater discharge zone of subtropical forest catchments emit substantial amounts of CH4 during monsoonal summers Well‐drained hillslope soils contribute a small CH4 sink or source depending on climatic conditions Despite small area of their groundwater discharge zones, subtropical forested catchments can act as a net CH4 source in a wet year