Humid Subtropical Forests Constitute a Net Methane Source: A Catchment‐Scale Study

Longfei Yu,Jan Mulder,Jing Zhu,Peter Dörsch,Xiaoshan Zhang,Zhangwei Wang

doi:10.1029/2019jg005210

Abstract

AbstractCommonly, 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.

Highlights

Methane (CH4) is an important greenhouse gas, and its global warming potential is 28 times that of carbon dioxide (Ciais et al, 2013)
Our findings suggest that subtropical forests under projected climate change could contribute a net CH4 source with consequences for the regional CH4 balance
While small fluctuating CH4 fluxes at the well‐ drained hillslope and foot of hillslope indicated a balance between moderate CH4 uptake and production, significant CH4 emissions were found in the wetter valley bottom and water‐saturated groundwater discharge zone

Summary

Introduction

Methane (CH4) is an important greenhouse gas, and its global warming potential is 28 times that of carbon dioxide (Ciais et al, 2013). The sink strength of soils for CH4 varies greatly (Kirschke et al, 2013) and most soils can turn into transient CH4 sources under warm and wet conditions (Bridgham et al, 2013; Melton et al, 2013), making estimates of global CH4 budgets highly uncertain. Regardless of the common understanding of forest soils as net CH4 sink, CH4 budgets of forest soils are balanced by two antagonistic biogeochemical processes, CH4 oxidation, and production (Le Mer & Roger, 2001). Conventional methanotrophs are believed to prefer soil pH ~ 7 (Le Mer & Roger, 2001), the evidence of CH4 uptake has been reported for a number of acidic forest soils, characterized with “high‐affinity” methanotrophs (Kolb et al, 2005)

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Discussion

Conclusion