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

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.

Responses of Soil N2O Emission and CH4 Uptake to N Input in Chinese Forests across Climatic Zones: A Meta-Study

“…In Northwest China, after a moderate N addition of 20 kg N ha −1 yr −1 , Song et al [55] reported a clear switch of CH 4 sink to source in the temperate forest soils, although the sink strength was small (about 6 µg CH 4 m −2 h −1 ). In a N-saturated subtropical forest located in Southwest China, Yu et al [22] reported significant soil CH 4 emissions during monsoonal summers that was comparable with wetland soils. Therefore, the potential contribution of methanogens should not be overlooked in future research in upland soils from Chinese forests.…”

“…This could be explained by the mechanism that enhanced NH 4 + concentration intensifies the competition with CH 4 for CH 4 mono-oxygenase catalyzing CH 4 oxidation [32,33], and methanotroph bacteria prioritize NH 4 + oxidation, resulting in reduced soil CH 4 uptake. In a N-saturated forest in China, Yu et al [22] even reported positive CH 4 fluxes from upland soils. This confirms that CH 4 uptake by methanotrophs is strongly dampened due to excessive N presented in soils.…”

“…During nitrification, N 2 O is mostly produced as a by-product from the intermediate hydroxylamine, thus resulting in small emission potentials [20]. By contrast, a number of studies have shown that soil N 2 O fluxes are positively related to soil moisture in Chinese forests, indicating denitrification as an important source of N 2 O [21][22][23][24]. Therefore, N 2 O emission in forest soils is likely to be controlled by a series of climatic and edaphic factors, including temperature, moisture, soil pH, N substrates and carbon nutrient levels [25].…”

Topography-related controls on N2O emission and CH4 uptake in a tropical rainforest catchment

“…Such discrepancy may be explained by more complete denitrification to N 2 in periodically submerged soils (Koehler et al, 2012) and illustrates the importance of topographic positions on N cycling and thus N 2 O emission (Wexler et al, 2014;Yu et al, 2016). Previous works have also revealed topographic control on CH 4 fluxes (Kaiser et al, 2018;Warner et al, 2018;Yu et al, 2019b). For example, Warner (2018) identified the transition zone between upland and valley bottom as a hotspot of CH 4 uptake in a temperate forests landscape.…”

“…One possible mechanism is that increased NH 4 + in soil competes with J o u r n a l P r e -p r o o f CH 4 for the reactive site of methane monooxygenase, the key enzyme for CH 4 oxidation (Bedard and Knowles, 1989). In water-saturated forest soils, net CH 4 emissions have been reported, pointing to a role of methanogenesis for net CH 4 flux in forest soils under a wet climate (Itoh et al, 2009;Kaiser et al, 2018;Yu et al, 2019b). Soil CH 4 production, i.e.…”