[en] Highlights: • Hourly CH₄ fluxes from Lake Baihua have been measured using floating chamber method. • CH₄ emission in the lake exhibited remarkably diel patterns. • Solar radiation is likely a key driver to CH₄ in lakes and should be focused on. • Floating chamber method is recommended for CH₄ flux observations in small lakes. • Averaging results at sunrise and at sunset as daily values can reduce uncertainty. Lakes are significant sources in global methane (CH₄) budgets. However, estimations of the magnitude of global CH₄ emissions from lakes may be highly biased owing to the uncertainties in data originating from observation times, methods, and parameterizations of the gas transfer velocity (k). Here, we conducted continuous 48-hour measurements of CH₄ fluxes using the floating chamber method seasonally at Lake Baihua, a small reservoir in southwestern China, and compared the results with estimates derived from boundary layer models. Results showed that there was a weak dependency of k on wind speed, indicating that wind speed was not the major factor regulating gas exchange in such small lakes. It is thus concluded that the wind speed-dependent boundary layer model method is not suitable for CH₄ flux observations in small and medium-sized lake, and that the floating chamber method is recommended for use instead. The measured CH₄ fluxes displayed remarkably diurnal patterns, therefore the use of single observations to represent daily average values comes with unacceptably large uncertainties. A reasonable alternative is averaging observations made at sunrise and at sunset to represent daily values, which has a much smaller uncertainty (ranging from 0.8% to 13.6%). The coincident peaks of CH₄ and chlorophyll concentrations in the subsurface indicate that CH₄ originated mainly from aerobic methanogenesis. Solar radiation is likely one of the major factors regulating CH₄ production and emissions in the lake through enhancing CH₄ production, inhibiting CH₄ oxidation, and probably changing hydrodynamics conditions. Therefore, irradiation should be taken into consideration as a key factor in observing CH₄ fluxes in lakes. As sampling times are limited, observations during both sunny and cloudy weather should be proportionally included. This is the first time, to the best of our knowledge, that solar radiation has been proposed as a key driver of CH₄ emissions from lakes.

[en] Highlights: • S in soils formed from granite is derived mainly from decomposing litter. • Cold/dry climate results in S accumulation at the surface. • Warm/wet climate results in S retention in the subsurface. • Pedogenic Fe/Al minerals play a key role in retaining soil S. • A conceptual model of S dynamics in soil profiles of different climate is proposed. Sulfur (S) dynamics in soils formed from granite remain poorly understood despite its importance as an essential plant macronutrient and component of soil organic matter. We used stable S isotope ratios to trace the sources and biogeochemical processes of S in four forest soil profiles developed on granite under contrasting climate conditions. The soil S is derived mainly from decomposing litter; no significant geogenic contribution to its content is noted as a result of the low S concentration of the granite (~ 5 μg/g). Colder/drier climate results in high organic S retention at the surface due to weak mineralization of organic S. Although warmer/wetter climate increases the S mineralization and leaching loss, SO₄²⁻ adsorption is an important S retention process in the subsurface. The vertical distribution of S isotope compositions in the soil profiles across the four sites indicates (i) a downward increase in δ³⁴S values in the upper profiles due to continuous mineralization of organic S with an occasional decrease in δ³⁴S values in the subsurface due to dissimilatory sulfate reduction (DSR), (ii) constantly high δ³⁴S values in the middle profiles due to the low water permeability, and (iii) a downward decrease in δ³⁴S values in the low profiles due to increased contribution of bedrock with depth. Regardless of the variation in soil depth and climate, the total S concentration is proportional to the pedogenic Fe/Al minerals, suggesting the important role of secondary Fe/Al minerals in retaining S in soils. This study provides an integration and synthesis of controls of climatic and edaphic variables on S dynamics in forest soil profiles developed on granite.