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Karst environments are an important but often overlooked global sink of atmospheric CH 4. Little is known about methanotrophic bacterial communities in karst soils; consequently, the effects of land use change (LUC) and soil management practices on them and how they function as part of the soil CH 4 sink are unclear. In this study, we compared net soil CH 4 fluxes, kinetic parameters (K m and V max) of CH 4 oxidation, and the pmoA gene of CH 4 oxidizers at five test soil plots in karst areas that have different soil types and land use types. The maximum atmospheric CH 4 uptake rate (−5.62 ± 3.14 kg ha-1 y-1) occurred in reverting scrublands just 10 years after they had been utilized for crop cultivation; followed by natural forest (−4.30 ± 4.02 kg ha-1 y-1) in limestone soils. The cultivated soils in the cropland emitted CH 4 into the atmosphere. The difference in net CH 4 fluxes (−8.21 kg ha–1 y–1) between soils in cropland and soils in reverting scrubland from cropland was 4–8 times higher than the global average values, suggesting that reforestation on limestone soils in karst cropland areas has much greater potential to enhance soil CH 4 oxidation than in non-karst areas. Soil moisture is the most likely environmental factor to trigger the enhancement effect of reforestation. The remarkable corresponding relationships between K m , net soil CH 4 fluxes, and methanotrophic compositions in these plots suggest that bacteria play an essential role in regulating the effects of LUC on soil CH 4 oxidation rates. It is type Ⅰ methanotroph USCγ and JR3 (USCγ sensu lato), not type II methanotrophs USCα as in previous studies, that dominate in enhancing soil oxidation of atmospheric CH 4 since reforestation on limestone soils. The specific unstable habitats in karst settings favor USCγ who, as r-strategists, can change rapidly to adapt to new conditions formed from LUC and then grow presumably at an exponential rate. This coupling of environmental and microbial factors can well explain the great and fast response of CH 4 oxidation in limestone soils. Our study provides new insights that reforestation of cropland in karst areas may be a noticeably more efficient way to potentially mitigate atmospheric CH 4 build-up and thus should be strongly encouraged. [Display omitted] [ABSTRACT FROM AUTHOR]

Abstract: The consumption of CO2 due to carbonate weathering is thought to be an important terrestrial sink for atmospheric CO2. However, the reaction and fate of weathering products may significantly affect the magnitude of the sink. Carbon mass balances were determined in Hongfeng Lake, a small artificial lake located in the carbonate plateau of SW China. This was accomplished by measuring the concentrations of DIC (dissolved inorganic C), DOC (dissolved organic C), POC (particulate organic C), and CO2 partial pressure in lake waters, inflows and outflows, and in surrounding spring waters. Results show that maximum DIC (mainly derived from carbonate weathering by CO2) to DOC ratios in the catchment export load reach ∼21, implying that as much as ∼80% of the exported primary production is in the form of , and, therefore, the CO2 consumed by carbonate weathering and subsequent DIC export is a critical component of the C balance in this ecosystem. This exported subsidizes C cycling of aquatic ecosystems in the catchment, and 32% (excluding carbonate-derived C) returns to the atmosphere as CO2 evasion through the fluvial and lacustrine water–air interface and 10% is stored in lake sediment. This lake has a small CO2 emission rate (13.2±3.9mmolm−2 d−1, or 0.097molm−2 a−1 relative to catchment area) compared to mean values for many reservoirs found in temperate and tropical climatic zones. This is likely due in part to low organic C loads supplied by the karst-dominated catchment, and low lake water DOC concentrations. The estimate indicates that more C (39%) returns to the atmosphere and much less (13%) is stored in sediments, implying that the C sink of carbonate weathering may not be as significant as once thought. [Copyright &y& Elsevier]

Presents a study which investigated the microbiological and physical-chemical characteristics of subtropical forest, grassland and cropfield soils from the karst areas of Southwest China. Review of related literature; Methodology of the study; Results and discussion.