Great and fast increase in soil CH₄ uptake after reforestation in karst cropland area is linked to the environmental and microbial factors

Karst environments are an important but often overlooked global sink of atmospheric CH4. 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 CH4 sink are unclear. In this study, we compared net soil CH4 fluxes, kinetic parameters (K-m and V-max) of CH4 oxidation, and the pmoA gene of CH4 oxidizers at five test soil plots in karst areas that have different soil types and land use types. The maximum atmospheric CH4 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 CH4 into the atmosphere. The difference in net CH4 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 CH4 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 Km, net soil CH4 fluxes, and methanotrophic compositions in these plots suggest that bacteria play an essential role in regulating the effects of LUC on soil CH4 oxidation rates. It is type I methanotroph USC gamma and JR3 (USC gamma sensu lato), not type II methanotrophs USC alpha as in previous studies, that dominate in enhancing soil oxidation of atmospheric CH4 since reforestation on limestone soils. The specific unstable habitats in karst settings favor USC gamma 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 CH4 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 CH4 build-up and thus should be strongly encouraged.