142M471B3J Carbon Dioxide Journal Article Methane OP0UW79H66 Research Support, Non-U.S. Gov't eddy covariance random forest CH4 and CO2 fluxes Eddy-covariance Research Support, U.S. Gov't, Non-P.H.S. Soil artificial neural network comparison of gap-filling techniques generalized additive modeling global change greenhouse gas balance hyperseasonal savanna lags machine learning marginal distribution sampling methane methane flux mutual information net ecosystem carbon balance predictors support vector machine synthesis time scales tropical wetlands