Modeling methane emissions from paddy rice fields under elevated atmospheric carbon dioxide conditions

Methane (CH4) emissions from paddy rice fields substantially contribute to the dramatic increase of this greenhouse gas in the atmosphere. Due to great concern about climate change, it is necessary to predict the effects of the dramatic increase in atmospheric carbon dioxide (CO2) on CH4 emissions from paddy rice fields. CH4MOD 1.0 is the most widely validated model for simulating CH4 emissions from paddy rice fields exposed to ambient CO2 (hereinafter referred to as aCO(2)). We upgraded the model to CH4MOD 2.0 by: (a) modifying the description of the influences of soil Eh and the water regime on CH4 production; (b) adding new features to reflect the regulatory effects of atmospheric CO2 upon methanogenic substrates, soil Eh during drainages, and vascular CH4 transport; and (c) adding a new feature to simulate the influences of nitrogen (N) addition rates on methanogenic substrates under elevated CO2 (hereinafter referred to as eCO(2)) condition. Validation with 109 observation cases under aCO(2) condition showed that CH4MOD 2.0 possessed a minor systematic bias in the prediction of seasonally accumulated methane emissions (SAM). Validation with observations in free-air CO2 enrichment (FACE) experiments in temperate and subtropical climates showed that CH4MOD 2.0 successfully simulated the effects of eCO(2) upon SAM from paddy rice fields incorporated with various levels of previous crop residues and/or N fertilizer. Our results imply that CH4MOD 2.0 provides a potential approach for estimating of the effects of elevated atmospheric CO2 upon CH4 emissions from regional or global paddy rice fields with various management practices in a changing climate.