Methane emission from Texas rice paddy soils. 1. Quantitative multi-year dependence of CH4 emission on soil, cultivar and grain yield

Methane emissions at different rice productivity levels were observed from Texas rice paddy soils during the years 1991--95. Analysis of field measurements showed that seasonal methane emission (E) was strongly dependent on soil, cultivar, and rice grain yield. The relationship can be quantitatively described by E (g m-2) = 0.048 xSI x VI x GY. SI is a soil index to characterize the relative effectof soil texture on emission and is linked with soil sand percentage.VI is a variety index to identify the intervarietal difference in methane emission and is related to the amount of methane emission per unit grain yield. GY is grain yield (g m-2). Constant 0.048was derived from the measurements of 10 cultivars planted in 1993. Computed emission applying the relationship is well matched with measured data. The comparison of computed with measured seasonal methane emission over an 80-day period using a total of 32 data sets yields a correlation coefficient r2 of 0.800. In addition, the ratio of seasonal methane emission to net primary productivity was calculated on a carbon to carbon basis, which produces an average value of 2.8%, ranging from 1.2% to 5.4%. A further investigation indicated that the ratio is soil and variety dependent and can be quantitatively explained by C[CH4]/C[NPP] (%) = 3.21 x SI x VI + 0.12 (r2 = 0.738, n = 32). Under the condition of 30% soil sand, this ratio is (approx.) 3% for the majority of cultivars. [ABSTRACT FROM AUTHOR]