Quantifying methane emissions from rice fields in the Taihu Lake region, China by coupling a detailed soil database with biogeochemical model

Abstract. As China has approximately 22% of the world's rice paddies, the regional quantification of CH₄ emissions from these paddies is important in determining their contribution to the global greenhouse gas effect. This paper reports the use of a biogeochemical model (DeNitrification and DeComposition or DNDC) for quantifying CH₄ emissions from rice fields in the Taihu Lake region of China. For this application, the DNDC model was linked to a 1:50 000 soil database derived from 1107 paddy soil profiles compiled during the Second National Soil Survey of China in the 1980s–1990s. The simulated results showed that the 2.3 Mha of paddy rice fields in the Taihu Lake region emitted the equivalent of 5.7 Tg C from 1982–2000, with the average CH₄ flux ranging from 114 to 138 kg C ha⁻¹ y⁻¹. As for soil subgroups, the highest emission rate (660 kg C ha⁻¹ y⁻¹) was linked to gleyed paddy soils accounting for about 4.4% of the total area of paddy soils. The lowest emission rate (91 kg C ha⁻¹ y⁻¹) was associated with degleyed paddy soils accounting for about 18% of the total area of paddy soils. The most common soil in the area was hydromorphic paddy soils, which accounted for about 53% of the total area of paddy soils with a CH₄ flux of 106 kg C ha⁻¹ y⁻¹. On a regional basis, the annual averaged CH₄ flux in the Taihu Lake plain soil region and alluvial plain soil region were higher than that in the low mountainous and hilly soil region and the polder soil region. The model simulation was conducted with two databases using polygons or counties as the basic units. The county-based database contained soil information coarser than the polygon system built based on the 1:50 000 soil database. The modeled results with the two databases found similar spatial patterns of CH₄ emissions in the Taihu Lake region. However, discrepancies exist between the results from the two methods. The total CH₄ emissions generated from the polygon-based database is 2.6 times the minimum CH₄ emissions generated from the county-based database, and is 0.98 times the maximum CH₄ emissions generated from the county-based database. The average value of the relative deviation ranged from −20% to 98% for most counties, which indicates that a more precise soil database is necessary to better simulate CH₄ emissions from rice fields in the Taihu Lake region using the DNDC model.