Effect of soil conditions on the decomposition rate of azolla

Azolla can be cultivated and used as green manure in most rice fields in China. Because the decomposition rate of azolla is an important factor determining its effect on rice growth, a series of decomposition experiments, using the carborundum tube method, were conducted at four sites in different climate zones in 1980. The climate zones were southern subtropic (Guangzhou), northern subtropic (Wuxi), warm temperate (Tianjin), and temperate (Gongzhuling). The mean annual temperature in the four zones ranged from 21.8 to 4.9 deg C., and mean annual precipitation was from 1,623 to 572 mm. The soils in Tianjin and Gongzhuling are calcareous, those in Guangzhou acidic, and those in Wuxi neutral. Under waterlogged conditions, the azolla decomposition rate was highest in Tianjin soils, lower in Guangzhou, and lowest in Gongzhuling. Under upland conditions, the decomposition rate of azolla was highest in Guangzhou soils and lowest in Wuxi soils. These results may seem contradictory to the general belief that the decomposition rate of plant residues doubles for every incremental temperature increase of 10 degrees Celsius. This apparent anomaly is explained by the fact that the soils at the four sites differed greatly in acidity, texture, clay mineral, and, in the case of upland, soil water regime. It is well known that plant residues decompose more rapidly in calcareous soils than in acidic soils, and that water shortage limits microbial activity. It was concluded that the decomposition rate of azolla in soils was governed not only by climate but by soil properties as well. Under all conditions, however, the decomposition rate of azolla was lower than that of rice straw under the same conditions, because azolla has a much higher lignin content (22.0 + - 6.6%). It is well demonstrated that the humification coefficient of plant residues correlates significantly with their lignin content, irrespective of climate and soil type.