Organic matter decomposition of 14C-labelled rice straw in tropical soils [Philippines]

Uniformly 14C-labelled rice plants were grown up to maturity in a specially designed growth chamber. Chopped labelled straw (equivalent to 5 t/ha) was incorporated into the top soil of open cylinders set in the soils down to 45 cm. Each treatment was replicated 8 times. Soil cores were taken out at appropriate intervals and each year the entire soil done to 40 cm of 2 replicates was sampled. The soil samples were freeze dried, combusted by an automatic oxidizer, and analyzed for radiocarbon by scintillation counting. All soils were sampled as undisturbed monoliths and installed in the research farm at IRRI [International Rice Research Institute, Los Banos, Laguna, Philippines] with an average air temperature of T max = 31.3 plus or minus 1.6, T min = 22.4 plus or minus 1.0. The average rainfall is 1040 mm per year with a distinct dry season from January - May. The monoliths remained fallow. Data have been achieved for 4 years. Submergence retarded decomposition of 14C - labelled rice straw only slightly in neutral and alkaline soils. In acid soils, no significant difference was found. The decomposition curves follow a logarithmic function in all soils regardless of water regime. It seems to be that only the initial decomposition differs while the remaining compounds being rather similar irrespective of soils and water regimes. The half life of the more resistant residues and metabolites is about 2 years only. The initial decomposition rate followed in the order of: Tropept (1.6% OM, 37% clay, pH 7.6 = Tropept (1.6% OM, 43% clay, ph 6.0 4.1% phi 46% clay, pH 6.0) = Aquult (4.1 % OM, 46% clay, pH 4.8). The high initial mineralization rate and short half life in submerged soils (Eh -50 mV) is most likely caused by constant soil temperatures of around 27-30 degrees Centigrade, constant pH of 7, low bulk density of puddled clayey soil (0.5 gram per cubic centimeter) and high biological activity of aquatic invertebrates (Mollusces, Oligachaeta, Crustaceae).