Easily decomposable organic matter in paddy soil

Abstract

We have investigated quantitative relationships between the microbial decomposition of soil organic matter and the oxidation-reduction reactions in the submerged soils which are affected by soil temperature. Seven soil samples of manured and non-manured plots with varying contents of organic matter, active iron, active Mn, etc. were used either after air-drying or without air-drying. They were incubated anaerobically in truncated syringes for 12 weeks at 15, 20, 30, and 40°C, and Eh as well as the contents of Fe(II), Mn(II), volatile fatty acids (VFA), dissolved gaseous components (CO₂, CH₄, N₂O, and O₂+Ar) and sulfides were determined. Air-drying pretreatment and high incubation temperature were found to enhance the development of the reduction state in the submerged soils. Both Mn(II) and Fe(II) contents increased rapidly and reached maximum levels which did not change afterwards. These maximum values were specific for each soil and were not affected by either the incubation temperature or the air-drying pretreatment, which indicates that the amounts of biologically reducible Mn-oxides and Fe-oxides were characteristic for each soil. VFA accumulated temporarily and then disappeared when active CH₄-formation started. The amount of VFA was found to be large in the air-dried soil of manured plots incubated at 40°C for 4 weeks. The following quantitative relationships were recognized: 1) Rapid decomposition of organic matter in the early period of incubation was mainly associated with Fe(II) formation (66–84%). 2) Gradual decomposition of organic matter in the later period of incubation was associated with VFA and CH« formation (53–74%). 3) The same substrates may be utilized for both CH4-formation and SO₄ ^2--reduction. On the basis of the relationship between Fe(II) formation and the amount of total organic carbon, it was inferred that soil organic matter was composed of two fractions; one susceptible to manuring and the other not affected by manuring.