Effect of flooding on Chemistry of Paddy soils: A Review

An understanding of the influence of flooding on electrochemical and chemical properties of paddy soils provides insight needed in their management for rice production.When an aerobic soil is submerged, its redox potential decreases during the first few days and reaches a minimum; then it increases, attains a maximum, and decreases again asymptotically to a value characteristic of the soil, after 8-12 weeks of submergence. In submerged soils the organic matter is decomposed by facultative and obligate anerobes, the decomposition and assimilation process are slow resulting in the accumulation of plant residues. Redox potential oscillation due to paddy management control community structure and function and thus short-term biogeochemical processes. After flooding microbial reduction processes sequentially use NO3, Mn,Fe, SO4 as electron acceptors, accompanied by the emission of trace gases N2O, N2, H2S, CH4 and due to reduction-induced increasing pH-NH3. This results in N losses and low N fertilizer use efficiency. However, transport of atmospheric O2 to the roots via the rice plant aerenchyma modifies conditions in the rizosphere leading to nitrification and methane oxidation and precipitation of Mn and Fe oxides. The large accumulation of soil organic matter observed in some, but not all paddy soils is considered to be due to high input of plant residues and charred materials associated with retarded decomposition underanaerobic conditions. A specific feature of paddy soils is the coupling of organic matter turnover with mineral transformations and fluxes which seem to be intensified by alternating redox conditions. Bioavailability of soil organic N isstrongly coupled to soil organic matter cycling and is a crucial parameter determining crop