Characterizing redox status of paddy soils with incorporated rice straw

Abstract

Redox status is one of the more difficult soil quality criteria to characterize especially in paddy soils. The soil system chosen in this study was an ongoing paddy field trial on alternative practices of incorporating rice straw instead of traditional burning of straw. Upon flooding, rapid changes in redox potential (Eh) occur in paddy soil due to the decomposition of soil organic matter (SOM), including rice straw. This investigation evaluated three methods of characterizing redox status: (i) Eh, a conventional method using Pt black electrode; (ii) terminal electron-accepting processes (TEAPs), a method of diagnosing microbially mediated electron acceptor (oxidized species) consumption, intermediate product concentration of H 2 and accumulation of final products (reduced species); and (iii) oxidative capacity (OXC), a comprehensive analysis of the algebraic sum of oxidized and reduced species into a single descriptive parameter. There was a need to develop a sampling technique for anaerobic soil pore water in rice paddy (without exposure to the atmosphere) to measure all the redox parameters required by these methods. The setup comprised of a capped piezometer (sampling well) back-filled with sand and sealed with bentonite, and vacuum extraction of pore water containing dissolved constituents. The pore water containing dissolved gases was collected into previously evacuated 400-ml two-port PVC bags from which gaseous phase sub-samples were withdrawn with a syringe into 10-ml vacutainers previously flushed with N 2 gas for H 2 and CH 4 determinations in the laboratory. Unstable water quality parameters such as DO, Eh, pH and S 2− were measured on site using fresh samples, while other parameters such as NO 3 −, SO 4 2−, EC, Fe(II) and Fe(III), and Mn(II) were analyzed in the laboratory. The above three methods of evaluating redox status were applied to monitoring the paddy pore water over 3 years of field observations (1997–1999). Eh is a simple measure, but it gives at best only qualitative assessment of redox status because this electrode may not respond to many of the important redox couples. The adaptation of TEAPs to assess non-equilibrium redox conditions in flow pathways in large-scale ground water systems to small-scale rice rootzone was not entirely successful because of differences in the two systems and the significant overlapping among electron acceptors in paddy soils. OXC, a non-equilibrium capacity-type redox parameter, clearly identified geochemical redox classes of oxic, post-oxic, sulfidic and methanic conditions in the paddy pore waters during the course of the rice-growing season. Based on data observed for 3 years, it is concluded that straw incorporation does enhance more reducing conditions development compared to that without straw incorporation. We conclude that OXC provides a better characterization of redox status in paddy soils.