Changes in soil chemical properties with flooding: A field laboratory approach

Phenolic compounds, which can affect the availability of soil nitrogen (N) for plants, may accumulate in the soil under flooded conditions. The objective of this study was to determine if short-duration floods (i.e., 3 vs. 5 weeks) with varying flow rates (i.e., stagnant vs. flowing) would affect soil inorganic-N or polyphenolic content under field conditions. Our study site was an outdoor research facility used for evaluating flood tolerance of various plant species. Flood conditions (i.e., timing, depth, duration, and flow rate) at this facility can be manipulated making it ideal for a field-based investigation of the effects of flooding on soil chemical and environmental properties. Four flood treatments (non-flooded control, 3-week-flowing flood, 5-week-flowing flood, and 5-week stagnant flood) were implemented in May-June, 2005. Soil redox potential, pH, temperature, volumetric water content and dissolved oxygen were monitored over the course of the flood treatments with automated sensors. Pre- and post-flood soil samples were collected and analyzed for inorganic-N content, total N, total organic C, and total soluble polyphenolics. Sensor data revealed a decrease in redox potentials and dissolved oxygen with flooding; however, differences between flood treatments were not significant. Both 5-week flood treatments developed anaerobic soil conditions within 12 days of inundation; anaerobic conditions were maintained for these treatments until dry-down. Soil conditions in the 3-week-flowing treatment remained generally in the suboxic range. Significant differences in soil temperature, volumetric water content and pH were not observed. Flooding affected soil inorganic-N in that NO₃-N decreased under 5-week flood treatments and increased under 3-week-flowing and control treatments. Losses in soil NO₃-N were nearly 4 times greater for the 5-week-stagnant treatment than the 5-week-flowing treatment. Trends included all flood treatments (including the control) showing increases in soil NH₄-N and most flood treatments (all except the 5-week-flowing) showing decreases in soil polyphenolic content. Changes in soil NH₄-N and polyphenolics, as well as TN, TOC and C:N ratio with flooding were not significant. Short-duration floods in this facility, therefore, did not result in the accumulation of polyphenolics in the soil. Other changes in soil chemistry, such as TN and TOC were minimal and not significant despite development of anaerobic soil conditions.