Flooded Rice Crop Rotation May Slow Carbon Loss, Subsidence in Everglades Organic Soils

South Florida’s muck soils store carbon built over thousands of years. However, decades of drainage for agriculture have resulted in a shift in redox condition resulting in soil, organic matter, and subsequent carbon losses. A new University of Florida Institute of Food and Agricultural Sciences (UF/IFAS) study examines this oxidation-reduction (redox) process. The findings provide new insights into how flooded rice crop rotation might slow organic matter oxidation.

Much of the Everglades Agricultural Area (EAA) sits on organic Histosols that form under waterlogged conditions. When drained for crops like sugarcane and winter vegetables microbial oxidation accelerates, releasing carbon dioxide and causing gradual soil loss through subsidence.

Irrigation floods a field in the Everglades Agricultural Area to prepare for a flooded rice crop rotation. (Photo provided by Xue Bai)

One strategy gaining traction in the EAA is summer flooding for rice cultivation after sugarcane or vegetable harvest. This reintroduces water and may slow the redox process by shifting soils from aerobic to anaerobic conditions (with oxygen to without). But in practice, achieving an anaerobic state is more complex.

“I was surprised by how persistently aerobic the surface soils remained, even under flooding,” said Xue Bai, a recent soil, water and ecosystem sciences (SWES) Ph.D. graduate whose research led to this published study. “It shows how important irrigation duration, depth of water and soil structure are to actually shift redox conditions.”

From Anecdotal Evidence to Experimentation

Farmers in the EAA have been crop rotating with flooded rice since 1977. However, a comprehensive study on optimum irrigation conditions to stop or slow redox was lacking. Bai, her dissertation adviser, associate professor of soil, water and ecosystem sciences Jango Bhadha, and colleagues set up field trials over multiple years. The team planted rice at the UF/IFAS Everglades Research and Education Center (EREC) during the summer fallow period. They continuously irrigated from 21 to 35 days after planting until drain-down before harvest.

A monitoring device stands in a field of flooded rice. (Photo provided by Xue Bai)

They measured dissolved oxygen, redox potential, pH, soil temperature and flooding depth at 15-minute intervals. Soil sampling before planting and after harvest focused on organic matter and active carbon content. Bai analyzed the data to relate irrigation on and off, environmental controls and redox dynamics.

“We found that flooding the soil did not make it fully anaerobic,” said Bai. “Even under continuous inundation, the surface muck soil often stayed in aerobic to moderately reduced ranges.”

It took extended flooding events, such as seven to 10 days of continuous inundation, to have an impact. That resulted in dissolved oxygen and redox potential dropping more substantially toward moderate anaerobic conditions. Still, increased saturation did not consistently slow organic matter oxidation.

“In the second year, even though dissolved oxygen was lower with deeper flooding and more stable water levels, the redox potential average increased,” said Bhadha. “This suggests other chemical transformations/reactions within the vadose zone such as the mineralization of organic matter in the saturated soil occurred to sustain redox.”

Implications

As far as developing best management practices in the future, the study shows dissolved oxygen is the strongest predictor of redox potential. Flooding depth, temperature and pH all have smaller but still significant influences.

Overall, total organic matter stayed stable, while active carbon declined by about 15 percent each year across the season. This shows that while the flooding regime slowed decomposition, it did not eliminate it entirely.

“We didn’t see a net gain in organic matter,” said Bai, “but preserving the active carbon pool is a promising sign that flooded rice can slow oxidation relative to fallow.”

The study suggests flooded rice crop rotation can help manage oxidation when properly controlled. Some key takeaways:

Deeper, more stable flooding is essential to push oxidizing conditions downward.
Longer irrigation periods, when feasible, may increase reductive periods.
Good field infrastructure and land preparation are critical at large scales.
Crop rotation is a great way to conserve soil loss.

“This work highlights redox potential in terms of irrigation design in tropical organic soils,” said Bhadha. “You should balance carbon conservation, yield and greenhouse gas tradeoffs. You can’t just flood; you must flood smart.”

Small, green shoots emerge from the soil in a flooded rice crop rotation system. — Small, green rice shoots emerge from the soil. (Photo provided by Xue Bai)

Future Studies

The team members said more research is needed, including multiyear field trials and greenhouse gas assessments. Those findings could give a better understanding of how irrigation practices affect redox conditions and atmospheric outcomes. The additional research would represent a stronger step toward data-driven, adaptive water management and long-term stewardship for Florida’s organic soils.

The full article, “Sensitivity of Redox Conditions to Irrigation Practice and Organic Matter Decomposition in a Rotational Flooded Rice (Oryza sativa) Cropping System,” is available on the Journal of Environmental Quality website. The U.S. Department of Agriculture Natural Resources Conservation Service provided funding for this research through the Conservation Effects Assessment Project Watersheds Assessment Studies award (NR203A750023C015).

Featured image of a flooded rice field provided by Xue Bai, Ph.D.