Daily Cyclic Changes in Water pH of Flooded Rice in the Mississippi Delta and Black Belt

Mississippi (MS) Delta is the primary rice-producing area in the state of Mississippi. The soil type in these fields (Sharkey soil), often has a pH of around 8.2. How does rice grow in such high soil pH? We selected two rice paddies (Figure 1) in the RR Foil Plant Science Research Center, Mississippi State University, situated on the MS Black Belt, and a soil pH of 8.2 for pH measurements. A cyclic pH change in a single day was found in the waters used for flooding the rice paddies. The RR Foil Plant Science Research Center is a 725-acre rowcrop farmland in the northeast region of Mississippi State University. It has a typical soil of the Mississippi Black Belt with Selma Chalk outcrops at its south and north boundaries. The thickness of the soil is no more than 2m on the Selma chalk basement. The soil has high native fertility and usually contains carbonate particles and montmorillonite.

A pond on the farm acts a source of irrigation for the farm (Figure 1). Near the southeastern corner of the pond, two rice paddies, R1, and R2 (Figure 1) were selected for pH measurements. Besides rice, the primary weed is ducksalad. The water for flooding the rice comes from the pond and has a pH of around 9. The paddy and pond waters were sampled in 20mL vials with caps at 7:30am, 12:30pm, 5:00pm, and 7:30pm in August 2019. The pHs were measured by a SympHony B10P pH meter in lab within one hour after sampling. The muddy water samples near the soil-water boundary were centrifuged, and the supernatants were decanted into 20mL vials for pH measurements. Soil pH was measured following the USDA protocol [1].

Figure 1:The irrigation pond at RR Foil Plant Science Research Center, Mississippi State University, provides water for the adjacent rice paddies, R1 and R2. The pond water is supplemented with groundwater with a pH of 8.4-8.5, and precipitation with a pH ~4.2. The current pond water pH is around 9.

The changes in the rice paddy during the daytime (Figure 2) is generally parallel to the trend in changes in temperature during daytime; with the lowest in the morning, highest in the early afternoon, and then dropping down in the evening. This correlation might reflect the negative correlation of CO₂ solubility with temperature, which is approximately linear. The highest temperature in the early afternoon leads to lowest dissolved CO₂, and hence highest pH around the day, or vice versa. The lab measured soil pH was 8.2, and the rice paddy flooding water came from the pond with a pH of around 9. The pH of the flooding water in the rice paddies was lower than both the soil pH and the original pond water pH, except at noon when the temperature was high. The primary reason for the lowered pH is soil CO₂ production from respiration. Xu et al. [2] used isotope technique and found that root-derived respiration contributed 85-92% of bulk soil respiration in rice paddy [2]. Hence, the rice and ducksalad root respiration, and minor non-rhizomicrobial respiration supplied CO₂ to lower the paddy water pH in this case.

Figure 2:The pH of the water used for flooding the rice paddies changes in the daytime. The pH in both rice paddy 1 and 2 showed a similar trend, with the lowest pH in the morning and highest pH in the early afternoon, and then dropping down in the evening.

The water depth in the rice paddy was around 3 inches, with little changes in pH being observed along the water column. This suggests that convection along the water column is present except at the bottom. Even when the paddy water pH was at 8.7 around noon, the bottom water around the soil-water boundary remained at a pH of 7.6. The primary CO₂ source was from root respiration in the soil. The transport of CO₂ from roots to the water column depends on diffusion [3], which requires a CO₂ concentration gradient. The hence higher CO₂ concentration is expected in the rice rhizosphere. Since the lowest pH of paddy water was 7.45 (in the morning), the rice rhizosphere pH is expected to be lower than 7.45. Oh et al. [3] estimated soil CO₂ concentration as 10-100 times of air CO₂ concentration due to soil respiration [2]. For this case if we assume rice rhizosphere CO₂ concentration as 41,000ppmv (air CO₂ concentration is taken as 410ppmv) and the rice rhizosphere air pressure as 1atm, then CO₂ partial pressure PCO₂ is 0.041atm.

The solubility product K_sp of CaCO₃ is taken as 10-8.35, the water autoionization constant is taken as 10^-14 and the Henry’s law constant K_H is taken as 10-1.495. From the carbonate equilibrium, the rice rhizosphere pH is calculated as 6.87. Although the highest CO₂ concentration [2] is used, Oh et al. [3] referred to bulk soil CO₂ concentration, and did not consider CO₂ diffusion gradient around rhizosphere. Considering rice roots are the main CO₂ source in this case, the rice rhizosphere in-situ pH of 6.87 is more reasonable. After 24 hours or longer in lab to reach equilibrium with air CO₂, all the rice paddy waters showed a pH of 8.3-8.5; the pond water pH was 8.5-8.7. Both showed oversaturation with calcite whose equilibrium pH (with air CO₂) is 8.3. This indicated that both sets of waters were potential sinks for CO₂. The pond water was absorbing air CO₂ as no abundant plants grew in the pond and provided CO₂ from their respiration. The rice and ducksalad root respiration with minor non-rhizomicrobial respiration contributed to the CO₂ levels in the paddy waters.

The pH of rice paddy flooding water was found to change in a single day with the lowest pH in the early morning, and the highest pH in the early afternoon. From carbonate equilibrium, pH is negatively correlated with dissolved CO₂, while dissolved CO₂ is controlled by temperature which is negatively correlated with CO₂ solubility. Hence, the pH fluctuates with temperature on the same day. The paddy flooding water pH could be lower than the soil pH and the initial flooding water pH because of higher CO₂ supply from the paddy soil than the air CO₂. The primary source of the soil CO₂ is rice and weed root respiration with minor non-rhizomicrobial respiration. High pH was observed around noon/high temperature because CO₂ was released from water due to lower solubility. Even around noon, the pH at the soil-water boundary was still as low as 7.5-7.6. The in-situ pH of the rice rhizosphere was even lower due to higher CO₂ concentration at the rhizosphere forming a diffusion gradient to the flooding water. The pH was possibly around 6.87 in this case. Derivation of the rhizosphere in-situ pH is important because the in-situ pH is critical to rice (crop) health and yield, and it is different from the lab measured soil pH due to higher CO₂ concentration and partial pressure.

The authors appreciate Dr. Ling Li in the Department of Biological Sciences and Dr. Zhaohua Peng in the Department of Biochemistry, Molecular Biology, Plant Pathology and Entomology at Mississippi State University for allowing us to measure pHs in their rice paddies.

The authors declare that there is no conflict of interest.

1. USDA (2014) Kellogg soil survey laboratory methods manual. Soil Survey Investigations Report No. 42 Version 5.0.
2. Xu X, Kuzyakov Y, Wanek W, Richter A (2008) Root-derived respiration and non-structural carbon of rice seedlings. European Journal of Soil Biology 44(1): 22-29.
3. Oh NH, Kim HS, Richter DD (2005) What regulates soil CO₂ concentrations? A modeling approach to CO₂ diffusion in deep soil profiles. Environmental Engineering Science 22(1): 38-45.