Incidence of chalky rice grain in farmers’ paddy fields is influenced by soil temperature

Context

Chalky rice grains (CRGs), a visible symptom of heat-induced degradation in rice quality, have become an increasingly serious issue for rice production in Japan under rising summer temperatures. While CRG incidence is primarily driven by high air temperatures during the grain-filling period, previous studies have shown that soil and water temperatures in flooded paddy fields can also modulate CRG occurrence. As water management varies across individual rice paddies, resulting differences in soil and water temperatures may influence CRG incidence. However, no studies have explicitly investigated the effects of inter-field variability in soil and water temperatures on CRG incidence.

Research questions

This study aims to quantify the inter-field variability in soil temperature and CRG incidence across farmers’ paddy fields, and to determine whether differences in soil temperature can explain the variation in CRG incidence among fields.

Method

We conducted field observations over three consecutive years (2019–2021) in more than 20 farmers’ rice paddies within a 10 km radius of Tsukuba City, Japan. We quantified the inter-field variability in soil temperature and CRG incidence and analyzed their relationship.

Results

Inter-field variability in soil temperature during August reached approximately 0.5 °C, a magnitude comparable to the inter-annual variability in air temperature. CRG incidence also showed substantial inter-field variability, with a standard deviation of 4.3 % across fields—exceeding its inter-annual variation. A significant positive correlation was observed between soil temperature during the 20 days after heading and CRG incidence, indicating that higher soil temperatures increase CRG incidence.

Conclusion

These findings suggest that localized differences in thermal environments, particularly soil temperature, can significantly affect rice grain quality even within small geographic areas with similar air temperatures.

Implication

Identifying the field-level factors contributing to soil temperature differences could support the development of targeted, site-specific adaptation strategies to mitigate CRG incidence under future climate change scenarios.