Agronomic performance and nitrogen management of continuous rice systems following a year-long fallow

Abstract

Erratic precipitation events, including winter droughts and spring rains, are increasing in California, challenging the feasibility of continuous rice (CR) (Oryza sativa L.) mono-cropping and increasing fallows. Exposure of CR soils to extended aerobic periods increases soil nitrogen (N) availability, but region-specific agronomic guidelines have yet to be developed. Yield response to N fertilization and disease severity were evaluated in a 3-year field study for two treatments—CR and fallow rice (FR—rice following a year-long fallow). Maximum observed yields did not differ between treatments, averaging 14.0 Mg ha⁻¹ in 2021, 12.6 Mg ha⁻¹ in 2022, and 9.6 Mg ha⁻¹ in 2023. Based on quadratic regressions of yield response to N, the agronomic optimum nitrogen rate was higher for CR in all years. Where no fertilizer N was applied, FR yielded higher than CR, averaging a difference of 2.9 Mg ha⁻¹. The yield differences at 0 kg N ha⁻¹ can be attributed to soil N availability, where FR averaged 31.6 kg N ha⁻¹ more N uptake than CR at maturity. Fertilizer recovery efficiency (FNRE) did not differ between treatments and averaged 59.8%. Stem rot, caused by Sclerotium oryzae, was more severe in CR than in FR, having averaged severity indexes of 3.7 and 3.1, respectively. Based on differences in soil N uptake and FNRE, the N rate can be reduced by approximately 50 kg N ha⁻¹ for fields following a fallow compared to CR fields, allowing growers to maintain yields with lower inputs.