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

Authors: ZHANG, ZHENGLIN - University Of California, Davis; Olk, Daniel; ESPINO, LUIS - University Of California, Davis; ROEL-REZK, VALENTINA - University Of California, Davis; LINQUIST, BRUCE - University Of California, Davis

Submitted to: Agronomy Journal
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 8/28/2025
Publication Date: 9/27/2025
Citation: Zhang, Z., Olk, D.C., Espino, L.A., Roel-Rezk, V., Linquist, B.L. 2025. Agronomic performance and nitrogen management of continuous rice systems exposed to a year-long fallow. Agronomy Journal. https://doi.org/10.1002/agj2.70182.
DOI: https://doi.org/10.1002/agj2.70182

Interpretive Summary: Soil contains large amounts of bound nitrogen, which is released slowly into forms that are available for crop uptake. The rate of release can be slowed when the soil is flooded. Recent droughts in California have caused many rice fields to be left unplanted, in aerated conditions, creating uncertainty over the effects on soil nitrogen availability. We found that rice grown after an unplanted season took up more soil nitrogen than did rice grown every year under flooded conditions. This benefit allowed the rice following unplanted to reach maximum grain yield at a lower rate of nitrogen fertilizer than for continuous cropped rice. But it did not increase the maximum potential grain yield, as the continuous rice still reached the same maximum yield if more nitrogen fertilizer was applied. We also found the rice following unplanted season had less incidence of a fungal disease. Based on these results, we calculated by how much a farmer could reduce nitrogen fertilizer application after an unplanted season and still reach desired grain yields. These results will assist rice farmers in maintaining their grain yield levels and adjusting the need for nitrogen fertilizer in future droughts. They will be of interest to rice scientists who strive to understand the chemical processes that control availability of soil nitrogen.

Technical Abstract: Erratic precipitation challenges the default of continuous rice mono-cropping in California. Both droughts and late spring rains cause increased fallows. Exposure of continuous rice soils to extended aerobic periods has been shown to increase soil nitrogen (N) availability, but region-specific agronomic guidelines for fallow management have yet to be developed. Yield response to N fertilization and stem rot severity were evaluated in a three-year field study for two treatments - continuous rice (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 9.58 Mg ha-1 in 2023. Based on quadratic regressions of yield response to N, the agronomic optimum N rate (AONR) 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 had 31.6 kg N ha-1 more soil N uptake than CR at maturity. Apparent fertilizer nitrogen recovery efficiency (FRNE) 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 average FNRE, N rate can be reduced by by roughly 50 kg N ha-1 for fields following a fallow, allowing growers to maintain yields with lower inputs.