Introduction of a fallow year to continuous rice systems enhances crop soil nitrogen uptake

Authors: ZHANG, ZHENGLIN - University Of California; Olk, Daniel; LINQUIST, BRUCE - University Of California

Submitted to: European Journal of Soil Science
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 1/2/2025
Publication Date: 1/28/2025
Citation: Zhang, Z., Olk, D.C., Linquist, B.A. 2025. Introduction of a fallow year to continuous rice systems enhances soil nitrogen uptake. European Journal of Soil Science. https://doi.org/10.1111/ejss.70046.
DOI: https://doi.org/10.1111/ejss.70046

Interpretive Summary: Soil contains large amounts of bound nitrogen, which is released slowly into forms that are available for crop uptake. Previous work has shown that this release can be slowed in rice soils where crop stalks and roots decompose after harvest under flooded conditions, compared to aerobic decomposition. 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 large benefit in soil nitrogen uptake was associated with increased rice grain yield, and also with a decrease in soil abundance of phenols, which are the building blocks of wood, likely from rice roots and straw. These results confirmed previous work that suggested soil phenols might bind soil nitrogen into forms that are not available to the crop. 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: Of total U.S. rice production, 20% is from California, where it is typically grown in monoculture. Recent droughts have forced rice farmers to fallow much of their land, causing extended periods of soil aeration and creating knowledge gaps in nitrogen (N) cycling dynamics. A two-year field study was conducted to test the hypothesis that a fallow rice (FR) crop, that is, rice cropping following a fallow season, would have greater N uptake compared to a continuous rice (CR) crop. Crop uptakes of soil and fertilizer N were quantified through a 15N trial, where 15N-enriched ammonium sulfate was applied in microplots embedded within larger plots, and plant content of 15N was presumed to represent fertilizer uptake. For both seasons when 150 kg N ha-1 N was applied as a preplant fertilizer, the FR treatment had higher grain yield than did the CR treatment, with yield differences averaging 1.97 Mg ha-1. Crop uptake of soil N at harvest was 16.8 kg N ha-1 higher in the FR treatment than in the CR treatment. In contrast, fertilizer N uptake was similar between FR and CR. Based on previous studies elsewhere, we hypothesize that the decreased soil N uptake was due to soil N stabilization by soil phenols that accumulate under continuous rice cropping. Here we found that soil phenols were indeed enriched in CR compared to FR. Complementing this rigorous field study, a regional survey of nine paired farmers' fields also showed that CR soils had greater phenol levels compared to FR soils. Our results support previous findings that continuous rice systems characterized by anaerobic residue incorporation and prolonged flooding result in reduced N uptake. By introducing aeration to continuous rice soils, in this case a year-long fallow, productivity can likely be enhanced due to increased soil N uptake. Future work should identify the extent of soil phenol accumulation necessary to impair soil N cycling under CR and the extent of aeration needed for its reversal, as well as any roles of soil microbial populations in these soil N cycling patterns.