Introduction of a Fallow Year to Continuous Rice Systems Enhances Crop Soil Nitrogen Uptake

Abstract

Rice grown in California constitutes 20% of total U.S. rice production and is typically grown in a continuous rice monoculture system. In recent years, growers have been forced to fallow their lands often due to winter droughts leading to water restrictions or spring rains leading to prevented planting. Increased soil aeration due to fallowing creates knowledge gaps in soil nitrogen (N) availability. A two-year field study was conducted to evaluate differences in crop N uptake between rice cultivation following a fallow season, fallow rice (FR) and continuous rice (CR) systems. Crop uptake of soil N (N uptake_soil) and fertiliser N (N uptake_fertilizer) were quantified using ¹⁵N-enriched ammonium sulfate applied in microplots as a preplant (150 kg N ha⁻¹) or topdress (30 kg N ha⁻¹) application. In both seasons when N was applied as a preplant fertiliser, the FR treatment had a higher grain yield than did the CR treatment, with yield differences of 2.3 Mg ha⁻¹ in 2021 (p < 0.05) and 1.7 Mg ha⁻¹ in 2022 (p < 0.05). Examining the sources of crop N uptake for preplant applied N, on average, N uptake_soil in the FR treatment was 16.7 kg N ha⁻¹ higher than the CR treatment at maturity (p < 0.05). In contrast, N uptake_fertilizer was similar between treatments. Additionally, comparable soil and crop fertiliser N recoveries in CR and FR preplant N suggested that the pathways and magnitudes of fertiliser N losses were similar in both systems. These results indicate that N uptake_soil was primarily responsible for lower N uptake in CR. Similar results were found when N was applied as a topdress, where FR had increased N uptake_soil in both years. We further investigated the reason for lower rates of N uptake_soil in CR. Soil phenols, which have been documented to accumulate in continuously flooded rice systems and stabilise soil N, were quantified in the field study. Complementing the rigorous field study, a regional survey study that incorporated nine paired fields was conducted to quantify regional phenol levels. In both the field and the regional survey studies, soil phenols were higher in CR than in FR fields. Together, higher phenol levels and lower N uptake_soil in CR provide mechanistic evidence that the introduction of a season-long fallow to continuous rice systems enhances soil N availability by reducing organic substrate recalcitrance. Future work should identify the duration needed for soil phenol accumulation to impair soil N cycling under continuous rice cultivation, as well as any roles of soil microbial populations in these soil N cycling patterns.