A research team has found that warming changes where nitrogen is held in flooded rice soil. In a three-year field experiment, warmer conditions increased both nitrogen uptake by rice and emissions of nitrous oxide, a greenhouse gas. Yet nitrogen also accumulated in soil aggregates, where it is less exposed to rapid breakdown. The findings suggest that protecting these soil nitrogen pools could help sustain fertility as rice-growing regions warm, while showing why nitrogen management must account for increased gaseous losses.
Flooded rice fields rely on soil nitrogen to support crop growth. Researchers have extensively studied how climate change affects soil carbon, but its effects on nitrogen are less clear. Warming can speed up the microbial processes that release plant-available nitrogen, while rising carbon dioxide can change how plants and microbes compete for it. Nitrogen may then leave the soil through crop uptake or gas emissions, or remain in organic matter protected by aggregates and minerals. Understanding which pathways prevail is essential for anticipating changes in soil fertility.
A study (DOI: 10.48130/nc-0026-0010) published in Nitrogen Cycling on 30 June 2026 by Pil Joo Kim's team, Gyeongsang National University, reports that warming increased nitrogen losses through rice uptake and nitrous oxide emissions while also promoting nitrogen retention in protected soil fractions.
To investigate these competing pathways, the team grew rice for three years in open-top field chambers under three conditions: ambient climate, a temperature increase of 2 °C, and the same warming combined with a carbon dioxide increase of 200 parts per million. All plots received the same fertilizer and water management. The researchers measured nitrogen in harvested grain and straw and monitored nitrous oxide emissions over two growing seasons. They also sampled soil during the third season, separating its nitrogen into freely available particulate material, particles enclosed within aggregates, sand-associated material, and material associated with silt and clay. Measurements of naturally occurring nitrogen isotopes helped them examine how nitrogen in each fraction had been processed. Finally, they assessed root biomass, nitrogen released from roots, and the abundance of nifH, a microbial gene associated with the capacity to fix atmospheric nitrogen. Warming alone increased rice nitrogen uptake and nitrous oxide emissions relative to ambient conditions. Adding carbon dioxide did not consistently increase crop uptake further; uptake under the combined treatment was closer to ambient levels. Despite the greater nitrogen outputs, total soil nitrogen was maintained or higher in the warmed plots. Nitrogen enclosed within soil aggregates increased under both warmed treatments, whereas nitrogen associated with silt and clay showed little change in quantity. Both protected fractions showed changes in their nitrogen-isotope signatures consistent with greater microbial processing, although nitrogen loss could also have contributed to those signatures. Root biomass and measured root exudates did not differ significantly among treatments. By contrast, nifH abundance increased under warming, including when carbon dioxide was also elevated. That gene result points to a change in microbial nitrogen-fixing potential; it does not establish how much nitrogen the microbes fixed.
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