Nitrogen and Water Stress Interact to Influence Carbon‐13 Discrimination in Wheat

The impact of interactions between water and N stress on 13C isotopic discrimination (Δ) is not well understood. The objective of this study was to determine the impact of N on Δ in wheat (Triticum aestivum L.) grown under low, moderate, and high water stress. In a field study located near Havre, Montana, USA (48° 30′ N lat. and 109° 22′ W long.), wheat grown under three different water stress environments (low, moderate, and high) was fertilized with three different N rates (none, moderate, and high). Each treatment was replicated four times. The grain N fertilizer use efficiency increased as water stress decreased. A differential response of Δ to N was observed. In general, if plants were grown under high water stress and N increased yield, then adding N to N‐deficient plants reduced Δ (−0.01‰ for every kg of N added); and if plants were grown under low water stress and N increased yield, then adding N had little or no impact on Δ. The break point between N impacting or not impacting Δ was ∼17.45‰. Under non‐N limiting (moderate and high N) conditions the equation relating Δ to yield was, yield (kg ha−1) = −11000 + 884 Δ, r = 0.92**. Wheat grown under N‐deficient conditions (0N treatment) did not fit this curve. By accounting for the impact of water and N stress on Δ, this variation could be explained. Results from this study suggest that Δ can be used to characterize N and water stress at different landscape positions in watershed studies.