Nitrogen and water stress interact to influence carbon-13 discrimination in wheat

The impact of interactions between water and N stress on C-13 isotopic discrimination (A) is not well understood. The objective of this study was to determine the impact of N on A in wheat (Triticum aestivum L.) grown under low, moderate, and high water stress. In a field study located near Havre, Montana, USA (48 degrees 30 ' N lat. and 109 degrees 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 A 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 Delta (-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 A. The break point between N impacting or not impacting Delta was similar to 17.45%.. Under non-N limiting (moderate and high N) conditions the equation relating Delta to yield was, yield (kg ha(-1)) = - 11000 + 984 Delta, r = 0.92**. Wheat grown under N-deficient conditions (ON treatment) did not fit this curve. By accounting for the impact of water and N stress on A, this variation could be explained. Results from this study suggest that A can be used to characterize N and water stress at different landscape positions in watershed studies.