The effects of differentiated water supply after anthesis and nitrogen fertilization on delta15N of wheat grain.

The delta(15)N signature of plants integrates various processes in soil and plant. In this study, the effect of different water regimes applied during the period of grain growth of winter wheat on grain delta(15)N was examined in a 4-year field experiment. The treatments comprised water shortage (S), an ample water supply (W), and rain-fed crop (R). Zero fertilization (N0) and 200 kg N.ha(-1) in mineral fertilizer (N1) treatments were studied. The grain (15)N was determined during grain growth and at maturity. The water regime, nitrogen application and year had a significant effect on mature grain delta(15)N (p < 0.001). Water and nitrogen explained 54.6% of the variability of delta(15)N in the experiment, the year accounted for 10.7% and the interactions for another 19.6% of the total variability. The analysis of non-mature grain delta(15)N showed significant effects of N and year but not of water. Nitrogen fertilization reduced the delta(15)N of mature grain in years by 0.7-6.3 per thousand in comparison with N0 plants; the reduction was more pronounced under stress (average reduction by 4.1 per thousand) than under rain-fed (2.4 per thousand) and ample water supply (2.2 per thousand). Water stress decreased the grain delta(15)N in fertilized wheat, by 0.1-2.1 per thousand and 0.6-3.6 per thousand in experimental years, on average by 1.30 per thousand and 1.79 per thousand in comparison with the R and W water supply, respectively. The effect of water supply was not significant in non-fertilized wheat. A significant negative linear relationship between grain N concentration and delta(15)N in maturity or during the grain growth (R(2) = 0.83, R(2) = 0.76, respectively) was found. The observed sources of grain delta(15)N variability should be taken into consideration when analyzing and interpreting the data on the delta(15)N signature of plant material from field conditions.