Influence of nitrate and ammonium nutrition on the growth of giant foxtail (Setaria faberi)

The objective of this study was to evaluate growth and seed production of giant foxtail under different N sources (NO3 and NH4) and N fertilizer application rates. Nitrate and NH4 fertilizers plus nitrification inhibitor were applied at 56, 112, and 225 kg N ha−1 under field conditions, and in the greenhouse four N rates (1, 5, 10, and 25 mM N) were applied in three NO3: NH4 ratios (100 : 0, 50 : 50, 0 : 100). Growth of giant foxtail was affected by N rates under both greenhouse and field conditions. In 1993, abundant rainfall in May and June allowed a rapid and earlier uptake of N by giant foxtail, resulting in larger plants with greater N accumulation and higher numbers of heads and seeds than in 1994. Total dry weight increased with increasing N rates; however, seed production reached a maximum at approximately 150 kg N ha−1. Nitrogen translocation efficiency decreased with increasing N rates. Giant foxtail did not show any preference to N form; however, seed production was reduced when the high N rate was applied as NH4 compared to NO3. These results suggest that NH4 fertilizer applications with a long-term nitrification inhibitor could reduce the seed production of giant foxtail and its contribution to the soil seedbank for subsequent growing seasons.