Wheat Root Growth Responses to Enhanced Ammonium Supply

Abstract

AbstractSlowing nitrification with a nitrification inhibitor (NI) provides an enhanced NH4 supply (EAS), which has improved N uptake efficiency and crop yield. A physiological response to EAS has been demonstrated in container experiments with restricted rooting volumes and uniform N distribution, but less is known about root responses to EAS when distributions of N forms are not uniform. Greenhouse and 2‐yr field experiments were conducted to determine how the root system of wheat (Triticum aestivum L.) responds to EAS. Three N sources: (i) calcium nitrate (NO3); (ii) urea with a nitrification inhibitor, nitrapyrin (NH4 + NI); and (iii) 50% NO3 + 50% NH4 + NI were uniformly incorporated in the field into the top 20 cm of a Shano silt loam soil (Coarse‐silty, mixed, superactive, mesic Xeric Haplocambid). Urea with a NI increased NH4 in the top 30 cm of soil and increased root surface area densities in this soil layer at the boot stage in both years. Rooting was either decreased or not affected by NH4 + NI relative to NO3 in the 30‐ to 90‐cm subsoil. In a greenhouse experiment, NO3, NH4 + NI, or NH4 + NI + CaCl2 were uniformly mixed into the top 20 cm of soil (diffused) or banded at the 20‐cm depth in rooting‐boxes (5 by 17 by 70 cm). Nodal root density at boot stage increased in the fertilized zones with NH4 + NI, mostly in the presence of CaCl2, compared with NO3, while seminal roots in the fertilized zones were only enhanced by banded NO3. Wheat root types responded differentially to N forms and the associated differences in vertical distribution.