Wheat root growth responses to enhanced ammonium supply

Slowing nitrification with a nitrification inhibitor (NI) provides an enhan ced NH4 supply (EAS), which has improved N uptake efficiency and crop yield . A physiological response to EAS has been demon strated in container exper iments 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 inhib itor, nitrapyrin (NH4+M); and (iii) 50% NO3 + 50% NH4 + NI were uniformly i ncorporated in the field into the top 20 cm of a Shano silt loam soil (Coar se-silty, mixed, superactive, mesic Xeric Haplocambid). Urea with a NI incr eased NH4 in the top 30 cm of soil and increased root surface area densitie s in this soil layer at the boot stage in both Sears. Rooting was either de creased or not affected by NH4 + NI relative to NO3 in the 30- to 90-cm sub soil. In a greenhouse experiment, NO3, NH4 + NI, or NH4 + NI + CaCl2 were u niformly 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 stag e 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.