Yield Formation and Plant Metabolism of Spring Barley in Response to Locally Injected Ammonium

This study was conducted to elucidate the crop physiological basis for yield differences frequently observed in experiments comparing top‐dressing of N fertilizers with injection of ammonium or ammonium/urea solutions into the soil. The effects of diammonium phosphate (NH₄‐N) injected at the two‐leaf stage, calcium nitrate (NO₃‐N) broadcasted and incorporated before sowing, and a control without N fertilization (‐N) were assessed from measurements of growth, N‐uptake and N‐partitioning, light interception, gas exchange, leaf anatomy and the activity of key enzymes of N‐metabolism. The experiment was performed with spring barley (Hordeum vulgare L.) grown in 80‐l containers in a vegetation hall in Braunschweig, Germany. The plants in the NH₄‐N treatment produced a 20 % higher grain yield than those in the NO₃‐N treatment. The grain yield superiority of the NH₄‐N plants was attributable to a higher number of ears per plant (+13 %) and more grains per ear (+6 %). The NH₄‐N plants exhibited lower concentrations of inorganic cations than plants supplied with NO₃‐N. In the NH₄‐N treatment, the light penetrated more deeply into the crop canopy and the NH₄‐N plants exhibited a higher leaf carbon exchange rate at the different leaf layers than the NO₃‐N plants. It is concluded that as opposed to predominantly nitrate nutrition, provision of a persistent source of ammonium enables plants to take advantage of the positive yield effect of mixed N nutrition.