SEEDLING RESPONSES TO BAND APPLIED NH4OH RATES AND TO N-FORM IN 2 MAIZE HYBRIDS

Growth of maize seedlings can be improved by enhanced ammonium nutrition, but placing fertilizer anhydrous ammonia close to seedlings introduces the risk of ammonia toxicity. In this study, growth and root elongation response to rates of closely placed NH4OH bands were investigated in two contrasting maize hybrids. Seven rates of NH4OH, ranging from 0 to 200 mg N kg-1 soil were injected into the center of each pot. A single rate of Ca(NO3)2-N was included to compare hybrids for N form preference at a moderate N rate. Three seedlings per pot were planted 5.7 cm from the injection point. Hybrid B73 x LH51 produced a quadratic response in shoot growth to NH4OH rates, whereas LH74 x LH123 exhibited a significant linear decline in response to NH4OH rate. Root length density sampled from the fertilized zone declined linearly in response to NH4OH rate while a slight increase in root length density in unfertilized zones was observed at intermediate NH4OH rates. Hybrids did not differ in root length density in either zone. The hybrid with greater tolerance of NH4OH rates (B73 x LH51) also showed a preference in shoot growth for NH4- over NO3-N at 66.7 mg N kg-1 compared to LH74 x LH123. On average across hybrids, nitrate concentrations of xylem exudate collected from detopped plants were 14.5 mmol g-1 for Ca(NO3)2 treatments and 1.5 mmol g-1 for NH4OH treatments, indicating that contrasting N-form nutrition resulted from fertilizer treatments. Malate concentrations were higher in the NH4OH treatment indicating that this organic acid anion may substitute for the negative charge of nitrate during enhanced ammonium nutrition in maize. The results suggest that potentially useful genetic variation exists in maize for N form preference and for tolerance to increasing ammonical-N rates.