Importance of the nitrogen source in the grass species Brachiaria brizantha responses to sulfur limitation

Plant responses to S supply are highly dependent on N nutrition. We investigated the effect of S status on metabolic, nutritional, and production variables in Brachiaria brizantha treated with different N forms. Additionally, N-15 and S-34 root influx were determined in plants under short- and long-term S deprivation. Plants were submitted to soil fertilization treatments consisted of combinations of N forms [without N, ammonium (NH4 (+)), nitrate (NO3 (-)) or NH4 (+)+NO3 (-)] at S rates (0, 15, 30, or 45 mg dm(-3)). N and S influx capacity was determined in hydroponically-grown plants. Shoot production due to S supply increased 53, 145 and 196 % with NH4 (+), NH4 (+)+NO3 (-) and NO3 (-) treatments, respectively. No or low S impaired protein synthesis and led to high accumulation of N-NO3 (-) and asparagine in NO3 (-)-fed plants, both alone and with NH4 (+). Proline accumulation was observed in NH4 (+)-fed plants. Short- and long-term S deprivation did not promote considerable changes in N-15 influx. S-34 absorption decreased depending on the N form provided: NH4 (+)+NO3 (-) > only NH4 (+) > only NO3 (-) > low N. Including both NH4 (+) and NO3 (-) forms in fertilizer increases N and S intake potential and thereby enhances plant growth, nutritional value and production.