LAUSR

Abstract The aim of this work was to evaluate the vermicompost humic acid (HA) stimulatory effects on the N-uptake kinetics and the N metabolism of rice plants under high and low N-NO3- or N-NH4+ supply in nutrient solution. Plants were grown in a growth chamber, and at 21 days after germination, they were submitted to N deprivation for 96 h, followed by HA treatment or no treatment. After 96 h of N deprivation, the plants received 0.2 or 2.0 mmol L−1 N-NO3- or N-NH4+. Changes in pH, NO3- or NH4+ net influx, fresh weight, soluble nitrogen fractions and sugars in both the roots and shoots were evaluated. Pretreatment of rice plants with HA stimulated NO3- uptake, preserved plant metabolic status and increased fresh weight. On the other hand, plants submitted to N-NH4+ facilitated the accumulation of this N form, promoting symptoms of toxicity and leading to a reduction in fresh weight. The results suggest that HA pretreatment modifies the net influx of NO3- or NH4+, which cause differences in plant physiology. Vermicompost humic acids could be part of biotechnology packages for the purpose of increasing the nitrogen nutrition of rice plants.