Influence of the temporal and spatial variation of nitrate reductase, glutamine synthetase and soil composition in the N species content in lettuce (Lactuca sativa)

• Temporal variation of nitrate reductase and glutamine synthetase activities in lettuce.
• Accumulation of NO3−, NH4+ and organic-N during lettuce growth.
• Mobility of inorganic nitrogen within the groundwater–soil–plant system.

The variation of nitrate reductase (NR), glutamine synthetase (GS) and N content in lettuce was evaluated at 5 stages of lettuce growth. Soil physicochemical properties and its N content were also assessed to elucidate the soil-to-plant transfer of inorganic N and potential leaching to groundwater. A decrease of NR activity and an increase of NO3− and N-Kjeldahl content in lettuces were observed during plant growth, whereas GS activity and NH4+ increased during the first few weeks of lettuce growth and then decreased. Although the temporal variation was similar in lettuces grown in different soils, quantitative differences were observed, indicating that high NO3− content in soil caused a higher NO3− accumulation in lettuce despite the higher NR activity during the initial stage of plant growth. Higher levels of NO3− and NH4+ were correlated with higher levels of N-Kjeldahl in lettuce suggesting a positive effect of these N species in the biosynthesis of organic forms of N. Soil physicochemical properties influenced the mobility of inorganic N within the groundwater–soil–plant system. Sandy soils with low OM content allowed NO3− leaching, which was confirmed by higher NO3− levels in groundwater. Therefore, lettuces grown in those soils presented lower N content and the inputs of N to the environment were higher.