Uptake of and Preference for Nitrate, Ammonium and Amino Acids by Understory Species in Deciduous Forests

Abstract: AbstractThe uptake of different nitrogen forms by understory species was studied in field and laboratory. Species of the understory in deciduous forests have previously been thought to take up and assimilate only inorganic nitrogen sources. However, species also have the opportunity to assimilate amino acids, usually in low rates compared with ammonium, according to solution culture studies, but may have relatively high uptake rates in soil, as indicated by measurements of uptake rates with 15N-labelled compounds, at least of the amino acid glycine. The uptake of amino acids increased after a drying-rewetting cycle, whereas uptake of ammonium radically decreased, indicating a shift from inorganic nitrogen to organic nitrogen uptake after plants were exposed to stress. The hypothesis that less acid-tolerant species of the understory take up nitrate to a higher degree than acid-tolerant species was investigated in the field by measurements of the potential nitrification activity in the rhizosphere and bulk soil, and an investigation of the natural abundance of 15N in the aboveground biomass. Concerning nitrification, less acid-tolerant species usually exhibited higher nitrification rates in their rhizosphere than in the bulk soil, and also showed higher nitrification in their rhizosphere than other species at the same site. Differences in nitrification rates were interpreted as a relatively higher uptake of nitrate. The less acid-tolerant species also showed lower values of natural abundance of 15N, which was interpreted as a relatively higher uptake of nitrate as nitrate should have lower values than ammonium, but differences could be due to other reasons. The understory seldom makes any large contributions to the biomass of the ecosystem, but the turnover of nutrients is faster than for the trees. Concerning uptake of nitrate (measured as the activity of the enzyme nitrate reductase), the understory in oak forests makes a considerable contribution during spring, but could also be of importance during summer as the same uptake of nitrate per unit area was observed by the tree and the understory layer at some sites. The importance of the understory is of special interest as the risk of nitrate leaching is highest during spring, when trees are inactive.