Adaptations to nitrogen form: comparing inorganic nitrogen and amino acid availability and uptake by four temperate forest plants

Abstract

There are few examinations of the relative availability and plant uptake of inorganic N and amino acid N in temperate forest regions. We determined the availability of amino acid N and inorganic N in soils under two shrub species ( Vaccinium ovalifolium Sm. versus Rubus spectabilis Pursh) on three sites near Jordan River, British Columbia, over a growing season. We compared biomass production of the two shrubs and two conifers ( Picea sitchensis (Bong.) Carr. and Pseudotsuga menziesii (Mirb.) Franco var. menziesii) when given inorganic N (20:80 or 80:20 NH4+–NO3–) or organic N (glycine and glutamic acid) and assessed short-term uptake (24 h) of 15N-labelled NH4+, NO3–, glycine, or glutamic acid by the four species. Water-extracted soil concentrations of NH4+ were up to 1.5 times greater than NO3– averaged across sites. Concentrations of amino acid N and inorganic N were similar on soils under Rubus , but the amino acid N to inorganic N ratio was up to 2.4:1 in soils under Vaccinium . Soils dominated by Rubus had up to twice the NO3–-N and two thirds the amino acid N concentrations of soils dominated by Vaccinium, averaged across sites and Rubus had relatively high short-term 15NO3– uptake. The dry biomass of conifers was approximately four times greater when supplied mainly with NH4+ compared with NO3–, but biomass of the two shrub species was similar in both inorganic N treatments. All plants had comparable rates of short-term 15N uptake from amino acids and inorganic N, suggesting that amino acids could contribute to the N nutrition of these temperate species; however, dry biomass of all four species grown with amino acids was less than one half that of plants grown with inorganic N.