Causes of differences in response of plant species to nitrogen supply and the ecological consequences

Summary

Plant species differ in response to nitrogen (N) supply, depending on environment. Mechanisms of NH₄ ⁺ and NO₃ ⁻ uptake, reduction of NO₃ ⁻ to NH₄ ⁺, and NH₄ ⁺ incorporation into amino acids, depend on light energy which drives photosynthesis, providing reductant, ATP and carbohydrates. Amino acids are used in protein synthesis, which is the basic process determining development and growth. The mechanisms are similar, but differ quantitatively, between species. Genetic factors (the ‘genetic potential’) regulate protein synthesis and metabolism and control organ growth and composition, but rates of processes depend on temperature and supply of N and other nutrients, water etc. Species differ in biochemical composition, size and growth rates: these determine requirements for, and responses to, N etc, under different conditions. With N supply below that required to meet the genetic potential, maximum growth is not achieved: with N in excess, growth is maximal. Interaction between genetic potential and N supply results in differences in growth, production, survival and reproductive success of species and thus their occurrence (‘richness’) in complex ecosystems. Slow growing, low capacity species of nutrient-depleted soils are adapted to limited N. Rapidly growing, large capacity plants requiring large N-supply for maximal growth are associated with nutrientrich habitats. With increased N-deposition from pollution, plants able to accumulate N and grow and reproduce rapidly, out-compete slow growing species requiring little N, and expand their range. However, there is considerable interaction with other nutrients, pH etc. Thus, there is a biochemical explanation for changes in species composition and production of ecosystems with increasing N-pollution. Examples of N-effects on moorland and pasture ecosystems are given.