Soil NH4+/NO3? nitrogen characteristics in primary forests andthe adaptability of some coniferous species

Abstract

In terrestrial ecosystems, soil nutrient regimes at a plant s living site generally represent the plant s nutrition habitat . Plant species frequently well adapt to their original nutrition habitat  during a long process of evolution, and the apparent preference for ammonium or nitrate nitrogen source (NH₄⁺ or NO₃^?) might be an important aspect of the adaptation. Plants typically favor the nitrogen form most abundant in their natural habitats. Nitrate has been recognized as the dominant mineral nitrogen form in most agricultural soils and the main nitrogen source for crops, but it is not usually the case in forest ecosystems. A large number of studies show that the nutrition habitats  associated with primary forest soils are typically dominated by NH₄⁺ rather than NO₃^?, generally with NO₃^? content much lower than NH₄⁺. Low levels of NO₃^?in these forest soils generally correspond to low net rates of nitrification. The probable reasons for this phenomenon include: 1) nitrification limitations and/or inhibitions caused by lower pH, lower NH₄⁺ availability (autotrophic nitrifiers cannot successfully compete for NH₄⁺ with heterotrophic organisms and plants), or allelopathic inhibitors (tannins or higher-molecular-weight proanthocyanidins) in the soil; or 2) substantial microbial acquisition of nitrate in the soils, which makes net nitrification rates substantially less than gross nitrification rates even though the latter are relatively high. Many coniferous species (especially such late successional tree species as Tsuga heterophylla, Pinus banksiana, Picea glauca, Pseudotsuga meziesii, Picea abies, etc.) fully adapt to their original NH₄⁺-dominated nutrition habitats  so that their capacities of absorbing and using non-reduced forms of nitrogen (e.g., NO₃^?) substantially decrease. These conifers typically show distinct preference to NH₄⁺ and reduced growth due to nitrogen-metabolism disorder when NO₃^? is the main nitrogen source. The physiological and biochemical mechanisms that account for the adaptation to NH₄⁺-dominated systems (or limited ability to use NO₃^?) for the coniferous species include: i) distribution and activity of enzymes for catalyzing nitrogen reduction and assimilation, generally characterized by lower nitrate reductase (NR); ii) greater tolerance to NH₄⁺ or rapid detoxification of ammonium nitrogen in the roots; iii) lower capacity of absorption to NO₃^? by roots that might be controlled by feedback regulations of certain N-transport compounds, such as glutamine; iv) relations and balance between nitrogen and other elements (such as Ca²⁺, Mg²⁺, and Zn²⁺ etc.). Some NH₄⁺-preferred conifers might be more adapted (tolerant) to lower base cation conditions; v) NO₃^?2+ nutrition, rather than NH₄⁺, that may lead to the loss of considerable quantities of organic and inorganic carbon to the surrounding media and mycorrhizal symbiont and probably contribute to slower growth; and vi) the metabolic cost of reducing NO₃^? to NH₄⁺2+ that may make shade-tolerant conifers favor the uptake of reduced nitrogen (NH₄⁺). The adaptation of late successional conifers to NH₄⁺-dominated habitats has profound ecological implications. First, it might be an important prerequisite for the climax forest communities dominated by these conifers to maintain long-term stability. Second, primary coniferous or coniferous-broadleaved forests have been widely perturbed because of commercial exploitation, where the soil ammonium nitrogen pool tends to be largely transformed to nitrate after disturbance. In such a situation, the coniferous species that were dominant in undisturbed ecosystems may become poor competitors for nitrogen, and the site will be occupied by early successional (pioneer) plants better adapted to nitrate utilization. In other words, the implicit adaptation of many conifers dominant in undisturbed communities to ammonium nitrogen will cause difficulties in their regeneration on disturbed sites, which must be taken into account in the practical restoration of degraded temperate forest ecosystems.