Soil NH4 +/NO3 − nitrogen characteristics in primary forests and the 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.