Production and consumption of NH ⁺₄ and NH₃ in trees

Abstract

Of all the nutrients acquired from the soil for plant growth, nitrogen (N) is generally required in the greatest amount. The availability of N is dependent on the global N cycle, which relies on the formation of combined inorganic N from atmospheric N₂, as very little N is made available to soils from weathering of substratum (Sprent 1987). Climate and soil organisms interact with soil building processes in such a way that the N cycle is very dynamic and has an important impact on habitat development and plant succession. Plants form a critical part of the dynamics of the N cycle in that they act as a large store for N, but also release much of their N back to the global cycle as tissues senesce and decay. However, as plant succession progresses to climax, more N is locked up in relatively larger and longer-lived species and N becomes a growth-limiting nutrient in most habitats. In such a situation, the N economy of a plant is likely to be under strong selection pressure. In the case of trees, they are faced with balancing acquisition of new or primary N, against retention and recycling of secondary N from old/storage tissue to new growth. Thus in senescing deciduous trees the maximum re-absorption of N for recycling is about 70%, with a slightly smaller value for evergreen species (Aerts 1996).