Nitrogen is often limiting and always a central nutrient in forest growth. As a consequence it has been extensively studied and thus makes a good example nutrient in which to delve more deeply into its dynamics.
Inert nitrogen gas (N2, dinitrogen) is the most abundant element in the atmosphere forming around 78%. Unfortunately nitrogen in this form is not directly usable by plants. As Galloway et al. (2003) put it: '... more than 99% of this N is not available to more than 99% of living organisms'. The reason for this seeming contradiction is that plants and animals use nitrogen mostly in various inorganic forms: nitrate (NO3), nitrite (NO2) and ammonia (NH3), usually found in the soil as ammonium ions (NH4+). Nitrogen is fixed by the reduction of gaseous nitrogen to an inorganic form by nitrogen-fixing microbes or lightning and to a lesser extent by cosmic radiation and meteorites, and from volcanic eruptions (cumulatively around 10% of all fixation). The nitrogen-fixing microbes, including free-living (asymbiotic) bacteria such as Azobacter and Clostridium and blue-green algae (cyanobac-teria) such as Nostoc and Calothrix, produce ammonia by splitting the N2 and combining it with hydrogen ions. These nitrogen-fixers can be free-living, often in the rhizosphere of roots or in symbiotic association with a plant species (such as Rhizobium bacteria found in the root nodules of legumes - see Section 5.5.1 - and Frankia actinomycetes in other families, notably alders). Ammonium ions produced by the microbes are rapidly oxidized to nitrite and then to nitrate in the process of nitrification by fungi and particularly the aerobic bacteria Nitrosomonas (ammonium to nitrite) and Nitrobacter (nitrite to nitrate). This happens particularly at neutral or higher pH. Lightning and the other physical processes combine N and oxygen (O) to form nitrates directly. The opposite process of denitrification happens when oxygen is absent (anaerobic conditions) and nitrate is converted back to gaseous nitrogen by different fungi and bacteria (e.g. Pseudomonas). Nitrogen is also held in organic compounds in the soil. These are converted into readily usable inorganic forms by the process of mineralization (see Chapter 7). Nitrogen is taken up by plants not only as nitrate and ammonium but also as organic compounds such as amino acids (see Section 8.4.2). Nitrate is readily stored or moved in a plant but ammonium has to be incorporated into organic forms in the roots because of its toxicity.
Nitrogen can be gained and lost by the forest. Nitrogen fixation in young ecosystems can be substantial (more than 100 kg of nitrogen per hectare each year) but over time it is the recycling of the accumulated nitrogen that supplies the majority of ecosystem needs. Nitrogen-fixing microbes are normally responsible for around 90% of all the gains of nitrogen in a forest. Of this, 70% comes from symbiotic bacteria (see Box 8.2) and only around 30% is fixed by free-living bacteria and actinomycetes, but can reach high levels in those trees that do not readily form symbiotic relationships. For example, in eastern white pine Pinus strobus this can be in the order of 50 kg ha-1 y-1 and in red alder Alnus rubra up to 320 kg ha-1 y-1. Experiments to measure this in eastern white pine (see Knops et al., 2002) were done using trenched plots to sever any roots coming into the plot to ensure there was no 'root mining', bringing in nitrogen from the surrounding area. The inferred input of the free-living microbes is confirmed by the fact that the trees would have had to mine
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