Nutrient cycling

8.3.1 Losses, gains and cycling

Plant and animal tissues are composed largely of carbon, oxygen and hydrogen which flow through an ecosystem as part of large global gaseous cycles. A number of other nutrients, making up at most 5% of a plant's bulk, are also crucial to growth. As described in Chapter 2 these essential mineral nutrients are divided into those required in larger quantity (the macronutrient elements -N, P, K, Ca, S and Mg) and those needed in much lower amounts (the trace or micronutrient elements - see Figure 2.3). The surface layers of the Earth hold large amounts of these chemical elements but a high proportion are locked up in minerals and sediments (or atmosphere in the case of nitrogen) as part of large biogeochemical cycles. Forests are part of these cycles and interact with them through the small losses and gains of nutrients made over time but the bulk of the nutrients are recycled within the ecosystem.

Most nutrients do not have a gas phase under normal conditions and so go through a strictly sedimentary cycle with inputs to living organisms being through uptake from the soil. The exception is nitrogen: this is usually the nutrient element needed in highest quantities (typically ten times greater than phosphorus and several hundred times greater than a micronutrient like zinc), is usually most limiting, and is the one element that has a large interaction with the gases of the atmosphere (sulphur can appear as gaseous hydrogen sulphide under certain circumstances and so is a minor exception). Much of the nitrogen used by plants has its origin directly from the atmosphere. The processes that control the availability of nutrients are of obvious interest to forest ecologists in understanding how the forest functions.

The main sources of nutrients in a forest are chemical weathering, biological fixation and atmospheric deposition. Chemical weathering of rocks releases a number of nutrients, particularly phosphorus, magnesium and calcium, the amounts available depending upon the rock type. Plants can make an important contribution by roots physically breaking up rocks and by root exudates accelerating the weathering process. Biological fixation primarily involves the conversion of atmospheric nitrogen into a form useable by organisms - this is discussed further under nitrogen below. Atmospheric deposition is the input of particles and gases carried or dissolved in rain, snow and fog as wet deposition, and by dry deposition as dust and fragments carried by the wind. Such inputs, especially from dust carried from other soils, can be very important in forests with poor, highly weathered soils. Chadwick et al. (1999) show that after four million years of weathering, the soils of the oldest Hawaiian islands are very nutrient poor and the rich rain forests flourish on cations (positive ions, such as calcium) supplied in sea spray carried by the wind and on phosphorus arriving in dust blown from central Asia over 6000 km away. Saharan dust is similarly important to parts of Africa. Lichens can make a considerable difference to atmospheric deposition. Knops et al. (1996) examined the lichen Ramalina menziesii on blue oak Quercus douglasii in the fog belt along the Californian coast and found 3.8 kg of lichen per tree, totalling 590 kgha-1 for the woodland, equivalent to over half of the oak leaf biomass (958 kgha-1). The extra surface area of the lichen intercepted significant amounts of many nutrients from wind and rain. It is estimated that the lichens intercepted an extra 2.85 and 0.15kgha-1 y-1 of nitrogen and phosphorus, respectively, against background levels of 0.88 and 0.06 kg ha-1 y-1.

Agriculture and industry are together putting increasingly polluting amounts of some nutrients into the atmosphere, particularly nitrogen and phosphorus. In areas of North America and Europe the atmospheric deposition rates of nitrogen are up to 40 times the normal background rate of less than 1kgha-1 y-1. The implications of such high levels are discussed in Section 11.4.

Loss of nutrients from a forest is usually by leaching to groundwater and consequent loss to streams, and gaseous loss to the atmosphere. Loss by leaching partly depends upon variations in nutrient supply and to demand; if a nutrient is available in excess to what organisms can immediately use (and so store), it is more likely to be lost by leaching. Loss also depends upon the form of the nutrient and its chemical reactivity with the soil. These both help explain why nitrate is more readily leached from soil compared with ammonium which is rapidly taken up by microbes and held by the soil cation exchange complex. Phosphorus, by contrast, is captured in the soil in an insoluble form and is less readily leached than some forms of nitrogen. Gaseous losses of nutrients are possible for those elements with a gaseous phase such as sulphur as hydrogen sulphide and nitrogen as methane and several other forms.

Once nutrients are in an ecosystem, they can be cycled a number of ways. In forests, the dominant path is in litter-fall, decomposition to mineral form and uptake by plants (discussed in detail in Chapter 7). Central in this is the role of microbes (fungi and bacteria). The co-evolution of fungi and trees over vast periods of time has resulted in extensive mutual interaction that influences this cycling; indeed Rayner and Boddy (1988) consider there is constant interaction between them via chemically communicated feedback systems and dynamic interactions between boundaries. Mycorrhizas (see Box 5.1 for more detail) protect the tree against toxins and forage for nutrient elements which they supply to trees; the trees in exchange provide the fungus with photosynthate. Endomycorrhizal or arbuscular (AM) fungi are able to garner extra supplies of phosphorus for their host principally by increasing the soil volume exploited by the fungal mycelium. Ectomycorrhizas become most important with increasing latitude as nitrogen becomes more limiting and are able to exploit extended soil volumes and by accessing nitrogen and phosphorus in forms otherwise unavailable to the host tree. Ericaceous mycorrhizas are beneficial due to their ability to utilize nitrogen and phosphorus from complex organic sources. Mycorrhizas are also capable of degrading proteins and transferring amino acids directly to the host plant. Mycorrhizal fungi additionally provide contact between the tree and the roots of adjacent members of the same species. Different species of mycorrhizas provide a link between trees and other plants. The success rate of tree seedlings is greatly enhanced when their roots form mutualistic associations with appropriate fungi (Section 5.4.1).

8.3.2 Deficiencies and proportions

Deficiencies in any of the essential nutrients lead to characteristic symptoms such as discolouration of leaves, and changes in growth such as size, number of leaves and lengths of internodes. Such deficiencies can be related to availability in the underlying bedrock but more usually are a product of soil pH. As shown in Fig. 2.3, acidic soils tend to be deficient in nitrogen, phosphorus, calcium, magnesium and potassium. Equally important, acidic conditions lead to greater solubility and increasingly toxic amounts of aluminium and iron. There is increasing evidence that nutrients are used more efficiently on nutrient-poor soils (Paoli et al., 2005): organisms grow with lower tissue nutrient concentrations.

As noted in Chapter 7 nitrogen is usually thought of as the nutrient most limiting in temperate forests. Outside these well-studied temperate forests, especially on soils of great age, phosphorus may well be the limiting nutrient. It may seem strange that this is not definitively known. The main problem is in knowing how much of the total amount of a nutrient like phosphorus (which may appear abundant) is actually available to the plant, and how the conditions in the rhizosphere can influence and change its availability at the site of uptake. It is certain that in most soils the amount of soluble P is a very small fraction of the total phosphorus content. To add further confusion, Carline et al. (2005) found that when red deer Cervus elaphus were excluded from birch woodlands for 14 years the limiting nutrient changed from nitrogen to phosphorus. This appeared to be due to increased nitrogen mineralization (see Section 8.4.1) making N readily available and so leaving P in short supply; quite why the exclusion of deer should cause this is open to speculation.

The relative proportion of nutrients available (covered by the term stoichio-metry) is a more useful way of looking at nutrient supply than concentrating on a single deficiency. Plants and animals need a balance of nutrients and this goes a long way in explaining why adding nitrogen or phosphorus to a forest soil may have little effect. In a forest the addition of a single element may often elicit some response but usually it also reveals the existence of other limiting deficiencies. The interactions between different quantities of nutrients can be very complex as they interfere (perhaps competing for uptake sites on roots) or synergistically interact with each other.

The characteristic ratios between carbon, nitrogen and phosphorus (C/N/P stoichiometry) are widely used in marine ecosystems for investigating productivity, but less so in terrestrial ecosystems. Forests tend to have relatively higher levels of carbon than marine systems due to the need for high investment in structural and defensive compounds (see McGroddy et al., 2004). C/N/P ratios of foliage and litter are globally variable but C/nutrient ratios in litter are consistently higher than in foliage (particularly C/P), suggesting that resorption of nutrients is a globally important mechanism of conserving nutrients that are in short supply (see Section 7.5.2).

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