Ecology. Michael Begon

by contrast, are much less adept at acquiring organic nitrogen, and compete strongly with soil microorganisms for ammonium sources. Nitrates are therefore the major source of nitrogen for most higher plants (Bloom, 2015).

      The acquisition of nitrogen by plants is facilitated both by molecular transporters at the root surface and by root architecture (Kiba & Krapp, 2016). Of all the major plant nutrients, nitrates move most freely in the soil solution and are carried from as far away from the root surface as water is carried. Hence nitrates will be most mobile in soils at or near field capacity, and in soils with wide pores, and they will be captured most effectively by wide ranging, but not intimately branched, root systems. Their RDZs will themselves be wide, and those produced around neighbouring roots are likely to overlap such that the roots compete for the same nitrogenous molecules.

As we discuss in more detail in Section 13.9, the roots of most terrestrial plants are colonised by specialist fungi, forming mycorrhizas. In fact, it is these intimate, ‘mutualistic’ associations between the two (beneficial to both parties), rather than simply the roots alone, that are responsible for nutrient acquisition (as well as providing a series of other benefits to the plants). The fungi, for their part, are reliant on the plants for carbon. The advantages to plants of having mycorrhizal fungi are most apparent in the case of less mobile nutrients (see below), but even with nitrogen, mycorrhizas may have some role to play (Jin et al., 2012). Of arguably greater significance to the nitrogen economy, many plants form intimate mutualistic associations in their roots with nitrogen‐fixing bacteria, overcoming the shortage of available nitrogen in the soil by harnessing the microbes’ ability to convert free nitrogen in the atmosphere into ammonia, nitrate and other compounds. The most important example is the association between leguminous plants and rhizobia. These are discussed in detail in Section 13.11.

      phosphorus

      In many habitats, the phosphorus levels available to plants are limiting to growth, even though phosphorus itself may be abundant. It forms inert complexes, notably with iron and aluminium, and even the free phosphorus in soil solutions is relatively immobile, much of it being tightly bound on soil colloids from which its release is difficult. In contrast to nitrogen, therefore, it pays plants foraging for phosphate to explore the soil intensively rather than extensively, and the RDZs tend to be narrow. Roots or root hairs or threads from mycorrhizas will only tap common pools of free phosphorus (that is, they will compete with one another) if they are very close together.

Indeed, mycorrhizas play a crucial role in facilitating most plants’ acquisition of phosphate, producing branched mycelial threads up to 100 times longer than root hairs, as well as having physiological capabilities that increase the phosphate flow (Javot et al., 2007). At the very lowest levels of phosphate availability, however (either because of its near‐absence in the soil or because it is especially tightly bound) a number of plants lack mycorrhizas, using instead an alternative strategy, namely the production of citrate and other carboxylates in their roots, often specialised, very finely divided structures called cluster roots. The carboxylates mobilise phosphate from its tightly bound (unavailable) state, such that cluster root species can make better growth at low levels of phosphorus supply than mycorrhizal species (Lambers et al., 2015).

      potassium

      Potassium is another key mineral in plant nutrition, often abundant in the soil, but again, strongly adsorbed to soil particles and hence of potentially limiting availability. The role of mycorrhizas in potassium acquisition is relatively poorly understood but is becoming increasingly apparent (Garcia & Zimmermann, 2014).

      It is clear even from these few examples that different mineral ions are held by different forces in the soil, that plants with different shapes of root system, with different root system properties, and with different mycorrhizal associations may therefore tolerate different levels of soil mineral resources, and that different species may deplete different mineral resources to different extents. This may be of great importance in allowing a variety of plant species to cohabit in the same area. We deal with the coexistence of competitors in Chapter 8.

3.6 Oxygen – and its alternatives

      Oxygen is a resource for both animals and plants as the final electron acceptor in the process of aerobic respiration that provides the energy that drives their metabolism. In above‐ground terrestrial environments it is rarely in limited supply, but its diffusibility and solubility in water are very low and so it can become limiting much more readily in aquatic and waterlogged environments. Because oxygen diffuses so slowly in water, aquatic animals must either maintain a continual flow of water over their respiratory surfaces (e.g. the gills of fish), which often have very large surface areas relative to body volume, or they may have specialised respiratory pigments (e.g. diving mammals and birds, see Mirceta et al. (2013)), or may continually return to the surface to breathe. The roots of many higher plants fail to grow into waterlogged soil, or die if the water table rises after they have penetrated deeply. Even if roots do not die when starved of oxygen, they may cease to absorb mineral nutrients so that the plants suffer from mineral deficiencies.

      However, it would be wrong to adopt a higher‐organism centred point of view in which respiration is predictably aerobic, reliant on oxygen as a resource that is equally predictably available. On the contrary, there are environments where oxygen is simply absent – often described as ‘extreme’, such as hot springs or deep in the ocean – and many others in which oxygen levels are depleted by biological activity at rates that cannot be counteracted by diffusion or by the activities of photoautotrophs. This is the case, for example, when organic matter decomposes in aquatic environments, and microbial respiration makes a demand for oxygen that exceeds the immediate supply. It is true, too, in water bodies that suffer eutrophication (see Section 21.1.3) when they are overly enriched with nutrients, particularly nitrates and phosphates, often as pollutants, inducing excessive growth of plants and algae that may again deplete oxygen faster than it can be replaced. Many microorganisms living in all these types of environment respire anaerobically, using alternative resources to oxygen as the final electron acceptor in the respiratory process: nitrates, sulphates, CO₂, ferric iron and many others. Of course, where oxygen is absent altogether, all those respiring actively must do so anaerobically.

      anaerobic respiration: widespread and varied

Anaerobic respiration is generally far less efficient (produces much less energy) than aerobic respiration. Hence, when oxygen is in ready supply, aerobes thrive and anaerobes are little in evidence. However, the balance within ecological communities can change rapidly. One reason is that many microbes are facultatively anaerobic – capable of both aerobic and anaerobic respiration. We see an example in Figure 3.27a, where pitcher plants (carnivorous plants that trap their prey in pitcher‐shaped modified leaves) contain a digestive liquid that supports a community of microbes. Natural communities of pitchers of the northern pitcher plant, Sarracenia purpurea, from Vermont, USA, were compared with pitchers that had been enriched through the repeated addition of finely ground insects (without a microbial community of their own) similar to the plants’ natural prey. Such excess loading of organic material leads to an increase in microbial activity and hypoxic (low oxygen) conditions within the experimental pitchers, as can happen naturally in pitcher plants, and as it does in much large water bodies such as ponds and lakes. The microbial activity within the communities of the control and experimental pitchers were very different, as judged by the peptides within them, which could be extracted, identified and assigned to the types of microbes producing them (Figure 3.27a). When oxygen was readily available as a resource, most of the peptides were contributed by aerobic bacteria. But when oxygen was in very short supply, most came from facultative anaerobes that could rapidly switch their metabolism from aerobic to anaerobic respiration. It is also notable, therefore, that in neither case was there a major contribution from bacteria that were obligatory anaerobes.