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macronutrients and trace elements

It takes more than light, CO₂ and water to make a plant. Mineral resources are also needed and these must be obtained from the soil or, in the case of aquatic plants, from the surrounding water. These include macronutrients (i.e. those needed in relatively large amounts) – nitrogen (N), phosphorus (P), sulphur (S), potassium (K), calcium (Ca), magnesium (Mg) and iron (Fe) – and a series of trace elements, for example, manganese (Mn), zinc (Zn), copper (Cu), boron (B) and molybdenum (Mo) (Figure 3.25). Many of these elements are also essential to animals, although it is more common for animals to obtain them in organic form in their food than as inorganic chemicals. Some plant groups have special requirements. For example, aluminium is a necessary nutrient for some ferns, silicon for diatoms, and selenium for certain planktonic algae.

Figure 3.25 Periodic table of the elements showing those that are essential resources in the life of selected organisms.

Green plants do not obtain their mineral resources as a single package. Each element enters the plant independently as an ion or a molecule, and each has its own characteristic properties of absorption in the soil and of diffusion, which affect its accessibility to the plant even before any selective processes of uptake occur at the root membranes. All green plants require all of the ‘essential’ elements listed in Figure 3.25, although not in the same proportion, and there are some quite striking differences between the mineral compositions of plant tissues of different species and between the different parts of a single plant (Figure 3.26).

Figure 3.26 The mineral compositions of different plants and plant parts are very different. (a) The relative concentration of various minerals in whole plants of four species in the Brookhaven Forest, New York. (b) The relative concentration of various minerals in different tissues of the white oak (Quercus alba) in the Brookhaven Forest. Note that the differences between species are much less than between the parts of a single species.

Source: After Woodwell et al. (1975).

foraging for nutrients

There are strong interactions between water and nutrients as resources for plant growth. Roots will not grow freely into soil zones that lack available water, and so nutrients in these zones will not be exploited. Plants deprived of essential minerals make less growth and may then fail to reach volumes of soil that contain available water. There are similar interactions between mineral resources. A plant starved of nitrogen makes poor root growth and so may fail to ‘forage’ in areas that contain available phosphate or indeed contain more nitrogen. Again, plants may make both strategic and tactical responses to heterogeneities in nutrient availability (Hodge, 2004).

nitrogen

Nitrogen is the element that organisms require in the greatest amounts after carbon, hydrogen and oxygen. It is no surprise therefore that nitrogen availability often limits overall productivity in an ecosystem. Higher plants acquire nitrogen through their roots in inorganic form – as ammonium and nitrate salts – and in organic forms as urea, peptides and amino acids. This is true, too, of microorganisms, but they are best adapted to use the organic sources, followed by ammonium and then nitrate. Phytoplankton, fungi, cyanobacteria and bacteria, therefore, usually only assimilate nitrate in the absence of organic nitrogen and ammonium. Higher plants, 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.