3.1.2  Root architecture and uptake of nutrients

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Figure 3.3 Root length density in relation to depth in the soil for a wheat crop (circles) and a jarrah (Eucalyptus marginata Sm.) forest (squares). Both have a dense population of roots near the surface but wheat roots barely penetrate below 1 m, whereas jarrah roots penetrate to well below the 2.5 m shown here, often to 20 m (Based on Carbon et al. 1980 (jarrah); and F.X. Dunn, unpublished data (wheat))

Total length of root per unit volume of soil (root length density, L, expressed in km m–3) is often large in surface layers of the soil and typically decreases with increasing depth. Commonly, hundreds of kilometres of root per cubic metre of soil are observed near the soil surface (Table 3.1). Figure 3.3 shows L as a function of depth in a wheat crop in early spring, and under a Eucalyptus stand, also in spring. Dense root proliferation near the soil surface probably reflects an adaptation of plants to acquisition of nutrients such as phosphorus, potassium and cationic micronutrients such as zinc and copper. Such nutrients do not move readily in soil, hence roots branch prolifically to ensure close proximity (a few millimetres) between adsorbing surfaces and these soil-immobile ions. Roots of jarrah are also concentrated near the soil surface (Figure 3.3) but some roots penetrate very deeply to tap subsoil moisture. Such water escapes the wheat roots and finishes up as groundwater.

Many plants are mycorrhizal (Section 3.4). They form symbioses (mycorrhizas) with certain fungi which obtain fixed carbon from the host plant, in turn supplying the host with poorly mobile nutrients, especially phosphorus. This is achieved by proliferating their hyphae to provide a much greater surface area for nutrient uptake than could be provided by roots alone. Another adaptation, common in the Proteaceae, and also occurring in some species of lupin, is proteoid roots, clusters of tiny rootlets that greatly enlarge the available surface area for ion uptake and which are inducible by low levels of phosphorus (see Case study 3.1).


Figure 3.4 Excavated root system of wheat plants whose roots were provided with a concentrated band of ammonium sulphate fertiliser at the head of the arrow. This band is toxic at first but roots start to proliferate in its vicinity and eventually form a dense cylindrical cluster as they progressively take up the fertiliser (Photograph courtesy J.B. Passioura)

Nutrients are distributed unevenly in soil, generally being concentrated in the topsoil and also dispersed elsewhere in pockets. Surface enrichment arises from diverse sources such as dead fauna, urine patches and localised application of fertiliser. Root systems respond to enriched zones of nutrients by proliferating in their vicinity. Figure 3.4 shows an example of such a proliferation; the dense cluster of roots in the centre of the figure is a response by the row of wheat plants to application of a large pellet of nitrogen fertiliser (see arrow). Such proliferations ensure plants garner nitrogen ahead of loss to competing plants or as leachate into subsoil.

Young roots absorb nutrients more rapidly than old roots. New roots supply annual plants with abundant sites for nutrient uptake, especially during establishment. A feature of the roots of perennials is that they have a large turnover of the fine, high-order lateral roots that emerge from the secondarily thickened framework each year. This turnover draws heavily on photoassimilate, equivalent to between 40 and 90% of the standing biomass of temperate forests and half the CO2 fixed in desert succulents (cited in Waisel et al. 1996). Production of fine (and often ephemeral) roots ensures uptake of nutrients over many years.