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Plant nutrients represent only a tiny fraction of plant biomass, but fulfil multiple roles as either catalytic or structural components of all living cells. Over 90 elements are known to exist in nature, but only 14 or 15 appear crucial to plant growth and reproductive development. Moreover, these few essential elements have to be acquired from a huge soil volume relative to plant volume, and against an uptake gradient. Root system proliferation, fine root turnover, ongoing maintenance and energy-dependent uptake of key nutrient resources all contribute to a substantial biological cost.

Nutrient acquisition is thus expensive in terms of both energy and carbon, and draws upon a sizeable fraction of photoassimilate. On nutrient-rich sites, vascular plants might expend less than a third of their fixed carbon on root-zone operations, but on nutrient-poor sites where the cost:benefit ratio is less favourable, at least two-thirds or even more of their carbon resources are so utilised.

Nutrients often constitute a sparse resource, and in contrast to young soils in a country such as New Zealand, soils in much of Australia are relatively old in world terms due to a lack of geological activity that produces new rock. Such old, highly weathered soils tend to be infertile. They represent a strong selection pressure for evolution of adaptive features in vascular plants that enhance nutrient acquisition and economise on subsequent use via recirculation in vivo. As evidence, slow growth, sclerophylly and internal recycling, coupled with biotic associations below ground, are widely represented in Australian species native to depauperate regions. Some cases are discussed later. Crop plants have contrasting needs, and have to be sustained by high inputs to realise their genetic potential for yield. Meeting those needs on nutritionally poor sites calls for major additions of key elements which in turn carry adverse consequences for soil biology.

Clearly then, native vegetation in much of marginal Australia has a lower order of resource requirements compared with intensive industries such as the agriculture, plantation forestry and grazing enterprises that displaced it. Australia’s ancient landscape has a complex geology and biology and is subject to climatic extremes. Distinctive ecosystems evolved as resilient and genetically diverse assemblages. As a consequence, once land has been used for high-input rural industries, it is well-nigh impossible to rebuild a functional ecosystem that resembles a native state. South Africa and parts of India share aspects of Australia’s Gondwanan heritage and offer parallels to our complex geology, biology and climate.

Are these new rural industries sustainable? Until 200 years ago, Australia did not have a cultivated agriculture but previous human manipulation of the environment is evident in fire ecology (Section 19.4). Native species survived that impact without modern supplements. Present-day agriculture and forestry have expanded in response to an increase in human population and its need for food and fibre, but at a price. Agriculture inevitably results in an ecological and species simplification of plants and animals, and this expedient is imposed upon vast areas of land. Crop species lack diversity, and are predominantly annuals. By contrast, native vegetation is predominantly perennial, heterogeneous and patchy with asynchronous dynamics. Native plants have also co-evolved with a wide array of biotic associations for nutrient acquisition. As a consequence, agricultural and pastoral developers first encountered stable landscapes with well-buffered ecosystems that had attained a stable equilibrium with soil nutrient resources.

Low availability of soil nitrogen and phosphorus limits productivity of most agricultural plants and has shaped adaptive features of many native species. In addition, perennial native species usually have more efficient mechanisms for nutrient acquisition. Moreover, they are less dependent on annual nitrogen and phosphorus uptake, and have more modest seasonal demands due to slower growth rates. Native plant communities have a great array of species which fix nitrogen
biologically and contribute to soil resources. Both nitrogen-fixing and non-fixing species have a wide range of mycorrhizal associations that are missing in cropped land.

In fertile ecosystems, mineralisation of organic nitrogen to ammonia and subsequent oxidation to nitrate are major restrictions on nitrogen supply to plants. However, with infertile ecosystems, measured rates of net microbial production of inorganic nitrogen are often less than half the observed rates of nitrogen acquisition. Plants must therefore acquire nitrogen from two sources, an inorganic source which forms after release by mineralisation and an organic pool of unmineralised nitrogen. Polyphenols predispose plants towards retention of organic nitrogen in root zones which can then be acquired by native plants or their mycorrhizal associates. Research on uptake processes by rainforest tree seedlings implies that plants so adapted can take up and metabolise phenol-bound organic nitrogen thus sourcing a previously unrecognised form of plant-available nitrogen (Case study 16.3).

Native vegetation is highly adapted to cope with sparse nutrients, and in heterogeneous communities different species draw on different resources at critical times. Native plants thus operate according to a broader seasonal tempo compared with crop plants where requirements of a given planting are uniform in time and space.

With European settlement, rural practices displaced the original self-sustaining ecosystems with high-input enterprises. Sustainability in rural areas then becomes an issue. Judicious and costly inputs must be integrated with a farming business to maximise returns on investment but minimise landscape deterioration. An understanding of nutrient physiology and soil–plant nutrient ecology aids that process (Feature essay 16.1). Standard abbreviations for chemical elements have been adopted throughout this chapter and are as follows: nitrogen, N; phosphorus, P; potassium, K; calcium, Ca; magnesium, Mg; sulphur, S; iron, Fe; boron, B; zinc, Zn; manganese, Mn; copper, Cu; cobalt, Co; molybdenum, Mo; chlorine, Cl; sodium, Na; silicon, Si; Nickel, Ni; cadmium, Cd; selenium, Se; chromium, Cr; iodine, I; aluminium, Al; lead, Pb.