Phosphorus, Sustainability, and Advancing Nutrient Management in Cropping Systems
Pacific Agriculture & Natural Resources 2017, Volume 7, Number 1
Phosphorus (P) plays critical roles in all living cells, including the intracellular energy transport via the coenzyme adenosine triphosphate (ATP), cell wall integrity via phospholipids, and the biochemistry of deoxyribonucleic acid (DNA) and ribonucleic acid (RNA). Agricultural crop and livestock systems frequently receive inputs of P as fertilizers and feed mineral supplements because P availability is often a major factor limiting production. Excessive importation of P into agricultural systems may result in undesired export of this non-renewable resource to ecologically sensitive surface waters, thereby resulting in eutrophication. The paradox of ongoing global depletion of minable P reserves while economically wasteful inputs of excess P to agriculture in developed countries lead to pollution and agriculture in tropical developing countries continues to be frequently severely constrained by P deficiencies has thrust P management into the public limelight to a greater extent than any other mineral. This brief review of the complex nature of P management in cropping systems identifies and synthesizes P related sustainability issues in agriculture and outlines strategies for advancement.
At present, almost 800 million people are undernourished, with the vast majority living in developing regions (FAO, 2015). About 70% of the undernourished people globally live in Asia, predominantly in India and China (FAO, 2015). Sub-Saharan Africa with about 200 million undernourished people is another critical region of the world suffering from malnutrition (Sanchez, 2002). According to Barrett (2010), most food insecurity is associated with chronic poverty, resulting in the lack of food availability, access, and utilization. As Tscharntke et al. (2012) argue, poverty instead of the quantity of food produced is the major underlying reason for hunger. In their study, Tscharntke et al. (2012) also highlight the fact that 90% of farmers worldwide are small producers with <2 ha, concluding that smallholders rather than large-scale commercial farmers are the most significant contributors to food security in the developing countries, of which most are located in the tropics. Across the world's croplands, major nutrient imbalances exist, which depending on economic development, range from inadequate inputs in many developing countries to excessive applications in developed and rapidly growing economies (Vitousek et al., 2009). The maintenance of soil fertility is critical to long-term crop production (Dawson and Hilton, 2011), yet crop yield and quality from >60% of presently cultivated soils worldwide show mineral disorders, including toxic concentrations of aluminum (Al), manganese (Mn), and sodium (Na), and deficiencies of nitrogen (N), phosphorus (P), potassium (K), sulfur (S), iron (Fe), and zinc (Zn; Cakmak, 2002). Soil mineral disorders derive from soil degrading processes that occur naturally, but are frequently accelerated by human activities. Examples of soil degradation processes that place major limitations on crop productivity include erosion-related soil losses and leaching of nutrients. Particularly, soils developed under humid tropical climates are subjected to intense chemical weathering and rapid microbial turnover of organic matter (Jörgensen, 2010). Agricultural practices, such as crop harvesting and plant residue removal deplete soil nutrients, in particular N and P (Vitousek et al., 2009). Furthermore, conventional tillage affects soil nutrient status through accelerated organic matter decomposition, stimulated emissions of climate active gases (nitrous oxide, N2O; methane, CH4), and increased surface runoff and soil erosion (Busari et al., 2015; Krauss et al., 2017). Consequently, in order to sustain soil productivity and crop yields, the amount of nutrients that are removed with harvest and lost to the environment must be reduced, replaced through biological N2 fixation and inputs, such as animal manures and mineral fertilizers, or otherwise mitigated (Vitousek et al., 2009; Millar and Robertson, 2015).
The growing global demand for agricultural crop products, including food, feed, and fuel, is causing the large-scale conversion of forests, peatand grasslands, and savannas into cropland and pasture across much of the developing world (Fargione et al., 2008; Gibbs et al., 2010; Foley et al., 2011). Yet, in order to keep key ecosystem processes intact, natural terrestrial ecosystems must be spared from agricultural conversion. In order to achieve this, crop production must be intensified per unit of existing cropland (Cakmak, 2002). During the second half of the last century, substantial crop yield gains were accomplished through a development referred to as the Green Revolution (GR). The main strategies behind this progress included utilization of improved high-yielding cultivars, and application of mineral fertilizers, synthetic pesticides, and irrigation technology (Goulding et al., 2008; Malézieux, 2012; Pingali, 2012). Despite a tripling of cereal crop yields, the GR failed to be successful in the marginal production areas where the majority of the poorest population is located (Pingali, 2012). Moreover, excessive use of external inputs and fossil fuels has led to severe environmental problems, which are now raising concerns about the sustainability of intensive high-input agriculture (Pretty, 2008; Pretty and Bharucha, 2014).
Despite the already existing environmental impacts associated with food production, such as land clearing, habitat fragmentation, biodiversity loss, soil degradation, eutrophication of marine and freshwater ecosystems, and about one-quarter of global greenhouse gas emissions, the global population is estimated to further grow and reach 9 billion by 2050 (Vitousek et al., 1997b; Dirzo and Raven, 2003; Godfray et al., 2010). According to major projections, food production must be doubled to meet the demand of the growing world population (Cakmak, 2002; Tilman et al., 2002). Most of this increase will (have to) occur in developing nations, especially countries in South Asia, Africa, and Latin America (Graham et al., 2001; Chen et al., 2011). A recent article by Schmidhuber and Tubiello (2007) reviewed existing literature that estimated the effects of climate change on global food security. The authors concluded that between 5 and 170 million additional people will likely be at risk of hunger by 2080 as changes in temperature and precipitation patterns associated with continued emissions of greenhouse gases will bring changes in land suitability and crop yields (Schmidhuber and Tubiello, 2007).
Phosphorus is after N the most important yield determining mineral nutrient, yet its limited availability in the rhizosphere due to slow diffusion and high fixation tendency deem P major limiting nutrient for plant growth on many soils across the world (Shen et al., 2011). Hence, crop production relies on heavy applications of finite phosphates which cause severe environmental damage when transported to sensitive surface waters (Vitousek et al., 1997b). The objective of this paper is to concentrate on P as a major sustainability bottleneck of agricultural intensification worldwide and to discuss potential ways to improve P use in agriculture.
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