Global trends in nitrate leaching research in the 1960–2017 period

Highlights

• A rising interest in last decades in nitrate leaching research worldwide

• Most research dealt with nitrate leaching from agroecosystems and farmlands

• Dominance of soil nitrogen cycle, fertilizer use and water quality research topics

• Most research on nitrate leaching conducted in the United States and China

• Increasing relevance of research with maize, wheat and grasses from 1990

Abstract

Nitrate leaching is the process whereby the nitrate (NO₃⁻) anion moves downwards in the soil profile with soil water. Nitrate leaching is commonly associated with chemical nitrogen (N) fertilizers used in agriculture. Nitrate leaching from different sources and contamination of surface and groundwater is a global phenomenon that has prompted social and political pressure to reduce nitrate leaching and contamination of water bodies. This bibliometric study analyzed global trends in nitrate leaching research. The results showed a rising interest in the last decades in this topic; given the growth tendency over the last years, it was envisaged that the importance on nitrate leaching research will continue increasing in the future. Knowledge on nitrate leaching was mostly disseminated through scientific publications (90% of total documents recovered), both as journal articles and reviews, classified in the Scopus database in the Agricultural, Biological and Environmental Sciences areas. Most publications dealt with soil nitrogen losses from agroecosystems and farmlands and the associated impact on the environment; they were published in journals with a focus on the influence of anthropogenic and soil-crop-animal systems in the environment, and on how such changes in the environment impact agroecosystems. Most documents published on nitrate leaching were indisputably from the United States, followed by China, the United Kingdom and Germany. An analysis of the main keywords showed an overall dominance of the soil nitrogen cycle, fertilizer use in agriculture and water quality aspects. The evolution of main crop species involved in nitrate leaching research showed a rising relevance of research conducted with maize, wheat and grasses from 1990 onwards. The most productive institutions in terms of number of documents dealing with nitrate leaching research, h-index and total citations, were located in the United States, China and the Netherlands. The United States Department of Agriculture stood out, followed by the Chinese Academy of Sciences and Wageningen University and Research. There were clusters of institutions with intercontinental interaction, on nitrate leaching research, between institutions from Europe, Asia and South and North America. Overall, this study has highlighted, from a bibliometric perspective, the rising concern on nitrate leaching. Progress in this field has been made particularly on the impact of the soil-plant-animal system on the environment and agroecosystems, and on fundamental and applied aspects of plant-soil interactions with an emphasis in cropping systems.

Introduction

Nitrogen (N) is an essential element for all life process in plants (Hester et al., 1996); it is a structural component of all proteins, including enzymes involved in photosynthesis, growth and development, and is an important component of nucleic acids and chlorophyll (Gianquinto et al., 2013; Lawlor et al., 2001). At the same time, N is one of the major limiting nutrients in most ecosystems and agricultural soils (Vitousek et al., 1997), which commonly contain between 0.1% and 0.6% N in the top 15 cm, depending on the soil type (Cameron et al., 2013). Soil N is present in four major forms: (a) organic matter, such as plant material, fungi and humus; (b) soil organisms and microorganisms; (c) ammonium ions (NH₄⁺) held by clay minerals and organic matter, and (d) mineral N forms in soil solution, including NH₄⁺, nitrate (NO₃⁻) and low concentrations of nitrite (NO₂⁻) (Cameron et al., 2013; Hester et al., 1996). However, any N in the soil that is available to plants is likely to be present as NO₃⁻, or as NH₄⁺, which microbes of the soil soon convert to NO₃⁻ (Hester et al., 1996). Mineral N forms are mainly prone to losses through: (a) ammonia (NH₃) volatilization (i.e., the loss of gaseous NH₃ from the soil surface), (b) denitrification and gaseous losses of nitrogen (mainly as dinitrogen gas (N₂) and nitrous oxide (N₂O)), and (c) leaching (i.e. removal in drainage water) (Cameron et al., 2013; Gillette et al., 2018). Nitrogen losses by leaching occur mainly in the NO₃⁻ form but some leaching of NH₄⁺ may occur in sandy soils (Moreno et al., 1996). It is leaching of the NO₃⁻ anion that is analyzed in this article.

Fig. 1 summarizes the nitrogen cycle and the nitrate leaching process, whereby the NO₃⁻ anion moves downwards in the soil profile with soil water (Gianquinto et al., 2013; Hester et al., 1996). Nitrate is completely soluble in water and is prone to be leached, because the negatively-charged NO₃⁻ anion is repelled by negatively charged surfaces of clay minerals and soil organic matter. This keeps nitrate dissolved in the soil solution and moves freely in the soil by percolating rainfall or irrigation (Gianquinto et al., 2013; Hester et al., 1996).

Nitrate leaching is commonly associated with chemical fertilizers used in agricultural crops (Cameron et al., 2013; Fowler et al., 2013; Lemaire and Gastal, 1997; Pratt, 1984), but some of the soil nitrate that is vulnerable to leaching is produced by microbes that break down plant residues and other nitrogen-containing residues in the soil (Hester et al., 1996). Localized sources of nitrate leaching can be animal organic waste effluents; some of these being dairy shed effluent, dairy pond sludge, pig slurry or sewage sludge (Di and Cameron, 2002; Power and Schepers, 1989). Published data indicate that nitrate leaching losses typically would follow the order: forests < cut grassland < grazed pastures < arable cropping < ploughing of pastures < horticultural and vegetable crops (Cameron et al., 2013; Di and Cameron, 2002). Nitrate leaching losses are generally lowest from forest systems because there is usually zero or only low rates of N fertilizer applied, and the N is cycled efficiently through the forest ecosystem (Di and Cameron, 2002). However, logging and burning of forests can release large amounts of N that can be leached or washed off slopes through soil erosion (Cameron et al., 2013). In grassland systems, NO₃⁻ comes from fertilizers (i.e., mineral or urea-based fertilizers) or from mineralization of soil organic N. Grasslands that are mown or cut for hay or silage have very low nitrate leaching losses, because grass and pasture plants are usually very efficient at taking up the N applied in fertilizer or N fixed by legumes such as clovers that are grown in the pasture sward (Cameron et al., 2013). The nitrate leaching potential increases when grassland is grazed rather than harvested. This is because a large proportion of the N ingested by the grazing animals is excreted back to the soil in the small concentrated areas of urine and dung patches (Minet et al., 2018). However, the low animal stocking density of extensive systems means that the whole of the grazed field is not covered by urine patches (Cichota et al., 2018). The overall nitrate leaching loss is thus somewhat diluted by the lower leaching loss from the inter-urine patch areas (Cameron et al., 2013). Nitrate leaching losses are generally greatest from horticultural crops because of the higher rates of N fertilizer that are used in these crops, the shallow root systems of horticultural plants and the low nutrient use efficiency (Fereres and Goldhamer, 2003; Goulding, 2006; Meisinger et al., 2008; Pratt, 1984; Thompson et al., 2007b). Indeed, intensive vegetable production systems are commonly associated with significant nitrate leaching loss worldwide (Goulding, 2006; Min et al., 2011; Ramos et al., 2002; Thompson et al., 2017; Zotarelli et al., 2007).

Nitrate leaching losses from soil into water not only represent a loss of soil fertility but also represent a threat to the environment and to human health (Cameron et al., 2013; Hester et al., 1996; Watanabe et al., 2018). Nitrate leaching from different sources and contamination of surface and ground water is a global phenomenon (Ju et al., 2006; Prakasa Rao and Puttanna, 2006; Pulido-Bosch et al., 2000). Nitrate that enters rivers or lakes can contribute to eutrophication, which may result in algae blooms and loss of fish (Cameron et al., 2013). A critical factor related to nitrate leaching from irrigated lands is the subsequent use of drainage waters or waters composed significantly of drainage waters. The problem of nitrate leaching to groundwaters is naturally more crucial in areas where high-value crops with high water and high N demands are grown and where municipalities and irrigation districts are both using the underground supplies (Pratt, 1984). In addition, there is a public concern about nitrate as a health hazard (Hester et al., 1996). This arises from two medical conditions that have been linked to nitrate: methaemoglobinaemia (or the ‘blue-baby syndrome’) in infants, and stomach cancer in adults. Both are serious conditions that are not caused by NO₃⁻ itself, but by the reduction of NO₃⁻ to NO₂⁻; nitrate itself seems to be harmless (Hester et al., 1996).

Methaemoglobinaernia or the ‘blue-baby syndrome’ can occur when an infant ingests too much NO₃⁻ in drinking water. Microbes in the stomach convert NO₃⁻ to NO₂⁻and when this reaches the blood-stream it reacts with the haemoglobin. Normal oxyhaemoglobin becomes methaemoglobin, greatly lessening the capacity of the blood to carry oxygen (Hester et al., 1996). A link between stomach cancer and NO₃⁻ consumption in drinking water has been suggested (Hester et al., 1996). Nitrite produced from reduction of NO₃⁻ could react in the stomach with a secondary amine coming from the breakdown of meat or other protein, to produce an N-nitroso compound. The N-nitroso compounds are carcinogenic, so the reaction could result in stomach cancer (Hester et al., 1996).

Nitrogen fertilizers are commonly required in large amounts in modern agriculture to guarantee high crop yields (Fowler et al., 2013; Lemaire and Gastal, 1997). However, only part of the N applied is recovered by crops (Ter Steege et al., 2001; Vitousek et al., 2009), and much of the excess is lost as nitrate leaching beyond the rooting zone. Traditionally, a secure fertilization strategy, based on application of N quantities larger than those strictly required for maximum yield, was used to ensure profit (Thompson et al., 2007b). However, this secure fertilization strategy cannot be longer used (Lawlor et al., 2001). Protection of the environment and improved N management become a necessary constraint for sustainable agriculture (Ter Steege et al., 2001). Solving the problem of nitrate leaching starts with the optimization of N fertilization with respect to the plant demand and the soil supply capacity (Agostini et al., 2010; Ju et al., 2009; Neeteson et al., 1999; Ter Steege et al., 2001). The surest way of avoiding nitrate leaching is to ensure that as little NO₃⁻ as possible is in the soil at any time (Hester et al., 1996). However, nitrate leaching is not only related to N inputs but also to the interaction between N processes and the water balance in the soil (Moreno et al., 1996; Pratt, 1984; Ter Steege et al., 2001). In fact, nitrate leaching is mainly determined by NO₃⁻ concentration in the soil during the drainage period (Cameron et al., 2013; Ter Steege et al., 2001) and the amount of water that moves through the soil (Cameron et al., 2013; Pratt, 1984). With the exception of a few areas where irrigation waters are almost salt-free, irrigated lands must be leached periodically to maintain the rooting zone free of excessive soluble salts (Moreno et al., 1996; Pratt, 1984). In many areas leaching takes place as a result of rains; in some areas the rainfall is so small or so erratic that management must provide sufficient irrigation water to leach the soil profile. In irrigated lands, the leaching process is a result of the combination of relatively large inputs of N and ample irrigation that move drainage waters beyond the root zone (Pratt, 1984).

In addition to soil NO₃⁻ concentration and drainage volume, many other factors such as the nature of the crops, the type of soils or the cropping techniques are also responsible for the nitrate leaching potential (Di and Cameron, 2002; Pratt, 1984; Ter Steege et al., 2001). Soil properties have an influence on the nitrate leaching because they affect how the water is moved. The nitrate leaching losses are usually less from fine-textured soils than from coarse-textured soils, because of slower drainage and greater potential for denitrification (Di and Cameron, 2002). The depth of the vadose zone, i.e. the part of the soil that comprises the unsaturated zone beyond the roots and above the groundwater or zone of saturation, is also an important factor, with nitrate reaching the groundwater quicker in shallow soils than in deep soils (Di and Cameron, 2002).

Concerns over human health and environmental impact associated with nitrate leaching have prompted social and political pressure to reduce contamination of water bodies with nitrate originating from agriculture. For example, in the European Union (EU), two pieces of legislation, the Nitrate Directive 91/676/EEC (EEC, 1991) and the Water Framework Directive 2000/60/EC (Council of the European Communities, 2000), require all farmers in areas sensitive to nitrate leaching, to adopt improved N management practices. Several organizations have set NO₃⁻ concentration limits for drinkable water: the World Health Organization and the EU imposes a limit of 50 mg L⁻¹ (EEC, 1991; World Health Organization, 2011), the United States Environmental Protection Agency (EPA, 2007) and the Water and Air Quality Bureau of Canada (Health Canada, 2013) set the limit at 45 mg L⁻¹.

Scientific publication is the end product of research activity. The scientific productivity of researchers can be assessed by a quantitative and qualitative description of their production. This in turn can be extended to the institutions and countries to which they belong. For bibliometric analysis, extensive bibliographic information is required (Hood and Wilson, 2001; King, 1987). A bibliographic database is usually used for this purpose (Rojas-Sola and Aguilera-García, 2015). These databases are made up of a set of records with bibliographic information (author, title, name of the source, date of publication, keywords, citations). Bibliometric studies consists of the use of tools and methodologies aiming at analyzing scientific production and trends in a research area (Cobo et al., 2015). Thanks to these tools it is possible to identify trending topics since the development of the research field and assess the current state of research, as well as the contributions of institutions and countries in the given field.

The present bibliometric study aims to analyze global perspectives in nitrate leaching research in the 1960–2017 period using the Scopus database. The existence of two major databases, Web of Science (Clarivate Analytics, Philadelphia, PA, USA) and Scopus (Elsevier B.V., Amsterdam, The Netherlands), poses the important question of the comparison and stability of statistics compiled from different data sources (Salmerón-Manzano and Manzano-Agugliaro, 2017). The overlap between databases and the impact of using different data sources for specific fields of research on bibliometric indicators has been measured by several research studies, revealing a greater number of journals indexed by Scopus when compared to Web of Science (Mongeon and Paul-Hus, 2016). With respect to the overlap, 84% of Web of Science titles are also indexed in Scopus, while only 54% of Scopus titles are indexed in Web of Science (Gavel and Iselid, 2008).