Nutrient Imbalances in Agricultural Development
Nutrient additions to intensive agricultural systems range from inadequate to excessive—and both extremes have substantial human and environmental costs.
Abstract
Nutrient cycles link agricultural systems to their societies and surroundings; inputs of nitrogen and phosphorus in particular are essential for high crop yields, but downstream and downwind losses of these same nutrients diminish environmental quality and human well-being. Agricultural nutrient balances differ substantially with economic development, from inputs that are inadequate to maintain soil fertility in parts of many developing countries, particularly those of sub-Saharan Africa, to excessive and environmentally damaging surpluses in many developed and rapidly growing economies. National and/or regional policies contribute to patterns of nutrient use and their environmental consequences in all of these situations (1). Solutions to the nutrient challenges that face global agriculture can be informed by analyses of trajectories of change within, as well as across, agricultural systems.
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References and Notes
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Nutrient budgets can be calculated at different spatial scales and levels of completeness. The agronomic budgets discussed here are based on managed inputs of nutrients and outputs in harvested crops, the most widely available information. More detailed budgets include other nutrient fluxes (atmospheric deposition, emissions of ammonia and other trace gases, denitrification, and leaching) across ecosystem boundaries and changes in nutrient pools within agricultural soils (11, 23).
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Maize and stover yields in 2004 were determined in 90 farms randomly selected from 1000 households in Siaya District, Western Kenya (24). Nutrient contents of maize and stover are from the same area of Western Kenya (25). Fertilizer applied is from household surveys conducted in the same area in 2005. Small quantities of N and P (relative to fertilizer) in manure are added to this and the other systems; these applications largely represent the recycling of nutrients rather than inputs.
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Crop yields are based on county-level National Agricultural Statistics Service data averaged for 1997–2006 for seven east-central Illinois counties. Fertilizer inputs are from 1997–2006 sales to the state of Illinois, with the county sales estimated from Census of Agriculture expenditures on fertilizers. Estimates of N and P in harvested grain and N2 fixation follow methods in (26).
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McIsaac G. F., David M. B., Gertner G. Z., Goolsby D. A., J. Environ. Qual. 31, 1610 (2002).
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Science
Volume 324 | Issue 5934
19 June 2009
19 June 2009
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Copyright © 2009, American Association for the Advancement of Science.
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Published in print: 19 June 2009
Acknowledgments
This work is based upon discussions at the Aspen Global Change Institute—supported by U.S. National Aeronautics and Space Administration, the William and Flora Hewlett Foundation, and the David and Lucille Packard Foundation—and at a Scientific Committee on Problems of the Environment (SCOPE)—sponsored meeting of the International Nitrogen Initiative in Paris. G. Billen made helpful comments on an earlier draft.
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Response to E. Ongley and Y. Tao's E-Letter
E. Ongley and Y. Tao do not disagree with our point, based on thorough experimental research summarized by X.-T. Ju et al. (1), that additions of N and P to double-cropped fields in the North China Plain (NCP) and elsewhere are both excessive and environmentally damaging (2). Their disagreement is with unreferenced analyses by "Chinese scientists" concluding that the majority of N and P in surface water in a particular catchment of the NCP derives from agricultural systems. Ongley and Tao suggest that excessive fertilizer use there pollutes groundwater rather than surface water, and that excessive withdrawals of that polluted groundwater have broken the connection between groundwater and base flow in regional rivers.
We agree that understanding the distribution of excessive nutrient additions in particular sites and regions requires knowledge of local conditions; as Ju et al. demonstrate (1), the fates of these nutrients differ substantially in double-cropped systems in North versus South China. However, regardless of this variation in loss, agri-environmental policies must focus on the fundamental problem—additions of nutrients in excess of crop requirements. Where farmers add more than 200 kg ha-1 year-1 and 50 kg ha-1 year-1 of N and P, respectively, in excess of what can be used by crops (1), environmental degradation is inevitable. That degradation may be experienced in soils, groundwater, surface water, regional air quality, and/or greenhouse gas accumulation; the answer will typically be "all of the above, but to differing extents depending on site conditions and management practices" (3). We doubt that surface waters in the NCP are largely free of agricultural influence. The volatilization and atmospheric deposition documented by Ju et al. (1) and base flow from groundwater (however reduced by withdrawals), as well as the runoff from animal systems and occasional storms mentioned by Ongley and Tao, are all likely to cause elevated N and P concentrations in surface waters. However, the larger point is that we should focus on the source of excessive nutrients in agricultural systems, not on just one of their several fates.
Peter M. Vitousek
Department of Biology, Stanford University, Stanford, CA 94305, USA.
References
1. X.-T. Ju et al., Proc. Natl. Acad. Sci. U.S.A. 106, 3041 (2009).
2. P. M. Vitousek et al., Science 324, 1519 (2009).
3. J.-W. Erisman, M. A. Sutton, J. Galloway, Z. Klimont, W. Winiwarter, Nat. Geosci. 1, 636 (2008).
Agricultural N and P Runoff: A Major Pollutant of NCP Surface Water?
P. M. Vitousek et al's. Policy Forum on fertilizer use and potential environmental impacts, at least for the North China Plain (NCP) ("Nutrient imbalances in agricultural development," 19 June 2009, p. 1519), has the potential to skew policy priorities by misidentifying causes and effects. Chinese scientists claim that agricultural N and P contribute a substantial component of total river pollution loads (1). We believe that methodological problems result in substantial overestimation (1). Increasing aridity and capture of surface water on the fields means that during many years there is little or no runoff. N pollutes groundwater, but without runoff, there can be no fertilizer contributing to surface water pollution. It is not known whether decadal rainfall events that produce major runoff from fields are significant for receiving waters, whether these respond more to smaller but more frequent events, or whether frequent runoff events are important relative to the large loadings on a continuous basis from urban and industrial sources. If the first is true, then agricultural impact must consider nutrient accumulation on the field; if the second or third is true, then there may be little effect on receiving waters. The only clear link between agriculture and surface water quality on an annual basis is that of farm and village feedlot operations that are located on banks of canals to facilitate cleaning of (mainly) liquid wastes. The N levels in groundwater have no effect on surface water, given that NCP rivers receive little to no base flow due to excessive draw-down of the groundwater table. Chinese non-point source investigations routinely ignore atmospheric N input that may amount to as much as 25% of the total N in fertilizer (2). For lakes throughout China, point-source and non-point source nutrient loads remain poorly quantified, with large degrees of uncertainty (3).
Edwin Ongley
Environment Canada (retired), Montreal, Canada.
Yu Tao
Chinese Research Academy of Environmental Sciences, Beijing, China.
References
1. E. D. Ongley, Y. Tao, in The Management of Water Quality and Irrigation Technologies, J. Albiac, A. Dinar, Eds. (Earthscan Publications UK, 2009), pp. 7–39.
2. J. L. Shen et al., Environ. Pollut. (2009).
3. Y. Wang, in The Management of Water Quality and Irrigation Technologies, J. Albiac, A. Dinar, Eds. (Earthscan Publications UK, 2009), pp. 117–134.