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Managing the Cycles of Nitrogen and Phosphorus: Mitigation and Adaptation

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Session Description: 

The scale and scope of human alterations to the global cycles of nitrogen (N) and (to a lesser extent) phosporous (P) have been the subject of a number of recent analyses and syntheses, and most global change scientists now are
aware of the magnitude of these changes. The consequences of alterations to these biogeochemical cycles –
eutrophication of lakes and coastal waters, increases in the greenhouse gas nitrous oxide, oxidant air pollution,
interactions with the carbon cycle, losses of biological diversity – also are well-recognized in the global change
community. Public recognition of the importance of these changes has lagged behind scientific understanding,
and both scientific and policy engagement with managing the consequences of altered biogeochemical cycles has
lagged as well in comparison with the focused international effort to mitigate anthropogenic changes to the carbon

We propose that the Aspen Global Change Institute host a small working group focused on identifying and
developing the information that society needs to respond effectively to anthropogenic changes in the cycles of N
and P. We see no need for yet another assessment of the N and P cycles. Rather, we believe that it is time to
develop the information that will enable effective action to manage those changes. We have identified two topics
that we believe are fundamental to managing changes in N and P, and that may be ripe for progress; further, we
believe that a dedicated group focused on these issues can identify other potential topics in which progress is

Both of the areas which we have identified are agriculturally-related, as we should expect since agriculture is the
major cause of change in N and P. We should note that in keeping with the distributed nature of changes to the N
and P cycles, there will be no single global challenge to be met. For example, it is clear that in much of Africa,
agriculture doesn’t have access to enough supplemental N in contrast to much of North America, Europe, and
China, where the problem is one of too much N.

We suggest that one key topic in the N cycle is that there is a ceiling to the efficiency with which fertilizer N
(synthetic or organic) makes it into crops. Regional and global analyses demonstrate that little more than 10% of
the fertilizer N applied globally is actually consumed by humans. While there is substantial work designed to
increase that efficiency, there are striking limits to the efficiency that intensive agriculture can achieve. Even
under the best circumstances, rarely does 50% of the massive amount of N applied to agriculture go into crops.
There has been excellent work designed to increase the N efficiency of agriculture – but as long as there is an
efficiency ceiling near 50%, then at best, half of all N is going to places where it is wasted (from farmer’s
perspective) and potentially damaging to the environment. We do have ways to increase the efficiency of agricultural systems towards the ceiling – but are there paths towards increasing the efficiency ceiling itself? And if so, what are the environmental and policy implications?

For P, the challenge is different. P is relatively immobile in most soils, and much of the P applied to agricultural
systems stays in soils – with only a fraction available to crops. Continued P fertilization thus enriches soils as
well as crops, in contrast to N which is lost relatively quickly from ecosystems. As a consequence, it is often
possible to detect ancient agricultural sites to which P has been added based on enriched P in their soils. Massive
modern P applications enrich surface soils over a much wider area than did past systems, and associated modern
agricultural practices enhance erosion. Consequently, P-fertilized agricultural soils represent a substantial and
ultimately mobile source of P to downstream and downwind ecosystems, one that will continue to change
recipient ecosystems for decades and centuries even if application were soon to cease (as it cannot). What can be
done to reduce the accumulation of P in agricultural soils or the consequences of that enrichment? What can be
done to lessen the impact of the massive stores of P that already exist in the environment?

These analyses (and others) must be carried out in the context of massive ongoing changes to agriculture, in both
developed and developing countries. For example, meat and dairy demand continues to grow in the middle
income countries, encouraging the development of industrial livestock systems. Moreover, to the extent biofuels
increase as a component of developed world agriculture (as they likely will, both where they make sense and
where they do not), alterations to N and P will expand, intensify, and potentially take new forms that we ought to
try to anticipate. Dead zones in the Gulf of Mexico and elsewhere are one serious result from the intensification
of agriculture. A recent global assessment notes 400 coastal zones affected covering an area of more than
245,000 square kilometers. An interdisciplinary systems approach is required to develop sustainable pathways for
N and P management considering both mitigation and adaptation issues.