Area of Research: Forests, Farms and Materials

Consumers

The past century. In the United States, absolute timber consumption for products and fuel grew 70 percent between 1900 and 1993 (US Bureau of the Census 1975, 1996). Including the use of residues, consumption of wood for pulp exceeded that for lumber, which stayed nearly level. Fuelwood consumption nearly disappeared only to reemerge during the Depression and the oil shocks of the 1970s. Plywood and veneer consumption began to account for a little under 10 percent of the processed harvest in the early 1990s.

The annual percentage change of the national consumption of each of these products is approximately the sum of the small percentage changes in the first three components of equation 1. In the equation, consumers’ leverage the flow of forest products with their numbers (population), wealth (GDP per capita), and the amount of product consumed per dollar of GDP. Industrial ecologists call the amount of product consumed per dollar of GDP “intensity of use” (IOU). The difference between changes in population plus wealth on the one hand, and the national consumption on the other is the IOU. This factor gauges how large a part forest products played in national economic activity as represented by the GDP.

From 1900 to 1993, American population tripled, rising an average 1.3 percent per year. The annual growth of the GDP per person fluctuated, from -4 percent entering the Depression to more than +8 percent in the following decade; the annual average for the century was 1.7 percent. Combined, growing population and personal wealth raised GDP 16 times, an average of 3 percent per year. But the 70 percent rise in timber consumption from 1900 to 1993 corresponds to an annual increase of only 0.5 percent. The IOU, which represents the difference, thus fell an average of 2.5 percent per year.

What changes timber IOU? Constant IOU means that consumption rises in tandem with the rise of population and personal wealth. For example, 2 percent more people and 2 percent more GDP per person would raise consumption 2 + 2 + 0, or 4 percent. If paper use grows faster than the economy, however, and the IOU for paper goes up by 2 percent each year, then consumption rises 2 + 2 + 2, or 6 percent. If paper use slows relative to the economy and the IOU falls 2 percent per year, then consumption rises only 2 + 2 – 2, or 2 percent.

With rare exceptions, during the 20th century the IOU for solid wood products fell (fig. 1, p. 10). Steel, concrete, and plastic became substitutes for wood in certain structures, furniture, barrels, and cross ties. New preservatives for wood prevented fire and decay. Such changes helped lower the IOU for solid wood.

Early in the century US paper use grew dramatically, raising the IOU of pulp even during the Depression. Beginning in the late 1930s, the IOU for pulp fell sharply as the wartime economy lifted GDP faster than paper consumption. Following a burst after the war, the IOU for pulp fell and has been falling since. Because we calculated the IOU only from the industrial roundwood that millers used for solid wood products and paper, more efficient utilization by millers deserves some of the credit for lowering the IOU we calculated (see note in figure 1). By the early 1990s, the falling IOU of pulp actually overcame the growing population numbers and affluence to reduce pulpwood consumption.

Figure 1. Relative annual percentage changes in intensity of use (IOU) of roundwood used for solid wood, pulp, and fuel. IOU is amount of product divided by GDP. Sources: US Bureau of the Census 1975, 1996; USDA 1993. Note: The data represent the industrial roundwood used for the product itself. Intensity of use goes up with consumer demand but is moderated by increases in efficient utilization; the graph incorporates both effects.

Use of fuelwood fluctuated dramatically over the century. Early in the 1900s, the growing use of coal and oil lowered fuelwood IOU. Then, during the Depression fuelwood IOU soared, only to fall steadily until the mid 1970s, when oil shocks again encouraged the burning of fuelwood.

This next century. Population projections for the next 5 0 to 100 years in the United States range from 0.3 to 0.7 percent per year (US Bureau of the Census 1996). From 1985 to 1995 annual per capita GDP grew an average of 1.3 percent per year-a bit slower than the 1.7 percent average since 1900. The combined growth of population and GDP per person can reasonably be projected to be about 2 percent per year.

After considering trends in industrial production and consumer demand, the USDA Forest Service projects national timber consumption for products to grow about 0.6 percent per year from 2000 to 2040 (Haynes 1990). Assuming the 2 percent rise in population and GDP, the 0.6 percent projection implies a 1.4 percent annual decline in timber IOU. This compares with the average annual drop of 1.8 percent in IOU from 1985 to 1995 and the 20th-century average of -2.5 percent. If consumers continue the long-term 2.5 percent drop in IOU, they could counter the 2 percent rise in population and wealth and reduce timber consumption 0.5 percent annually.

Millers

The past century. In equation 1, millers exert their leverage by changing wood per product in the flow volumes depicted in figure 2 (p. 11). In 1993, 646 million cubic meters of wood and fiber entered the US forest product sector. Trees furnished 78 percent, recycling 10 percent, and imports 12 percent. Other removals (i.e., timber from thinnings or land-clearing operations) and logging residues never entered the commercial flow. Mills consumed 26 percent for solid wood products, 26 percent for paper, and 36 percent for fuel 10 percent was exported. The dotted lines in the center column show the flow of residues in mills, primarily for paper and fuel. To transform logs into lumber efficiently, Millers took advantage of sophisticated new scanning and cutting technologies and sharper, more stable blades (Haynes 1990). They raised the ratio of lumber to roundwood from 33 percent in 1970 to 42 percent in 1993. The volume of unused residue has fallen about 30 million cubic meters since the 1950s. Since 1970, the residues lost in making lumber fell from over 25 percent to 2 percent. Stretching the value of each log and each hectare, Millers also developed composite products like oriented strand board that use irregular wood shapes as well as previously unused tree species.

Figure 2. Flow of forest products, in millions of cubic meters, in the United States in 1993. We use volume instead of mass to eliminate variables like changing moisture content, mineral fillers, and synthetics in products; I cubic meter of wood is considered equivalent to 0.5 metric ton of paper. The dotted rules show the flow of residues in mills. Notes: Timber removals are based on the ratio of logging residues (15.1 percent) and other removals (6.6 percent) to all removals for 1991. Dashed lines represent recycled paper. Construction includes millwork, such as cabinetry and moldings. Other paper and board includes industrial uses, such as materials handling, furniture, and transport. Fuel: The ratio of end uses relies on Btu data from the Energy Information Administration; fuel includes 100 million cubic meters burned by paper mills for energy. Residential and commercial fuel includes electric utilities. Sources: Ince 1994; Energy Information Administration 1994; USDA 1993; US Bureau of the Census 1996; American Forest & Paper Association 1995; Smith et al. 1994; and data from the Engineered Wood Products Association, Tacoma, Washington; and the Western Wood Products Association, Portland, Oregon.

Including fuelwood, residues, and pulping liquors, 47 percent of the US timber harvest in 1993 became useful energy. In fact, US lumber and plywood mills generated over 72 percent of their energy internally-by 1991 (Energy Information Administration 1994). Outright harvest for fuel has remained a relatively steady percentage of harvest since rising to about 15 percent in the early 1970s. Currently, about three-quarters of fuelwood harvest comes from non-growing stock.

In the forest products sector overall, had the log-to-lumber efficiency remained at -1970 levels, meeting 1993 market demand would have required 48 million cubic meters more timber. Without composites displacing lumber, an additional 32 million cubic meters of roundwood input would have been required. Finally, improved efficiency in papermaking spared 34 million cubic meters of material input in satisfying 1993 demand. Their virtual elimination of waste since 1970 leaves millers with the challenge of transforming the near half of wood that is burned into more valuable products.

This next century. On top of raising their efficiency, millers can use substitutes for pulp, such as wastepaper and crops. The recovery and use of wastepaper have increased, and the first facilities are now becoming operational to make newsprint from such crops as kenaf (Kafus Industries 1999). Furthermore, by incorporating organic .and mineral fillers into products, millers can also lower the input of timber. Other substitutions offered by manufacturers and accepted by consumers include polyethylene for paper bags, concrete for rail ties, and steel beams for wooden rafters.

Foresters

The past century. The foresters’ parameter in equation I is the hectares of forest disturbed per cubic meter of wood harvested. A forester can maximize timber yields and minimize area disturbed in three ways: (1) harvesting more of each tree, (2) harvesting more trees on each hectare, and (3) increasing annual tree growth.

1. The roughly two-thirds of the dry matter above the stump in sterns generally sets the limit on how much of each tree is harvested. From 1952 to 1991 logging residues as a percentage of timber removals from growing stock fell from 10 percent to 7.5 percent for softwoods and from 22 percent to 12 percent for hardwoods (Haynes et al. 1995). Because removing vegetation also removes nutrients and because stems are poor in nutrients, harvesting more than stems can deplete a site. Considering the already high harvest index and the nutrient depletion from x raising the index further, foresters have little opportunity to get more wood by harvesting more of each tree.

2. Harvesting more trees on each hectare means less partial cutting, more clearcutting. In the 1980s, partial cutting accounted for three-fifths and clearcutting for two-fifths of the 4 million hectares affected by harvest nationally (W. Brad Smith 1997, pers. commun.). Because 1991 roundwood removals were about 500 million cubic meters, loggers obtained an average of roughly 125 cubic meters of timber for roundwood products for every hectare of timberland harvested. Compared with clearcutting, partial cutting affects appearance less, but harvesting the same volume of timber requires more roads to reach a wider area and counters the objective of disturbing less area.

Concentrating harvest on fewer hectares by cutting more per hectare shrinks the area disturbed nationally. The result is tree plantations, and such concentrations of single species can encourage pests. The attack by fungi and insects in Connecticut plantations of red pine brought from Europe is an example. But the epidemics that removed native chestnut, elm, and hemlock from heterogeneous forests in the same state show that heterogeneity is no guarantee against disaster. The values of homogeneous plantations for wildlife habitat and aesthetic experiences are another concern, but in northeastern Minnesota 53 conifer plantations that were 30 years old, the overstory produced the intended timber, and the understory resembled that beneath naturally regenerated and mature forests (Ohmann 1984).

Plantations allow concentrated harvests. In the South, which contains more than half of the nation’s industrial forestdand, one-eighth of all timberland is now plantations, and that proportion is expected to double by 2030 (Environmental Defense Fund et al. 1995). This expansion of plantations and clearcutting should increase the harvest per hectare and I shrink the area annually disturbed by harvesting. Changing social preferences in recent decades, however, have made the lever of harvesting more trees per hectare more difficult to apply.

3. Increasing the annual tree growth is forestry’s third way to shrink the area disturbed by harvest. At the present average growth rate for growing stock of 3 cubic meters per hectare per year, almost the entire 200 million hectares of American timberland is required to supply the 500 million cubic meters harvested. If foresters achieved annual productivity of 5.9 cubic meters per hectare on appropriately productive sites, they could provide the needed 500 million cubic meters on just 23 percent of US timberland.

Yields can be raised. On industry-owned forests, yields exceed by half those on national forests and private nonindustrial land (Haynes 1990 Powell et al. 1993). Nationally, although the forest industry owns only about one-seventh of timberland, it produces a fifth of national timber growth, a quarter of softwood growth, and about a third of the annual harvest. Here are two other statistics that indicate industry’s disproportionate contribution: Industrial foresters did more than 40 percent of the planting and 70 percent of the stand improvement in 1995 (Moulton et al. 1996).

This next century. Although the forest industry actively manages its own lands, private owners, who control more than half of all American timber land, may lack incentives to do so. Millions of acres await even rudimentary management measures. In the late 1970s, Burwell (1978) argued that millions of acres of forest were underutilized:

In this context the term “underutilized” refers to the practice of minimum management of privately owned forest for many millions of acres. Some significant fraction of this area is never harvested, and mature trees, both merchantable and cull, topple and decay. For a larger fraction of the acreage, merchantable stemwood is periodically harvested but not replanted, the woody residues and dead stemwood arc not removed, and growing cull trees are left to expand their area of coverage.

Burwell’s observation remains apropos today and can also apply to acres released from crops by rising agricultural yields since the 1970s. Given sufficient resources, conventional management of neglected stands offers foresters a great opportunity to raise average yields.

Foresters already possess the means to increase yields. Improving drainage of Southern pines at planting time increased their height after 25 years. About one-tenth of the additional height was due to bedding and weed control, a third to water control, and half to phosphate fertilization (Allen et al. 1990). Foresters can plant species that grow faster than the average annual 3 cubic meters. To illustrate the potential for higher yields, we note that red alder grows biomass as fast as 26 tons per hectare per year, yellow poplar 24, and Western hemlock 38 (Grier et al. 1989). A national consensus would encourage corporations and state and federal agencies to improve average yields with best management methods. The first step toward reaching that consensus is for diverse stakeholders to recognize the unique leverage that higher yields offer to reduce overall forest disturbance.

The forester’s parameter in the flow of forest products is wood per hectare of forest and thus the area disturbed per cubic meter of wood harvested. Among the three levers-harvesting more of each tree, cutting more trees on each hectare, and increasing the annual growth of trees-the best opportunity for reducing the area harvested lies in growing more wood per hectare.

Conclusion

By the end of the 19th century, population growth, increasing wealth, the Industrial Revolution, and expanding agriculture had shrunk the expanse of US forests about 30 percent from the pre-European settlement condition. Although population and wealth continued to multiply, the conservation movement and technological developments beginning early in the 20th century combined to halt the clearing of forests and allowed forest regeneration. Former pastures in New England and the upper Great Lakes states are now mature forests. Expanding exurbs, contracting agriculture, and new logging will all affect future forest clearing (Waggoner et al. 1996). Will consumers, millers, and foresters use their leverage to lessen such disturbances and continue the rebirth of the American forest?

Consumers spare forests by recycling fiber to paper mills, and more importantly, lowering national intensity of use. Millers already use about 120 million fewer cubic meters of roundwood a year than they did three decades ago; further innovations may allow millers to profit more from each unit of timber and spare still more hectares by channeling residues burned as fuel to producing composites and paper. Foresters have the most dramatic opportunity to reduce the hectares harvested-not by harvesting more of a tree or even more trees per hectare, but by increasing the growth rate of trees per hectare. They can narrow the gap between the potential of American timberland and the current low growth rate through improved management of neglected stands and intensive silviculture to raise yields.

American history hints that reducing wood demand as the economy grows, utilizing wood more efficiently, and raising yields can decouple the need for land from timber demand. A plausible 2.5 percent per year drop in intensity of use coupled with an annual I percent rise in yield would shrink the forest area disturbed by logging by 1.5 percent annually. Compounded, that 1.5 percent would reduce the spatial extent of logging by half in 50 years. The benefits of such sustained diligence include preserving a national treasure, sparing land for undisturbed nature, and sequestering carbon from the atmosphere.

Literature Cited

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AMERICAN FOREST & PAPER ASSOCIATION. 1995. 1995 statistics: Paper paperboard, & woodpulp. Washington, DC.

BURWELL, C.C. 1978. Solar biomass energy. An overview of US potential. Science 199:1041-48.

ENERGY INFORMATION ADMINISTRATION, 1994. Manufacturing consumption of energy. US Department of Energy Report DOE/EIA-0512(91). Washington, DC: US Government Printing Office.

ENVIRONMENTAL DEFENSE FUND ET AL. 1995. Economic considerations in forest management. White Paper No. 11. Washington, DC: Environmental Defense Fund.

GRIER, C.C., K.M. LEE, N.M. NADAKARNI, G.O. KLOCK, and P.J. EDGERTON. 1989. Productivity of forests of the United States and its relation to soil and site factors and management practices: A review. USDA Forest Service Report PNW-GTR-222.

HAYNES, R.W. 1990. An analysis of the timber situation in the United States: 1989–2040. USDA Forest Service General Technical Report RM-GTR-199.

HAYNES, R.W, D.M. ADAMS, and J.R. MILLS. 1995. The 1993 RPA timber assessment update. USDA Forest Service Report RM-GTR-259.

HYDE, WE 1997. Policies today and for the future. In Wood in our future. Washington, DC: National Academy Press and National Research Council.

INCE, P.J. 1994. Recycling of wood and paper products in the United States. Madison, WI: USDA Forest Products Laboratory.

KAFUS INDUSTRIES. 1999. Kafus options 147,000 acres in Arizona as proposed site for increased kenaf fibre production. News release. Available online at www. Kafus.com/pr/news.html.

MOULTON, R.J., F. LOCKHART, and J.D. SNELLGROVE. 1996. Tree planting in the United States 1995. Washington, DC: USDA Forest Service.

OHMANN, L.E 1984. Biomass in conifer plantations of northeastern Minnesota, USDA Forest Service Research Paper NC-247.

POWELL, D.S., J.L. FAULKNER, D.R. DARR, Z. ZHU, and D.W. MacCLEERY. 1993. Forest resources of the United States, 1992. USDA Forest Service Report RM-GTR234.

SMITH, W.B., J.L FAULKNER, and D.S. POWELL. 1994. Forest statistics of the United States, 1992. USDA Forest Service Report GTR-NC-168.

US BUREAU OF THE CENSUS. 1975. Historical statistics the United States, colonial times to 1970. Washington, DC: US Government Printing Office.

—–.1996. Statistical abstract of the United States. 116^th ed. Washington, DC: US Government Printing Office.

US DEPARTMENT OF AGRICULTURE (USDA). 1993. Agricultural statistics 1993. Washington, DC: US Government Printing Office.

WAGGONER, PE., J.H. AUSUBEL, and I.K. WERNICK. 1996. Lightening the tread of population on the land: American examples. Population Development and Review 22(3):53145.

WERNICK, I.K., RE. WAGGONER, and J.H. AUSUBEL. 1998. Searching for leverage to conserve forests: The industrial ecology of wood products in the US Journal of Industrial Ecology 1(3):125-45.

Iddo K Wernick (e-mail: iw4@columbia.edu) is associate research scientist, Earth Institute, Columbia University New York, NY 10027; Paul E. Waggoner is scientist, Connecticut Agricultural Experiment Station, New Haven; and Jesse H. Ausubel is director, Program for the Human Environment, Rockefeller University, New York. A more detailed analysis, on which this article is based, is available online at https://phe.rockefeller.edu/forests.

Nitrogen Fertilizer

In 1931, Karl Bosch received the Nobel Prize for making nitrogen fixation practical.  Like many new technologies, synthetic nitrogen fertilizer enjoyed strong growth, from 1.3 Tg nitrogen in 1930, to 83 Tg in 1998.  (One Tg equals a trillion grams or a million metric tons.  These data refer to the fertilizer year 1 July-30 June.  Thus, for example, 1998 data are for 1998-99.)  By “synthetic nitrogen fertilizer,” we mean commercial product, with almost all the nitrogen fixed by the Haber-Bosch process — in contrast to manure, guano, oil meals, and packing-house waste.

Is the use of nitrogen fertilizer growing exponentially?  No.  A constant percentage increase accelerates a curve exponentially.  This is not what is happening with nitrogen fertilizer.

After an explosive annual growth of 14% during the period 1945-56, global growth of nitrogen fertilizer use slowed to 8% as 1970 approached.  Growth then slowed further to 5% yearly during 1976-85, and to less than 1% during 1986-98.  In Europe and the United States, which adopted the new technology early, growth slowed even sooner.  Since 1976, use in Europe and the U.S. has grown 1% or less yearly, and sometimes has actually fallen.

Like numerous products that saturate their market, nitrogen fertilizer fits not an ever-rising exponential growth pattern, but an S-shaped logistic one (Frink et al. 1999).

Nitrogen Deposition

The nitrogen falling from the atmosphere, mostly as NO₃-nitrogen and NH₄-N (ammonium-N), can be measured by simply collecting it in the open as “bulk deposition.”

“Wet deposition” is collected only during precipitation.  “Throughfall” is nitrogen deposited on forest canopies and eventually falling or washing down in precipitation.  Nineteenth century agronomists, who were  concerned about deposition on crops, collected bulk deposition.  More recently, scientists concerned about acid rain (caused by sulfur and nitrogen) collected wet deposition, and ecologists concerned about accumulation of nitrogen in forests collected throughfall beneath trees.

In order to understand nitrogen deposition, we must compare it with some scale.  Synthetic fertilizer use has increased approximately 100 fold.  Concurrently, worldwide NO₃-nitrogen from high temperature combustion in car motors and power plants has risen 10-fold to about 20 Tg.

Farmers increase crop yields with 100 to 200 kg nitrogen ha^-1 (kilograms per hectare).    For prevention of eutrophication and 95% safety, Europe has established incremental critical loads of 3-10 kg ha^-1  (with standards of less than 3 and more than 10 in some places) (Posch et al. 1997).

In infertile soil made responsive by fertilization with other nutrients, increments as little as 10 kg, but sometimes not less than 54 kg, nitrogen ha^-1 decreased biodiversity (Tilman 1987).  Early in the 20^th century, bulk deposition in Europe and the U.S. was 4 to 7 kg ha ^-1.

Anyone smelling a barnyard knows local deposition of NH₄-N can be high.  In the Netherlands, a local throughfall of 100 kg ha^-1 attributed to animals will affect vegetation (Ivens, 1990).  The manure km^-2 in the Netherlands is five times that in the quintessential U.S. agricultural state of Iowa.   Locally, where animals are concentrated, throughfall can approach farmers’ fertilizer rates.

However, deciding whether widespread, rather than local, increase of nitrogen deposition accompanied the use of more nitrogen fertilizer requires measurements spanning decades at several places.  For a comparison that offers data over a period as long as the growth of fertilizer use, we must turn to the simple, robust measure of bulk deposition.  In Rothamsted, England, annual deposition rose a total of about 1 kg ha^-1 between 1888 and 1966.  Bulk deposition of 8.7 kg (estimated from wet deposition during 1987-96 at nearby Woburn) confirms an increase, perhaps 5 kg ha^-1 during the century.

In northern Netherlands, bulk deposition increased about 7 kg from 6.7 kg ha^-1 at Groningen during 1908-10 to 14.5 kg at nearby Kollumerwaard during 1994.  In Sweden, it changed little from the 5.1 kg ha^-1 at Flahult in 1909 to 7.1 kg in 1996-97 at three nearby stations (Frink et al. 1999).

At Geneva and at Ithaca, New York, during the first quarter of the 20th century, annual bulk deposition ranged from 4 to 8 kg ha^-1.  About 6 kg was also deposited at Mays Point and Huntington Forest, New York, during 1965-80 and also at Hubbard Brook, New Hampshire, during 1972-92 (Frink et al. 1999).  In the northeastern United States during the 20th century, bulk deposition changed little.

One should not be surprised that nitrogen fertilizer use and widespread increase of nitrogen deposition did not rise in tandem.  Except for escape of some ammonia fertilizer and dust and a little NO₂, no direct path puts fertilizer nitrogen in the air for deposit.  Also, even 80 Tg fertilizer nitrogen plus 20 Tg NO₃-nitrogen from high temperature combustion is insufficient to increase widespread deposition significantly; in the impossible case of all 100 Tg being deposited, it would average only about 2 kg ha^-1 on Earth.  The low concentration of nitrogen in Greenland precipitation (Mayewski et al. 1990) corresponds to deposition far less than that.

Given the moderate likely rise in nitrogen use that we now project, prospects seem good for any future increase in deposition to continue making the minor contribution to plant growth that Nadelhoffer et al. (1999) have calculated is now being made.  This should pose little hazard to biodiversity.

Prospects to 2070

After overcoming worries about exponential increase and widespread rising deposition, we still come to the lurking fear of burgeoning, richer humans eventually demanding farmers smother Nature with multiples of today’s nitrogen in order to grow more food.  Although nitrogen from fertilizer can scarcely increase widespread atmospheric deposition, its leaching and pollution of water justify conservation.

Foreseeing what humanity will eventually demand requires integrating forces that drive fertilization (Frink et al. 1999).  The change of fertilizer use (see Exhibit 1) is the sum of changes in four forces:  population, GDP/capita, crop production/GDP, and the Ratio of nitrogen in fertilizer to nitrogen in crops, which we capitalize to “Ratio.”  (We neglect the small change over time in the nitrogen composition of the crop mix.)  To foresee prospects to 2070, and a population of roughly 10 billion people, we assume a population growing at 0.8% and GDP per capita at 1.8%, to lift their sum, the total GDP, to 2.6% per year.

Although wealthy people eat better than the poor do, they do not eat more in fixed proportion to GDP.  Hence, the declining ratio of crops to overall GDP has mirrored the rise of world GDP.  Whereas the GDP/Cap data in Exhibit 1 are above the zero change line, the Crop/GDP are below the zero change line, signifying a falling ratio of crops to GDP.  Expecting that the world economy will continue to favor computer chips over potato chips, we project a long decline of 1.0%/year in crop/GDP.  The sum of GDP per capita rising 1.8%, and crop/GDP falling 1.0%, still raises the crop for all persons by 2070 to the level that rich countries enjoy now.

Farmers control the fertilizer per crop as they economize inputs.  Globally, the Ratio of fertilizer nitrogen to crop nitrogen recently plummeted 2.0%/year.  In the United States, it fell earlier, and since the 1970s has fallen some 1.0%/year.  For the long pull, a decline of 0.5%/year seems reasonable as farmers continue lifting yields faster than their nitrogen fertilizer use grows.  The sum of the changing forces (0.8 + 1.8 – 1.0 – 0.5%/year) would raise world nitrogen fertilizer use 1.1%/year to 2.4 times the 1990 use by 2070.

Concern for sparing only fertilizer would be myopic.  We integrate with the saving of fertilizer the sparing of land, which is preeminent for sparing Nature.  Accordingly, we project cropland taken as well as fertilizer used (see Exhibit 2).

As a baseline for prospects to 2070, consider the situation in 1990, when 79 Tg fertilizer and 11% of world land yielded the caloric equivalent of 1,900 kg ha^-1.  The 150% Ratio of fertilizer nitrogen to crop nitrogen shows that nitrogen from fertilizer, plus that from legumes and manure, far exceeds nitrogen incorporated into crops, indicating an opportunity for conservation.

Our first projection for 2070 (and assuming a population of ten billion people) is a situation in which farming has stagnated at 1990 levels in terms of yields and Ratio.  The consequent 284 Tg fertilizer nitrogen, or 3.6 times 1990 use, exceeds the 2.4 times projected above because the Ratio remains 150%.  Under this projection, cropland has expanded because population and crop per capita has grown, while yield has stagnated.  A scenario assuming use of 284 Tg nitrogen, and 38% of the land, offers the scary specter of population and wealth eventually demanding that farmers push Nature aside and smother the land in fertilizer.

Researchers can forestall that specter becoming reality with at least two levers:  Substitute nitrogen already on farms for nitrogen fertilizer, and raise yields, thus lowering the Ratio of fertilizer nitrogen to crop nitrogen.

Farms fix nitrogen in legumes and collect nitrogen in manure.  Although so-called alternative agriculture features nitrogen fixed by legumes, legumes take land and devour natural habitat.  Manure is another matter.  The estimated 80 Tg nitrogen in the world’s manure matches the 83 Tg fertilizer nitrogen used now and is significant compared to the projected 284 Tg.  Remembering that some manure nitrogen is already captured in crops, but that animals may multiply by 2070, one can envision 50 Tg of manure nitrogen substituting for fertilizer nitrogen, decreasing the fertilizer-nitrogen-to-crop-nitrogen Ratio to 124% with crops still using 38% of the land.

The ancient challenge of saving and storing nitrogen without odor continues.  Concentration of animals has focused attention on odor and increased the distances for hauling manure to fields.  It has also changed the goal from conserving nitrogen to denitrifying it and controlling odor.  A challenge for pollution prevention is replacing the negative tasks of regulating odor and returning N₂ to the air with the positive task of discovering profitable and practical ways to move more of the dilute nitrogen in manure into crops.

Conserving fertilizer can also lower the Ratio of fertilizer nitrogen to crop nitrogen.  Conservation includes testing soil nitrogen and adjusting application to each site by precision farming and splitting applications during the season to avoid leaching.  Because yields divide the Ratio, higher yields can lower the Ratio and spare land, too.  Increasing yields to lower the Ratio involves breeding better crop varieties, plus removing limitations such as other nutrients, water, and pests so that crops can exploit the nitrogen provided.

While words about lowering the Ratio of fertilizer nitrogen to crop nitrogen could be mere anecdotes, statistics show farmers are in fact doing it (see Exhibit 1).  During the 1980s and 1990s the worldwide Ratio fell.  In the United States since the 1970s, the Ratio of fertilizer nitrogen to crop nitrogen fell about 1%/year.  With regard to a specific locale and crop, the Ratio for Indiana, Iowa, and Nebraska corn fell 1% to 3% per year during 1980-96 (Frink et al. 1999).  Farmers lifting the world average yield of corn, rice, soybeans, and wheat by an annual average of 1.6 to 2.3% during 1961-2000 demonstrate that higher yields also are not mere anecdotes.

A projection involving slower lifting of yields, but conserved nitrogen, shows that raising yields 1% and lowering the Ratio 0.5%/year would take 19% of land, while lowering nitrogen fertilizer use to 192 Tg.  This is the 2.4-fold increase we deem probable.

With sustained lifting of yields and conserved nitrogen, raising yields 2% and still lowering the Ratio of fertilizer nitrogen to crop nitrogen would require the same 192 Tg of nitrogen, but would actually shrink cropland 144 million hectares, which is about half the area of India, or ten Iowas.

Today, fertilizer manufacturers have idle capacity.  This, combined with weak agricultural markets in Europe and the U.S., reveal how groundless are fears that fertilizer use will soon explode.  There is little prospect of a widespread downpour of nitrogen from the air, or Nature pushed aside to relieve hunger.

Outline for Future Action

For the long run, however, and to relieve “hot spots” of nitrogen deposition, several things need to be done.  On the grand scale, regulators need to monitor two simple environmental metrics:  land spared from cropland expansion and the Ratio of fertilizer nitrogen to crop nitrogen.

On the local scale, where action is taken, fertilizer manufacturers should offer information on how to make fertilizer go further while farmers, the most hard-pressed actors, struggle to survive.  Researchers, the actors with the most opportunity to effect change, should devise affordable ways for farmers to utilize manure (now seen as a nuisance) for higher yielding crops.

Meeting these challenges will temper the use of nitrogen fertilizer, while sparing more land for Nature.

Exhibits

Exhibit 1.  Annual Global Changes in Population, GDP per Capita, Crop Production per GDP, and the Ratio of Synthetic Fertilizer Nitrogen to Crop Nitrogen.

Exhibit 2.  Prospects to 2070.

References

Frink, C.R., Waggoner, P.E., & Ausubel. J.H. (1999).  Nitrogen Fertilizer:  Retrospect and Prospect.  Proceedings, National Academy of Sciences, 96:1175-1180.

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Moffat, A.S. (1998).  Global Nitrogen Overload Problem Grows Critical. Science 279:988.

Nadelhoffer, K.J., et al. (1999).  Nitrogen Deposition Makes a Minor Contribution to Carbon Sequestration in Temperate Forests.  Nature 398:145-148.

Posch, M., Hettelingh, J.-P., de Smet, P.A.M., & Downing, R.J. (1997).  Calculation and Mapping of Critical Thresholds in Europe. RIVM Report 259101007.

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Charles R. Frink (charles.frink@po.state.ct.us) is a soil expert and scientist emeritus at the Connecticut Agricultural Experiment Station in New Haven.  Paul E. Waggoner (paul.waggoner@po.state.ct.us) is an agronomist and meteorologist, and past director of the Connecticut Agricultural Experiment Station.  Jesse H. Ausubel (ausubel@mail.rockefeller.edu) is an industrial ecologist and director of the Program for the Human Environment at The Rockefeller University in New York City (https://phe.rockefeller.edu).