Sustainability Issues

Sustainability refers to the capacity of agroecosystems to maintain their productivity and economic profitability into the foreseeable future. The major threats to sustainability of agricultural production, as suggested by various researchers and writers, are:

• Agrichemicals (Pesticides and fertilizers)
• Soil erosion
• Dependency on fossil fuels
• Declining biodiversity

Many of the issues involved with declining biodiversity were discussed in the section on diversification of agroecosystems. This section discusses the issues associated with agrichemicals, soil erosion, and dependency on fossil fuels.

Agroecology has been characterized as "the science of sustainable agriculture" by Miguel Altieri, Stephen Gliessman, John Vandermeer, and other agroecologists. Agroecologists thus focus not only on the problems of modern, "industrial" agriculture, but also attempt to discover how a better understanding of ecological processes operating in agroecosystems can lead to development of solutions to these problems.

Agrichemicals

Pesticides

Problems associated with pesticide use were explored in the unit on species interactions and won't be repeated here. During the 1950s and 1960s pesticides were widely viewed as the "magic bullet" that would solve agricultural pest problems forever. Not discussed at length in the earlier unit was the issue of externalities-costs incurred outside agroecosystems associated with pesticide use within agroecosystems. Public concern over externalities results in increased restrictions on the availability and use of pesticides, providing incentives for the development of alternative pest-control measures.

Pimentel et al. (1992) attempted to estimate the total costs and benefits of pesticide use:

Benefits:

$16 billion/year reduced crop losses

Costs:

$ 4 billion/year cost of pesticides

$ 8 billion/year externalities

These externalities include:

• Human health effects (hospitalization, outpatient treatment, long-term cancer treatment, deaths), estimated at $787 million/year.
• Animal poisonings and contaminated products: $30 million
• Destruction of beneficial natural predators and parasites: $520 million
• Pesticide resistence in pests: $1.4 billion
• Bee poisonings and reduced pollination: $320 million
• Crop and crop product losses: $942 million
• Ground water and surface water contamination: $1.8 billion
• Fishery losses: $24 million
• Government regulations to prevent damage: $200 million
• Wild bird losses: $2.1 billion

Fertilizers

Humans have added significant amounts of nitrogen to the global biogeochemical cycle through production and application of fertilizers, by planting N-fixing crops, and by combustion of fossil fuels.

N fertilizers are produced by combining N₂ and H₂ to produce ammonia (NH₃) by the Haber-Bosch process. Smil (1991) argues that the Haber-Bosch process is "the most important invention of the twentieth century" since it has increased the capacity for global food production by surpassing yield limits imposed by recycling of wastes and BNF.

Nitrogen saturation is a global change issue that has received little attention compared to global warming or stratospheric ozone depletion. Nitrogen saturation refers to the human-mediated increase of inorganic nitrogen to the global biogeochemical cycles (Vitousek et al., 1997).

Approximately 140 Tg y^-1 of anthropogenic N are added to soils, the atmosphere, and the oceans; approximately 20 Tg y^-1 result from fossil fuel combustion, 40 Tg y^-1 come from BNF by crop plants, and about 80 Tg y^-1 come from industrial fixation, that is, fertilizer production.

[A Tg (teragram) = 10¹² gram = one million metric tons]

These additions of N to the global N cycle are having several effects, including:

• Increased concentration of N₂0 (nitrous oxide), a greenhouse gas.
• Increased concentration of NO and NO₂, reactive gases which contribute to stratospheric ozone depletion.
• Loss of soil cations through increased leaching.
• Increased rates of soil acidification.
• Increased N loading of estuaries and coastal waters.
• Accelerated loss of biodiversity.
• Forest dieback.

Nitrogen deposition is probably increasing the productivity of forest ecosystems, but the effects may be short term. Aber et al. (1998) have found that productivity initially increases but then declines with additions of N to N-limited forests.

Smil (1991) projected global fertilizer N-use to increase to 115 Tg y^-1 by 2000 and to 145 Tg y^-1 in 2010, which would likely further stress many global ecosystems. The driving forces behind these projected increases include:

• Population increase (increasing demographic pressure);
• Declining yield responses to higher N-fertilization levels;
• Increases in per capitata protein consumption (increasing socioeconomic pressure); and
• Decreasing arable land per capita.

Nitrogen fertilizers are used relatively inefficiently in crop systems:

• N recovery from fertilizers varies from 20-66%; less than 50% in most agroecosystems.
• ¹⁵N studies showed 65% recovery in spring wheat in India from KNO₃, 33-44% with urea; in corn in Nebraska recovery estimated at 24-35%.
• Data from rice studies shows 20-30% N recovery.
• Recovery often (but not always) inversely related to N fertilizer levels.

In reality, N does not "cycle" very well. Losses of N from agricultural systems are high. Smil (1991) provided the following estimates for the global N budget of croplands:

Increasing the efficiency of N use involves attempting to manage immobilization, mineralization, and nitrification. N should be immobilized in the absence of a growing crop and mineralized during periods of crop uptake. Additions of high C:N residues, for example, can create an "N-trap" to reduce losses by leaching during the fallow season.

Cover crops are often used to "scavenge" N. A cover crop is a crop planted during the normal fallow season. Several studies suggest that much of the N lost by leaching occurs during the fall and winter after harvest. The N lost by leaching may be mostly N mineralized from crop residues and SOM and nitrified to nitrate. Winter cover crops can take up much of this N, thus decreasing leaching.

Would it help to increase BNF? That is, is nitrogen fixed by legumes used more efficiently than nitrogen applied in fertilizers? Is BNF associated with lower leaching and higher organic N levels? There is still little understanding of the relative efficiencies of fertilizer N versus N provided by BNF.

Methods to increase BNF include:

• Greater use of crop rotation with legumes (green manures, forage crops, and pulses (grain legumes), although the latter sequester much of the fixed-N in their seeds).
• Develop more efficient strains of Rhizobium for inoculation in the soil; better match particular rhizobial symbionts with particular host crops.
• Greater use of legume cover crops.

Rice farmers have been reluctant to use legume cover crops and other green manures due to the low cost of N fertilizers.

Most use of cover crops to date has been to reduce soil erosion (see below). Tian et al. (2000) point out that legume cover crops have both direct (BNF) and indirect beneficial effects. The latter include: improved microclimate, enhanced soil microbial activity, and increased P availability (through effects of cover-crop root exudates).

Effective cover crops should have the following traits: rapid seedling growth, winter hardiness, delayed flowering and fruiting (to reduce N sequestered in seed). The USDA is concentrating efforts on varieties of subterranean clover (Trifolium subterraneum) and hairy vetch (Vicia villosa).

• Transfer ability to fix N to cereals?

Nodulation-restriction genes may be highly strain specific. Selected and/or engineering Rhizobium (and other microbes cultured in the laboratory) often have a low "fitness" in the soil, that is they do not compete well with the existing microbial community and often fail to become established--a phenomenon known as "microbiostasis" (Shantharam & Mattoo, 1997)

One early projection for the use of genetic engineering was the idea of producing cereals capable of biological nitrogen fixation. This would presumably eliminate the need for rotations with legumes. The wisdom of this strategy has been challenged on ecological grounds.

GMO cereals with BNF would have to increase their allocation of C to roots to support the microbial symbionts; this would have the effect of decreasing yields.

Because of these decreased yields, genetically-engineered cereals would probably be grown in monoculture. (If the grower is going to use a crop rotation, why use GMO varieties and take the yield penalty?) Costs associated with continuous monoculture include soil erosion, and reduced pest and weed control.

In other words, the growers depending on such GMOs would not benefit from rotation effects associated with improved soils, pest control, weed control, and reduced autotoxicity.

To date little progress has actually been made in producing such GMOs.

As has been stressed repeatedly in this course, crops must consume resources (including N) to grow and produce a yield. Is there any way to reconcile (1) the need to provide crops with enough N to produce sufficient food to feed the world's population, with (2) the negative environmental effects of global nitrogen saturation? Ultimately, the answer might be to increase the recycling of nutrient N back from the harvested yield to the agroecosystem (how might this be accomplished?), reintegrate crop and livestock production, and decrease the consumption of animal protein.

Soil Erosion

Estimated loss of topsoil in USA--1/3 over the past 200 years (Edwards 1990). About 0.5 billion ha of agricultural land worldwide has been lost to erosion in the past 40 y (Giampietro et al., 1999)-about 1/3 of the currently cultivated arable land.

Soil Erosion removes the upper soil horizons-organic layers and A horizon ("topsoil"), resulting in:

• Decreasing fertility
• Decreasing water-holding capacity

That is, soil erosion decreases the levels of resources available to crops.

Externalities (costs to society) of soil erosion; annual costs estimated at about $30 billion (Uri & Lewis, 1999):

• Decreased water storage capacity of reservoirs
• Damage to commercial fisheries
• Degradation of drinking water supplies
• Siltation of waterways impeding navigation
• Damage to outdoor recreation (fishing, boating, etc.)

USDA sets maximum tolerant soil loss yields as:

• 5 t acre^-1 y^-1 (11.2 Mg ha^-1 y^-1) on deep soils
• 1 t acre^-1 y^-1 (2.3 Mg ha^-1 y^-1) on shallow soils

Actual erosion rates:

• 15.4 Mg ha^-1 y^-1 averaged over all US Cropland in 1965
• 113-227 Mg ha^-1 y^-1 on steep slopes in Palouse Region
• Worldwide rates from Pimentel et al. (1987) given in handout
• Average erosion rates in US have fallen from 18 Mg ha^-1 y^-1 in 1982 to 11.7 Mg ha^-1 y^-1 (Uri & Lewis, 1999)

Soil Erosion vs. Soil Formation:

• Soil eroding at 13 Mg ha^-1 y^-1 will lose 1 cm of topsoil in 10 years
• Weathering of parent materials occurs at rates of 1 cm per 80-400 years
• Loomis and Conner (1992) suggest that rates of soil formation following disturbance are likely to be 10 × higher

Causes of soil erosion are addressed by the Universal Soil Loss Equation (USLE), an empirical model based on 10,000 plot-years of data collected at 49 locations:

A = R K L S C P

where:

• A = Annual soil loss per unit area
• R = Rainfall factor [amount, intensity, seasonal distribution]
• K = Soil erodibility [texture, structure, SOM]
• L = slope Length factor
• S = slope Steepness factor
• C = Crop system factor [relative to continuous fallow plowed up and down slope; continuous production of row crops contribute to erosion]
• P = Conservation practices factor

Soil conservation practices include:

• Minimum tillage/conservation tillage (where crop residues are left on surface.
• Crop rotation.
• Cover crops.
• Contouring.

Reduced tillage has had a marked impact in reducing erosion rates on cropland in the USA. Leaving crop residues on the surface, instead of incorporating them into the soil, changes the environment in several ways:

• Increases weed population. This reduction in mechanical control has been accompanied by increased herbicide applications in no-till systems.
• Decreases soil temperatures.
• Shifts the composition of microbial communities--relatively fewer bacteria and more fungi--which affects nutrient cylcing processes.
• Favors conservation of VAM hyphal network.
• Increases infiltration rates and storage of water.

The weed seedbank and weed flora (species composition of the seedbank) are affected by management, particularly use of herbicides and tillage. For example, a study by Zanin et al. (1997) in maize-soybean-small grain rotations found that both the number of biennial and perennial weed species and the total number of weed plants of all species increased with reduced tillage.

Is soil erosion really a problem?

Many agronomists and soil scientists believe that concerns about soil erosion are overstated. For example:

• Boul (1995): "sustainable soil use is threatened by erosion only when unsuitable soil material is exposed by that erosion ..."
• CAST (1992): "present evidence indicates that soil erosion is not now a serious threat to the long-run productivity of the nation's land resources."

These views are based in large part on the writings of Pierre Crosson of Resources for the Future, who argues that:

• Yields have increased in recent decades in spite of erosion (due to fertilizer applications, improved varieties).
• Soil removed from cropland may be deposited on other cropland.
• Another 100 years at current erosion rates will reduce crop yields only 3-10% (assuming increased fertilizer applications and continued genetic yield increases). Alt et al. (1989), however, estimated that 25% of US cropland will have to increase applications of fertilizer and lime just to offset yield losses due to erosion.

Crosson compares soil erosion estimates produced by the USLE with sediment loads measured in rivers and streams:

• USLE predicts total erosion of about 2.5 billion tons y^-1.
• Sediment yield estimates for US streams are estimated at 0.5 billion tons y^-1.
• Thus approximately 2.0 billion tons y^-1 are "missing".

The basis for the viewpoint that soil erosion is not a serious threat is essentially that fertilizers can substitute for topsoil loss. It suggests that the only important role of soil is in providing a medium of support for plant roots--that fields essentially can be managed as hydroponic systems. This ignores the externalities involved with nitrogen saturation, and implicitly assumes that fertilizers will always be available in high supply at low cost.

Dependency on Fossil Fuels

US production of petroleum peaked (predictably) in 1971.

Shortages caused by political instability (the oil "shocks" 1973, following Yom Kippur War; and 1979 following fall of Shah of Iran) demonstrate how rapidly petroleum prices can rise when supply fails to meet demand.

There is little agreement on when global petroleum production will peak:

• Many economists suggest that oil consumption will continue increasing past the year 2050
• DOE projects that consumption will continue increasing up to the decade of 2020
• Petroleum geologists are less optimistic. Campbell (1997) projected a global peak in petroleum production in 2001.

Ultimate recovery of conventional oil (cumulative production + reserves + yet-to-find) estimated to be 1.8 Tb by Campbell. 784 Gb were produced globally by 1996; 1.6 Tb were discovered by end of 1996.

[1 Tb = terabarrel = a trillion barrels = 1000 Gb (gigabarrels)].

Conventional oil includes secondary or enhanced recovery in Campbell analyses. Nonconventional oil includes tarsands, oil shales, oil in very small fields. Perhaps 500 Gb of nonconventional oil are recoverable (Campbell 1997).

The key point is that the world may be running out of certain fossil fuels (petroleum and natural gas) much more rapidly than its leaders, policy makers, and the general population believe. The consequences of demand permanently and finally exceeding supply are (1) increased cost and (2) decreased consumption; decreased consumption will affect all areas of the economy, including agriculture.

Fossil Fuel Energy Use in Crop Production

Energy from fossil fuels is used in crop production in many ways, directly in the form of gasoline and diesel fuels, and indirectly in the form of electricity, fertilizers, machinery, pesticides, irrigation systems, etc.

For example, ammonia for N fertilizers is manufactured via the Haber-Bosch Process using natural gas:

N₂+ 3 H₂ --> 2 NH₃

The hydrogen comes from fossil fuels, mostly natural gas (methane):

CH₄ + H₂0 + 0₂ --> C0₂ + H₂

Coal is used for about half of the ammonium production in China. Coal, which has much larger reserves, produces more CO₂ per unit of ammonia produced compared to natural gas.

The energy cost of producing urea is 65-80 MJ kg^-1 N (Smil, 1991). Approximately 2/3 of the economic costs of producing ammonium in the period 1990-1996 was the cost of natural gas (Jenkinson, 2000).

Agriculture now accounts for about 5% of total energy consumption in industrial countries, 10% in China, and 20% in Egypt.

An energy budget analysis attempts to describe how much energy is used in crop production. All production inputs need to be expressed as their gross energy requirements (the amount of energy required to produced the input). USA maize energy budgets show that major inputs are for: nitrogen, liquid fuels, drying, machinery.

Major historical changes include:

• Substitution of human labor by draft animals and later substitution of draft animals by tractors.
• Fuel use peaked ca. 1979. Gas tractors were replaced by diesel tractors, and smaller tractors replaced by larger tractors.
• Energy devoted to fertilizer use has risen more rapidly than yields, possibly due to declining soil fertility or decreased use of rotations.
• Increased energy for irrigation, probably associated with the shift of maize production to center-pivot Great Plains sites, e.g. Nebraska.

Overall, fertilizer N accounts for 50-65% of energy use in most humid-zone agricultural systems (Smil, 1991).

Energy budgets for crop production in arid lands, show that energy devoted to irrigation (fuel or electricity for pumps, wells, distribution system) is the single major input.

Conservation of Energy

Why is modern, western agriculture so inefficient in its use of energy (as measure by energy output/energy input ratios)? "Efficiency" can be defined in terms of each of the major economic inputs:

• Land-Mg grain/ha
• Labor-Mg grain/man-hr
• Water-Mg grain/cm water
• Energy-Mg grain/MJ consumed

Production systems tend to optimize their efficiency with respect to the most expensive input-labor for countries in the developed world; land for countries with very high population densities. Energy costs in real dollars have been very low in the last half of the 20^th Century.

Strategies for reducing energy consumption in agriculture include:

• Reduce fertilizer inputs: increased use of legume-based rotations, and increased use of legume cover crops.
• Reduce machinery use, through minimum tillage or no-tillage practices.
• Reduce irrigation levels: more efficient delivery systems; lower water-use crops.

No practices that substitute labor for energy would be economical given current energy prices. However, under economic conditions characterized by rapid increases in energy costs (early in the 21^st Century?) development and adoption of energy-conserving practices should increase.

On-farm Fuel Production.

Another option for growers is to produce their own fuel on the farm. Options for doing this include:

• Ethanol by fermentation. This is not an energy-efficient process, despite its high priority in the USA.
• Methanol by gasification. This is a capital intensive, large-scale technology that would require grower cooperatives. Agricultural wastes (straws, residues) can be used as feedstocks; use of such materials would decrease soil fertility in the long run, however.
• Methane by anaerobic digestion. This is a household-scale technology; the gas produced could be compressed for use by machinery; residues can be returned to the field.
• "Biodiesel" (vegetable oils) by extraction, filtration, and transesterification. This technology is now appropriate to small farm or farm cooperative.

URL: http://ag.arizona.edu/~spmcl/lecturenotes/Sustainability.Issues.htm