3.
Arizona's Agricultural Ecosystems
Authors
The report in this chapter was written by the following individuals:
o Janick F. Artiola, Department of Soil and Water Science, University of Arizona
o Jim Dubois, Arizona Department of Environmental Quality
Introduction
Agricultural activities began to emerge in the history of mankind more than 10,000 years ago. In Arizona, irrigated agriculture can be traced back to the Hohokam people.
In order to stabilize food production, humans began to set aside tracts of land that were, and are, intensely managed with several inputs:
o Manual, animal and mechanical labor
o Surface and groundwater sources of irrigation
o Animal and human residues
o Chemical fertilizers and pesticides
o Processed wastes additions
The intense management by humans of these tracts of land has completely changed the nature of the original "natural" systems. Thus, these food production systems are uniquely human. Now, agricultural ecosystems exist in their own right and are indispensable to the survival of modern human societies.
In agricultural ecosystems, humans control as much as possible of the physical, chemical, and biological processes that lead to a predictable and successful plant- based food, fiber or forage production. Agricultural ecosystems are dependent solely on humans for their maintenance and survival.
Because they are human-created, these systems would progressively revert to a more random state if left unmanaged.
Agricultural ecosystems are intensely managed biosystems designed to optimize food, fiber and forage production. These biosystems consist primarily of soil, plants (crops) and animals that require an infrastructure for support. These biosystems also require multiple inputs such as energy, water, pesticides/herbicides and fertilizers, as well as non-native plant species.
Until recently, the primary goal of agricultural ecosystems was to produce as much food as possible at the lowest cost without regard to short or long-term effects to the environment. Increasingly however, humans are recognizing the need to understand and predict effects and perturbations to the environment and its ecosystems. Healthy, sustainable agricultural ecosystems are still as important to the survival of humankind as is the preservation of natural ecosystems.
Arizona's Agricultural Ecosystem
According to the National Resources Inventory (NRI), Arizona dedicated 1.8 percent, or about 1.3 million acres of the total 72.7 million acres as cropland and pastureland in 1987. (See Map B-9 (Agricultural Lands of Arizona), Appendix B.)
Since the 1950's, the active cropland acreage has been reduced from about 1.6 million acres to nearly 1 million acres by 1992. More than 95% of all Arizona cropland is irrigated (NRI).
However, irrigated cropland has declined steadily in response to reduced water availability, increasing costs, and decreasing commodity prices.
Pastureland has been, and is, a very minor but constant component of agricultural ecosystems in Arizona, totaling about 80,000 acres in 1987.
Stressors on Agricultural Systems
The impact of agricultural activities on other ecosystems is discussed in each of the other ecosystems reports in this section.
Environmental Stressors and Agricultural Ecosystems
Accidental Releases
No information is available on accidental spills and toxic releases in agricultural ecosystems.
When accidental spills and toxic releases do occur, however, they are likely to be very localized. Small spills may occur during the transportation or handling of chemicals prior to and during field applications.
Agricultural ecosystems regularly sustain large inputs of chemicals such as fertilizers and pesticides.
Fertilizers
Table 3.1 illustrates the volume of fertilizers sold for a recent 5-year period. The table assumes 1 million acres of agricultural land.
Table 3.1 Fertilizer sales in Arizona, 1987 to 1992
---------------------------------------------------------------- | Fertilizer Element | Tons per Year | Average Pounds | | | | per Acre per Year | ================================================================ | Nitrogen (N) | 80,000 to 90,000 | 190 | ---------------------------------------------------------------- | Phosphorus (P) | 25,000 to 31,000 | 62 | ---------------------------------------------------------------- | Potassium (K) | 1000 to 2000 | 3.3 | ----------------------------------------------------------------
The total amounts of nitrogen, phosphorus, and potassium have remained fairly constant during the last five years.
Pesticides
Estimated minimum pesticides sales in Arizona during the 1987 to 1991 period are shown in Table 3.2 on the following page. The table assumes 1 million acres of agricultural land.
Table 3.2 Pesticides Used in Arizona Soils, 1987 to 1991
------------------------------------------------------------------------------------------------------------------------------------------------------------------ | PESTICIDE | 1987 | 1988 | 1989 | 1990 | 1991 | Average Pounds | | | | | | | | | | (Active Ingredient Concentration Varies | (Pounds x 1000) | (Pounds x 1000) | (Pounds x 1000) | (Pounds x 1000) | (Pounds x 1000) | per Acre per Year | | with Product) | | | | | | | ================================================================================================================================================================== | Insecticides (over 18 pesticides in this | 1344 | 1008 | 1011 | 567 | 1003 | 4.9 | | category) | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | Herbicides (over 11 pesticides in this | 727 | 246 | 416 | 404 | 492 | 2.3 | | category) | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | Fungicides and Bactericides (over 5 | 169 | 25 | 69 | 115 | 169 | 0.55 | | pesticides in this category) | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | Defoliants and Growth Regulators | 625 | 486 | 567 | 555 | 827 | 3.1 | | (over 5 pesticides in this category) | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | Fumigants (over 3 pesticides in this | 96 | 39 | 224 | 104 | 41 | 0.5 | | category) | | | | | | | ------------------------------------------------------------------------------------------------------------------------------------------------------------------ | TOTALS | 2961 | 1804 | 2287 | 1745 | 2532 | 11.4 | ------------------------------------------------------------------------------------------------------------------------------------------------------------------
Although the majority of these chemicals are assimilated by plants, degrade, leach below the root zone, volatilize or are transported by sediments and run off into the surrounding environments, some remain and accumulate in the soil.
o Examples of these include:
Arsenic-based defoliants
Stable organic herbicides and insecticides that are absorbed by soil organic matter and do not degrade
Metals
Salts
Presently, pesticides used in Arizona agriculture must meet strict criteria designed to protect groundwater. Thus, in general, pesticides used now have half-lives of less than 30 days, and degrade quickly in the soil. They have moderate to low toxicities to animals and humans.
o DDT and Toxaphene: Residual levels of DDT plus Toxaphene are known to be present in Arizona agricultural systems, the result of their extensive use prior to 1970.
Evidence of DDT and Toxaphene contaminated soils and sediments in the Phoenix metropolitan area of west central Phoenix and along the Gila River is well documented (ADEQ 1992, SCS 1991). However, the total extent of residual levels of DDT in Arizona agricultural soils is not known.
Residual levels of other refractory pesticides or by-products of banned pesticides may still be present in agricultural soils. Examples include dioxins, endrin, lindane and other chlorinated pesticides which sorb readily to soil particles and degrade slowly. No data are available on the effects of the residual concentrations of these pesticides on agricultural biosystems.
Municipal Sludges and Wastewaters
Agricultural ecosystems are increasingly being used for the disposal/treatment and the beneficial re-use of municipal sludges and secondary treatment wastewaters.
o Municipal sludge has been applied to Arizona agricultural soils since the 1960's near urban centers such as Tucson and Phoenix at increasing rates.
o Municipal sludges from California are also applied to Yuma area agricultural soils.
o Recently, solid sludge imported from the east coast has been applied to agricultural land in the Willcox-Bowie area.
In 1992, 122,000 acres of Arizona farmland had approved plans and likely received sludge applications. This equals about 12 percent of total active land (ADEQ 1993).
Municipal sludge re-use in agricultural lands is regulated at the state and federal levels. These regulations determine maximum annual and lifetime applications of sludge product on land, based on estimates of nutrient crop requirement and maximum allowable accumulation of metals in the plow layer of the soil. Therefore, metals accumulate in the soil when they are not taken up by plants or when they do not leach below the root zone.
While the short and medium term effect of these metal accumulations are well known, long-term effects are more difficult to predict. The Arizona Department of Environmental Quality compiles annual data on sludge applications and metal accumulations in Arizona soils.
Repeated sludge applications tend to improve soil structure and result in marginal but measurable increases in soil organic matter in the plow layer (Artiola and Pepper 1992). However, any specific changes that may occur in the microbiological composition of sludge amended soils are not known.
Similarly, no data are available on the possible impact and/or accumulation in sludge amended soils of refractory organic chemicals such as the previously discussed chlorinated pesticides that are known to be present in trace amounts in all municipal sludges.
Currently about 50 percent, or approximately 110 acre-feet per year of all reclaimed water from secondary wastewater treatment plants is also being used in the irrigation of farmlands that are located mostly in Maricopa and some in Pima counties (Lieuwen 1990). However, this amount constitutes only 23% of total agricultural water use in the last 10 years.
Although irrigated agriculture in Arizona is declining, effluent-irrigated land is projected to increase. Potential pollution problems associated with the use of reclaimed water include excessive total dissolved solids in the form of nitrate and/or salts accumulation in the soil profile.
Effluent irrigation has rarely been associated with soil metal or organic chemicals pollution problems. This is because these compounds are seldom found above Arizona Aquifer Water Quality standards and are often below laboratory detection limits in reclaimed water.
A ten-year research study of agricultural use of secondary effluent in California (Engineering Science 1987) found no additional contributions to residual soil metal content from effluent, and no difference in plant tissue heavy metals between well water and effluent-irrigated crops.
Outdoor Air Pollution
Urban air pollution has the potential to affect nearby agricultural ecosystems.
Although the two major counties of Arizona have air quality monitoring programs in the Tucson and Phoenix urban centers, no information is presently available on the effects of urban pollutants on Arizona agriculture.
In the case of the Tucson metropolitan area, historical air quality data show that air pollution levels have been generally well below federal standards since 1982 (Pima County Department of Environmental Quality 1994).
Originating from car emissions, air pollutants such as ozone (O3) and sulfur dioxide (SO2) are known to damage plants. However, no federal standards exist for their protection. Also, the pollutant impact is plant-specific and difficult to quantify.
Studies conducted elsewhere on ozone injury to cotton showed yield reductions of up to 48 percent using intermittent ozone exposures of 0.2ppm (Bruck 1990) which is twice that found in downtown areas of Tucson at peak times (hour average) (AQ 1994).
In general, most crops will experience significant damage and yield reductions at ozone levels just twice as high as the annual national standard (NAAQS=0.05 ppm) for air quality (Pima County Department of Environmental Quality 1994, Bruck 1990).
The acidifying effect of sulfur dioxide when combined with water is likely to be negligible in Arizona agricultural soils, as most are alkaline.
Industrial air pollution sources (smelters) are usually located far away from agricultural centers. No information is available on the potential impact of industrial air pollution on Arizona agricultural systems. However, smelter sulfur dioxide emissions, like car emissions, may temporarily damage some plants as fumigation of foliage converts to sulfuric acid within the leaf.
Dust control is routinely implemented on unpaved access roads near agricultural fields. The effect of dust deposition on leaves and crops in general is hard to quantify, as it decays rapidly away from the source (road).
Degradation of Built/Cultural
Not Applicable.
Ecosystem Physical Alteration
Grazing
Not applicable. No data.
Highways
Not applicable. No data.
Energy Production
Not applicable. No data.
Fire Suppression
Not applicable. No data.
Mining Extraction
Not applicable. No data.
Timber Management
Not applicable. No data.
Interbasin Water Transfers
Not applicable. No data.
Channelization
Not applicable. No data.
Water Diversion
Not applicable. No data.
Impoundments and Dams
Not applicable. No data.
Recreation:
Not applicable. No data.
Physical Alteration of Agricultural Ecosystems
According to the National Resources Inventory (NRI Data 1982, 1987), about 85% of Arizona's agricultural ecosystems are in prime farmland that offers the best physical and chemical characteristics for food production. The same report estimates that over 55% of the irrigated cropland needs to improve its water and irrigation management.
Table 3.3 illustrates the NRI report (1982) estimates for erosion from Arizona cultivated cropland.
Table 3.3 Estimated Annual Erosion in Arizona Cultivated Cropland (Tons per Acre)
---------------------------------------------------------------------------------------------------------- | Land Class | Wind Erosion | | Sheet and Rill | | Total Erosion | | | | | | Erosion | | | | ---------------------------------------------------------------------------------------------------------- | | x1000 | Mean * | x1000 | Mean * | x1000 | Mean * | | | Tons | | Tons | | Tons | | ========================================================================================================== | Class I (Prime | 2,123 | 3.1 | 361 | 0.5 | 2,484 | 3.7 | | Farmland) | | | | | | | ---------------------------------------------------------------------------------------------------------- | Classes II, V, III | 1,564 | - | 167 | - | 1,723 | - | ---------------------------------------------------------------------------------------------------------- | Totals | 3,688 | 3.6 | 520 | 0.5 | 4,207 | 4.1 | ---------------------------------------------------------------------------------------------------------- | * Tons per acre | | | | | | | ----------------------------------------------------------------------------------------------------------
Wind Erosion
Wind erosion provides over 90% of the physical soil loss, and is the single largest contributor to soil physical degradation from Arizona agricultural ecosystems. More than 50% of the wind erosion occurs primarily in intensively farmed Class I prime farmland during idle and low plant cover periods.
Other classes of farmland also suffer significant wind erosion losses due to their uneven topography and longer idle periods. It is estimated that the top six inches (the plow layer) of top soil weighs about 2000 tons per acre. Therefore, assuming four tons per acre annual losses, it may take as little as 500 years to lose the top six inches of soil from Arizona agricultural soils.
Nonetheless, according to the NRI report, in 1982 about one million acres of active cropland and pastureland sustained some conservation practices. However, the report fails to discuss the efficacy of these conservation practices in reducing physical degradation of these lands (NRI 1982).
Salinity
The same NRI report (1982) lists about 100,000 acres of farmland and pastureland that have salinity (EC greater than 4.0 mmhos/cm) and sodicity (ESP greater than 15 percent) problems. While some salt accumulations occur naturally in soils from semi- desert environments, these may have resulted from poor irrigation management or abandonment. Salt-affected agricultural lands can be safely reclaimed with a combination of chemical amendments and irrigation management.
Retired Farmland
Arizona experienced a progressive decrease in farmland, with more than 20 percent lost, since the 1940's until 1983 (Jackson 1994).
In 1991, there were about 300,000 acres of retired farmland in Arizona (UA Department of Arid Lands 1991). No estimates about wind and water soil erosion losses from these areas are available. However, abandoned farmland can have higher wind erosion losses than cultivated lands. This is because plant cover is slow to start without human intervention. Located near urban areas, much of the abandoned land has been absorbed by urban growth and can be considered permanently lost. However, the abandoned farmland located away from urban centers with limited water resources remains idle and continues to degrade.
Since the desert ecosystem was fragmented during the construction of agro- ecosystems, channelized surface water and groundwater sources are not available to sustain plant growth in abandoned farmland (Jackson 1994).
Other complex changes that have occurred in the physical, chemical and biological properties of soils during years of intense farming make the task of revegetation very difficult to accomplish (Jackson 1994). These changes may include loss of ground cover and near complete removal of native plant species, loss of soil microbial diversity, and loss of organic matter and soil structure.
Biological Alteration of Ecosystems
Grazing
Not Applicable.
Hunting
Not Applicable.
Fishing
Not Applicable.
Illegal Collection
Not Applicable.
Species Introduction
Agricultural ecosystems are changing continuously as new plant species and varieties are introduced. Such introductions are carried out in response to changing market demands and as new more productive, disease-tolerant plant varieties are developed. Thus, this practice is not considered damaging or detrimental to agricultural ecosystems.
However, extensive tillage and use of chemicals may have significant effects on the soil microorganism biodiversity. Nonetheless, it appears that the basic biological nutrient cycles continue in soils even after repeated physical manipulation and chemical additions. Little is known about the biological alteration of Arizona soils in response to agricultural practices.
Predator Control
Uncontrolled plant pests can seriously affect yields in agricultural systems. This happens because agricultural ecosystems do not have sufficient plant or animal biodiversity.
Unfortunately, monocultural practices usually result in serious imbalances in plant predators, imbalances which lead to the extensive use of artificial means for predator control, usually in the form of pesticide applications. The impact of these chemicals on agricultural biosystems was previously discussed.
Food and Drinking Water Contamination
Agricultural ecosystems make use of large quantities of chemicals to insure consistent food production, and residual levels of pesticides are known to remain in crops. The use of pesticides in food production is extensively regulated by state and federal agencies such as the EPA and the Department of Agriculture. The impact of these chemicals to the agricultural systems themselves was previously discussed.
Agricultural ecosystems can be responsible for the contamination of drinking water sources.
In Arizona, a number of pollutants affect drinking water sources (Idso and Kimball 1992), including:
o Nitrates (most common)
o Pesticides
o Petroleum hydrocarbons
o VOAs (volatile organics)
o Chlorinated solvents
o Bacteria
o Natural pollutants such as radionuclides, metals, and salts
The Arizona Department of Environmental Quality has a pesticide groundwater protection program that regulates the use of pesticides that can impact groundwater resources. The use of contaminated water for agricultural production (irrigation) has never been studied. However, it is improbable that the use of pesticide and/or nitrate (NO3) or even TCE (trichloroethylene) contaminated water has any measurable negative impact on these systems. This is because the bulk of nitrate and pesticides is applied directly to agricultural fields for plant fertility and pest control needs.
Global Climate Change
Not addressed.
Stratospheric Ozone Depletion
Not addressed.
Natural Hazards
Agricultural ecosystems are prone to floods, since historically these tracts of land have been located in sedimentary flat lands of river basins within 100-year flood plains. More than 90% of Arizona's agricultural lands are located in the Santa Cruz, Gila, Salt, and Colorado river basins.
The NRI report (1982) lists about 112,000 acres of cropland and pastureland that are located in flood-prone areas. However, Arizona has extensive flood control systems on or near the major river basins.
In the last 100 years, at least 17 major floods have occurred on one or more of the major drainage areas (Dubois and Parks 1981). These have been caused by unusually heavy rains or snow and snow-melt that have overwhelmed food control systems such as dams, resulting in lowland floods.
Although estimates of flood-related losses may exceed $0.5 billion dollars, no estimates are available on the actual damage to, and complete loss of, agricultural land (Dubois and Parks 1981).
o For example, recent floods in the Yuma area resulted in the loss of millions of dollars in crops, but less than 400 acres of farmland appear to have been permanently lost ( Dr. Sanchez, Yuma Agricultural Research Center, pers. comm.).
Groundwater Contamination
Large portions of aquifers within agricultural regions of the state show groundwater quality effects characteristic of agricultural impact (ADEQ 1993). Affected areas include the East and West Salt River Valley, the Casa Grande area, an area along the Middle and Lower Gila River, Marana and parts of the Avra Valley, and the Green Valley vicinity.
Constituents of concern are nitrates and total dissolved solids (TDS), a gross measure of the salts concentrated in water. Pollutant concentrations typically are higher in shallow groundwater than at deeper levels of the aquifers.
Nitrate concentrations up to 30 mg/1, which is three times the primary drinking water standard, can be found in portions of the groundwater beneath land that has been intensively used for agriculture. These pollutants are discharged when they are leached from soil which has accumulated fertilizers and salts. However, in terms of negative impact on agricultural ecosystems, only TDS represents a threat, since rising levels of salt in groundwater will eventually render it unusable for crop irrigation. Rising nitrate levels in groundwater do not represent a threat to agricultural ecosystems.
Effective operation of irrigated agriculture in arid and semiarid climates requires practices to prevent salt build-up in surface soils. For this purpose, agricultural lands are usually leached of salts at some point prior to planting.
Salts that move downward eventually accumulate in the groundwater, and, if groundwater conditions are relatively static, the groundwater will increase in salinity with time (see, for example, Muller, 1973). This effect does not directly alter the agricultural ecosystem, per se, but it does change the quality of the water resource necessary to sustain the ecosystem.
While some crops, like cotton, are relatively salt-tolerant, others, like alfalfa, show dramatically declining yields even with small increases.
Boron is another constituent in water with potential detrimental effects on crop yields. It, too, may concentrate through leaching, as salts do. However, little data is available regarding boron buildup in Arizona groundwater.
Impediments: Lack of Environmental Education and Awareness
Little is known by the urban public about issues that affect the health of agricultural ecosystems. Such issues as increased water costs, the state of abandoned farmland, and the conversion from agriculture to urban land use constitute stressors on agricultural ecosystems.
Recent newspaper articles have focused on the use and importation of municipal sludges in Arizona agricultural soils. While these articles have increased the level of public awareness on this particular topic, media attention has not provided altogether fair and balanced information about the cost-benefits-risks of this practice related to agricultural ecosystems and society as a whole.
Farmers must also increase their level of environmental knowledge and adopt agricultural practices that minimize the chances for deleterious impacts to agricultural and other surrounding ecosystems.
o For example, the State of Arizona's nitrogen Best Management Practices for agriculture, if adopted, will help reduce nitrate impact to groundwater resources.
Other Factors Contributing to Loss of Agricultural Ecosystems.
Present economics of water supplies and availability favor urban use rather than agricultural use.
Groundwater pumping costs are increasing due to diminishing groundwater resources. Agriculture uses approximately 80% of the groundwater pumped in Arizona (Marsh 1994). Surface water available with Central Arizona Project (CAP) may or may not be economical for traditional agricultural use in Arizona.
Cotton production varies from year to year, but recently has been decreasing steadily.
o From 1988 to 1993, cotton production acreage has decreased from 478,000 acres to 376,000 (Arizona Agricultural Statistics 1993, 1994). Cotton still represents the major crop planted in Arizona surface-wise, followed by hay crops (180,000 acres in 1992).
The increasing groundwater pumping costs and the high cost of CAP water likely will result in decreasing cotton production in Arizona in the next few years. Present commodity prices do not appear to favor the traditional Arizona agricultural practices dominated (acreage-wise) by the production of cotton, hay and grains.
Urban expansion can be considered the most important variable in the continuing loss of agricultural land in some areas of Arizona. (See Map B-10 (Urban Lands of Arizona), Appendix B.)
The urban impact on agriculture is particularly visible in the area of Phoenix, which shows more than 50 percent urban encroachment in the Phoenix agricultural basin. Since Phoenix has one of the highest urban growth rates in the nation, it is likely that most of the agricultural bio-systems in the Phoenix area and nearby surroundings will be lost within the next 20 years.
Conclusions
The decline of agricultural ecosystems in Arizona will probably continue in the future, primarily due to a combination of diminishing groundwater resources, increasing water costs, low commodity prices, and economic and physical pressures of urban expansion.
Agricultural ecosystems that remain in production in Arizona continue to have significant inputs of fertilizers and chemicals to sustain or increase existing crop productions.
It is also very likely that municipal sludge and reclaimed water inputs to agricultural ecosystems will increase significantly in the near future, since more municipal sludge will be produced as urban growth increases.
It also seems very likely that local farmers will be able to reduce fertilizer costs by importing more economically advantageous sludges from outside of Arizona. Subsequently, this practice may continue to raise the levels of some pollutants in Arizona soils, but long-term impacts to agricultural ecosystems are presently very difficult to predict. Long term soil and soil-pore water monitoring programs are needed to evaluate possible irreversible deleterious impacts from sustained applications not only of sludges, but also of fertilizers and pesticides to agricultural ecosystems.
Physical degradation of agricultural ecosystems will continue to be driven primarily by wind erosion and farmland abandonment. This impact may be limited by developing new crop and soil management practices. Additionally, new programs are needed to restore abandoned farmland to a more stable natural state.
Arizona farmers likely will continue to diversify their agricultural base in response to changing market demands and diminishing water resources. More efficient use of water resources is the key to the future sustainability of agricultural ecosystems in Arizona.