Jon K. Piper CONTENTS
Introduction: Environmental Problems Associated with Modern
An Agriculture Modeled on the Prairie Ecosystem Elements of the Prairie Model Perennials as Grains Species Diversity Research Agenda and Findings That Support the Model
Question 1: Can a Perennial Grain Yield As Well As an Annual Grain?
Question 2: Can a Perennial Polyculture Overyield?
Question 3: Can a Perennial Polyculture Provide Its Own Nitrogen
Question 4: Can Perennial Polyculture Manage Weeds, Herbivorous Insects, and Plant Pathogens? Weeds Insect Pests Plant Disease Community Assembly Concluding Remarks Acknowledgment References
INTRODUCTION: ENVIRONMENTAL PROBLEMS ASSOCIATED WITH MODERN AGRICULTURE
"One Kansas Farmer Feeds 101 People and You," proclaims a billboard alongside Interstate 135, near Salina, KS. Modern agriculture has been overwhelmingly successful in terms of output per farmer, acre, or hour worked. Agricultural productivity has steadily increased as a result of technological advances in machinery, fertilizer, and pesticides coupled with the intensive use of plant genetic diversity to improve yield through plant breeding. For example, yields of corn and sorghum increased severalfold in the U.S. between the 1930s and the 1980s (Jordan et al., 1986).
In terms of return on labor, industrial agriculture based on monocultures of annual grains is unquestionably a highly productive form of food and feed production. This productivity has arisen largely through simplifying agroecosystems into monocultures and tailoring them to maximize yield. In the process, however, many of the links between organisms and the soil that serve to regulate natural communities are ignored or disrupted. The following account surveys some of the more severe environmental effects deriving from large-scale monocultures.
With the publication of Silent Spring 35 years ago (Carson, 1962), the public began to become aware of unforeseen environmental consequences of modern agriculture and to question or not whether increasing agricultural production alone was a worthy goal. Three of the most obvious environmental consequences of high-production agriculture are fossil fuel dependency (and its consequent contribution to global warming), contamination of soil and water with toxic chemical residues, and rates of topsoil loss that exceed the natural rates of soil formation. Additional important consequences include the net depletion of aquifer water for irrigation and the loss of biodiversity from crops, land races, and crop wild relatives.
A general consequence of our modern agricultural system is dependency on fossil fuel-based energy. Pimentel et al. (1995) estimate that 10% of all energy used in U.S. agriculture is expended to offset the losses of soil nutrients and water caused by erosion. Over the last few decades, it has taken increasingly more fossil fuel energy to produce a unit of grain in the U.S. (Pimentel, 1984; Cleveland, 1995), with a recent ratio of total energy expended in agriculture (including transportion and processing) to food energy consumed in the U.S. of about 10:1 (Lovins et al., 1995).
Another consequence resulting from decades of chemical application on agricultural soils is contamination of surface waters and groundwaters by toxic chemicals. Particularly troublesome are unsafe levels of nitrate derived from applied fertilizer and residues from pesticides aimed at harmful insects, weeds, and pathogenic fungi.
Nitrate concentrations in groundwater are strongly correlated with overlying land use (Singh and Sekhon, 1979; Hallberg, 1986). Crops often do not take up all nitrogen applied before it leaches below the zone of biological activity in soil. This excess nitrogen in agricultural soils can slowly leach into deep aquifers, even years after fertilizer application ceases.
By the early 1960s, the properties of such long-lived, low-toxicity, and bioac-cumulating substances as DDT began to emerge (Carson, 1962). In temperate regions, DDT has a half-life of 59 years. Once bioconcentrated in such top predators as carnivorous fish and bald eagles, DDT sharply reduces the reproductive potential of these species. Long-term exposure has been associated with mutagenesis and carcinogenesis. DDT was banned in 1969; other long-lived pesticides were banned in the 1970s. They were largely replaced with several types of short-lived, acutely toxic compounds (e.g., organophosphates). Residues of many of these subtances are still present all over the planet. Concentration of DDE (a DDT metabolite) is increasing in some Great Lakes and Arctic species (Hileman, 1994).
It was not until 1979 that routine agricultural use of the new generation of pesticides was linked to groundwater contamination. Researchers discovered that some of these apparently short-lived, unstable compounds can become extremely persistent once below the soil biologically active zone, where the usual biological degradation does not occur (Zaki et al., 1982; Cohen et al., 1995). Just as with nitrate, complete cessation of pesticide use would not immediately halt the increasing presence of pesticides in groundwater.
Many of these chemicals are threats to human health, especially among farmworkers who are exposed directly and rural families dependent upon drinking water from wells. Accumulating epidemiological evidence suggests that agricultural chemicals are associated with increased risks of many types of cancers (Blair et al., 1992; Zahm and Blair, 1992).
An additional unintended consequence of widespread and constant pesticide application is evolution of pesticide resistance in target organisms. The result is a need for higher application rates of some pesticides as well as continuous research to develop new substances to control the targeted pests. This phenomenon has been termed the pesticide treadmill; we work harder and harder to stay in the same place but with ever-increasing costs to environmental quality.
Despite the enormous problems presented by chemical contamination and fossil fuel dependency, the most serious problem for the long-term sustainability of agriculture is soil loss. Soil erosion is the primary conservation problem on much of U.S. cultivated cropland, and occurs mostly during short intervals of heavy rain or high wind and when the surface is not protected by a mulch or crop canopy (Larson et al., 1997). During the last few decades, about one third of the world arable land area has been lost through soil erosion, and this loss continues at an estimated annual rate exceeding 10 million ha/year (Pimentel et al., 1995). On average, soil on about 90% of U.S. cropland is being lost faster than it is being formed. Because the effects of erosion on some soil physical attributes are irreversible, erosion rates alone may not be good indicators of soil degradation and, consequently, soil quality can decline faster than the erosion rate. Once virgin soil is cultivated, organic carbon rapidly decreases (Campbell and Souster, 1982), large pores crucial for soil function are destroyed, changes in some physical properties increase the rate of erosion, rates of nutrient leaching can increase (Blank and Fosberg, 1989), and populations of such beneficial invertebrates as earthworms decline (Edwards and Lofty, 1975). Prairie soils can lose 30 to 60% of their organic carbon, 30 to 40% of nitrogen, and up to 25% of phosphorus from the A horizon after only a few decades of cultivation (Anderson and Coleman, 1985; Schoenau et al., 1989; Woods, 1989). Many of the consequences of soil degradation, such as reduced crop productivity, have been offset to by improvements in fertilizer and irrigation technology and the development of new, higher-yielding varieties.
The industrialization of agriculture, typified by widespread annual monocultures, has led to such profound problems as soil loss, loss of genetic diversity in cultivars, fossil fuel dependency, depletion and contamination of water supplies, pesticide poisoning of farmworkers and nontarget wild species, and development of pesticide resistance in pests. Modern agricultural methods, while highly productive in the short run, are sustainable only as long as topsoil is intact, fossil fuel supplies are affordable, and effective pesticides are available. This may be justified when fossil fuels are cheap and environmental costs can be ignored, but such practices make us vulnerable over the long run.
Definitions of sustainability abound. In view of the issues listed above, a sustainable agriculture for the Great Plains should address simultaneously several key environmental problems of modern agriculture. It should feature reduced or eliminated soil erosion, efficient use of land area and soil nutrients, improved water use efficiency, reduced reliance on synthetic nitrogen fertilizer, decreased risk of pest and disease epidemics, effective chemical-free weed management, reduced fossil energy requirements, reduced chemical contamination of soil and water, and the opportunity for farmers to hedge their bets among several agricultural products. A good working definition of sustainable agriculture is one that includes grain production with (1) no chemical contamination of the environment (via pesticides or fertilizers), (2) no dependence on nonrenewables (e.g., fossil fuel, fossil water), and (3) no net soil loss. This working definition is limited in that it leaves out such important considerations as sociology, economy, and justice. But it provides a beginning point for a biological research agenda.
By using this definition of sustainability, what type of system could simultaneously satisfy all three criteria? Natural grassland ecosystems provide appropriate models of long-term sustainability because they run on sunlight and rainfall, resist pests, weeds, and disease epidemics, and most importantly because they do not lose soil beyond the natural rate of formation. Some of these aspects are explored in depth in the following sections.
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