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VI. Watershed, Regional, and Global Issues
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curred in the winter for both agricultural land uses. Nitrate losses from southeastem U.S. agricultural watersheds tended to be greater than losses from forested
ones, whereas P loads were similar or slightly less than those coming from the agricultural watersheds (Lowrance et al., 1986). Kilmer et al. (1974) measured consistently higher N and P concentrations in discharge from a more heavily fertilized
watershed than one with lower fertilizer inputs. Hallberg et al. (1983) tracked increases in nitrate-N draining from an agricultural watershed in northeastern Iowa
as agricultural practices changed following the introduction of N fertilizer. However, Thomas and Crutchfield (1974) observed that the "background" nitrate levels from small Kentucky watersheds with crop, pasture, and forested land uses
were highly variable. A strong relationship existed between the geology of the watersheds and the P contents of the stream flow rather than the land use. Stream P
levels corresponded closely to those compiled 50 years earlier when few nutrient
inputs were available to agriculture.
The aerial extent of intensive agricultural operations has been related to nitrate
observed in groundwater (Beck et af., 1985; Pionke and Urban, 1985) and surface
water (Ritter, 1984). Patterns of elevated nitrate in a southwest Georgia study were
coincident with an area of intensive cropland (Beck et af., 1985). Nitrates in
groundwater in a Pennsylvania watershed were approximately four times greater
in wells under cropland than under forest (Pionke and Urban, 1985). The mixing
of recharge from the two areas diluted the nitrate concentration before the flow entered the surface water stream. Nitrate concentrations in drainage from watersheds
in Delaware with >60% cropland were more than 30% greater than those with
<60% cropland (Ritter, 1984). Klepper (1978) determined that both N fertilizer
use for corn and the fraction of watersheds in row crops (primarily corn and soybeans) were significant factors in explaining the variation in nitrate concentration
in surface waters from a central Illinois watershed.
The watershed studies suggest that making meaningful observations of nutrient
cycling in a sustainable agriculture at aggregated geographic scales is difficult.
Lowrance et af. (1986) suggest that watershed-level studies are essential in relating management to external environmental impacts. However, the relationships
may not be simple nor sufficiently sensitive for most decision making. Small plot
and field results do not seem to aggregate well to the higher levels of spatial resolution where the land use patterns are mixed and complex. Furthermore, there must
still be some connection between the activities and the responsibilities (for both
the farmer and the beneficiaries) before changes can be promoted. The appropriate level for observation and for responsibility remains a question. Background
concentrations and differential, as well as relatively slow nutrient transfer rates
may limit the usefulness of observations at the real or perceived scale of the problem. Perhaps an indicator at another level of resolution will be useful. The German guidelines for organic agriculture have selected the farm as the unit to limit
off-farm inputs of both fertilizers and feeds as a means to meet the expectations
they associate with organic agriculture (Nolte and Werner, 1994).
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
The introduction of clover in Europe during the 17th through 19th centuries
(Kjaergaard, 1995) is a historical illustration of the impact a change in the potential nutrient stocks can have on a region beyond the scale of an individual farm or
watershed. For example, the input of N to cultivated land doubled with the introduction of clover to Denmark during this period and an ecological dilemma of declining yields and degrading landscapes was averted. Grain yields doubled and cattle numbers increased by one-third in less than 35 years. Even the vast increase in
potato production has been linked to the newly available quantities of N. The new
clover-based system relied heavily on animals to utilize the legume forage crop
and provided an opportunity to retain P and K by marketing animal rather than
crop products. Individual cultivators influenced the flow by the allocation of land
resources to the N “collectors,” but it was the introduction of a production factor
from outside the original system that made the change feasible for the individual
Nutrient balance information at other spatial scales, such as county, state, andor
country level, can be used to identify where clustering of nutrients may occur when
inputs exceed outputs and/or utilization potential, assess potential nutrient impact
on water resources, and provide a basis for future decision making (Barker and
Zublena, 1995). Almost 20% of North Carolina counties were found to have nutrients in excess of crop requirements with excess P more common (18% of the
state) than excess N (3% of the state) (Barker and Zublena, 1995). When Keeney
(1979) estimated the N budget for Wisconsin, he identified N fixation by legumes
and N fertilizer as significant sources of N for the state (54 and 22%, respectively). However, approximately 40% of the N inputs could not be accounted for and
were assumed to be lost to the environment through denitrification and leaching.
Based on estimates of total N use in all kinds of farming in The Netherlands and
the total removed from farms in produce, 75% of the N is wasted (Whitmore and
van Noordwijk, 1995). Granstedt (1995) estimated that 74,60, and 74% of the N,
P, and K, respectively, imported onto the farms in Sweden are not exported in food
products. Of the nutrients actually taken up by plants only 20,25, and 5% of the
N, P, and K, respectively, are actually exported from the farm to the community
food system. Isermann (1990) recognized that the surpluses of N and Pat the country level in Western Europe are due to increasing levels of nutrient application not
resulting in similar increases in nutrient exports in farm products.
1. Intercontinental Flows
Cooke (1989) highlighted the significance of intercontinental plant nutrient
transfers. Not only is there a concern for soil mining at the farm level, but these
concerns should be nationwide. The value of nutrients lost may not be offset by
the prices paid for the commodities. This transfer affects the countries producing
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the commodities, which are often developing countries, and the countries importing the commodities. Furthermore, he estimated that developed countries import
12% more N and more than 60% more K than they export, whereas comparable
depletions exist in developing countries. Only in Pdo the developing countries appear to be gaining while the developed countries are losing. Nutrients in a single
animal feedstuff, casava, imported from Thailand to The Netherlands were equivalent to more than the K fertilizer used in The Netherlands; one-third of the P fertilizers used; and slightly less than 10% of the N fertilizers used. Of course, these
data do not represent the whole range of conditions from various countries. Symptoms of the importing countries may be excessive nutrient accumulation that will
have adverse environmental impacts. In 1983, 40, 72, and 59% of the total imported N, P, and K, respectively, to The Netherlands could be attributed to feedstuffs. Cooke (1989) suggested that all countries need to develop national nutrient
balances as policy guideposts for the future prosperity of their countries. Issues of
sustainability, especially the contribution of maintaining the integrity of the resource base, either from depletion or enrichment, to intergenerational equity
should be a major consideration. Part of the difficulty in implementing policies that
might be derived from such analyses is that the controllers of the nutrient flow may
not correspond with the authority of national jurisdictional boundaries and that future generations and other considerations are routinely underrepresented in the
evaluation of impacts of a wide array of decisions.
2. Finite Geologic Deposits
Although global reserves of P and K are large, the deposits will eventually be
exhausted. It is estimated that, at current rates of mining, the known global reserves
of phosphate and potash will last for approximately 200 years (Louis, 1993). Even
though there are larger “potential” reserves that may extend the time span for mining these nutrients, the rate of growth of demand for P and K may work to limit
the increase in time until deposits are exhausted. Regardless of the time until reserves are depleted, the extent of the potential supply must be compared to the ability of society to sustain the pattern of nutrient flow. Gains in efficiency of P and K
utilization may extend the life of the geological reserves, but if the connections in
the patterns of nutrient flows are not sustainable devoting management and other
resources to enhanced efficiency in order to prolong the reserves may contribute
little to true sustainability.
C. ENERGYUSEAND NUTRIENT
Patterns of nutrient flow depend on the energy contributed to the process from
a variety of sources. Energy use and flow is the central purpose of the food chain
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
as solar energy is captured in primary production and moves along to the succession of consumers. Ultimately, energy is dissipated as heat, becomes part of decomposer organisms, or remains in the undecomposed residues. Animals consume
solar energy that was recently concentrated by plants. In addition to relying on capturing current solar energy, agricultural production systems also utilize historic solar energy, recovered from geologic reserves, to create inputs. The inputs have a
variety of functions. These are generally recognized as substituting for labor, for
reducing the deficiency of production inputs such as crop nutrients or water, or creating conditions that could not otherwise exist. However, substituting animal power or machine inputs in corn production for human labor generally reduces the
amount of solar energy captured per unit of expended energy (Pimentel and
Burgess, 1980). Nitrogen fertilizer can eliminate a N deficiency and actually increase the potential for crop production, but its use also tends to decrease the energy captured per unit of expended energy.
Energy accounting can be used to identify the various forms and the amounts of
energy used in agricultural production. The accounting may be conducted for enterprises such as corn, tomatoes, alfalfa, or animal production. Nitrogen fertilizer
is typically a major energy input to cereal production such as corn. This one input
may account for 20% of the total energy in the production inputs and fuel can be
over 40% (Pimentel and Burgess, 1980). For legume crops such as alfalfa the total energy requirements can be approximately 60% of corn production (Heichel
and Martin, 1980).There are usually no N fertilizer inputs to an established stand,
and the fuel requirements for cultural practices become approximately 98% of the
total energy used. Animal agriculture benefits from the controlled environment
conditions provided by modern housing systems (Spedding, 1982). Energy inputs
to animal production vary widely with species and geographic location, but the ratio of energy inputs to outputs is generally much greater than for common crops
(e.g., <0.5 MJ MJ- for cereal crops and 2.8 and 10 MJ MJ-' for milk and broilers, respectively; Spedding, 1982).
Farms are rarely single enterprises and it is essential to evaluate energy consequences of the entire system instead of focusing only on one aspect such as nutrients. For example, changing crop sequences from continuous corn to corn-soybeans in a dairy farm simulation resulted in a more dramatic decrease in energy
requirements (36%decrease) than when a corn-alfalfa rotation was selected (Vinten-Johansen et ul., 1990.The greater reduction in N fertilizer requirement for the
corn-alfalfa rotation was partially offset by the greater requirement for fuel to
manage the forage crop.
The structure of contemporary agriculture has significant implications for the
energy requirements to sustain the connections among the various components. Pimentel(1980) estimated that farm input supplies travel an average of 650 km and
require 257 kcal kg-' for transport. With emerging regional and global markets
(Lanyon, 1995)since Pimentel's original calculation, the energy consumed for this
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purpose is likely to have increased. A further link in nutrient flow that is seldom
considered is the energy requirements to “return” the nutrients to some former location. Identifying the relevant controls and the responsible parties is likely to be
an essential part of acting on an understanding of the role of nutrient cycling in a
sustainable agriculture. Thus, the ultimate energy concern regarding the nutrient
flow pattern may have less to do with farm practices that replace fertilizer inputs
than with the extent of the connections between sources of nutrients, production
of cash crops that are transferred to animal production locations, and even the
transfer of animal products to consumers. Another important issue is that the deposits of the two main types of fossil fuels used in production have different projected lifetimes. For example, the natural gas reserves to produce N fertilizer may
well outlast those for the liquid fuels that are the keystone of modern agriculture
and its associated transportation networks (Lanyon, 1995).
The significance of energy use in maintaining the pattern of nutrient flow can
be evaluated using several criteria. For instance, there are environmental consequences that develop as energy is depleted from reserves, as energy and mineral
extraction takes place, as nutrients are used with decreasing efficiency in crop production, or as nutrients accumulate in areas of animal concentration. Consideration of proposed changes will need to include these criteria and to develop relative
priorities for addressing them. Also, if energy scarcity is a concern, then evaluating both the relative magnitude of competing needs and the flexibility for change
may need to be considered. Production agriculture used <3% of the total U.S. energy consumption in the 1970s and was <20% of the energy in the food system
from field to table (Fluck and Baird, 1980).
Reforming the pattern of nutrient flow is likely to involve a reconsideration of
the strategic direction of agriculture. Thus, putting energy use into perspective
with the competing uses across the spectrum of energy uses in society may be a
first step. From this perspective will emerge a new set of questions and associated
technical challenges. Questions such as who decides the characteristics of the new
pattern of nutrient flow, who benefits from the new formulation, and who pays for
the new arrangement are likely to all become part of the debate.
Isermann ( 1 990) cites the long-term possibilities of balancing nutrient applications and use at the field and farm levels by reintegration of animal and crop production, improved animal feeding programs, modified animal housing to reduce
potential ammonia volatilization, increased emphasis on N and P recycling, and
even changes in human diets toward less animal protein. He projects a degree of
success in reducing emissions if these fundamental changes are implemented within 20-30 years. Kaffka and Koepf (1989) suggest that not just a site-adapted pro-
NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS
duction program but a mixed plant and animal husbandry are prerequisites for the
success of an approach to sustained production and balanced fertility. This view is
supported by Granstedt (1995) because he suggests that a good balance between
animal production and crop production at local, regional, and national levels is required to reduce the losses of nutrients from agriculture. He envisions that certain
regions of Sweden would emphasize animal production less and animal production would be decentralized in other areas in order to achieve ecologically based,
resource-conserving agriculture. He also believes nutrient balancing can be
achieved through cooperation among farmers with different enterprises and
through returning nutrients from the community to the farm. Stopes (1995) supports the notion of fundamental changes in the food production and delivery system and the human diet if widespread adoption of organic farming systems (as one
form of an agriculture presumably with nutrient cycles) is to become a reality.
There is tension apparent between the value of animal agriculture in conserving
nutrients on the farm and the perceptions that less animal production would be desirable.
Land application of human waste has been viewed as one mechanism to return
nutrients to farms. Witter and Lopez-Real (1 987) suggest that nutrients in sewage
sludge are only 3% of the fertilizer requirement in Britain, but that with increased
emphasis on reducing waste and emphasizing return flows 40% would be possible. The impact of industrial pollution of the waste stream and a variety of policies that keep fertilizer prices low and emphasize productivity will not encourage
“recycling.” Although between 40 and 60% of the N, P, and K of food products in
Sweden is in slaughterhouse and domestic wastes that could be recycled with little risk (Granstedt, 1995), it would require extensive change in the relations between agriculture and the rest of society.
Loehr ( 1969)recognized animal waste management as a “national” problem and
highlighted increasing reliance on readily digestible ration ingredients as a practice to reduce the costs of animal waste management. Increased digestion of nutrients supplied in the rations would reduce the quantity required for the same production. Cromwell et d.(1993) project considerable reductions in required P
supplementation of corn- and soybean-based rations with the use of an enzyme
that increases P availability from these sources. These approaches also are likely
to provide only incremental, short-term improvements. The challenge is to recognize that apparent farm-based solutions may not be that at all if they do not address
the appropriate scale of the problem, which is the transfer of nutrients between specialized farms with different ecosystem elements.
Increasing the efficiency of feed nutrient utilization through changes in the ration components or better balancing of rations has also been suggested. The effect
of livestock production on nutrient balance can be managed by exporting fewer
animal products (cutting production) or reducing losses of nutrients by on-farm
management practices. A biodynamic farm was found to be self-sufficient in nu-
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trients because the livestock rate was adjusted to the carrying capacity of the farm
crop production (Granstedt, 1995). Organic farms in Germany cannot purchase
more than 10% of the feed for cattle (Nolte and Werner, 1994), and their production is, therefore, less than that of farms with unlimited potential to purchase feeds.
Sustainable livestock production is an issue of manure management, effects on soil
erosion and soil organic matter, and the export of nutrients in products sold (de Wit
et al., 1995). Postharvest and postplowing N losses have been suggested as a focus for reducing nitrate losses when legume crops are rotated to nonlegume crops
(Kopke, 1995). Other losses from manure application and manure “heaps” need to
be considered. Sustainability must be determined for various agroecological criteria in a system-specific analysis (de Wit et al., 1995).
There are controls on flow patterns that are exerted at different scales. In general, controls exerted at the regional, national, and global levels have ramifications
that influence the choices and decisions made at the farm level (Table IV). The
control that the farmer then exerts over the direction of the farm influences the
choices and decisions made regarding practices in particular fields.
One of the main factors influencing farm decisions affecting nutrient flows is
the perceived economic benefits of particular management options. The growth of
specific crops or raising of animals, the amount of purchased feed for animals vs
farm-grown feed, and the enterprise mix affects the farm nutrient balance. The use
of fertilizers at rates recommended by reputable soil testing laboratories is believed
to help ensure maximum profits from crop sales. Reliance on off-farm feeds frequently enhances the economic performance of an animal farm. In some circumstances, dairy operations may have greater net returns if off-farm feeds are purchased rather than if the dairy cows are fed only from on-farm sources (Westphal
et al., 1989). However, purchased feed arrangements potentially create a gap between the location of crop production and the location of animal production. Because cost is a major factor in purchased feed decisions, rather than the biology of
production, the least costly feed may come from widely dispersed sources. “Return’’ of nutrients to these locations is not considered as part of the cost when they
are purchased. Nor are the full costs of growing and transporting the crops necessarily paid for when feeds are purchased. A variety of government programs, actions, or subsidies commonly mask the true costs of water (as in California), of
transportation (such as the construction of the interstate highway system), or of
maintaining a low-cost energy supply (such as military expenditures to help maintain a flow of oil from the Middle Ease under favorable terms). Consequentially,
there is no economic incentive to maintain biological integrity in the transaction.
Profit maximization is not the only farmer goal influencing the pattern of nutri-
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
Summary of Controls and Their Outcomes on Nutrient Cycling and Flows
at Different Geographic Resolutions
Political process, corporate
geologic reserves of nutrients
and energy, transportation
Government programs, international
trade patterns, externalization of
+ certain costs, corporate decisions
about where to locate facilities
and how to organize their
supply of agricultural products.
Farmer orientation (outlook).
Enterprise mix (types of crops and
economics (potential sale
animals raised), products exported
price - costs), consumer
+ off-farm, products imported,
animal density relative
infrastructure and preferences
to land area, etc.
Amounts and forms of
nutrients added, method
and timing of nutrient
additions, tillage system and
surface residue status. types
of crops grown, portion of
crop removed, etc.
K- Soil structure, runoff and erosion,
nutrient stock levels, bioligical
diversity and activity, amount of
active SOM, amount of
total SOM, etc.
ent flow to and from a farm, especially on animal production farms. Although rates
of fertilizer application are likely to be reinforced by the economics of crop production (Legg et d.,
1989),sometimes N fertilizer use in addition to the available
N from legume residue and manure is thought to be a risk-reducing management
tactic for livestock farmers.
Farm-scale nutrient flows are influenced by other factors in addition to individual farmer goals such as profit maximization in crop production or intensive animal production, risk aversion, or some alternative ideology. Government programs
that create incentives for particular practices also influence farm-level nutrient
flows. Actions taken by large corporations to ensure their profits have transformed
the very nature and structure of agriculture. For example, the production of broilers in the United States is almost 100% controlled under production and marketing contracts, with a limited number of integrators or they are raised under integrated corporate ownership (Welsh, 1996).For the most part, the direction of the
hog industry is following poultry, with the decisions as to where animals are raised
and what they are fed coming increasingly under the control of a small number of
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Although not shown in Table IV, there can be feedback mechanisms that move
from the field or farm scale upwards and may influence farmer as well as governmental decisions. For example, when soil structure declines, a farmer might
choose to change tillage, crop rotation, and /or the farm enterprise mix. Widespread problems with soil erosion and water pollution have also been recognized
nationally with the implementation of government-financed soil conservation programs aimed at decreasing soil and nutrient loss to water.
If changes in the pattern of nutrient flow are to be made to more closely approximate a cycle for a sustainable agriculture, consideration of the character of
the existing linkages and the incentives behind them will be essential. Nutrient cycling must become a priority in farmer decision making before it can play a significant role in farm performance. For instance, a nearly closed nutrient cycle is a
goal for individual farms in Germany for them to be recognized as organic (Nolte
and Werner, 1994).However, widespread adoption of such a goal cannot be based
solely on resources and personal perspective of individual farmers. It must be a
goal that is supported in the messages communicated to the farmer from the “surroundings.” Efforts to associate nutrient cycling with sustainable agriculture or
management of nutrient flow to meet any criteria other than those present in marketplace transactions will have marginal results unless they are strongly endorsed
outside the farm and then communicated to the farmer.
VII. PROMOTING A MORE SUSTAINABLE
AGRICULTURE THROUGH CHANGES INFLUENCING
NUTRIENT CYCLES AND FLOWS
A sustainable agriculture is not simply an assemblage of numerous possible onfarm techniques, but one that will have to confront the realities of functioning within dynamic social and economic constraints on agricultural activities. The decisions to structure farms in particular ways are shifting from resource-based
constraints to emerging choices based on a variety of social factors. Stopes (1995)
suggests that a new orthodoxy of multiple goals for agriculture that are not limited to production but rather that include environmental and social outputs and a
greater degree of “sustainability” will emerge. Will mainstream society consider
the diversity of approaches that could be utilized to meet production, efficiency,
and consumption goals (Whitmore and van Noordwijk, 1995)?
Many changes in the way farms are managed and in the way society at large interacts with the agricultural sector have been suggested in order to enhance sustainability. A number of these approaches either directly involve nutrient flows and
cycles or have important implications for these flows. For the long-term sustainability of agriculture, ways need to be found to decrease the dependence on fossil
NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS
fuels to manufacture fertilizers or other amendments and to transport fertilizers
and feeds over long distances. With the fertilizer industry (especially N) so intimately tied to the price of energy, a sharp increase in the cost of fuel will make
agriculture especially vulnerable. Ways also need to be found to reduce nutrient
pollution of surface and groundwater. Enhanced efficiency of nutrient cycling and
uptake by plants can increase individual cash crop farm profitability by decreasing the amount of purchased inputs.
Finally, although there are large reserves of K- and P-bearing mineral deposits,
the pace of mining these nonrenewable deposits may increase, thus decreasing the
time needed to exhaust the deposits. This indicates that the nutrients should be used
carefully and efficiently.
Changes in agriculture and society that are related to nutrient flows and that
might promote a more sustainable system include those that can happen quickly
as well as those that will take longer to accomplish (Fig. 9). Some of these suggestions appear to hold more promise than others. In general, short-term changes
help to “tighten up” the efficiency of the use of nutrient sources and the cycling of
nutrients in the field setting. Those changes that might occur at the farm level will
take longer to put into effect but will have a more long-lasting impact. However,
farm-based changes may not turn out to be appropriate if the scale of the problem,
such as the transfer of nutrients between specialized farms, is not addressed. Fi-
4c 6 6
Figure 9 Relationship between spatial scale of possible changes affecting nutrient flows and estimated time needed to complete changes (see text for explanation of numbers).