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II. Framework for Evaluating Nutrient Dynamics
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crops through the raising of forages, especially N,-fixing clovers, allowed continuous cropping to take the place of the alternate year or every third year fallow systems (Bairoch, 1973). It also permitted the integration of livestock into cropping
systems and ended nomadic husbandry. The enhanced productivity of the land allowed a significant increase in the annual agricultural production over the needs
for farm family consumption (Bairoch, 1973). Although the industrial revolution
began in England during the last half of the 18th century, it reached other countries in Europe and the United States only during the 19th century. Through much
of the 19th century, and well into the 20th century in pockets, most agricultural
products were consumed on the farm where produced. This was a common feature
of temperate region agriculture in what eventually became the advanced economically developed countries. In the less developed temperate and tropical regions,
with the important exception of plantation crops such as sugar and bananas, subsistence farming has been common through much of the 20th century, with only
small amounts of products exported off the farm.
In the diversified subsistence farming systems that developed in Europe and the
United States before the industrial revolution, most of the plant products were either consumed directly by people on the land or were consumed by animals that
were then consumed by humans (Fig. la). In this example the three parts of the
pyramid are physically connected and residues and waste products can easily return to the land.
The development of large cities and transportation systems to move food long
distances in the United States and the industrializing countries of northern Europe
created the first modern widespread physical break in the production-consumption chain. Crops and animal products were sent from the countryside to urban areas and even to other countries, decreasing the potential for on-farm nutrient cycling (Fig. 1b). In the last half of the 20th century, rapid urbanization has also been
occurring in most developing countries (usually without commensurate economic development), and this, together with the development of an “advanced” commercial agricultural sector oriented toward exports, has also had a significant negative impact on nutrient flows in those countries. Concern about the consequences
of interrupting the cycling of nutrients was expressed in the last century:
Capitalist production, by collecting the population in great centers, and causing
an ever increasing preponderance of town population . . . disturbs the circulation of matter between man and the soil, i.e., prevents the return to the soil of
its elements consumed by man in the form of food and clothing; it therefore violates the conditions necessary to lasting fertility of the soil.’’ (Marx, 1887; originally published in German in 1867)
Another physical break in the trophic pyramid resulted from the transformation
of animal agriculture based on small diversified farms to large specialized production units separated by long distances from the farms that produce feeds (Fig.
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
Figure 1 Changes in the spatial relationships of the trophic pyramid relating plants and animals
to humans. (a) Early agriculture (eighteenth to mid-nineteenth century); (b) urbanizing agriculture
(mid-nineteenth to mid-twentieth century); (c) industrial agriculture (mid- to late-twentieth century).
lc). The availability of low-cost N fertilizers after WW I1 rendered forage legumes
superfluous on farms producing grain crops. There was no longer the need to raise
animals to utilize the forages. In the United States, the conversion to enormous
production units is essentially complete for poultry, far advanced for beef cattle,
and well under way for hogs. This phenomenon has further exacerbated environmental problems associated with agriculture. The heart of the issues resulting from
the geographic separation of crops and animals can be summarized as two sides of
the same coin: (i) the decline of SOM and nutrients on crop farms (requiring the
application of large quantities of synthetic fertilizers as well as other inputs to compensate for organic matter depletion, and (ii) the simultaneous overabundance of
nutrients and organic matter at animal production facilities (with the resulting pollution of surface and groundwaters).
Clarification of the definitions of some of the key terms that we will use will be
helpful for the discussion of issues and problems of crop nutrient management.
Stocks-Stocks refer to the quantity of nutrients within a defined part of a system. The total stock of nutrients may be of interest for many assessments. However, from the point of view of plant nutrition the maintenance of a sufficient stock
(pool) of nutrients that are either available or easily transformed into an available
state is essential for crop productivity. At the same time, available nutrient substocks must be low enough to moderate potential environmental effects of agriculture. Flows will both contribute to and be subject to the magnitude of the various stocks.
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There are numerous biological and chemical reactions that change the state of
nutrients to more or less available forms. These transformations convert nutrients
from one stock of the element to another but do not change the quantity of the total stock of a nutrient. Although the total stock of a particular nutrient may be important for long-term sustainability, it will not usually be of interest for the shortrun concerns of soil fertility unless the net rate of transformation to an available
form is also known.
The size of a stock may exert an influence on susceptibility for nutrient flow.
For example, large stock of inorganic Nor of soluble P will permit significant flows
of these nutrients with leaching or runoff waters.
Flows-The flow of nutrients in an ecosystem is the most basic concept of nutrient movement. Nutrient flows represent linkages among various pools (or
stocks). Measurements of various types of nutrient flows can suggest control
mechanisms and indicators of system performance.
Some nutrient flows are managed pathways, where the purpose of the operation
entails the intentional addition or removal of nutrients. Managed flows occur when
fertilizer is applied to meet an estimated crop need, when manure is applied to certain fields, when a crop is harvested and sold, when animals graze on pastures, etc.
Although other flows, such as leaching of nitrate or nutrient losses in runoff waters, are not purposely managed, their magnitude is strongly influenced by management practices such as tillage systems, rotations, fertilizer application rates,
manure application rates and application methods, and animal stocking density.
Cycles-A nutrient cycle is an example of a closed loop pattern of flow in which
a particular atom ends up back in the same location from where it started. Where
a boundary is drawn surrounding the extent of the system has a significant impact
on deciding whether a true cycle or rather another pattern of flow is occurring.
Transformations-There are numerous processes that determine the “state” or
form in which nutrients occur in soils. These include mineralization from organic
matter, immobilization of inorganic ions by microbial uptake, precipitation of lowsolubility compounds, various oxidation reactions such as nitrification, various reduction reactions such as denitrification, dissolution from solid forms, etc.
The particular form that a nutrient is in influences its availability for plant uptake as well as susceptibility to leaching or gaseous losses. When a nutrient undergoes a transformation to another form, it is not usually considered a flow because the transformation normally occurs in place. However, one transformation,
biological N, fixation, is also a flow. Because soil N, is in equilibrium with the atmosphere, N, moves into the soil as N, fixation occurs, and the stock of total soil
N is increased. For purposes of discussion in this chapter we will refer to N, fixation as a flow of N rather than a transformation.
Boundaries-When discussing nutrient flows and cycles it is essential to define a boundary around the system of interest. The boundary becomes a reference
point for evaluating relative movement of nutrients. Different objectives may
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
cause one to define a boundary to be around a certain portion of the soil or a field,
farm, state, region, watershed, or country. If global-scale cycles are of interest, the
boundary then includes the entire earth.
When discussing nutrient transformations, flows, and cycles it is important to
take into account implications of spatial scale, ecosystem relations, seasonal patterns, and landscape position. These various considerations can either influence
the nutrient flows and transformations themselves or our perception of them.
1. Spatial Scale and Ecosystem Relations
The extent of the system under consideration has a huge impact on how we view
and understand flows and cycles. The emphasis in the literature on nutrients has
been placed on the field scale because most tactical and operational management
decisions are field based. When viewing processes and flows at this scale, the issue of applying fertilizers or manures is relatively simple. When a specific nutrient application is believed necessary some is applied and this is a flow into the field
from somewhere outside. Likewise, when the crop is harvested, it seems to be a
simple flow of nutrients out of the field. However, the crop may be consumed on
the farm or leave the farm. Also, the nutrients in manure may come from inside
the farm (if animals are fed farm-grown feedstuffs without imported fertility
sources) or from off the farm (if animals are fed only imported feeds) or some mix
of the two (if farm-produced feeds are grown with imported fertilizers).
A greatly simplified diagram of a natural soil-plant-animal ecosystem (Fig. 2)
can aid the discussion of scale of consideration and nutrient cycling and flows in
agriculture.In this figure, the only input flows into the soil come from atmospheric
deposition while the only output flows result from erosion, leaching, and gaseous
losses. There are three stocks of nutrients (boxed in Fig. 2): in the soil (including
all living organisms), in living plants above ground; and in aboveground animals.
Nutrients are taken up from the soil by the plant as it grows and plant residues are
returned to the soil to complete a soil+plant+soil or a soil+plant+animal+
In general,cycling of nutrients is very efficient under natural ecosystems (Crossley et af., 1984). In most undisturbed natural systems such as forests and grasslands, there is a high degree of synchronization of the supply of available nutrients with the uptake needs of plants. This results in a low level of nutrients in the
soil solution at any one time, promoting an efficient soil+plant+soil cycling of
nutrients. Continuous soil cover with little disturbance helps promote water infil-
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Figure 2 Simplified natural system nutrient cycle and flows in the soil-plant system.
tration and maintain low rates of soil erosion. There may be some spatial discontinuity between where nutrients are taken up by plants and where they are deposited in residues, such as when leaves fall on the forest soil surface while roots
may take up nutrients at 10 or 20 cm or greater depth. However, soil organisms,
such as earthworms, beetles, and termites, and leaching help to reintroduce the nutrients into the root zone.
Plants in natural systems sometimes appear to use different nutrient cycling
“strategies” to their own advantage. It is hypothesized that through an evolutionary selection process some species of plants developed characteristics that enhance
the fitness of their environment for themselves at the expense of other plant species
(van Breeman, 1993, 1995). For example, fast-growing species tend to have
residues that decompose and turnover nutrients rapidly. On the other hand, slowgrowing species often have residues that are high in lignin and secondary metabolites that slow microbial decomposition and, thus, reduce competition from fastgrowing species that require high levels of available nutrients.
Compared to a natural ecosystem, a managed agricultural ecosystem has greater
amounts of nutrients flowing in and out, less capacity for nutrient storage, and less
nutrient cycling (Hendrix et al., 1992). There are now inputs of nutrients from a
variety of animal feeds, synthetic fertilizers, inorganic amendments, manures, and
composts (Fig. 3). In this example, the boundary has been drawn around a plant
and the soil below to the bottom of the root system. A major nutrient output from
the field is harvested plant material, which is fed to an animal or used in another
NUTRIENT CYCLING, TRANSFORMATIONS,AND FLOWS
* erosion,I leaching,
Figure 3 Simplified managed system nutrient cycle and flows in the soil-plant system.
manner. In general, nutrient losses by runoff, erosion, volatilization, and leaching
are far greater in an agroecosystems than in a natural system. Compared to natural systems, there is normally a greater quantity of soluble nutrients present in
agroecosystems and more soil disturbance and longer times during the year when
the soil is not covered with living vegetation. These agroecosystem characteristics
stimulate SOM breakdown and lead to more compact soils with less porous infiltrative surfaces and more runoff and erosion than in natural ecosystems.
When looking at the soil-plant system level, it is difficult to tell whether or not
an input is completing a true cycle where the nutrients removed from that particular area of soil are being returned to the same location. For example, is the origin
of the nutrients in manure the location under consideration or is it another field or
farm? Thus, it is necessary to look at both field- and farm-level flows and cycles
to determine whether or not true cycles are occumng.
When looked at regionally (or globally), the location where the nutrients are
produced or mined and refined or incorporated into plants or animals and where
the agricultural products are shipped to, processed, and consumed all become important considerations in understanding intraregional and interregional flow patterns. These may be as important to a sustainable agriculture as field- and farmlevel flows. Nutrients commonly travel significant distances, as when fertilizer is
shipped from the manufacturer to the farm or when feed grains are transported
from the Midwest to the dairy farms in the Northeast, vegetables are shipped from
California to New York, or wheat is transported from the Northern Plains and the
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Northwest of the United States to China. In these situations the flow is all one way
and there is no realistic means for the nutrients to cycle back to the farms and fields
from where they came.
2. Seasonal Patterns
Nutrient transformations and flows do not happen at a uniform rate during the
year. Mineralization of nutrients from organic matter is usually very slow during
the winter and at a standstill when soil is frozen. Peak rates of mineralization in
temperate region soils coincide with the warming in the spring and are probably
significantly enhanced by freezing and thawing over the winter (Magdoff, 1991a;
DeLuca et al., 1992). When soils dry down during the field season and are then
rewetted, there is also a burst of mineralization caused by the conversion of a certain portion of SOM to forms that are more susceptible to microbial attack.
Significant leaching and runoff losses of nutrients in most temperate annual
cropping systems are confined to the late fall, winter, and early spring when precipitation exceeds evapotranspiration and recharge requirements (Fig. 4).During
the summer season evapotranspiration is usually greater than precipitation and
leaching and runoff are usually minimal because of the drier soil conditions.
Managed flows also occur during distinct times of the year (Fig. 4).Large quantities of lime, fertilizer, and manure are normally applied when the crop is not in
the field-in the spring before the crop is planted or in the fall after the last crop
is harvested (some application during crop growth as side-dress and top-dress is
also common). The flow of nutrients leaving the field with the harvested crop usually occurs at a distinct time of the year-determined by climate, species and cultivar, and other management practices.
Thus, nutrients may be applied in the fall, taken up by plants during the following growing season, and removed from the field as the crop is harvested 10 or
11 months after application. Also, some portion of the applied nutrients may be
held by the soil so that they are taken up by plants and removed from the field only
years after application.
There are also changes in nutrient stocks that operate over decades and even
longer. Soil stocks of N in many midwestern soils were drawn down over decades
as organic matter was depleted (Hass et al., 1957). Also, the buildup of nutrient
levels by a few decades of heavy fertilizer and/or manure application by many
farmers has made it difficult to even find low P and low K soils in certain areas
(Engelstad and Parks, 1976; Sims, 1993).
3. Landscape Position
By increasing the scale of attention from the soil-plant system to the field and
then to the farm and watershed or subregion, issues relating to position in the land-
NUTRIENT CYCLING, TRANSFORIMATIONS, AND FLOWS
M I A
N I D
Figure 4 Seasonal aspects of nutrient flows into and out of fields for a northern hemisphere temperate region annual crop.
scape become apparent. For example, soil eroded from the slope of a field may or
may not leave the field or farm. The sediments may be deposited in a low-lying
depression in the field or in an adjacent field. Sediments might also flow from a
field to a stream and from there into a lake. In the first situation, there is only a redistribution within a field or a flow from one field to another. It is not the same net
loss to the field or farm that usually occurs after sediments enter a stream.
111. SOILPLANT SYSTEM
AND Son, NUTRIENT
Within the soil, for each plant nutrient of interest there are three main types of
stocks that can potentially supply nutrients in forms that are available to plants: (i)
nutrients in the soil solution in forms that can be taken up by plants, usually as simple ions; (ii) nutrients associated with organic matter by being adsorbed on negative exchange sites or present as part of organic molecules; and (iii) nutrients as-
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sociated with soil minerals, either adsorbed on exchange sites or as part of the
structure of the inorganic mineral.
Mineralization of organic compounds as well as cation exchange, solubilization,
desorption, and dissolution of minerals convert the soil nutrient stocks listed in (ii)
and (iii) into forms that can be immediately used by plants. Nutrients are also
added to the soil in a number of forms, such as fertilizers, manures, and crop
residues from other fields, in precipitation and dry deposition, and in the special
case of N by biological N, fixation.
1. Satisfying Short-Term Fertility Needs of Crops at
the Soil-Plant Level
To satisfy short-term needs of crops during the growing season the amount of
available nutrients must be greater than or equal to the uptake needs of the crop
(see soil-plant flow labeled 7 in Fig. 5). Using the numbering system in Fig. 5,
Solution stock + (1-2)
+ ( 3 4 ) + (5-6) + (10-13) 2 7,
where solution stock is the quantity of nutrient in soil solution at start, 1-2 are the
net mineralization, 3 4 are the net desorption from SOM, 5-6 are the net desorp-
[+output (flow)V-v I
Figure 5 Simplified nutrient cycle, flows, and transformations in the soil-plant system with inputs and outputs indicated.
NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS
tionholubilization from minerals, 10-13 are the net addition to the solution from
the outside, and 7 is the flow of nutrient to plant.
For most nutrients, leaching, runoff, or other such losses are normally small
enough during the growing season to be omitted from the equation. However,
NO, -N losses by leaching during heavy rains may be an important issue, especially on sandy soils.
There are two contrasting examples relating nutrient availability to plant uptake,
with most real-world situations somewhere in between. For one example, external
inputs create a very large stock of available nutrients at the beginning of the growing period in comparison to the initial solution stock and potential resupply from
nonavailable stocks. The external input, for all practical purposes, satisfies the entire crop need. This occurs when N fertilizer is used to supply crop N needs on a
light texture soil with little organic matter. The addition to the solution (10) is %[initial solution stock (1-2)
( 3 4 ) + ( 5 - 6 ) - 131 and Eq. (1) then becomes
10 2 7.
A different situation arises when there is a very small quantity of a nutrient in
the soil solution, but sufficient replenishment from nonavailable stocks occurs during the growing season so that external fertility sources are not required to satisfy
crop needs. This is common in the case of P and also can occur for N if leaching
and denitrification over the fall, winter, and early spring reduce solution N to very
low levels but there is enough mineralization from active SOM to supply plants.
Solution stock is then 4 [( 1-2) + (3-4) + (5-6) - 13)] and the equation becomes
+ ( 3 4 ) + ( 5 - 6 ) - 13 2 7.
Enhancing mineralization and desorption from SOM and minerals and/or decreasing immobilization, adsorption and precipitation, and leaching, erosion, and
gaseous losses promote a larger quantity of nutrients available for uptake by the
plant. Adding nutrients in available forms (or that are easily transformed to soluble forms) also enhances short-term nutrient availability. However, this may not
be necessary for many years in naturally fertile soils such as the tallgrass prairie
or in soils in which large quantities of external inputs have built up high total nutrient stocks. In these situations mineralization and/or desorption and dissolution
may be able to supply nutrient needs for many years. However, the decrease of the
total stock of individual nutrients cannot go on indefinitely because the supply of
potentially available nutrients is finite.
2. Maintaining Long-Term Soil Fertility at the Soil-Plant Level
Maintaining soil fertility and nutrient availability over the long term presents a
different perspective and challenge than when considering the short-term nutrient
needs of crops. Building up and maintaining high levels of SOM is essential to the
long-term fertility and productivity of soil (Magdoff, 1993) although this may not
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be the solution to low levels of every nutrient. Loss of nutrients by erosion of organic matter-enriched topsoil is an important consideration in the long term although erosion that occurs in any one year is usually of little concern for nutrient
availability in that year. The equation that describes buildup and maintenance of
Additions of organic materials 2 losses of organic matter
where 8 is the crop residue return, 11 is the addition of other organic residues, 1 is
the mineralization, and 12 is the loss of organic matter due to erosion.
Use of cover crops and additions of large amounts of crop residues and/or manures adds organic matter to the soil. Decreasing mineralizationby reduced tillage
and decreasing erosion slow the depletion of SOM.
When considering long-term changes in the total stocks of a particular nutrient
instead of SOM, the equation for maintenance and buildup becomes
Additions of nutrient 2 losses of nutrient
10 11 2 12 13 14,
where 10 is the additions of available forms of nutrient, 11 is the addition of nutrient in organic residues from off field, 12 is the loss of nutrient in organic matter
and minerals due to erosion, 13 is the loss of available nutrient by leaching, erosion, and gaseous loss, and 14 is the nutrient removal by harvest.
For the long-term consideration of N stocks, N, fixation (9) is also important.
Unavailable nutrients, such as much of the Pin rock phosphate and K in rock dusts,
are sometimes added to soils and will also contribute to the buildup of the total
If the export of nutrients off the field in harvested crops (14) and erosion, leaching, and gaseous losses (12 and 13) are low, it may be possible to maintain high
levels of nutrient stocks for years without using supplemental sources of nutrients
from off the field. However, in the face of high annual losses from the field, approaches that work in the short term, such as enhancing net mineralization by
plowing and relying exclusively on mineralization and desorption and mineral dissolution for particular nutrients, will inevitably lead to the long-term decline of nutrient stocks (Hass et al., 1957; Bray and Watkins, 1964).
If removal of nutrients in crops and/or by erosion or other losses is moderate to
large, the implication of Eq. (5) is that the only way to maintain or build up nutrient stocks over the long term is to add supplementalnutrients originating from outside the field. The only questions are in what forms will the supplemental nutrients be added to soils and where will they come from? This addition may occur as
N, fixation (there is no analogous reaction for the other nutrients), importing of
animal feeds from off the farm, adding of synthetic fertilizers and soil amendments, transferring nutrients from other fields on the farm in the form of crop