II. Framework for Evaluating Nutrient Dynamics

6



FRED MAGDOFF ETAL.



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



7



b



a



nutrients



II



t



Acnnsumers



I



primary producers

(plants)



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).



B. DEFINITIONS

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.



8



FRED MAGDOFF ETAL.



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



9



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.



C. IMPLICATIONS

OF SPATIAL

SCALEAND ECOSYSTEM

RELATIONS,

SEASONAL

PATTERNS,AND LANDSCAPE

POSITION

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+

soil cycle.

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-



10



FRED MAGDOFF ETAL.



residue



8



~



atmospKem

deposmn



erosto; kaching.

g~~~

LOSS



I



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

4



1I



crop removed



PLANT



* erosion,I leaching,



'



fertlltzers.

manures, lime,

organlc residues,

ahospherlc



deposition



+



gaseous loss



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



12



FRED MAGDOFF ETAL.



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



13



managed

flows

fertilizer



fertilizm



harvest



J



F



M I A

M

denitrification

leaching



J



J



A



S



0



N I D

denitrification

leaching



rUnOff



emion



I



unmanaged

flows



runoff



erosion



1



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



A. PLANTNUTRITION

AND Son, NUTRIENT

STOCKS

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-



14



FRED MAGDOFF ETAL.



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,



(1)



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



fertilizers,



atmospheric

deposltlon



Figure 5 Simplified nutrient cycle, flows, and transformations in the soil-plant system with inputs and outputs indicated.



NUTRIENT CYCLING, TRANSFORMATIONS, AND FLOWS



15



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.



(2)



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



(1-2)



+ ( 3 4 ) + ( 5 - 6 ) - 13 2 7.



(3)



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



16



FRED MAGDOFF ETAL.



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

SOM is

Additions of organic materials 2 losses of organic matter

8+1121+12,



(4)



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,



+



+ +



(5)



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

stocks.

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