measured, as with N, availability can only be
roughly gauged. Nitrogen mineralization and
availability are the result of numerous uncon-
trolled, unforeseen, and unknown factors that
affect the presence and activity of the micro-
organisms responsible for N transformation. Thus,
soil tests provide only rough approximations of
potential total nutrient supply.

During the first few years after loblolly
pine is established on sites without a forest
floor, most of the nutrients required for tree
growth and accumulation of the forest floor are
supplied by the mineral soil. In a loblolly pine
plantation established on an old field in the
South Carolina Piedmont, N decreased markedly
throughout the surface 24 inches (60 cm) of mineral
soil during the first 15 years (fig. 7). During
the next 5 years, there was little decline. De-
clines over the first 15 years were also observed
for extractable K, Ca, and Mg. Extractable P
decreases were noted only to a depth of 3 inches

350F

300

250

SOIL NITROGEN (p/m)

5 10 15 20
STAND AGE (years)

Figure 7.--Changes in N content of mineral soil in an old-field
loblolly pine plantation over a 20-year period.

16


Despite many limitations and unknowns,
nutrient budgets may provide a method for com-
paring effects of alternative forestry practices
on ecosystem nutrients (table 7). Although only a
budget for N is presented here, budgets can be
prepared for other nutrients and the implications
similarly studied. Some nutrient losses during
harvest and regeneration are unavoidable, but the
quantities removed depend largely on the manage-
ment techniques adopted and the skill with which
they are carried out. For example, employing
three short rotations rather than a single 60-year
rotation increases stem yield from 87 to 144 tons/
acre (195 to 323 t/ha) with only proportionate
increases in nutrient removal (Switzer and others
1978). If, however, the entire aboveground por-
tion of the loblolly stand is harvested, N removal
increases from 155 to 634 lb/acre (174 to 711
kg/ha) and P from 12 to 59 Ib/acre (13 to 66
kg/ha) or more than fourfold during the 60-year

Table 7.--Effects of stem and whole-tree harvest of loblolly pine
and site-preparation practices on nitrogen demands at rotations of
16 and 32 years, in pounds per acre

16 years 32_years
Practice or process Whole tree® Stem “Whole tree® Stem

DEMANDS OF N

Harvest 230 103 382 208
KG, windrow, disk 209 336 209 336
Erosion
Kg, w, disk (Piedmont) 28 28 28 28
Chop (Piedmont ) 5 5 5 5
Prescribed burn 60 60 120 120
Slash burn 0 120 0 136
Leaching
Disk or bed 32 32 40 40
Chop or herbicide 16 16 20 20
INPUT OF N
Atmospheric deposition 7 7 154 154
Nitrogen fixation
No seeding 32 32 46 46
Seeded legumes 400 400 400 400

*Excluding roots.

37


NITROGEN LEVEL ———>

NITROGEN LEVEL ———>

Fi
te
cor
Lor
(A

N Surplus
HIGH INTENSITY

Growth
Requirement

— Recycled

“= wy peticit
N. Surplus

Precip. & Fixation Inputs

Mineralization

LOW INTENSITY

Growth
Requirement

, “X— Recycled

N Surplus

Mineralization

Accumulated N

Precip. & Fixation Inputs

a

TIME ———-

gure 9.--Theoretical effect of site-preparation intensity on long-
rm N reserves. High-intensity site preparation (above) includes
petition control and mixing of the forest floor and mineral soil.
w-intensity treatment (below) includes only competition control.
dapted from Burger and Kluender 1982.)

33


Literature Cited

Burger, J.A.; Kluender, R.A. Site preparation-Piedmont: In:
Symposium, loblolly pine ecosystem. 1982 December 8-10; Raleigh,
NC. 1982:58-74. [Available from North Carolina State University,
Raleigh]

Kodama, A.E.; Van Lear, D.H. Prescribed burning and nutrient
cycling relationships in young loblolly pine plantations.
Southern Journal of Applied Forestry 4:118-121; 1980,

Morris, L.A.; Pritchett, W.L.; Swindel, F.G. Displacement of nu-
trients in windrows during site preparation of a flatwood
forest. Soil Science Society of America Journal 47:591-594;
1983,

Ralston, C.W.; Hatchell, G.E. Effects of prescribed burn on physi-
cal properties of soil. In: Prescribed burning symposium pro-
ceedings. 1971 April 14-16; Charleston, SC. Asheville, NC: U.S.
Department of Agriculture, Forest Service, Southeastern Forest
Experiment Station; 1971:68-85,

Switzer, G.L.; Nelson, L.E. Nutrient accumulation and cycling in
loblolly pine (Pinus taeda L.) plantation ecosystems: the first
twenty years. Soil Science Society of America Proceedings
36:143-147; 1972.

Switzer, G.L.; Nelson, L.E.; Hinesley, L.E. Effects of utilization
on nutrient regimes and site productivity. In: Proceedings,
Complete tree utilization of southern pine. 1978 April; New
Orleans, LA. Forest Products Research Society; 1978:91-102.

Switzer, G.L.; Nelson, L.E.; Smith, W.H. The mineral cycle in

forest stands. In: Forest fertilization--theory and practice.
TVA, Muscle Shoals, AL; 1968:1-9.

42


140
— |
STANDS a INDIVIDUAL TREES
120- / 20
my Stems
B wok / ik
3 /
2
= sok
a / Branches -——] ter
o -
° 60- va
E o.8F
2
4oh
Foliage oak
20
¥. Foliage
an 1 1 n n ° 1 1 1

oO 10 20 30 40 5O 60 ° 10 20 30 40 50 60
AGE (years) AGE (years)

Figure 4.--Accumulation of Nin fractions of loblolly pine stands
and trees on good sites. (Adapted from Switzer and others 1968.)

in the mineral soil and its low availability
explains why N fertilization can often increase
growth even though there is an abundance of N on
the site. The proportions of P, K, and other ele-
ments that are extractable vary widely with site,
and reliable measures of their nutritional avail-
ability have not been developed.

Needles have relatively little of a tree's
biomass, but they have the highest concentrations
of nutrients. Among tree parts, stemwood has the
lowest concentrations of nutrients. Nutrient con-
centrations in root biomass could be more accu-
rately described if roots were separated into two
categories: permanent woody roots for anchoring
the tree and providing channels for conduction,
and nonwoody ephemeral roots for collecting nutri -
ents and moisture required for growth. The latter
contain relatively high nutrient concentrations
compared with the former.

10


portion is recycled one or more times into short-
lived nonwoody feeder roots. Little is known
about these roots concentrated in the lower forest
floor and upper mineral soil, except that they
appear to have several cycles of biomass increase
and decline during a year. Even less information
is available on whether or not nutrients are
translocated from the emphemeral roots to more
Permanent structures to be used as reserves when
a new growth cycle is begun.

Of the 48.5 Ib (54.4 kg) of N translocated
into the aboveground portions of the tree, 4.7 1b
(5.4 kg) are retained in the stem biomass, and
43.8 Ib (49.1 kg) are initially moved into
branches to be augmented by 15.2 1b (17 kg) trans-
located from older needles just before their fall.
Of the total branch input of 59.0 Ib (66.1 kg),
4.0 (4.5) are lost through branch death, 5.9 (6.6)
are retained for branch growth, and 49.1 1b (55
kg) are used for the production of new leaves.
The 15.2 1b (17 kg) translocated from old needles
before fall, if used for new needles, will provide
about one-third of the N needs. During the next
growing season, the cycle is renewed. New leaves
and other components are grown, leached, aged, and
lost, and the cycle begins again. As the stand
ages, component sizes and their transfer rates
change. Stem N will continue to gain until stand
breakup, whereas foliage N will decrease slowly.
As the stand ages, the annual nutrient needs
decrease and the accumulated deficit in the
mineral soil should be gradually restored. é

Silvicultural Practices
Affecting the Nutrient Cycle

The most intensive forest management in North |
America is practiced in the Southeast, where lob-
lolly pine is grown in 20- to 30-year rotations.
Site preparation, thinning, fertilization, and
various degrees of harvesting all affect the
cycling of nutrients and the ability of the site

20


period. When nutrient changes of this magnitude
can occur as a result of a management decision,
the long-term ability of the ecosystem to support
this yield must be examined. If the conclusion is
negative, there are three basic remedies: (1)
apply fertilizers, (2) change the rotation length,
and (3) harvest less biomass. If one or more of
these remedies are not adopted, yield reductions
in future rotations must be anticipated.

Other causes of nutrient outflow froma site
may be as great or greater than that of the har-
vest. Intensive site-preparation practices such
as rootraking and windrowing are one such cause;
prescribed burning is a second. Surprisingly,
erosion, although conspicuous, seldom causes major
nutrient losses, and these losses can be minimized
by converting from intensive to conservative site
Preparation. Similarly, losses from leaching can
be reduced by using conservative site-preparation
treatments.

By examining various forest management syS-
tems and their associated nutrient removals, some
idea can be ohtained of their overall impact on
the ecosystem and its ability to provide nutrients
for sustained yield. Several systems with widely
differing N removal rates are shown in table &
Included are both intensive and conservative man-
agement systems. Even the most conservative sys-
tem listed, the 16-year stem-only harvest followed
by chopping, causes a small net loss of N without
legumes. With a 32-year rotation and chopping,
annual loss would be similar, 1 Ib/acre. These
data indicate that when stem harvest is the
largest contributor to N removal, rotation age
does not influence nutrient loss on an annual
basis. Nutrient losses associated with the stem
are uniform because stem biomass of the stand
increases fairly uniformly to maturity. If the
entire tree is harvested, nutrient losses of the
stand allocated over the lengthened rotation are
reduced due to stand foliage and branch biomass
peaking at an early age.

38


ABSTRACT

The nutrient cycle, which includes the input of
nutrients to the site, their losses, and their move~
ment from one soil or vegetation component to another,
can be modified by site preparation, rotation length,
harvest system, fertilization, and fire, and by using
soil. We report how alternative procedures affect
site nutrients, and we provide general principles that
can be followed to enhance long-term productivity of
loblolly pine.

Keywords: Pinus taeda, nitrogen, phosphorus, potas-
sium, rotatTon Tength, nitrogen fixing, whole-tree
harvesting.

One key to sustained yield in forestry is the
maintenance or improvement of soil fertility
through good soil management. Such management
requires understanding of nutrient cycling, which
is the circulation of nutrients in the forest eco-
system. In each ecosystem, circulation of nutri-
ents follows specific pathways. Cycling patterns
can only be described in broad terms because rates
and directions of movement vary with stand con-
ditions. Some general processes regulating nutri-
ent flow within a forest system can be identified,
however: (1) uptake by higher plants, (2) trans-
location and use within the plants, (3) return to
the soil and forest floor, (4) mineralization,
immobilization, and leaching of the returned nutri-
ents, (5) inputs of nutrients from atmospheric,
geologic, and biological sources, and (6) losses
in streamflow, harvesting, and volatilization to
the atmosphere.

Concerns about potential declines in forest
productivity through loss of soil nutrients are
not new. Over a century ago, experiments deter-
mined that excessive removal of nutrient-
containing litter by Bavarian peasants was
detrimental to tree growth. When this adverse
effect was clearly established, litter gathering
was discontinued.


One of the most intensive systems for site
preparation is shearing, rootraking, and wind-
rowing followed by disking for final competition
control. This system tends to concentrate surface
soil, roots, forest floor, and logging debris in
windrows and to mix any organic residues between
the windrows with the mineral soil. On infertile
soils, such nutrient displacement can seriously
affect the growth of the present rotations as well
as of successive ones. The result is superior
growth of trees adjacent to windrows compared with
those farther from the windrows. In a flatwood
site in Florida, Morris and others (1983) found
that although windrows occupied only 6 percent of
the site, these areas contained from 10 to 40 per-
cent of the total or extractable nutrient reserves
in the system. These nutrient displacements were
larger than were the removals assocjated with har-
vesting the stand. N displacement into windrows
was six times the 5.3 Ib N/acre (5.9 kg/ha) re-
moved in the bolewood harvest. Displacements of
P, K, Ca, and Mg into the windrows also equaled or
exceeded those in the harvest removal.

Erosion following soil disturbance trans-
ports nutrients in both dissolved and solid forms.
Losses from erosion may be greater than those
estimated from average nutrient concentrations in
sediments due to the selective transportation of
organic and mineral-organic particles. Only on
sloping sites and those without protective cover,
however, should overall nutrient reserves be sig-
nificantly affected by erosion. Local areas
affected by improperly constructed or located
roads and skid trails may be severely impacted.
Leaching losses of all elements except N are usu-
ally small.

Site preparation can significantly affect
biological activity and associated nutrient avail-
ability. Removing the protective plant canopy and
forest floor by shearing, piling, and disking can
raise soil temperatures 3.6 to 9 °F (2 to 5 °C)

31


(8 cm), possibly because below this depth the
extractable P concentrations were extremely low.
Nutrient changes in the mineral soil of loblolly
pine stands between 20 and 40 years of age appear
to be small. During this time, the forest floor
can provide the majority of nutrients required for
growth. Lack of further nutrient drawdown in the
mineral soil may also be due to the depletion of
readily available reserves.

Little information is available on the re-
charge of mineral soil depleted of nutrients
during the first stages of a stand's growth. If |
recharge occurs, it would be after about 40 years |
of age (figs. 4, 5). At this time, uptake of
nutrients by vegetation may be more than balanced
by a loss in dying vegetation. The excess
nutrients released by the decomposing roots and
forest floor could aid in the preparation of the
mineral soil to become a nutrient source when a
succeeding stand develops.

Declines of nutrients in the mineral soil of
old fields after pine planting illustrate the
importance of the soil's nutrient supply during
plantation establishment. What is not answered,
however, is whether this old-field situation is
representative of what occurs when natural stands
or plantations with a forest floor are regen-
erated. Treatments that diminish the forest
floor as a source of nutrients may prolong the
stand's reliance on the mineral soil. On infer-
tile or intensively harvested sites, declines
related to nutrient deficiencies may take place
unless fertilizer is applied.

The Loblolly Pine Nutrient Cycle

Within the stand, nutrients move between
and within the three major components: the living
vegetation, the forest floor, and the mineral soil.
Some factors that determine from which source nu-
trients are obtained include age and stage of
stand development, and amount and availability of
nutrients in the component or subcomponent.

17


quantities of soil N declined, soil N would be
exhausted after 200 to 300 years of whole-tree
harvesting. However, only a small proportion of
mineral soil N becomes available for plant use
annually. If the percentage of total N made
available is constant, then available N will
gradually decrease until a balance between growth
demands, inputs, and available site N is reached.
Depletion and the accompanying decline in tree
growth may be prevented by applying fertilizer,
by adjusting rotation length, or by limiting the
removal of nutrient-rich biomass in harvests.

Although the nutrient drain by loblolly pine
over a rotation appears large, the annual removal,
compared with that of agricultural crops, is rela-
tively small (table 5). Average annual production
of pulpwood over a 16-year span is 3.2 tons of

biomass/acre (7.2 t/ha).
kg) of N, 0.8 Ib (0.9 kg)
K, and 5.7 1b (6.4 kg) of
annually. Harvesting the
roots, doubles or triples

Averages of 5.8 Ib (6.5
of P, 4.5 Ib (5.0 kg) of
Ca/acre (ha) are removed
entire tree, including

annual removal, but only

Table 5.--Comparison of the average annual yield
and nutrient removal by a 16-year-old loblolly
pine plantation with that of agricultural crops

Crop Yield N P K Ca
Tons/
acre - - - Pounds/acre - - -
Loblolly pine,
whole tree 5.2 15.7 21 W.2 114
Loblolly pine,
pulpwood 3.2 5.8 0.8 4.5 5.7
Corn (grain) 4.2 116 26.4 33.2 --
Soybeans (beans) 1.1 129 13.2 41.5 -
Alfalfa (forage) 3.6 189 20.7 165.8 67

23



Dry Precipitation
Fallout 4.8

25 >|
12
49.2 Ol A
Nofaves —— eaves 32
49. 27
ry
49.1
Bi he, 15.2
ranches |[“*——__—
53.5 4.0
ry
43.8
Stem
103
ry
485 0.9
imal Fixation
Forest |}<-———
a3 Bie
Roots [Js
57 93.3 Soll
1564 }——>

Leaching
0.6

Figure 8.--The N cycle in a 16-year-old loblolly pine plantation.
Values in boxes indicate N storage in individual components.
Values outside boxes indicate annual N transfer. All values are
in pounds per acre.



above those that occur with low-intensity treat-
ments such as chopping. Fewer plants on prepared
sites also reduce transpirational losses and
result in higher soil moisture. The warmer and
wetter conditions coupled with the mixing of for-
est floor and soil promote greater biological
activity. Increased activity is especially impor-
tant in the conversion of organic N to nitrate N,
which is subject to leaching and denitrification.
In late summer, the nitrate pool in the surface
soil of a sheared, piled, and disked area on the
Piedmont of North Carolina was 10 Ib/acre (11
kg/ha) compared with less than 1 1b for a chopped-
only treatment. Differences of similar magnitude
were also found in soil solutions at the 28-inch
(70 cm) depth. Use of herbicides and the elimina-
tion of vegetation greatly increased nitrate in
both surface soil and at the 28-inch depth, adding
LG the loss of site N.
Dh

Nutrient losses from site preparation, espe-
cially those of N, may be reduced by maintaining
an intact forest floor and by allowing the planted
site to revegetate normally, controlling only the
most troublesome competing vegetation. These soil
covers will moderate the soil temperature and
moisture increases associated with intensive site
preparation that promote high biological activity.
The forest floor and organic residues can also
immobilize mineralized N, and the small plants use
any excess mineralized N. Nitrogen retained in
these pools may be utilized for tree growth later
in the rotation.

The mineralization rate of organic N, as
influenced by site-preparation intensity, has
important implications for tree growth. Models
relating N release to the intensity of site prep-
aration and N growth requirements are shown in
figure 9. After both high and low intensities of
site preparation, there is an initial excess of
mineralized N--more than the amount taken up by
the small root systems of the young trees. Under

32


N, nutrient release (tables 2, 3). Differences
may occur, however, in the distribution of nutri-
ents within the forest floor layers and in the
proportions of nutrients released from the layers.
In a 19-year-old plantation, the majority of bio-
mass and nutrients were in forest floor materials
less than 8 years old (table 2). In a 40-year-old
stand, forest floor materials younger and older
than 8 years each held about half of the nutri-
ents. Except for N, however, the majority of
nutrients are released from forest floor layers
less than 8 years old, regardless of stand age
(table 3). Older forest floor layers in a young
stand accumulate rather than release elements. In
the 40-year-old stand, nutrients in the forest
floor layers older than 8 years of age are re-
leased, but more slowly than from younger mate-
rial. Thus, it appears that until forest floor
biomass is in equilibrium with litterfall, older
layers do not contribute important amounts of
nutrients for tree growth. Before about age 20,
the older layers act as nutrient sinks or reserves
for cycling at a later time.

Many factors control the accumulation rate
and the point at which forest floor biomass decom-
position and nutrient release come into equilib-
rium with litterfall. These equilibria differ

Table 2.--Components of the forest floor of two loblolly pine
plantations

Forest floor layer age (years)

T through 6 Over 8 AiL_ages

Component Plantation age (years)
1] Ly 1 mw

settee eee Pounds/acre = = -------

Organic matter 22,276 18,259 3,623 11,055 25,899 29,314

Nitrogen 283 212 71 159 354 371
Phosphorus 23.7 15.6 9.2 16.9 32.9 32.5
Potassium 14,3 12.3 3.8 13.5 18,1 25.8
Calcium 94,2 62.1 21.6 43.2 115.8 105.3
Magnesium 12.8 8.9 4.0 9.6 16.8 18.5

14


the older and more deeply weathered the soil, the
less will be the potential for nutrient release.
On highly weathered soils, atmospheric deposition,
rather than weathering, is the major source of
natural replenishment for the small leaching
losses that may occur.

Biological N Fixation

Biological fixation can be an important
source of N. N is fixed by legumes, nodulated
nonlegumes, and, in some instances, by lichens
that include blue-green algae. Except under
unusual circumstances, free-living organisms--
bacteria and algae--contribute only insignificant
amounts of N. In loblolly pine stands with closed
canopies, N fixation from all sources probably
does not exceed a few pounds per acre annually.
The major sources of this fixation are nodulated
nonlegumes such as southern bayberry (Myrica ceri-
fera L.) and many species of legumes. The latter
are especially important for brief periods after
fires. Fixation by free-living bacteria and algae
is limited by energy sources, low soil pH, and
nutrients; it usually contributes less than 1
pound per acre of N annually. Mycorrhizae have |
not been shown to fix N. The apparent fixation
sometimes associated with mycorrhizae is accom-
plished by free-living organisms in the soil areas
influenced by the roots and their mycorrhizal
associates.

Nutrient Pools and Cycling

Nutrient pools change rapidly after ecosystem
disturbances such as wildfire, harvesting, and
site preparation but again reach an equilibrium as
stands mature (figs. 2, 3, 4).

Understory Vegetation

The nutrient content of herbaceous biomass
peaks soon after plantation establishment. This


These quantities, both lost and transferred to the
mineral soil, had a very small effect on total
site nutrients, especially on an annual basis.
Thus, burning to rapidly release nutrients will
not alleviate site nutrient deficiencies caused by
infertile soils. Good prescribed burning for
hazard reduction and vegetation control will have
little influence on nutrient loss or transfer.

In addition to fire intensity and amount of
organic material consumed, soil properties often
determine the fate and effects of released nutri-
ents. Released nutrients may greatly affect pH and
nutrient availability in soils with low exchange
capacity such as sands, but their influence will
be limited in soils with high exchange capacity,
such as clays. Similarly, due to these properties
and the differential potential for leaching
losses, plant response to burning may be dramatic
and short-lived on sand but moderate and extended
on clay.

Properly applied hazard-reduction burns that
remove only a portion of the L layer and none of
the F and H, and do not expose mineral soil, do
not increase runoff. Nutrient loss via runoff or
erosion, therefore, is not appreciably increased.
Soil erosion can be the most obvious consequence
of burning. However, in only one of six burning
studies by Ralston and Hatchell (1971) (one in the
North Carolina Piedmont) did erosion greatly ex-
ceed a soil loss of 1 inch (3 cm) in 1,000 years
or an annual loss of 0.3 tons/acre (0.67 t/ha).
This rate is the soil-loss rate estimated to take
place as a normal geologic erosion process.

Burning may indirectly affect nutrient cy-
cling by altering the micro and macro flora and
fauna of the forest floor. In long-term pre-
scribed burning programs, the populations of
spore-forming microfungi or bacteria are not
reduced sufficiently to impair soil processes.
Intensive fires, however, may reduce populations

29


greater portion of the mineralized N is likely to
be taken up by the residual trees, rather than
lost to leaching and denitrification. N minerali-
zation and nitrate production may be less after
complete-tree harvests than after stem harvests.
The reason is that complete-tree harvests remove
from the site the needles and other high N-content
tree parts in addition to the low nutrient woody
material.

Fire

Two common types of prescribed fires used in
southern forests--underburns and site-preparation
burns--strongly influence nutrient cycling.
Underburns are used in existing stands for fuel
reduction, brush removal, range improvement, etc.
These burns, which are relatively cool, remove
only the most recently fallen litter and kill only
the smaller hardwood stems. Site-preparation
burns, which are relatively hot, are used for
slash and forest floor reduction, and competition
control in regeneration areas. The differences in
the intensities of the burns and the amounts of
materials consumed are major factors that produce
the varying effects on nutrients and their
cycling.

Burning can lead to nutrient losses by vola-
tilization to the atmosphere, by increasing ero-
sion, by ash removal from the site in air cur-
rents, and by leaching of elements that have been
converted to more available or soluble form.

The most obvious effect of fire is a reduc-
tion in the forest floor. In the Southeastern
Coastal Plain, 20 years of annual summer burning
reduced the forest floor to 3.5 tons/acre (7.8
t/ha) compared with 13.4 tons/acre (30 t/ha) on
unburned plots. Burning in the winter, after
allowing a litter buildup for 4 or 5 years, con-
sumed 3.2 tons (7.3 t) or 27 percent of the 12

on


floor, and vegetation. Each major component can
be further divided into smaller components or
pools (fig. 1). With data for each pool and
information on the influence of management prac-
tices, flow models can be developed that show
the rate of nutrient transfer from one pool to
another. Predictions of nutrient availability,
and thus production, can be made based upon
changes in the pool quantities and on the trans-
fer rates between pools.

A number of factors complicate such predic-
tions at any given time, however. For example,
relatively small additions by biological nitrogen

INPUT INTRACYCLE LOSS

Vegetation
Stem

Bark
Branches
Leaves

Precipitation p>
Solids = [-—}

Roots

Stemflow | 2
Throughfall | §

—————3] Forest Floor

Biological
N Fixation
Overland Flow
2-4" |_-eson]
- f
4-28 Subsurface
>2e" & Flow

Figure 1,--The nutrient cycle in a loblolly pine stand.



Despite that experience, the prevailing view
of silviculturists until recently was that mineral
elements would "take care of themselves" or that
the loss of nutrient elements through the exploi-
tation of timber was not very serious if leaves
and twigs were left behind. In 1955, an analysis
of the requirements for forest growth on a poor
site in England showed demands for nutrients
exceeded the supply. Few noticed that this forest
soil was similar to many forest soils in the South-
eastern United States. Only recently have south-
ern foresters recognized that conservation and
replenishment of nutrients are as important in
forest management as is determination of rotation
length or thinning schedule.

Pines are adapted to infertile soils in part
because they require smaller amounts of nutrients
than do other plants. Recent work, however, has
revealed some additional possible mechanisms: (1)
conservation of available nutrients hy minimizing
losses to deeper soil layers or to the atmosphere
(tight nutrient cycles), (2) minimizing losses
from the cycle by temporarily immobilizing nutri-
ents, primarily in the soil organic layers, (3)
collection and retention of nutrients from inputs
to the cycle (from the atmosphere and weathering),
(4) reuse of nutrients within the tree, and (5)
evolution of special mechanisms, such as mycorrhi-
zae, that obtain nutrients from sources not read-
ily available. Regardless of the mechanisms used,
the pine ecosystem, like those of other plants, is
sensitive to disturbances that might reduce its
ability to obtain, retain, or cycle nutrients.
Disturbances such as harvesting, fertilization,
burning, and changes in atmospheric inputs all
affect cycling. Understanding of these processes
and their interrelationships, therefore, is neces-
sary for maintenance of forest productivity.

Ecosystem Components

The forest ecosystem can be divided into
three major components--mineral soil, forest

2


Table 3.--Annual release (accumulation) of
nutrients from the forest floors of two loblolly
pine plantations

Forest floor layer age (years)

Tto 8 Over 6 ATI_ages
Nutrient Plantation age (years)

mu) Li 1g a0 1S Lis}

meee ee Pounds/acre - - - - - -

Nitrogen 13.1 91 2.5 16,6 15.6 25.7
Phosphorus 4.8 3.3 (0.4) 1.8 4.4 5.1
Potassium 11.2 10.1 (9.2) 1.2 11.0 11.3
Calcium 16.6 11.8 0.4 3.7 16.0 15.5
Magnesium 4.4 3.4 0.2 0.7 4.5 4.6

among nutrients. Understanding the agents con-

trolling decomposition and nutrient release will
lead to improved tree growth and a reduction in

the need for fertilization under intensive forest
management.

Mineral Soil

Mineral soil contains a majority of the eco-
system's nutrient reserves. In one Piedmont plan-
tation, about three-quarters of all the site's N
was in the mineral soil to a depth of 28 inches
(70 cm) (table 1). Comparisons of the propor-
tions of P, K, Ca, and Mg in biomass plus forest
floor and in the mineral soil are meaningless
because the bases for element measurements differ.
In the forest floor, total nutrients are measured;
in the soil, only extractable forms are measured.
Nutrient analyses of mineral soils have been
adapted from agriculture. While appropriate for
annual agronomic crops, these tests are often in-
adequate as indices of long-term nutrient supply
in forest ecosystems. The tests are not capable
of estimating the size of the mineral soil nutri-
ent reservoir. They indicate only the soil's abil-
ity to release nutrients under a particular set
of conditions. Even where total nutrients are


and particulates or as materials dissolved in wet
precipitation, (2) weathering of soil minerals,
and (3) biological fixation of N.

Atmospheric Inputs

Precipitation contributes significant amounts
of nutrients for growth and maintenance of lob-
lolly pine. Average annual inputs per acre (ha)
throughout the tree's range are 5.4 Ib (6 kg) of
N, 0.4 1b (0.4 kg) of P, 1.5 Ib (1.7 kg) of K, 6.4
lb (7.2 kg) of calcium (Ca), and 1.5 1b (1.7 kg)
of magnesium (Mg). Variation between areas may be
great; areas of high precipitation tend to have
more nutrient inputs than those with low precipi-
tation. Other factors may influence the chemical
composition of precipitation. Areas near the
ocean can receive marine aerosols that are high in
sodium, magnesium, chloride, and sulphate. Sites
near urban and industrial areas can receive in-
creased quantities of sulphate sulfur and nitrate
and ammonium N. Around smelters, quantities of
sulfur and minor elements may be sufficient to
adversely affect plant growth. At worst, all but
the most resistant vegetation may be destroyed.

Gaseous and particulate inputs in the absence
of precipitation are usually referred to as dry
deposition. Unfortunately, there are few quanti-
tative measurements of dry deposition. Mechanisms
related to dry deposition include filtering of
atmospheric particles by tree canopies, absorption
of water-soluble gases onto moist surfaces, and
the direct gaseous uptake via leaf stomata. Sul-
fur dioxide and gaseous ammonia may also be
directly absorbed by the soil.

Weathering Inputs

Weathering refers to nutrient release from
primary and secondary soil minerals by geochemical
and biogeochemical processes. In the South, re~-
Jease of nutrients depends on soil parent material
and may vary greatly between sites. In general,


to maintain its productivity. Nutritional rela-
tionships should be considered when management
alternatives such as rotation length, harvest
intensity, prescribed fire, site preparation, and
fertilization are selected to meet a particular
objective.

Effects of Harvesting on
Nutrient Removal

Past forestry practices of long rotations
and harvesting only the bole of the tree have led
silviculturists to ignore nutrient removal and be-
lieve that nutrients take care of themselves. New
practices are forcing silviculturists to think
again. Intensive harvesting, in which more than
the bole is removed and trees are grown in short
rotations, is contemplated as a means to substan-
tially increase biomass production for fiber,
fuel, and chemicals. It is apparent that substan-
tial quantities of so-called waste materials
(branches and foliage) are available and could be
utilized. More complete utilization does more
than increase the biomass supply; it also can
improve site esthetics, reduce site preparation
and planting costs, and decrease the danger of
wildfires or the need for prescribed burning.
However, there are negative aspects that require
examination. Increased nutrient removals along
with the increased biomass removals may cause a
decline in site productivity. Residual nutrients
may also be lost more readily by leaching in
intensively managed stands.

There is concern that complete-tree har-
vesting combined with short rotations can deplete
the nutrient capital of a site and hence reduce
future production. The quantities of nutrients
removed are affected by the density of the par-
ticular stand, the age and stage of development,
and the biomass components that are harvested.
The N losses associated with aboveground biomass
harvests and stem harvest in 16- and 32-year-old

21



United States
Department of
Agriculture

Forest Service

Southeastern Forest
Experiment Station

General Technical
Report SE-37

A Loblolly Pine
Management Guide

Foresters’ Primer in
Nutrient Cycling

Jacques R. Jorgensen
Carol G. Wells



The Forest Service, U.S. De-
partment of Agriculture, is
dedicated to the principle of
multiple use management of
the Nation’s forest resources
for sustained yields of wood,
water, forage, wildlife, and
recreation. Through forestry
research, cooperation with the
States and private forest
owners, and management of
the National Forests and
National Grasslands, it strives
—as directed by Congress—to
provide increasingly greater
service to a growing Nation.

USDA policy does not permit discrimination because
of race, color, national origin, sex or religion. Any
person who believes he or she has been discrimi-
nated against in any USDA-related activity should
write immediately to the Secretary of Agriculture,
Washington, D.C. 20250.


vegetation, an important sink for nutrients, is
usually eliminated after stand closure, 5 to 8
years after establishment. Along with the resi-
dues from harvesting, it later provides important
quantities of nutrients to the developing loblolly
pine stand. In one plantation, aboveground her-
baceous vegetation contained 67 1b (75 kg) of N,
7 1b (8 kg) of P, 9 1b (10 kg) of K, 21 1b (23 kg)
of Ca, and 7 lb (8 kg) of Mg per acre (ha).

Overstory Vegetation

Rapidly growing young pines quickly accumu-
late nutrients and develop a forest floor at the
expense of the mineral soil and the herbaceous
components of the stand. Accumulation of nutri-
ents in the stand proceeds at a fast pace during
the first 20 years, slows through age 40, and
reaches an equilibrium thereafter (fig. 3). Maxi-
mum nutrient accumulation in foliage of loblolly
stands occurs sometime after stand closure, at
about 15 years of age, but before there is exten-
sive suppression mortality. As trees are sup-
pressed, foliar nutrient accumulation declines.

In the stand as a whole, stem and branch nutrients
accumulate relatively uniformly through age 40, at
which time a new equilibrium is reached. A dif-
ferent pattern of nutrient accumulation occurs in
individual surviving trees (fig. 4). For the sur-
vivors, foliage continues to accumulate nutrients
at a uniform rate throughout the rotation. Stems
and branches of these trees gradually increase
their wates of nutrient accumulation in contrast
to an equilibrium for the stand as a whole.

A vigorously growing loblolly pine plantation
can tie up a large portion of the site's nutrients
(table 1). Of 2,124 1b of N/acre (2,400 kg/ha) on
a site, 286 1b (321 kg) or 13 percent were in
16-year-old trees. An equal amount of N was in
the forest floor. Thus, 26 percent of the site N
was immobilized 16 years after planting loblolly
pine. The relatively large proportion of site N


50
45
40h
= 35-
2
(2)
x
P 30h System Total
°
=
KE 25
xr
oO
rT) if
= 20- / T
/ rees
> 1 ~——
a / /
a 5 va,
‘oa
Ji
4
10 / Forest
Y
. / Floor
_— / _——
5=—— _
a
Pa Herbaceous
ole & L !
O 5 10 15 20

AGE (years )

Figure 2.--Biomass accumulation in an unthinned loblolly pine
plantation during the first 20 years. (Adapted from Switzer

and Nelson 1972.)


Legumes may be an alternative source of N. How-
ever, their utilization is more complex than the
application of fertilizer. Seedbed preparation,
fertilization, selection of species, and compe-
tition are considerations if biologically fixed N
is to supplement or supplant N fertilization.

Quantities of nutrients added as fertilizer
to compensate for those removed from the site by
harvest or lost during regeneration are not of
equal value in maintaining site productivity. For
example, the N lost in erosion, windrowing,
burning, and harvesting is derived mostly from
organic compounds that are normally mineralized
and made available to higher plants over many
years. Organic matter and organic N are essential
to maintain the physical, chemical, and biological
Properties of the soil; therefore, fertilization
may be a poor and economically unsound substitute
for N losses, especially on infertile sites al-
ready low in organic matter.

Based upon the known deficiencies and proba-
bility of losses from the loblolly pine ecosystem,
the impacts of intensive management practices on
site nutrients are in the order of N > P > K >
Ca > other elements. Like N demands, those for P
and K under intensive forest management exceed
natural inputs. Phosphorus deficiency is less
difficult to correct than N because P persists in
the soil and trees are able to utilize it in
slowly soluble forms. Rock phosphate is a cost-
effective, slowly soluble source of P and, when
available, may be substituted for triple super-
phosphate and other soluble forms. When large
amounts of hiomass are removed and intensive site
Preparation causes accelerated leaching, K and Ca
deficiencies will eventually limit growth on many
soils. Potassium can persist because it can be
taken up in luxury quantities and cycled by the
vegetation with few leaching losses. Calcium,
however, is cycled more slowly and when accom-
panied by N fertilization or high mineralization

40


layer (nearly 60 percent) were approximately pro-
portional to the fraction of the L layer consumed
by fire (table 6). Only small nutrient increases
were measured in the F and H organic layers, and
it was concluded that most of the nutrients had
been lost from the site as ash in air currents and
smoke created by burning.

Recurrent burning over long periods can
transfer nutrients from the forest floor to the
mineral soil. A severe treatment, annual burning
for 20 years in the summer, reduced nutrient con-
tents of the forest floor by 12, 18, 89, and 19
lb/acre (13, 20, 100, and 21 kg/ha) for P, K, Ca,
and Mg, respectively. Roughly corresponding
increases in Ca and Mg were found in the mineral
soil, but no increases in P or K were recorded.

Table 6.--Quantities of nutrients in forest
floor layers of four loblolly pine plantations
before burning, and percentage change immedi -
ately after prescribed fire

Nutrient and L_layer F+H layer

condition Jb/acre % change Ib/acre % change

Calcium
Preburn 33.9 -- 80.3 --
Postburn 15.2 55 88.4 +10
Magnes ium
Preburn 13.4 -- 23.2 -
Postburn 4.5 66 24.0 +3
Potassium
Preburn 16.1 -- 37.5 --
Postburn 6.3 -61 36.6 -2
Nitrogen
Preburn 50.8 -- 262.5 --
Postburn 27.7 =45 245.5 -6
Phosphorus
Preburn 6.3 -- 17.0 --
Postburn 2.7 -87 18.7 +10

Source: Adapted from Kodama and Van Lear 1980.

28


tons/acre (26.9 t/ha) of litter present. The
winter burn volatilized 100 1b (112 kg) of the 300
lb/acre (336 kg/ha) of Nin the litter. Practi-
cally all of the organic N volatilized was con-
verted to molecular N (Nz), which cannot be used
directly by plants. Burning may convert some
organic N to ammonia, which can be adsorbed by the
soil in its gaseous state. Absorption of ammonia
on charred residues may help account for increases
in soil N under some burning conditions. Low-
intensity fires used in other studies have pro-
duced similar results. As fire intensity in-
creases, a larger proportion of the forest floor
will be consumed and the amount of N volatilized
will be increased. Sulfur may also be lost by
volatilization, but volatilization temperatures of
other elements are high and their loss in this
manner should be small.

Although much N can be lost in burning,
burning may influence compensating processes and
also influence N availability. Biological fixa-
tion by free-living micro-organisms can increase
after fires, but fixation by these organisms prob-
ably amounts to only a few pounds per acre. It is
severely limited by a lack of organic compounds to
supply energy and by poor overall environmental
conditions. A larger potential source of N may be
fixation by higher plants. Following fire, N-
fixing plants, both legumes and nonlegumes, are
often prominent in the understory growth flush.
Fixation by plants can range from a few to a
hundred or more pounds per year, with the amount
dependent on the plant density and the growth
environment.

The nonvolatile components of the burned
organic layers may either remain on the site,
filtering into the organic and mineral soil
layers, or be moved from the site hy the wind or
the convection currents set up by the fire. In
one prescribed fire in South Carolina (Kodama and
Van Lear 1980), nutrients lost from the litter (L)

27


(N) fixation, or nutrient losses by leaching, may
influence growth for several rotations of trees.
Therefore, extrapolation of the effects of exten-
sive treatments, such as fertilization, over long
time periods should be done cautiously and with
some knowledge of the potential for error.

Quantitative nutrient cycling data are
needed, however, to address some major forestry
problems with harvesting systems, fertilization,
and environmental protection. Currently, research
in fertilization is primarily directed toward
increased yield and toward delineation of sites
where trees will respond to fertilization. In the
long term, forest fertilization practices based
upon a knowledge of nutrient cycling will sustain
or improve yields.

Each nutrient or element has its own pecu-
liar system of cycling, but the systems have simi-
larities. N is largely in organic compounds and
requires biological action for its release. Phos-
phorus (P) is in both organic and inorganic mate-
rials and its availability is strongly influenced
by chemical reactions. Potassium (K) is almost
totally inorganic, and is in equilibrium between
the native, fixed, and exchangeable forms. Due to
these and other variations, there is no single
cycle: Rather, there are many cycles, one for
each element under each separate condition.

From this general introduction, it is evident
that cycling can be altered by modifications of
the forest environment. We shall examine the com-
ponents of the cycles and the influences of manage-
ment practi¢es thereon. Because it has been shown
that loblolly pine (Pinus taeda L.) responds most
readily to N and P fertilization, emphasis will be
placed on these two elements.

Nutrient Inputs to the Forest

In nature, nutrients are added to a loblolly
pine system from: (1) the atmosphere as dry gases


Nutrients are transferred from one component
to another by trees through litterfall, crown
leaching (throughfall), root death and exudation,
and translocation from one organ to another.
Other than movement from roots to crown, the most
common form of nutrient translocation is from
foliage to stems before leaf abscission. Trans-
location may also occur in stemwood where nutri-
ents in 1- to 2-year-old wood are conveyed to
newer growth. There is no information available
on translocation of nutrients out of branches and
roots before their death, but it appears--at least
in fine roots as in needles--that translocation is
an effective nutrient conservation measure.

Although each nutrient has unique component
pools, relative pool sizes, and transfer rates
between pools, the basic concepts of nutrient
transfers are similar. Figure 8 shows a simpli-
fied N cycle in a 16-year-old loblolly pine plan-
tation. Input of N to the system is primarily
atmospheric, with precipitation contributing 4.8
lb/acre (5.4 kg/ha) annually to the site. An
unknown amount of N is added through dry fallout
and by the direct absorption of ammonia by the
foliage and the forest floor. Leaching from new
and old needles by precipitation brings an addi-
tional 3.7 Ib (4.1 kg) to the forest floor in
throughfall, making a total input to the forest
floor of 8.5 Ib (9.5 kg) of dissolved N. At age
16, the forest floor contains 274 1b of N/acre
(307 kg/ha) and has annual inputs from the atmos-
phere, leaves, branches, and fixation that total
43.5 1b (48.9 kg). Additional inputs are 43.4 Ib
(49 kg) from roots in both the forest floor and
mineral soil. Total inputs to the forest floor
and mineral soil are 86.9 1b (97.6 kg), but uptake
by the trees amounts to 93.3 Ib (104.6 kg). Thus,
there is a net loss of N from soil and forest
floor of 6.4 lb (7 kg). Approximately half of the
N taken up by roots is translocated to the above-
ground portions of the tree. Most of the other

18


and the remainder is lost by leaching and volatil-
ization. Thus, for N, if inputs are needed to
balance losses, between 2 and 4 pounds of fer-
tilizer N would have to be applied for every pound
removed in management activities.

Phosphorus utilization during the first rota-
tion is similar to that of N. However, unlike N,
little P is lost from the site. Little is known
about the applied P availability during succeeding
rotations. No information is available on the
uptake-application ratios of other nutrients in
loblolly pine stands.

The point at which nutrient replenishment
is required for yield maintenance is not. known.
When replenishment of most nutrients is attempted,
small quantities of nutrients applied several
times during a rotation may be better than one
large application at the time trees are estab-
lished. Perhaps even greater knowledge and effort
will be required for efficient nutrient main-
tenance than for fertilization to alleviate
obvious nutrient deficiencies.

Soil-Improving Plants

Wild legumes and other N-fixing plants
help maintain the ecosystem's supply of N, and
as understory contribute to the cycling of other
nutrients from the lower into the upper soil
layers and the forest floor. At certain times
during the loblolly pine rotation, it may be
appropriate to establish soil-improving plants.
Ideally, these plants should not have severe com-
petitive or allelopathic effects on the pines,
should fix and release important quantities of
N, and should be tolerant of the severe site
conditions that are often found in loblolly
plantations.

In one young loblolly plantation on the North
Carolina Coastal Plain, trees were fertilized with

35


P and grown in stands of lespedeza (Lespedeza
cuneata (Dumont.) G. Don and L. thunbergii (DC
Naki) established to provide biologically fixed N.
Trees with the P plus lespedeza, at age 7, had a
volume of 10 cords/acre (89.6 m3/ha) compared with
8.4 cords (75.2 m3) for trees with P fertilizer
alone. During the initial years of lespedeza
development, foliage of trees in the lespedeza
stands contained higher concentrations of N than
did controls or trees receiving only P. As lespe-
deza biomass declined with stand closure, foliar N
concentrations, regardless of treatment, became
similar. The lespedeza was estimated to have
added 400 1b of N/acre to the site over its 4-year
period of maximum growth.

There may be instances when the secondary
effect of legumes may be as important as that of
the N fixed. Deep-rooted legumes may reduce
leaching losses by taking up nutrients from below
the rooting zones of young trees. Leguminous lit-
ter, high in N, may speed up forest floor decom-
position, hastening nutrient release and cycling.
Legumes may also be capable of improving the phys-
ical properties of soil, thereby increasing the
potential soil volume from which trees may extract
moisture and nutrients.

Intensive Forest Management
and Productivity

Barring severe erosion or soil and nutrient
displacement, it is unlikely that even the most
intensive harvesting and silvicultural procedures
will transform productive loblolly pine sites into
unproductive ones in a rotation or two. Existing
management systems, however, are based upon
sustained yield with the implication that produc-
tivity will be maintained. Yet, there is a dearth
of long-term data showing whether present manage-
ment is meeting sustained-yield objectives.

36


about 60 percent more biomass (mostly leaves,
branches, and roots of low commerical value) is
produced.

Even with as severe a system as complete-tree
harvest, nutrient removals are low compared with
those of annual agronomic crops. Corn, soybeans,
and alfalfa can deplete the soil at 10 times the
rate of loblolly pine harvest. The major reason
for the difference in nutrient removal of annual
and tree crops is the type of biomass harvested.
In agronomic crops, harvests are primarily seeds
or leafy materials high in nutrients. Soybeans,
for example, contain about 5 percent N. Loblolly
pine wood has an N concentration of less than 0.1
percent, and needles, a reservoir of high nutrient
concentrations, contain only about 1 percent.
Yields of agronomic crops are maintained by fer-
tilizing and good management practices. If nutri-
ents to supplement those in the soil are not
supplied, crop yields fall until availability,
input, and output are in balance. Soil nutrient
reserves and their availability control the rate
of decline in yield of unfertilized agronomic
crops. On poor sandy soils, the decline can be
precipitous. On fertile prairie soils, where
yields are often controlled by factors other than
nutrients, decades may pass before yields fall
below an acceptable level. Yield decline of agro-
nomic crops may be analogous to what can happen in
loblolly pine. On poor soils, short rotations and
complete harvest can, over a few rotations, lead
to unacceptably low production. On deep, fertile
alluvial soils, many short biomass harvest rota-
tions may pass before there are effects of nutri-
ent depletion on productivity. Therefore, the
harvest system used on any particular site should
be one that, in the long term, will minimize the
Toss of nutrient capital.

24




Jorgensen, Jacques R.; Wells, Carol G.
Foresters’ primer in nutrient cycling. A loblolly pine
management guide, Gen. Tech. Rep. SE-37. Asheville, NC:
U.S. Department of Agriculture, Forest Service, South-
eastern Forest Experiment Station; 1986. 42 pp.

The nutrient cycle, which includes the input of nutrients to
the site, their losses, and their movement from one soil or
vegetation component to another, can be modified by site
preparation, rotation length, harvest system, fertilization,
and fire, and by using soil-improving plants. Included is a
report on how alternative procedures affect site nutrients,
and provides general principles that can be followed to
enhance long-term productivity of loblolly pine.

Keywords: Pinus taeda, nitrogen, phosphorus, potassium,
rotation length, nitrogen fixing, whole-tree harvesting.

Jorgensen, Jacques R.; Wells, Carol G.
Foresters’ primer in nutrient cycling. A loblolly pine
management guide, Gen. Tech. Rep. SE-37. Asheville, NC:
U.S. Department of Agriculture, Forest Service, South-
eastern Forest Experiment Station; 1986. 42 pp.

The nutrient cycle, which includes the input of nutrients to
the site, their losses, and their movement from one soil or
vegetation component to another, can be modified by site
preparation, rotation length, harvest system, fertilization,
and fire, and by using soil-improving plants. Included is a
report on how alternative procedures affect site nutrients,
and provides general principles that can be followed to
enhance long-term productivity of loblolly pine.

Keywords: Pinus taeda, nitrogen, phosphorus, potassium,
rotation length, nitrogen fixing, whole-tree harvesting.



of native N is readily leached with the nitrate
ion. Its loss could reduce the productivity of
base-deficient sandy soils.

As a solution to the problem of excess nutri-
ent removal over that of input, it has been sug-
gested that an “ecological rotation" be matched to
site. In this system, rotation length and harvest
are related to nutrient input from the atmosphere
and from soil mineral weathering. While ecologi-
cally sound, this approach ignores the general
lack of primary nutrient-containing minerals in
soils of the South and the low and variable atmos-
pheric input rates of the different nutrients.
Furthermore, most rotation lengths would have to
be increased to such an extent that annual produc-
tion would fall. Forest managers would also lose
flexibility of forest production. The best system
for minimizing the potential for a long-term
decline in yield is a forest manager with a knowl-
edge of and the ahility and freedom to apply good
nutrient cycling principles.

se
AS

41


Table 1.--Distribution of tree biomass and nutrients in a 16-
year-old loblolly pine plantation in the North Carolina Piedmont

Component Biomass N pe Ke
Tons/acre - - - Pounds/acre (%) - - -
Trees
Needles 3.6 73 9 43
Branches 10.3 54 5 25
Stemwood 48.9 70 10 58
Stembark 6.8 32 4 21
Roots 16.2 57 15 54
Total tree 85.8 286 (13) 43 (11) 201 (34)

Forest floor 274 (13) 27 (7). 25 (4)
Mineral soil, 0-28 in. 1,564 (74) 331 (82) 360 (62)
Site total -- 2,124 401 586

*Values represent total quantities in vegetation and forest floor
and extractable amounts in mineral soil.

Forest Floor

Nutrients and organic matter accumulate in
the forest floor as a loblolly pine stand devel-
ops. Litter accumulates most rapidly between the
time the stand closes and the time when stand
foliage is at a maximum, about 15 years of age.

In one plantation, the accumulation rate was
nearly 2 tons/acre/year (4 t/ha) between ages 13
and 16. Accumulated litter reaches an equilibrium
of 13 to 16 tons/acre (30 to 35 t/ha) somewhere
between stand age 16 and 30 (fig. 5).

Nutrient contents of forest floors in lob-
lolly pine stands vary greatly, probably because
of differences in climate and soil. The forest
floor of a 15-year-old plantation in Mississippi
contained only about one-third as much N and P,
half as much K, and two-thirds as much Ca and Mg
as did that of a 16-year-old North Carolina plan-
tation. In the forest floors of five 15-year-old

11


120 160}
ORGANIC MATTER NITROGEN

90)

60

30)
a ote at 1 1 Tr 14
= o|! 34567 8 o:12345 678
o
E oof °r
& 80
~ 80] POTASSIUM PHOSPHORUS.
2
2 60) eo
Zz
<

40
Fa 4oh
& 20
_ 7
s o1 23 45678 o 12345 67 8
$ 129 120
=z

CALCIUM MAGNESIUM

90 gof

60] sob

30) 30h

ot L 1 ot ree:

o 12345678 o123 45678

PERIOD OF AGING (years)

Figure 6.--Percentages of initial organic matter, N, P, K, Ca, and Mg
remaining in the forest floor after 8 years of decomposition.

percent of the N, are lost. After 8 years of
decomposition, about two-thirds of the organic
material, 60 percent of the P, 90 percent of the
K, 67 percent of the Ca, and 79 percent of the
original quantity of Mg are lost. Net decrease of
N during this period is only 27 percent (fig. 6).

Forest floors of closed, growing loblolly

pine plantations, regardless of age, may be simi-
lar in weight, nutrient content, and, except for

13


Foresters’ Primer
in
Nutrient Cycling

Jacques R. Jorgensen
and
Carol G. Wells

Soil Scientists

Southeastern Forest Experiment Station
Research Triangle Park, North Carolina

A Loblolly Pine Management Guide


high-intensity preparation, the N surplus is
larger and is lost more rapidly than under low-
intensity preparation. Late in the rotation, this
loss of N reserves causes poor tree growth. Even
later, N reserves may be lacking for growth in
subsequent rotations. Thus, intensive preparation
that changes the rate and cycling pathways of
nutrients may provide for rapid early growth but
can result in reduced growth over the long term.

Fertilization

Forest fertilization has become a standard
practice for many forest products companies that
demand high productivity from loblolly pine sites.
Fertilization can modify nutrient cycling through
changes in the size of components of the cycle
(the pools), and in the transfer rates between
pools. For example, fertilization with a soluble
nutrient source may influence not only the
leaching rate of the applied element but also
those of other elements competing for exchange
sites in the soil. Additions of nutrients in
short supply may also increase the uptake of other
nutrients as tree growth accelerates. These two
simple examples, among many, can substantially
change nutrient cycling on a site, especially if
fertilization is accompanied hy modifications in
site preparation, rotation length, harvesting pro-
cedures, and other forest management practices.

In addition to alleviating naturally oc-
curring nutrient deficiencies, a second role for
fertilizer is to replace nutrients removed via
harvest or as a consequence of site disturbance.
Unfortunately, using fertilizers to replace nutri-
ents is more complex than simply substituting for
those removed in a one-to-one ratio. Information
on nutrient uptake by loblolly pine over long
periods is limited, but what exists shows overall
efficiency of fertilizer use is low. For applied
N, about one-quarter is taken up by trees, one-
quarter is held on the site in other components,

34


plantations are shown in table 4. Annual nutrient
removal is more affected hy intensity of harvest
than by rotation length. Whole-tree harvesting in
both 16- and 32-year rotations results in high
depletion rates, whereas stem harvest, regardless
of rotation length, is approximately balanced by
the N inputs from fixation and the atmosphere. In
the intensive harvest system, annual nutrient drain
is reduced by increasing rotation length. Nitro-
gen losses attributed to other management prac-
tices may be larger than those from harvest and
are discussed in other sections of this report.

Most upland Piedmont soils are low in N
(table 4). If all site N were completely avail-
able and productivity could be maintained as the

Table 4.--Changes in site nitrogen (N) with harvest intensity and
rotation length of loblolly pine

Harvest age

Pool or process Th years BZ_years
Whole-tree Stem  Whole-tree Stem
harvest harvest harvest harvest

------ Pounds/acre - - ~ - - -
Site reserves (immediately
before harvest)
Mineral soil, 0-28 inches 1,564 1,564 1,269 1,269
Forest floor 274 274 321 321
Total reserves 1,838 1,838 1,590 1,590
Harvest demand 230 103 382 208
Inputs (during rotation)
N fixation 32 32 46 46 |
Atmospheric min 77 154 154
Total input 109 109 200 200
Change due to harvest
Per rotation -121 +6 -182 -8 }
Per year (avg.) -8 #1 -6 -1



February 1986

Southeastern Forest Experiment Station
200 Weaver Boulevard
P.O. Box 2680
Asheville, North Carolina 28802


dramatically, temporarily sterilizing the soil.
This is most likely to occur in fires where slash
has been piled and burned for disposal.

Nitrogen fixation by free-living soil micro-
organisms has been found to increase after
burning. The amounts fixed range from about 1 1b/
acre/year to as high as 12.5 1b/year in some
areas. Comparable unburned areas fix less than
0.05 1b annually. Fixation rates by free-living
organisms are limited by a lack of an energy
source and available P in most forest soils.

Individual low-intensity prescribed fires do
not cause important losses of nonvolatile nutri-
ents. However, the long-term cumulative losses of
the volatile nutrients, especially N, can be suh-
stantial--over 100 pounds per acre during a rota-
tion. Nevertheless, this quantity is small in
relation to that removed in a complete harvest of
aboveground biomass plus the losses that accompany
intensive site preparation. In intense burns, as
in uncontrolled wildfires and those used for slash
reduction, important decreases of site N and sul-
fur through volatilization can occur. Losses of
nonvolatile elements caused by intense fires are
usually associated with ash carried from the site
rather than by postfire leaching. Losses of these
nonvolatile elements can be large, but even they
are smaller than can occur in harvest and site-
preparation activities.

Site Preparation

Site preparation controls competition, re-
moves impediments to planting, and modifies the
site nutritionally and physically for improved
tree growth. If properly handled, competition
control and the removal of planting impediments
will not adversely affect the nutrient cycle.
However, movement of organic matter and soil to
produce an immediate gain in seedling growth and
survival may have negative effects on nutrient
availability later in the rotation.

30


Table 8--Changes in system nitrogen with
various combinations of rotation length, biomass
removal, and site preparation in loblolly pine |
plantations |
Rotation |
length Biomass Site
(years) removal treatment Rotation Annual
Pounds /acre
16 Whole tree 1 -450 -29
16 Whole tree 2 82 -5
32 Whole tree 1 -579 -18
32 Whole tree 2 -225 -7
16 Stem 3 -15 -1
16 Stem 4 +233 +15
32 Whole tree 5 +27 +1
32 Stem 6 -148 5
Treatments:

1. kG-windrow, disk, prescribed burn (PB).
2. Treatment 1 plus legumes.

3. Chop.

4. Chop, slash burn, legumes.

5. Chop, PB, legumes.

6. Herbicide, PR, natural regeneration.

Site-preparation practices rank first or
second as causes of nutrient drain. Rootraking
and disking are sometimes combined with bedding
to impose severe nutrient demands on the site.
Although good survival and early growth can be
obtained with these methods, less nutrient-costly
alternatives may give equally good yields over the
long term. Chopping and herbicide applications
may substitute for intensive procedures, espe-
cially if site-to-site adjustments in planting and
preparation can be made.

Fertilization can offset nutrient losses, but
for N its effect is short term and the proportion
of the applied N recovered by trees is often low.

39


Influence of Harvesting on the
Cycling of Residual Nutrients

In addition to the direct removal of nutri-
ents in biomass, harvesting also influences
cycling of the remaining site nutrients through
chemical and physical site changes. Compared with
preharvest conditions, the postharvest site has
reduced evapotranspiration and the forest floor
receives more solar radiation and precipitation.
There is also a greater pool of nutrients from the
harvest residues available for transfer to other
components of the ecosystem than in the unhar-
vested stand. Secondary effects of the harvest
may include mixing organic residues with the
mineral soil, transfer of nutrient-containing com-
ponents from one area of the site to another,
exposure of soil surfaces to erosion, and degrada-
tion of soil physical properties.

All harvesting, regardless of the intensity,
will have some influence on cycling. After har-
vest, higher site temperature and moisture and
greater availability of tree residues stimulate
microbial activity. Organic residues decompose
more rapidly, releasing their nutrients. The
activities of nondecomposers, such as the micro-
organisms associated with N mineralization, are
also stimulated. In one area of the North Caro-
lina Piedmont, harvesting all trees increased N
mineralization and nitrate production threefold
and 36-fold, respectively, over an unharvested
stand. Increased N losses due to leaching and
denitrification may accompany increased nitrate
production, whereas organic and ammonia N is sub-
ject to little direct loss. If the stand is
thinned instead of clearcut, N mineralization will
be greater than in uncut stands but still less
than in clearcuts. In the thinned stand, the

25


250

200 J

(Ib /acre)

STAND NUTRIENTS

0 10 20 30 40 #495 60
STAND AGE (years)

Figure 3.--Accumulation of macronutrients in whole trees in loblolly
pine stands on good sites. (Adapted from Switzer and others 1968.)


20- 400- 40p-
ORGANIC MATTER niTrocen [7] PHOSPHORUS [
6h 300. NN sok.
°
§ N
3 200} 20-
2
2
z 109 10h
Sg
=
<
s ° o
3
=
Br r
9 POTASSIUM CALCIUM MAGNESIUM
<4 too} Nn
~ N 20) Ny
> L
GS 30p R || &§
8
8
a
2 eof
toh
toh
e ° 2 )
1316 2732 3439 13 1627 32 3439 1316 27 32 3439
rs) AGE (years) AGE (years)

(7 Jptantation 2 SSSS3 plantation 3

Figure 5.--Accumulations of organic matter, N, P, K, Ca, and Mg in the
forest floor of three plantations at various ages.

plantations in Virginia, there was slightly less
N, P, K, and Mg, but more Ca than in the North
Carolina plantation. Although the forest floors
of the three Piedmont North Carolina plantations
shown in figure 5 had approximately reached a
biomass equilibrium at age 16, nutrient contents
continued to change. Differences between plan-
tations were probably caused by the influences of
stand density and soil on initial needle com-
position and rate of decomposition.

Necomposition and nutrient release rates of
forest floor materials are highest in the first
year after litterfall, but each element or com-
ponent changes at its own rate. During the first
year, about 25 percent of the organic material, 50
percent of the P, 70 percent of the K, 25 percent
of the Ca, and 57 percent of the Mg, but only 10

12