11
Reprinted from SOIL SCIENCE SOCIETY OF AMERICA PROCEEDINGS Vol. 27, No.2, March-April 1963, pages 231-234 677 South Segoe Road, Madison 11, Wisconsin About This. File:
'
tion. ,
This file was created by sc nning the printed publica
ed;
correct
Misscans Identified by the software have been
however, some mistakes may remain.
i
i
i
Accumulation of Organic Matter and Soil Nitrogen Beneath
A Plantation of Red Alder and Douglas-Fir!
ROBERT F. TARRANT AND RICHARD E. MILLER2
ABSTRACT
-
Soil nitrogen accumulation beneath a plantation of red
alder and Douglas-fir was compared with that under a
pure fir segment of the sanik 30-year-old stand on the
Wind River Experimental Forest in southwestern Wash­
ington.
Beneath the mixed plantation, there were significantly
greater aniounts of nitrogen in the forest Hoor and in
the upper 24 inches of the mineral soil. Organic matter
content in the upper 12 inches of mineral soil was also
. greater and bulk density at 0 to 3 inches was significantly
'. less. Beneath the alder-fir stand, the carbon-nitrogen ratio
was less, both -in the forest Hoor and at 0 to 3 inches.
There were 938 pounds more N per acre to a depth of
36 inches under the alder-fir plantation. Consequently,
an average of 36 pounds more soil N per acre per year
has accumulated beneath the mixed stand than under
pure fir.
.
'Joint contribution from Pacific Northwest Forest and Range
Exp. Sta., Forest Service, USDA, Portland, Oreg., and College
of Forestry, University of Washington, Seattle. Presented be­
fore Div. V-A, Soil Science Society of America, St.Louis, Mo.,
Nov. 29, 1961. Received May 2, 1962. Approved Sept. 13,
1962.
'Soil . Scientist and Research Assistant, respectively. The
latter is now a Fulbright Scholar, Institute for Soil Science,
University of Goettingen, Hann Muenden, Gennany.
\
ED ALDER (Alnus rubra Bong.) is the most important
R hardwood
tree of Pacific coast forests from southern
Oregon to southeastern Alaska, in terms of not only com­
mercial value but also wood volume and distribution.
Apart from its economic value, red alder is of special
interest as a possible soil-improving tree in a region
where forests are predominantly coniferous.
Research,. principally in Great Britain and the Scan­
dinavian countries, has demonstrated that several species
of Alnus have the ability to fix atmospheriC nitrogen (N)
in root nodules, increase soil nitrogen; and' in turn im
prove the growth of associated plants. Scientists in gen­
eral agree that the increase in soil N under alder trees
results more from decomposition of enriched leaf litter
rather than from direct excretion by root nodules. These
conclusions are based mainly on studies of European. alder
(A. glutinosa (L.) Gaertn.) (2, 8, 8, 10, 15), speckled
alder (A. incana (L.) Moench) (8, 9, 15), and Sitka
alder (A. sinuata (Reg.) Rydb.) (4,6).
Recent studies indicate red alder, also, has the ability
to fix N and enrich the soil in which it grows. In a study
of 10 important forest trees of the Pacific Northwest, red
alder foliage was found to contain 40 % more N than
the next highest species (18). Where red alder was inter­
planted in a Douglas-fir (Pseudotsuga menziesii ( Mirb.)
Franco) plantation, soil N and foliage N of the fir was
increased substantially and growth rate of dominant fir
was improved ( 14). When European alder roots were
inoculated with an extract of red alder nodules gathered
in Oregon, the European alder roots developed typical
232
SOIL SCJENCE SOCJETY PROCEEDINGS 1963
nodules and growth of the young trees was accelerated.s
Therefore, We can be reasonably sure that the organism
in red alder nodules, although possibly not identical to
that in nodules of other species of Alnus, has a similar capacity for fixing atmospheric N.
A number of investigations have shown that hardwood
litter improves soil by hastening decomposition of organic
debris, reducing soil acidity, and stimulating nitrification
(7). Thus, where forests have been under management
for many years, hardwoods are frequently grown in mix­
ture with conifers. In the Pacific Northwest, however,
where forests are just beginning to be intensively man­
aged, we lack information on the possible value of grow­
ing broad-leaved trees in mixture with our important
conifers. Red alder, with its ability to. fix N, among other
attributes, represents a good candidate for initial studies.
This study was undertaken to assess differences in soil
N and organic matter' accumulation beneath a pure
Douglas-fir plantation and under a mixed plantation of
fir and alder. Corollary soil properties-car bon-nitrogen
ratio and bulk density-were also measured. We were
particularly interested in comparing the total soil N
supply beneath the two stands and in determining to
what depth differences in soil properti s might be found.
EXPERIMENTAL PROCEDURE
This study was made on the Wind River Experimental Forest
in southwes'tern vVashington along a generally north-facing
slope ranging from 2 to 60%. The area lies on the east edge
of the 294,000-acre Yacolt Burn over which wildfire swept in
1902, 1922, and again in 1929. Douglas-fir was planted im­
mediately after the 1929 fire, and in May 1933 a strip was
interplanted with 2-0 red alder seedlings as a trial firebreak.'
The resulting mixed plantation, averaging about 70 feet in
width, follows a north-south section line
, for most of its I-mile
length;c
The soil is a well-drained, silty clay loam derived from
andesite, basalt, and breccia materials apparently mixed by
mas,sive slumping of adjacent ridges. Total soil depth generally
exceeds 5 feet. Average annual precipitation is about 90
inches, of which most occurs during the October-March period.
During an earlier study (14), a series of paired 1!20-acre
plots was established. Each pair cons sted of one plot in the
center of the alder-fir plantation strip and another 132 feet
west, presumably outside. the alder inHuence but within the
original Douglas-fir plantation. Plots were rectangular (33 by
66 feet) and 12 pairs were sampled at a spacing of 132 feet.
Although the plantation was not designed originally for this
study, paired plots are comparable in topography and soil·
development. Therefore, within-block randomization of plots
was assumed so that treatment differences might be tested.
In June 1959, samples of the forest Hoor' and of the under­
lying mineral soil at 0- to 3- and 3- to 6-inch depths were
collected from each of the 24 plots at 10-foot intervals along
a NE-SW diagonal. Bulk density cores were taken at. the
midpoints of these layers. Loose samples from each depth
range were composited, thoroughly mixed, and subsampled.
Then, at a place nearest the midpoint of each plot where there
was no physical barrier to digging, a pit was opened to a
depth of 60 inches or to a lesser depth if rock prevented
further penetration. Loose soil samples were taken at 6 to 12,
12 to 18, 18 to 24, and 24 to 36 inches. Bulk density cores
were again taken at midpoints of the sample depth ranges.
Total nitrogen was determined by the Kjeldahl procedure;
'Correspondence with Dr. G. Bond, University of Glasgow,
Oct. 31, 1957.
'This firebreak represents one of the few instances where a
red alder stand was established through planting. Seed was
gathered at an elevation of only 50 feet above sea level.
Nursery-grown seedlings, however, were outplanted at an
average elevation of 2,000 feet where early examinations indi­
cated the trees were repeatedly damaged by cold. Thus, growth
of the alder component of the mixed stand was checked suf­
ficiently to allow the fir to compete successfully.
'All dead vegetable matter on the mineral soil surface. In­
cludes litter and unincorporated humus.
Table I-Content of organic matter in mineral soil be­
neath red alder-Douglas-fir (AF) and pure Douglas-fir (F) plantations. Sample depth
range, inohes
F
AF
11.59
7.44
4.17
2.43
1.21
0.65
0-3
3-6
6-12
12-18
18-24
24-36
•
P" Difference
Percent of dry weight of soil < 2
mm.
, 0.003
0.001
0.004
0.130
0.340
0.550
2.82
2.07
1.14
0.39
0.16
-0.04
8.77
5.37
3.03
2.04
1.05
0.69
P' fa the probablllty u.st a dlfference listed In the previous col\1Jllll could have
happened by chance If, In faot, the dlfferenoe were zero. Any probability < 0.05 Is
accepted a. being sta.tlsUcaUy Significant. See: Swartzendruber (12).
Table 2-Amount of forest Hoor organic matter (loss-on­
ignition) beneath red alder-Douglas-fir (AF) and
.
pure Douglas-fir (F) plantations.
Basis
Peroent of oven-dry material
Pounds/acre
•
AF
F
52.41
28,673
60.52
24,535
P' (See table 1.)
Difference
-8.11
4,138
P"
0.08 0.77 organic matter of the forest Hoor by1loss-on-ignition and of
other samples by the modified Walkely and Black technique;
and bulk density was determined on core samples of 100 cc.
volume (5). Carbon-nitrogen ratios were calculated on the
basis of carbon as 58% of total organic matter' and total N
(Kjeldahl) in percent.
RESULTS AND DISCUSSION Organic Matter Organic matter content of mineral soil, in percent, w as
, found to be greater to a depth of 12 inches beneath the
mixed stand (table 1). However, no significant difference
was found in weight of forest floor organic matter of the
two plantations (table 2).
Lack of difference in amount of forest floor organic
matter between b'eatments may appear puzzling since
one might reasonably expect that the more dense stocking
in the mixed plantation would be a greater potential
source of litter and, thus, organic matter. A probable
explanation lies in the relatively high N content of the
alder-fir forest Hoor (tables 3 and 4). Plant substances
high in N decompose more rapidly than those low in N.
Thus, N enriched litter beneath the mixed stand presum­
ably has broken down more rapidly leaving less surface
humus than has accumulated beneath the pUl;e fir plan­
tation.
Very likely, differences in densities of lesser vegetation
also tend to equalize accumulation of forest floor organic
matter in the two plantations. The mixed plantation has
a closed canopy with almost no ground cover, but the
more open-grown pure fir has a dense understory vegeta­
tion. Annual deposition of litter from understory vege­
tation in the pure fir plantation undoubtedly contributes
significantly to soil organic matter (11).
Total Nitrogen
Total N content, in percent, of both the forest floor
and the mineral soil to a depth of 24 inches was greater
beneath the alder-fir plantation (table 2). This difference
is attributed to enrichment by decomposed alder litter
which is high in N (13) and to more organic matter in
the soil under the mixed plantation.
.
Total weight of N per acre in fine earth to a depth of
36 inches was 938 pounds greater under the mixed plan­
tation (table 3). Thus, during its 26-year life span, the
red alder has contributed an average of 36 pounds of N
annually per acre to the soil under the mixed stand.
Other investigators have estimated that Alnus species
contribute an average of about 50 pounds of N per acre
annually (4, 6, 8, 9). The estimate from the present red
TARRANT AND MILLER: ORGANIC MATTER AND SOIL N BENEATH. RED ALDER AND DOUGLAS-Fill
Table 3-Content of total soil N beneath red alder­
Douglas-fir (AF) and pure Douglas-fir (F) plantations.
Sample depth
range, inches
AF
F
P"
Difference
Table 5-Soil C/N ratio beneath red alder-Douglas-fir
(AF) and pure Douglas-fir (F) plantations.
Sample depth,
Inches AF
, • P' (See table
0.79
0.25
0.16
0.12
0.08
0.06
0.03
0 •. 36
0.14
0.11
0.06
0.05
0.04
0.04
0:44
0.11
0.05
0.04
o.oa
0.02
-0.01
0.000
0.000
0.002
0.007
0.007
0.033
0.660
AF
F
Differ'ence
P"
lb. /acre-Inoh in soli < 2 mm. t
• P' (See table 1.)
X f. nitrogen.
_
Forest floor
0-3
3-6
6-12
12-18
18-24
24-36
39.45
27.66
26.57·.
21.27
18.82
12.76 ,
13.60
96.67
37.21
27.65
21.00
21.56
13.73
12.80
-57.22
-9.56
-0.98
0.27.
- .7.·
-0.98
0.80
.0.000 0.000
<i. 810
0.950
0.170
0.470
0.510
• P' (See table 1.)
Table 4-Weight of soil N beneath red alder-Douglas-fir
(AF) and pure Douglas-fir (F) plantations.
Forest floor
0-3
3-6
6-12
12-18
18-24
, 24-36
TotaJ.t
P" Carbon/nitrogen 1.)
Sample depth
range, iJlebes
Difference
F .
Percent of dry weight of soli < 2 mm.
Forest floor
0-3
3-6
6-12
U-18.
18"24
24-36
233
411.9
192.7
148.3
134.2
97.2'
70.3
43.9
3771.9
141.3
137.8
117.4
102.3
75.8
56.6
43.2
2833.5
270.6
54.9 .
30.9
31. 9
21.4
13.7
0.7
938.4
t 226.512Ib./acre-inoh of water X bulk density xf.
t:r; Pounds/acre-inch X lotal inches each layer.
0.000
0.022
0.129
0.060
0.088
0.136
0.513.
0.000
Table 6-Soil bulk density beneath red alder-Douglas-fir
.
(AF) and .pure Douglas-fir (F) plantations.
.
Sample depth,
Inches
F
'AF
g./cc •
1.5
4.5
9.0
15.0
21.0
30.0
0.62
0.77
0.84
0.84
0.88
0.95
0.72
0.80
'0.8,7
0.90
0.90
0.90
DUference
P"
-0.10
-0.03
-0.03
-0.06
-0.02
0.05
0.023
0.480
0.560
0.249
0.845
0.242
.
, P' (See table 1.)
soli < 2 mm.
alder study is probably conservative because it is based
on amount of fine earth rather than gross soil.volume, and
it does not include N held in the vegetation., ,
To check the influence on N content of differences in
soil depth and texture' between plots and treatments, a
regression analysis was made of total N in the uRper 36
inches of soil (table 4 ) and "effective soil depth" ( 36
inches less percent of material > 2 mm. in size). This
analysis indicated that N content and effe,ctive soil depth
were not related.
Effective soil depth averaged 24.25 inches beneath the
alder-fir plantation and 25.27 inches in the pure fir stand.
The difference is attributed larfely to two plots in the
mixed stand' in which lenses 0 coarse-textured material
were encountered.
Aside Jrom our general knowledge that red alder litter
contains substantially more N than Douglas-fir litter ( 13) ,
the source of N' enrichment in soil beneath the mixed
plantation was not determined. Turnover of the nutrient
capital may have been more rapid in the mixed stand be­
cause the entire. crop, of red alder foliage is deposited
annually in contrast to Douglas-fir which retains up to
eight. seasons of needle growth on the tree. That both
treatments had an equal amount of forest floor might be
taken also as an indication that the presumably greater
amount bf foliage and litter in the mixed stand is under­
going accelerated - decomposition with more rapid cycling
of N.
C/N· Ratio and Bulk Density
The carbon-nitrogen ratio was narrower in the aider­
fil; plantation in both the forest floor and the surface
mineral soil (0 to 3 inches) (table 5). Below this depth,
the ratios were substantially the same for both stands,
.approaching at 21 inches and deeper, values commonly
found in the surface of cultivated soils.
'
Mean values for bulk density were lower in soils be­
neath the alder-fir plantation to a depth of 24 inches, al­
though they were significantly different only in the 0- to
3-inch layer (table 6).
Foliage Nitrogen
In an earlier study, N content of Douglas-fir foliage in
the alder-fir plantation was found to be greater than in
the pure fir plantation (14). Although the present study
dealt mainly with soil properties, samples of current year's
foliage were collected in late August from the upper one-
third of the crown of one dominant and one codominant
Douglas-fir on a pair. of plots. Samples were split and
analyses for total N were made in duplicate.
. The dominant tree in the alder-fir plantation had 60 %
more foliage N than that sampled in the pure fir stand
(1.31 vs. 0.82%). The codominant tree of the mixed stand
had 57% more foliage N than its counterpart in the pure
stand (1.46 vs. 0.93%): Results o this small s mple, while
not subject to broad mterpretahon, agree WIth the pre­
vious finding that increased soil N beneath the mixed
plantation is taken up by the fir. Many observers have
noted, moreover, that the Douglas-fir in the mixed stand
has much darker green foliage with a more dense com­
plement of longer needles.
APPLICATION
Native red alder is. a formidable competitor with in­
tolerant Pacific Northwest conifers. However, use of an
off-site source Of alder, as in this study, might· allow
associated plantings to compete successfully with the
hardwood until- the superior height growth of the coni­
fers can dominate the alder (1). Another adaptation might
be to employ a less competitive species of alder such as
A. sinuata, which grows as a shrubby tree above 3,000­
foot elevation in the Douglas-fir region. Still another pos­
sibility might be to use other low-growing, N-fixing plants,
Lupinus spp. for example.
If an increase in soil N were the only objective in re­
. habilitating or improving a forest. site, applying commer­
cial fertilizer might be more economical than developing
a mixed stand. However, diversification of the soil organic
matter source through the use of N-enriched plants may
be of even more benefit in maintaining or improving forest
soil conditions. Certainly, further investigations seem war­
ranted in view of the possibilities suggested by this and
other studies of the growing of soil-improving hardwoods
in mixture with conifer plantations.
CONCLUSIONS
Comparison of a pure Douglas-fir plantati;n with an
adjacent mixed plantation of Douglas-fir and red alder
show d t at presence o! the red alder markedly improved
.
.
certam SOlI charactenshcs.
.
Prior to planting, the site had been deteriorated by a
series of wildfires. Twenty-six years after the alder had
been introduced, the upper 12 inches of soil under the
mixed plantation contained more organic matter than the
SOIL SCIENCE SOCIETY PROCEEDINGS 1963
234
soil at comparable depths under the pure fir plantation.
Total N was also higher' to a depth of at least 24 inches.
This increase represents an accumulation of about 36
pounds of N annually, attributable to the presence of red
alder. For. similarly burned or eroded sites, where the
primary objective is to reestablish a protective forest
canopy and restore soil organic matter and N, red alder
appears to be a valuable candidate as a pioneer tree.
LlTERATIJRE CITED'
1. Bemtsen, Carl M. A look at red alder-pure, and in
mixture with conifers. Soc. Amer. Foresters Proc. pp.
157-158. 1958.
2. Bond, G. An isotopic study of the fixation of nitrogen
associated with nodulated plants of Alnus, Myrica, and
Hippophae. J. Exptl. Botany 6(17):303-311. 1955.
. Evidence for fixation of nitrogen by root
3. .
nodules of alder ( Alnm ) under field conditions. New
Phytologist 55(2):147-153. 1956.
4. Crocker, Robert L. , and Major, Jack. Soil development in
relation to vegetation and surface age at Glacier Bay,
Alaska. J. Ecol. 43:427-448. 1955.
5. Forest Soils Coinmfttee of 'the . Douglas-fir Region. Sam­
pling procedures and methods of analysis for forest soils.
Univ. Washington Col. of Forestry. 1953.
6. Lawrence, Donald B. Glaciers and vegetation in south­
eastem Alaska. Am. Scientist 46(2):89-122. 1958.
Purchased by the
!
u.s.
7. Lutz, Harold J., and Chandler, Robert F., Jr. Forest Soils.
John Wiley & Sons, Inc., New York. 1946.
8. Mikola, Peitsa. Liberation of nitrogen from alder leaf
litter. Acta Forestalia Fennica 67:1-9. 1959.
9. Ovington, J. D. Studies of the development of woodland
conditions under different trees: IV. The ignition loss,
water, carbon and nitrogen content of the mineral soil.
J. Ecol. 44:171-179. 1956.
10. Redko,'c. 1. (Effect of Alnm glutinosa Gaerth. on the
productivity of Populm Canadensis Mnch.). Dopovidi.
Akad. Nauk Ukrain. R.S.R. 3:343-346. (In Russian with
English summary.) 1958.
.
11. Scott, D. R. M. Amount and chemical composition of the
organic matter contributed by overstory and understory
vegetation to forest soil. Yale Univ. School Forestry Bull.
62. 1955.
12. Swartzendruber, Dale. Procedure for cqmputing.· proba­
bilities in experimentation. Soil Sci. Soc. Am. Proc. 25:70­
71. 1961..
13. Tarrant, Robert F., Isaac, Leo A., and Chandler, Robert
F., Jr. Observations on litter fall and fqliage nutrient
content .of some Pacific Northwest tree species. J. Forestry
49:914-915. 1951.
; Stand development and soil fertility in a'
14. Douglas-fir-red alder plantation. forest Sci. 7: 238"246.
;
1961.
15. Virtanen, Artturi. Investigations on 'nitrogen fixation by the
alder: II. Associated culture of spruce and inoculated
alder without combined nitrogen. Physiologia Plant.
10:164-169. 1957.
Forest Service for Official use.