Microbial biomass C and N and mineraliza
PlantandSoil 175: 167-177, 1995.
(~) 1995KluwerAcadernicPublishers. Printedin the Netherlands.
167
Microbial biomass C and N, and mineralizable-N, in litter and mineral soil
under Pinus radiata on a coastal sand: Influence of stand age and harvest
management
D.J. Ross 1, G.P. Sparling 1'4, C.M. Burke and C.T. Smith3
lLandcare Research New Zealand, Private Bag 11052, Palmerston North, New Zealand, 2Landcare Research
New Zealand, Private Bag 3127, Hamilton, New Zealand and 3Forest Research Institute, Private Bag 3020,
Rotorua, New Zealand. 4present address: Department of Soil Science and Plant Nutrition, University of Western
Australia, Nedlands, WA 6009, Australia*
Received5 August 1994.Acceptedin revisedform21 February1995
Key words: coastal sand, forest, harvest management, microbial biomass, nitrogen mineralization, organic matter,
Radiata pine
Abstract
Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0-10
cm and 10-20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials
comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation,
and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation
trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management
stands were sampled in the fifth year after the second-rotation establishment.
In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter
and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of
P radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and
mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor
had been removed; they were higher in associated plots in which the original forest floor had been removed but
fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments,
viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added
slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C, and
especially total N, in 0-10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly
suggest that the microbial populations in mineral soil had a high metabolic activity.
On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil
were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in
the litter plus mineral soil (0-20 cm depth) after five years of second-rotation growth comprised 7.3% of the total
ecosystem N; values in the other treatments ranged between 5.6 and 6.0%.
Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous understorey species, in conserving pools of potentially available N during the early stages of tree development.
Introduction
Plantations of Pinus radiata (D. Don) have been successfully developed on low-fertility coastal sands in
* Fax no. correspondingauthor.+ 64 63559230
New Zealand, after initial stabilisation of the sands
with marram grass (Ammophila arenaria L.), followed
by the introduction of tree lupins (Lupinus arboreus
Sims) to increase total soil nitrogen (N) through symbiotic fixation (Gadgil, 1979; Gadgil et al., 1984).
The influence of lupins, fertilizer N, and stand man-
168
agement on P. radiata growth and nutrient cycling
has been investigated and the critical role of N clearly shown (Baker et al., 1986; Beets and Madgwick,
1988; Gadgil et al., 1984; Jackson et al., 1983). These
pine ecosystems are highly N-immobilizing (Baker et
al., 1986), particularly after canopy closure (Beets and
Madgwick, 1988), and responsive to fertilizer N (Gadgil et al., 1984). Rates of net N mineralization were
low in two 17-year old stands, with about 2200 stems
ha -~, when measured in an in situ incubation procedure (Dyck et al., 1987). The annual net amount of
mineral-N produced by litter and mineral soil (0-10
cm) totalled 9.4 kg ha- 1 in a stand that had been fertilized with N at least 7 years previously and 1.6 kg ha- 1
in an unfertilized stand (Dyck et al., 1987).
The release of plant-available N from litter and the
increasing pool of soil organic N during stand development is dependent on biological activity, with microorganisms playing a key role. Microorganisms can also
immobilize N, particularly in N-deficient sandy soils
(Miller et al., 1979), with the microbial biomass being
an important potential source of plant-available N during subsequent organism turnover (Binkley and Hart,
1989; Jenkinson and Ladd, 1981). In other conifer
ecosystems, soil microbial biomass has been found to
vary seasonally (Entry et al., 1986; Groffman et al.,
1993; Hunt and Fogel, 1983; Sundman et al., 1978),
and to be influenced by fertilizer (Arnebrandt et al.,
1990; Nohrstedt et al., 1989) and management practices (B~i[tth 1980; Ohtonen et al., 1992; Vitousek and
Andariese, 1986).
We here investigate the effects of tree age and harvesting methods on the pools of microbial C and N
in litter and mineral soil in a coastal sand in two trials comprising (a) stands of different age under normal forestry management, which allowed the rapid
re-establishment of volunteer herbaceous plants, and
(b) stands established after different harvesting procedures of first-rotation trees and where volunteer plants
were excluded (Dyck et al., 1991; Smith et al., 1994).
An estimate of potentially mineralizable N was also
made by an anaerobic (waterlogged) incubation procedure (Keeney and Bremner, 1966). Relationships of
the C and N in microbial pools to total C and N in
litter and mineral soil have been determined, and the
contribution of microbial N to total ecosystem N in the
harvest-management trial has been assessed.
Materials and methods
Sites and soil
Woodhill State Forest is located at 360 45' S and 1740
26' E on recent sand dunes and yellow-brown coastal
sands (Pinaki sand), about 50 km northwest of Auckland. Mean annual temperature is approx. 14.3°C and
mean annual rainfall approx. 1310 mm (Jackson et al.,
1983). Pinaki sand is classified as a Typic Udipsamment (Soil Survey Staff, 1992), and contains approx.
97% sand and 3% silt (McMurtrie et al., 1990). Some of
its pedological and chemical properties are described
by Cox (1977).
The sites had been initially planted with marram
grass (Ammophila arenaria L.), to stabilize the sand,
and lupin (L. arboreus Sims) to improve the soil N status (Gadgil, 1979). P. radiata was subsequently introduced and grown under a variety of management treatments. Two major variables were considered in this
study. One was the effects of stand age, and the other
of harvesting intensity.
In the first trial, seven different ages of trees were
selected on closely located stabilized dunes of similar
origin and aspect. The soil at all sites was also similar.
The ages of the trees ranged from 1 to 33 years. In the
study area examined, young first-rotation stands were
no longer available, and the five youngest stands were,
therefore, second rotation (Table 1). Volunteer grasses
and herbaceous plants, which were periodically grazed
by cattle, were abundant in the three youngest stands.
The understorey had been largely shaded out in the
other stands, although pampas grass (Cortaderia spp.)
was generally present. Pine litter was negligible under
the 1 to 4 year-old trees and was not collected. The
spatial distribution of litter on the forest floor at the
other stands was very variable, both in amount and the
relative proportions of needles and twigs and the more
decomposed FH material.
Details of the harvest-management trial have been
given by Dyck et al. (1991) and Smith et al. (1994).
Briefly, the site had been planted in P radiata in 1944
at a stocking rate of 2240 stems ha-I and thinned
in 1968 to 270 stems ha - l Before harvesting this
first-rotation crop in 1986, ecosystem nutrient pools
were determined; the basal area of the stems at harvest averaged 56 m 2 ha -z, with the understorey consisting almost entirely of evenly-distributed pampas
grass. Total above- and below- ground biomass then
averaged approx. 550 t dry weight ha- i. Total N content to 10 cm depth of mineral soil averaged approx.
169
Table 1. Site propertiesat the time of sampling
Site
Managementtreatment
Trees
Litter
Stand Age (years)
Stocking
Basal area Amount Composition
Current
Previous(stems ha-1) (m2 ha-l) a (t ha- l )
rotation rotation
Stands of different age
Second rotation
Second rotation
Second rotation
Second rotation
Second rotation
First rotation
First rotation
Cl
C2
C4
C9
C18
C22
C33
1
2
4
9
18
22
33
43
41
43
43
34
0
0
1100
1483
2417
983
333
775
233
0.7
1.1
6.4
23.3
32.9
40.3
40.4
ND
ND
ND
18(35)b
85(45)
35(86)
12(43)
FF
WT
SS
5
5
5
42
42
42
2283
2317
2300
13.3c
17.5b
15.3bc
DS
5
42
2200
15.0bc
FF(N)
5
42
2233
20.5a
9(38) Mainly needles
48(72) Varied
64( 11) MainlyFH
material
137(21 ) Nearly all FH
material
17(31) Mainly needles
C5
5
42
2317
ll.ld
37(81)
ND
ND
ND
Variedc
Varied
Varied
Varied
Harvest management; treatment after first rotation
Whole tree harvest; forest floorremoved
Whole tree harvest
Stem-onlyharvest; (singlelayer of
harvest slash)
Stem-onlyharvest; (doublelayer of
slash)
Whole-tree harvest; forest floor removed;
fertilized with urea-N
Current practice; not windrowed
Varied; some
grass litter
ND, not determined.
a Calculatedfrom diam. at breastheight (DBH, 1.4 m), except for C 1and C2 trees for whichdiam. at the base of the greencrown (0.10 m)
was used. Valuesfor the harvest-managementtrial were calculatedfrom the average DBH for all trees in each subplot;values not marked
with the same letter are significantly(p 5 mm diameter were excluded,
because they would have been difficult to assay for biochemical characteristics; these assays generally require
homogeneous samples of 1-20 g. No separation was
made of the L and FH horizons, because the properties
of the litter as a whole were the major interest in this
particular study.
In 1990, sampling was carried out in one compartment only for each tree age; three adjacent replicate
20 × 20 m plots were used. When present, litter was
collected in three randomly placed quadrats (20 × 20
cm) from each plot, to give a total of three samples.
Mineral soil was sampled at 0-10 cm and 10-20 cm
depths by taking 2.5 cm diam. cores at approximately
equal intervals in a diagonal transect across each plot.
Alternate cores from each of the three plots were combined to produce two pooled samples for subsequent
analyses, with each containing at least 15 cores from
each of the two depths.
In 1991, sampling was restricted to the inner 20
x 20 m of each plot. Litter was collected as above.
Mineral soil (0-10 and 10--20 cm depth) was again
sampled by taking 10 cores at approximately equal
intervals in a diagonal transect across each plot. Each
of these sets of 10 cores was then pooled to give one
sample per plot; three replicate pooled samples were
thereby obtained.
Samples were transported and stored at ambient
temperature (15-25°C) before sieving (
(~) 1995KluwerAcadernicPublishers. Printedin the Netherlands.
167
Microbial biomass C and N, and mineralizable-N, in litter and mineral soil
under Pinus radiata on a coastal sand: Influence of stand age and harvest
management
D.J. Ross 1, G.P. Sparling 1'4, C.M. Burke and C.T. Smith3
lLandcare Research New Zealand, Private Bag 11052, Palmerston North, New Zealand, 2Landcare Research
New Zealand, Private Bag 3127, Hamilton, New Zealand and 3Forest Research Institute, Private Bag 3020,
Rotorua, New Zealand. 4present address: Department of Soil Science and Plant Nutrition, University of Western
Australia, Nedlands, WA 6009, Australia*
Received5 August 1994.Acceptedin revisedform21 February1995
Key words: coastal sand, forest, harvest management, microbial biomass, nitrogen mineralization, organic matter,
Radiata pine
Abstract
Microbial biomass C and N, and anaerobically mineralizable-N, were measured in the litter and mineral soil (0-10
cm and 10-20 cm depth) of Pinus radiata plantations in two trials on a nitrogen-deficient coastal sand. The trials
comprised (a) stands of different age (1 to 33 years), with five of the seven stands studied being second rotation,
and (b) a harvest-management trial, with stands established after different harvesting treatments of the first-rotation
trees and understorey development controlled by manual weeding and chemical sprays. The harvest-management
stands were sampled in the fifth year after the second-rotation establishment.
In the stands of different age, the levels of microbial biomass C and N, and also mineralizable-N, in the litter
and mineral soil showed no relationship with tree age and were similar to those in the oldest (33 years) stands of
P radiata. In the harvesting trial, five years after establishment of the second rotation, levels of microbial N and
mineralizable-N in the litter and mineral soil were generally lowest where whole trees and the original forest floor
had been removed; they were higher in associated plots in which the original forest floor had been removed but
fertilizer N was regularly applied. No marked differences were then found between the other harvest treatments,
viz. whole-tree harvest, stem-only harvest with slash remaining on site, and stem-only harvest plus extra added
slash materials. In each trial, levels of microbial C and N and mineralizable-N were closely related to total C, and
especially total N, in 0-10 cm depth mineral soil, but not generally in litter. Respiratory measurements strongly
suggest that the microbial populations in mineral soil had a high metabolic activity.
On an area basis in the harvest-management trial, total tree N and microbial N in the litter and mineral soil
were lowest in stands where the original forest floor had been removed. In this particular treatment, microbial N in
the litter plus mineral soil (0-20 cm depth) after five years of second-rotation growth comprised 7.3% of the total
ecosystem N; values in the other treatments ranged between 5.6 and 6.0%.
Our results emphasise the importance of slash and litter, and probably volunteer shrubs and herbaceous understorey species, in conserving pools of potentially available N during the early stages of tree development.
Introduction
Plantations of Pinus radiata (D. Don) have been successfully developed on low-fertility coastal sands in
* Fax no. correspondingauthor.+ 64 63559230
New Zealand, after initial stabilisation of the sands
with marram grass (Ammophila arenaria L.), followed
by the introduction of tree lupins (Lupinus arboreus
Sims) to increase total soil nitrogen (N) through symbiotic fixation (Gadgil, 1979; Gadgil et al., 1984).
The influence of lupins, fertilizer N, and stand man-
168
agement on P. radiata growth and nutrient cycling
has been investigated and the critical role of N clearly shown (Baker et al., 1986; Beets and Madgwick,
1988; Gadgil et al., 1984; Jackson et al., 1983). These
pine ecosystems are highly N-immobilizing (Baker et
al., 1986), particularly after canopy closure (Beets and
Madgwick, 1988), and responsive to fertilizer N (Gadgil et al., 1984). Rates of net N mineralization were
low in two 17-year old stands, with about 2200 stems
ha -~, when measured in an in situ incubation procedure (Dyck et al., 1987). The annual net amount of
mineral-N produced by litter and mineral soil (0-10
cm) totalled 9.4 kg ha- 1 in a stand that had been fertilized with N at least 7 years previously and 1.6 kg ha- 1
in an unfertilized stand (Dyck et al., 1987).
The release of plant-available N from litter and the
increasing pool of soil organic N during stand development is dependent on biological activity, with microorganisms playing a key role. Microorganisms can also
immobilize N, particularly in N-deficient sandy soils
(Miller et al., 1979), with the microbial biomass being
an important potential source of plant-available N during subsequent organism turnover (Binkley and Hart,
1989; Jenkinson and Ladd, 1981). In other conifer
ecosystems, soil microbial biomass has been found to
vary seasonally (Entry et al., 1986; Groffman et al.,
1993; Hunt and Fogel, 1983; Sundman et al., 1978),
and to be influenced by fertilizer (Arnebrandt et al.,
1990; Nohrstedt et al., 1989) and management practices (B~i[tth 1980; Ohtonen et al., 1992; Vitousek and
Andariese, 1986).
We here investigate the effects of tree age and harvesting methods on the pools of microbial C and N
in litter and mineral soil in a coastal sand in two trials comprising (a) stands of different age under normal forestry management, which allowed the rapid
re-establishment of volunteer herbaceous plants, and
(b) stands established after different harvesting procedures of first-rotation trees and where volunteer plants
were excluded (Dyck et al., 1991; Smith et al., 1994).
An estimate of potentially mineralizable N was also
made by an anaerobic (waterlogged) incubation procedure (Keeney and Bremner, 1966). Relationships of
the C and N in microbial pools to total C and N in
litter and mineral soil have been determined, and the
contribution of microbial N to total ecosystem N in the
harvest-management trial has been assessed.
Materials and methods
Sites and soil
Woodhill State Forest is located at 360 45' S and 1740
26' E on recent sand dunes and yellow-brown coastal
sands (Pinaki sand), about 50 km northwest of Auckland. Mean annual temperature is approx. 14.3°C and
mean annual rainfall approx. 1310 mm (Jackson et al.,
1983). Pinaki sand is classified as a Typic Udipsamment (Soil Survey Staff, 1992), and contains approx.
97% sand and 3% silt (McMurtrie et al., 1990). Some of
its pedological and chemical properties are described
by Cox (1977).
The sites had been initially planted with marram
grass (Ammophila arenaria L.), to stabilize the sand,
and lupin (L. arboreus Sims) to improve the soil N status (Gadgil, 1979). P. radiata was subsequently introduced and grown under a variety of management treatments. Two major variables were considered in this
study. One was the effects of stand age, and the other
of harvesting intensity.
In the first trial, seven different ages of trees were
selected on closely located stabilized dunes of similar
origin and aspect. The soil at all sites was also similar.
The ages of the trees ranged from 1 to 33 years. In the
study area examined, young first-rotation stands were
no longer available, and the five youngest stands were,
therefore, second rotation (Table 1). Volunteer grasses
and herbaceous plants, which were periodically grazed
by cattle, were abundant in the three youngest stands.
The understorey had been largely shaded out in the
other stands, although pampas grass (Cortaderia spp.)
was generally present. Pine litter was negligible under
the 1 to 4 year-old trees and was not collected. The
spatial distribution of litter on the forest floor at the
other stands was very variable, both in amount and the
relative proportions of needles and twigs and the more
decomposed FH material.
Details of the harvest-management trial have been
given by Dyck et al. (1991) and Smith et al. (1994).
Briefly, the site had been planted in P radiata in 1944
at a stocking rate of 2240 stems ha-I and thinned
in 1968 to 270 stems ha - l Before harvesting this
first-rotation crop in 1986, ecosystem nutrient pools
were determined; the basal area of the stems at harvest averaged 56 m 2 ha -z, with the understorey consisting almost entirely of evenly-distributed pampas
grass. Total above- and below- ground biomass then
averaged approx. 550 t dry weight ha- i. Total N content to 10 cm depth of mineral soil averaged approx.
169
Table 1. Site propertiesat the time of sampling
Site
Managementtreatment
Trees
Litter
Stand Age (years)
Stocking
Basal area Amount Composition
Current
Previous(stems ha-1) (m2 ha-l) a (t ha- l )
rotation rotation
Stands of different age
Second rotation
Second rotation
Second rotation
Second rotation
Second rotation
First rotation
First rotation
Cl
C2
C4
C9
C18
C22
C33
1
2
4
9
18
22
33
43
41
43
43
34
0
0
1100
1483
2417
983
333
775
233
0.7
1.1
6.4
23.3
32.9
40.3
40.4
ND
ND
ND
18(35)b
85(45)
35(86)
12(43)
FF
WT
SS
5
5
5
42
42
42
2283
2317
2300
13.3c
17.5b
15.3bc
DS
5
42
2200
15.0bc
FF(N)
5
42
2233
20.5a
9(38) Mainly needles
48(72) Varied
64( 11) MainlyFH
material
137(21 ) Nearly all FH
material
17(31) Mainly needles
C5
5
42
2317
ll.ld
37(81)
ND
ND
ND
Variedc
Varied
Varied
Varied
Harvest management; treatment after first rotation
Whole tree harvest; forest floorremoved
Whole tree harvest
Stem-onlyharvest; (singlelayer of
harvest slash)
Stem-onlyharvest; (doublelayer of
slash)
Whole-tree harvest; forest floor removed;
fertilized with urea-N
Current practice; not windrowed
Varied; some
grass litter
ND, not determined.
a Calculatedfrom diam. at breastheight (DBH, 1.4 m), except for C 1and C2 trees for whichdiam. at the base of the greencrown (0.10 m)
was used. Valuesfor the harvest-managementtrial were calculatedfrom the average DBH for all trees in each subplot;values not marked
with the same letter are significantly(p 5 mm diameter were excluded,
because they would have been difficult to assay for biochemical characteristics; these assays generally require
homogeneous samples of 1-20 g. No separation was
made of the L and FH horizons, because the properties
of the litter as a whole were the major interest in this
particular study.
In 1990, sampling was carried out in one compartment only for each tree age; three adjacent replicate
20 × 20 m plots were used. When present, litter was
collected in three randomly placed quadrats (20 × 20
cm) from each plot, to give a total of three samples.
Mineral soil was sampled at 0-10 cm and 10-20 cm
depths by taking 2.5 cm diam. cores at approximately
equal intervals in a diagonal transect across each plot.
Alternate cores from each of the three plots were combined to produce two pooled samples for subsequent
analyses, with each containing at least 15 cores from
each of the two depths.
In 1991, sampling was restricted to the inner 20
x 20 m of each plot. Litter was collected as above.
Mineral soil (0-10 and 10--20 cm depth) was again
sampled by taking 10 cores at approximately equal
intervals in a diagonal transect across each plot. Each
of these sets of 10 cores was then pooled to give one
sample per plot; three replicate pooled samples were
thereby obtained.
Samples were transported and stored at ambient
temperature (15-25°C) before sieving (