In situ studies of nitrogen mineralization and uptake in forest soils; some comments on methodology

Soil Bdol. Biochem. Vol. 21,

NO. 3, pp. 423-429.

0038-0717,89 53.00 + 0.00

1989

Copyright 6 1989 Pcrgamoa Ress plc

Printed in Great Britatn. All rights reserved

flV SITU STUDIES OF NITROGEN MINERALIZATION AND UPTAKE IN FOREST SOILS; SOME COMMENTS ON METHODOLOGY M. A. ADAMS,P. J. POLGLASE,P. M. A~~WILL and C. J. WESTON School of Botany, University of Melbourne, Parkville, Victoria 30.52, Australia (Accepted

15 October 1988)

S~rn~~-A$~ts of the methodology of and i~te~retati~~ of results from. in si& studies of N-mineralization are discussed with reference to data cokcted from 17 eucalypt forests in south-eastern Australia during a S-year period. Results suggest that: (i) it is possible to maintain moisture of soils contained within corers at levels not significantly different from those of the surrounding soil; (ii) inorganic-N is not produced linearly over time under field conditions. nor should we expect it to be: {iii) minera~i~t~on rates are affected by all in situ methods. In each of the forests examined, the average rate of net N-mineralization decreased as the period of containment increased; (iv) shorter periods af containment (e.g. l-2 weeks) reduce artifacts due to containment and are therefore preferable to longer periods (e.g. 48 weeks); and (v) longer periods of containment cannot substitute for increased replication of sampling. Further, when annual or seasonal rates of N-mineralization and uptake are to be cahzulated, it is desirable to increase replication of the bufk soil sampling for the first and last sampling dates.

INTRODUCTION

of the rate of mineralization of nitrogen in agricultural soils generally aim at providing an index which can be correlated with productivity. The methods are mostly laboratory-based, and use either an aerobic incubation under standard (and usually optimum) conditions of temperature and moisture (e.g. Stanford and Smith, 1972) or an anaerobic incubation under standard (and usually optimum) conditions of temperature (e.g. Waring and Bremner, 1964). In situ methods have not been widely used in agriculture, although Eno (1960) suggested that the containment of agricultural soils in sifu (in plastic bags) may provide a better index of NO;-N production than those derived from incubation of soils in the laboratory. In contrast, much of the interest in forest soils is associated with studies of nutrient cycling, and in situ studies which aim to measure rates of N-mineralization under field conditions are gaining Measurements

wide appeal.

As far as we are aware, the first in situ

study of N-mineralization in forest soils was by Lemee (1967) who used inverted aluminium cans to isolate soil portions, but it took several years for these methods to be used more widely (Ellenberg, 1977; Rapp et nl.. 1979; Remacle, 1977). In situ methods have now been applied in studies of: (i) mature forests of differing productivity (e.g. Adams and Attiwill, 1986; Raison er al., 1987; Richards et al., 1985); (ii) forests along a successional gradient (Lamb, 1980); (iii) forests growing along a gradient of N-availability (e.g. Aber et al., 1985; Nadelhoffer er al., 1984, 1985; Pastor et af., 1984); and (iv) forests after disturbance (e.g. Matson and Boone, 1984; Polglase et al., 1986; Vitousek and Denslow, 1987). Results show that N-mineralization in mature forests ranges from
>800 kg ha-’ yr-’ in tropical forests. Correlations have been shown between the in situ rate of mineralization and other indices of soil N-availability, fine root production, PO:- -supply, N-turnover and forest productivity. An important feature of recent in situ methods is that, with appropriate assumptions, uptake of inorganic-N by forests can be calculated (Adams and Attiwill, 1985; Nadelhoffer et nt., 1984, 1985). Investigators use periods of in situ containment commonly of about 4 weeks; annual rates of net mineralization and uptake are derived by summation of the amounts of N mineralized or immobilized during each containment period. The most critical assumption made and one on which the validity of these measures rests, is that the method by which the soils are contained does not significantly alter the naturally-occurring rate of mineralization. The model of N transformations in soils is well known (Fig. 1). The ultimate step in production of inorganic-N from organic residues is not unidirectional but rather is one step in the “continuous internal cycle” of N in the microbial biomass (Jansson, 1958). Gross inorganic-N production is countered by immobilization and the resultant net increment in soil inorganic-N is rapidly depleted by plants. Thus, seldom are there large changes in soil inorganic-N concentrations from season to season, Soil microbial activity is regulated by pH, moisture, temperature and substrate quantity and quality. All in situ methods of containment developed to date alter the soil environment through (i) cessation of the carbon input from decomposing litter, and from fine-root turnover: (ii) increased carbon inputs from severed roots; (iii) modification of the moisture and temperature regimes relative to bulk soil; and (iv) accumulation of inorganic-N.

M.

424

A.

ADAMS er al

MOISTURE

DECOMPOSABLE SOIL ORGAMC MATTER

Fig. I.

A

model of nitrogen transformations in forests (adapted from Jansson, 1958)

Nadelhoffer et al. (1985) demonstrated how net rates of N-mineralization and uptake can be calculated from the in situ method, and these equations were again presented by Adams and Attiwill (1986) and Raison et al. (1987). The in situ methodology differs from study to study. Nadelhoffer er al. (1984, 1985) extracted soil cores, placed them in plastic bags and returned them to the sampling hole. Adams and Attiwill (1986) left the surface soil cores (O-5 cm) intact within capped plastic corers which were perforated around the circumference to allow moisture equilibration within the core. Raison et al. (1987) used intact cores contained within unperforated steel corers (either capped or uncapped) to a depth of 20 cm. Among their various studies, Raison er al. (1987) tested the effect on rates of N-mineralization of containing the core for periods of up to I3 I days. They found that accumulation of mineral-N was more or less linear with time, and concluded that containment periods of 30-90 days were appropriate.

Table Species. age (years, in parentheses) and location E. regnans forests (5) Toolangi. Victoria (IO) Toolangi. Victoria (41) Toolangi. Victoria (48) Toolangi, Victoria (51) Brittania Ck. Victoria (60) Mt Disappointment, Victoria (60) Mt Disappointment, Victoria’ (81) Toolangi. Victoria (250) Healesville. Victoria E. obliqua/E.

E. sideroxylon

(I 00)Heathcote.

of Eucol~prus Altitude (m)

forests

A”“Ul rainfall (mm)

Mea” diameter (cm) 4.18 II.2 32.3 42.9 59.5 60.7

740 720 860 780 340 650 650 800 820

1300 I300 1300 I300 I250 1100 II00 I300 I660

4

320

4 4 4

Live eucalypt stems ha-’

61.5 75.0

170 70

lo00

41.4

41.7

369

930 930 930

45.0 42.0 37.8

35.0 37.0 37.0

346 520 408

4

550 550 550 I40

800

60.9

35.4

169

4

100

750

40.8

21.4

256

Victoria

4

240

570

19.6

16.1

620

Victoria

4

260

570

16.5

19.7

690

Bluff. Tasmania

2 2 I 4 4 4 2 2

Dominant height (m)

65.7 350

regnctns

(I 00) Heathcote.

Raison ef al. (1987) also comment that the containment period should be long enough so that a significant change in the concentration of inorganicN can be measured, and should be variable to coincide with changes in environment such as temperature and drying and wetting cycles. There has, however, been no investigation of the most appropriate period of containment. We present here data from in sifu studies of N-mineralization during the period 1982-1987 in a total of 17 eucalypt forests in Victoria and Tasmania. Data for eight

19.000 I.490 550 320 210 230

Retreat. Tasmania E. obliqua forests (60) Mt Disappointment. Victoria (79) Mt Disappointment. Victoria (79) Mt Disappointment, Victoria’ (100) Emu Ground. Tasmania

E. microcorpo

I

(i) the moisture condition in soil contained within unperforated steel corers for long periods may not fluctuate in the same way as that in the forest soil. Raison ef al. (1987) do not provide such an assessment; (ii) we do not expect a linear rate of N-mineralization with time in forest soils (see discussion above and Fig. I).

12.6 16.4 43.1 53.7 58.5 48.8

(80)

E. amygdalina (100) Turquoise

I. Details

In siru containment period (weeks)

We wish to address two points:

‘Burnt by bushfire in November, 1982. &. regna”s is killed by fire, E. oblique recovers The& forests have been described by eolglase et (II. (i986)

after crown fire by epicormic

growth.

Measuring N-mineralization

forests in Victoria have been reported (Adams and Attiwill, 1986; Polglase et al., 1986). In this paper we reanalyse these data together with new data for six forests in a chronosequence in Victoria and three forests in a productivity sequence in Tasmania. A sampling scheme in which the periods of containment overlap (Adams and Attiwill, 1986) was used, and the period of containment ranged from 1 week in some of the forests and up to 4 weeks in others. The data are examined to test the assumption of all in situ studies that the methodology does not affect the rate of mineralization. .METHODs

Study sites

These studies were based in 0. I ha plots in eucalypt forests of Victoria and Tasmania, south-eastern Australia. The study sites included forests of Mountain Ash (Eucalyptus regnans F. Muell.), Messmate (E. obfiqua L’Herit.), Red Ironbark (E. sideroxylon A. Cunn.), Greybox (E. microcarpa Maiden) and Black Peppermint (E. amygdulina Labill.) forests. A brief description of the forests is given in Table 1. E. regnans forests are counted among the world’s tallest and most productive forests. In Victoria and northern Tasmania they are usually found at altitudes between 500-1000 m or on sheltered aspects where annual rainfall ranges from 900 to 1500 mm. E. obliqua forests are widespread; they occupy much of the foothills at altitudes < 500 m where annual rainfall ranges from 700 to 1OOOmm. E. sideroxylon and E. microcarpa forests grow in drier areas (annual rainfall 400-700 mm) of central Victoria, north of the Great Dividing Range. E. amygdufina is often found in association with E. obliqua or other eucalypts in drier forests in Tasmania (annual rainfall 600-900 mm). Sampling and analysis

Studies were confined to the surface 5 cm of soil in which organic-C, total-N and fine roots are concentrated. Plastic corers (10 cm length, 5 cm dia and perforated by a number of I cm dia holes over the entire length of allow moisture and temperature equilibration) were used for both sampling the bulk soil and for containment. The containment corers were driven by hand to a depth of 5 cm and capped with an inverted plastic Petri dish which was held in place by silicon putty. In our initial study (Adams and Attiwill, 1986) 10 replicates of the bulk soil were collected and 10 replicates of each containment were installed at random along the perimeter of each plot. In subsequent work 10 replicates were also used, but the replicates were bulked in twos to give 5 samples for analysis. Soil samples were brought to the laboratory in insulated containers and kept at 4% until analysed (usually within 7 days). Moisture content was determined gravimetrically and all results are expressed on a dry weight basis. Inorganic-N was extracted and measured by autoanalyser (Technicon, 1977a, b). In situ methodology and interpretation Our sampling protocol (Fig. 2) requires that, at the beginning of each period, the concentration of

z

in siru

425

t

Time

I number

of containment

periods

1

Fig. 2. The experimental protocol of overlapping containment periods used for the in situ study of N-mineralization (see Adams and Attiwill. 1986). B is the concentration of total inorganic-N in bulk soil. C is the concentration of total inorganic-N in soil which has been contained in situ in corers

for one (CI) or two (C2) containment periods of x weeks, where x is either I, 2 or 4 weeks. The study runs from 0 to T weeks. For the n th period, there are two independent estimates of the rate of N-mineralization, (C2, - Cl,_ ,) and (Cl, - B,_ ,).

inorganic-N (both NO;-N and total inorganic-N) in the bulk soil is measured (B). The concentration of inorganic-N is also measured in soil which has been contained in situ for 1 period (Cl) and for 2 periods (C2). Two independent measures of the rate of mineralization were thereby obtained for each period of containment (Fig. 2). In the 17 forests studied we have used 3 different periods of containment:

(i) containment

period = I week was used in two

E. regnans forests;

(ii) containment

period = 2 weeks was used in four

E. regnans forests;

(iii) containment period = 4 weeks (usually 1 calendar month) was used in 11 eucalypt forests. For the n th period (where n is the number of each period ranging from 0, the start of sampling, to T) we may calculate for l-period containments (see Fig. 2): N-mineralization

= Cl, - B, _ ,

N-uptake = Cl, - B,.

(1) (2)

If we ignore leaching losses and the inputs of inorganic-N in rainfall then these equations are identical to those used by Nadelhoffer et al. (1984), Adams and Attiwill (1986) and Raison et al. (1987). Denitrification is assumed to be similar in bulk and contained soils. Estimates of N-mineralization and N-uptake for the entire sampling time are then given by summation of all the rates [equations (1) and (2)] for T periods. It can be shown that total N-uptake differs from total N-mineralization only by the change in B (AB = B, - B,, Fig. 2): AB c 0: N-mineralization

< N-uptake

(3)

AB = 0: N-mineralization

= N-uptake

(4)

AB > 0: N-minerlization

> N-uptake.

(5)

If containment of the soil does not affect the rate at which net mineralization proceeds, it follows that

M.

426

A.

ADAMS

the rate of N-mineralization in each period of a 2-period containment should not differ from the rate of N-mineralization for the corresponding l-period containment. That is:

et

01.

Table

2.

which

has been contained

aged

Changes

41 yean.

in

moisture

Common

significantly

content

superscripts

(P < 0.05)

(see Fig. 2)

(6)

We have tested the strength of this relationship for containment periods of I,2 and 4 weeks by regression analysis. If the in situ method is not causing interference, the slope of the regression will be close to unity and we should be able to account for most of the variation between overlapping rates of mineralization. If we can remove that variation between overlapping rates caused by different moisture and temperature regimes, then residual variation is derived from inherent biological variation and artifacts induced by the containment. To simplify discussion we have presented detailed results only for the changes in the pool of total inorganic-N (“net mineralization”). In the most productive forests (E. regnans), NO;-N accounted for as much as 77% of total N mineralized. In most of the forests however, NOT-N was an insignificant proportion of N mineralized. RESULTS

Moisture and temperature equilibration

Our method using perforated corers (Rapp et al., 1979, also used perforated corers) resulted in ~5% difference in moisture contents between contained soils and bulk soils in all forests (see also Adams and Attiwill, 1986). For example, during a 4-week

Table

3. Regression

is the rate dependent

analysis

in soil which variable

for 2 periods

of rates

of mineralization

has been contained

is the rate for the same period

of x weeks (C2.

-

C.

_ , , seetext).

of the rate and

in oarentheses)

age

(2x

of data

period

uairs

Combined period

(250)

Combined E. regnonns (IO) period

Unburnt

(ii) Wetting

(5 I )

E. regnons

(60)

1985

41.6b

containment when the soil was either rapidly drying out (summer) or rapidly wetting up (winter) we can demonstrate a significant difference (P < 0.05) between moisture contents of the bulk soil from one month to the next, and an associated equilibration of moisture in the contained soil over that month (Table 2). Raison et nl. (1987) stated that containment methods, including ours, maintained soil at its initial moisture content throughout the period of containment. They commented further that “use of open and covered systems . . . enables assessment of the effects of moisture” and that “the core system is easiest to use in a continuously moist environment”. However, they present no data to test these statements. Our results (Table 2 and Adams and Attiwill, 1986) show that the in situ method using perforated corers, rather than maintaining the soil at its initial moisture content as Raison et al. (1987)

(pg g _’ dry one

period

wt day-‘).

The

of x weeks (Cl.

independent -

- x weeks) measured in soil whxh for

are independent

variable

B,_ , , see text). The has been contained

but overlapping

measures

the period

Slow

II>

SE

SE regression

0.502

0.516*‘*

I.465

0. I42

0.502****

I.039

0.163

0.189’

0.518

0.206

0.219.

0.472

0.171

0.183.

I.111

0.178

0.303”’

0.620

IO4

-0.384 0.535 0.397 0.574 0.356

0.087

0.140”

0.755

78

0.472

0.097

0.239*“*

0.772

7 14 I2

0.208 0.727 0.673

0.268

0.108”’

0.292

0.3 15

0.30s*

0.516

0.399

0.222”’

0.543

I4

0.836

0.245

0.492..

0.333

0.241

0.190

0.348”’

0.080

II

0.638

0.196

0.540..

0.110

I2

0.814

0.346

0.356.

0.164

12

0.190

0.514

0.014”’

0.504

microcorpo

(I 00)

I2

0.513

0.228

0.336.

0.250

99

0.577

0.102

0.249****

0.377 0.336

Burnt

forests

E. regnans

(60)

IO

0.71 I

0.317

0.4 19”’

E. obliqua Combined

(79)

14

0.591

0.445

0. I

24

0.724

0.192

0.392..

‘Levels

for

significance:

lP < 0.05.

l*P < 0.01.

l**P

soil

43.Ob

( 100)

(ii)

weight)

32.8’

1985

siderox$on

Combined

are not

30.P

0.369.

5

oven-drv

3?.6b

4 weeks

obliqua!E. regnons (80) obliqua (a0) oblique (79) obliqucr (100) omygdalim ( 100)

soil

forest

period

10 May 7 June

1985

forests

E. regnans

of

41.7’

1985

excluding

Containment

which

Contained

0.21 I

26 26 26 26

I)

if. regnms(8 I )

E. E. E. E. E. E. E.

(%

2 weeks

E. reggnans (IO)

and

periods

soil

0.221

I4 29

(i)

content

0.609 0.756 0.743

I5

(48)

Combined

values

within

I week

E. regnons

E. regnans

Bulk

February

The two variables

(5)

E. regncms (4

I

soil

period

4 January

of mineralization

E. regnanz

Containment

only

date

(i) Drying

bulk

Number

Species cvears.

Containment

for

Collection

denote

different

Moisture

C2,-Cl,_,=Cl,-B,_,

of

m corers for 4 weeks in E. regmms

< 0.001.

l***P

< 0.0001.

18”’

0.361 0.358

Measuring N-mineralization in

siru

suggested, is sensitive to fluctuating environmental conditions which affect the rate of net mineralization. The effect of containment period on mineralkation There was good correlation between overlapping rates of mineralization [equation (6)] in both of the forests where the containment period was 1 week (r2 = 0.502 for both forests combined, Table 3). Where the containment periods was 2 weeks, correlations between overlapping rates of N-mineralization were significant and positive for three of the four forests; the correlation for the fourth forest was negative [E. regnans (IO), Table 31 and we are unable to explain this. Even with this fourth plot excluded, however, r’ for the three plots combined was only 0.239 (Table 3). Where the containment period was 4 weeks, correlations for four of the nine unburnt forests were not significant (Table 3). In all of the 17 forests, the slope of the regression between the rate of N-mineralization in the second period of containment [C2, - Cl,_, , equation (6)] and the rate of N-mineralization for the overlapping first period of containment [Cl,, - B,_ , , equation (6)] was less than unity (Table 3). Furthermore, the data show both a decrease in slope and a decrease in correlation for the regression with increase in containment period from 1 to 4 weeks (Table 3). We have selected one plot for each of the three containment periods to demonstrate the general pattern of mineralization --(Fig. 3). The mean concentrations of inorganic-N (B, Cl, C2) for containment beginning with even numbers (time = 0, 2, 4, etc., Fig. 2) are overlapped with mean concentrations of inorganic-N for containments beginning with odd numbers (time = 1, 3, 5, etc., Fig. 2). We make the following points: (i) The rates of mineralization in overlapping containments are positively correlated as is demonstrated by regression analysis of rates for each period (Table 3) and by correlation between mean rates (Fig. 3). (ii) Mineral N does not accumulate in contained soils linearly with time (Fig. 3), nor do we expect it to. Rather, rates of accumulation or depletion fluctuate in response to changing environmental conditions. (iii) This fluctuation is most pronounced for the longest period of containment (4 weeks; Fig. 3, Table 3) and is typical for all the forests we have studied. It therefore does not follow that increasing the period of containment is necessary to produce “a statistically significant change in the pools of soil mineral-N” (Raison et al., 1987). (iv) Rates of mineralization in the second containment period (C2, - Cl,_,) are, on average, always less than the rate of the first containment period (Cl, - B,_ ,; Fig. 3, Table 4). There was only about a 4% and about a 30% difference between overlapping rates where the containment period was 1 week (Table 4). This discrepancy between rates tended to increase with period of containment so that differences of > 100% were encountered for five of the nine unburnt forests where the period of containment was 4 weeks (Table 4). The overall effect then, is for measured rates of net mineralization to decrease gradually as the period of containment increases.

0

1

T!me Inumber Fig. 3. Mean

2

3

of con~alnment pormds)

concentrations

of total inorganic-N in bulk contained in situ for Concentrations are calculated as proportions of the concentration in bulk soil. Data for three forests of E. regnans are shown, the period of containment ranging from I to 4 weeks.

soil (0). and in soil which has been either I period (W) or 2 periods (0).

(v) In five of the nine unburnt forests in which a 4-week containment period was used, N was immobilized (i.e. net mineralization < 0) in the second period (4-8 weeks) of containment (Table 4). (vi) The coefficients of variation for these rates were large (generally > loo%, Table 4). This is to be expected since rates of mineralization vary widely with time. Relatively high positive (net mineralization) or negative (net immobilization) rates can be encountered, and may change according to the prevailing conditions of moisture and temperature (assuming substrate quantity and quality to remain reasonably constant within sites). We might even suppose that time of day (e.g. morning or midafternoon) should be a consideration when sampling in situ containments. The coefficients of variation were in all cases relatively greater for mineralization rates in the second period compared to the first, and again this effect was least evident for the shortest period of containment. DISCLSSION

In undisturbed soil, gross mineralization is continuously opposed by immobilization. Paul and Juma (1981) state: “Generally, mineral-N does not accumulate in undisturbed grassland or forest sites since carbon input is high and nitrogen is the element limiting decomposition. Therefore it is difficult to measure even the net mineralization and immobilization rates.” When using the in situ method, we measure the concentration of inorganicN at discrete intervals; we measure the net balance between mineralization and immobilization. If

M. A. ADAMSel

428 Table (Cl,

4. Mean

rates of mmeralization

is the rate for that same period (C2,

in soil over

a period

B,_ , , see text) is the mean rate in soil which

-

Cl,_,

, see text).

The

(Lr

of x weeks

has been contained

- x weeks) measured

two variables

al.

in soil which

are independent for that

age (years,

Number in

has been contamed measures

day-‘,

C?,-Cl”_,

Reduction mean

E. WP”O”S (5)

I5

0.347

I75

0.332

E. re&ms

CVb(%)

Mean

CVb(%)

I4

0.251

767

0.181

W)

4

I83 II20

28

2 weeks

E. regnam (IO) E. regnonr (41)

26

0.467

I35

0.148

381

26

0.484

95

0.084

622

83

E. regnons (81)

26

0.952

I36

0.534

225

44

26

0.413

169

0. I32

550

68

E. regnam (250) Containment

period

(i) Unburnt

E. E. E. E. E. E. E. E. E. (ii)

forests

7

0.489

91

0. I29

220

74

I4

0.169

269

0.075

794

56

I2

0.362

II3

-0.104

563

I4

0.077

488

-0.018

2513

0.232

91

II

0.093

190

I2

0.080

179

I2

0.274

97

I2

0.107

309

IO

0.174

320

I4

0.104

218

5

0.030

291

-0.039 -0.025 -0.08 0.051

I

I28 123 87

398

I38

771

I31

450

130

569

52

forests

E. regnons (60) E. oblique (79) ‘Reduction

68

4 weeks

regnans (5 I ) regnans (60) oblique/E. regnans (SO) oblique (60) obliqua (79) obfiqua ( 100) amygdalh ( 100) sideroxvhn ( 100) microearpa ilooj Burnt

in

value’

I week

@I) period

of x weeks

dry wt basis)

Cl,-B,., Mean

Containment

for two periods

tint

second

of net mineralization

pairs period

The

The

of the rate of mineralization

of data

parentheses) Containment

measures. x weeks.

period Rate

and

independent one period

but overlapping

@gg-’ Species

by two for only

(%)

during

containment

calculated

as

[(Cl,-B,~,)-(C2,-CI._,)Ix

0.074 -0.174

709

58

214

267

100

(Cl.-B,.,) bCoefficient replicates

of variation. (CV

One data

point

omitted

from

data

set of

E. regnans (48) due to extreme

variation

between

> 40%).

soil could be contained in situ over a long period without affecting the rate of mineralization, we should observe the additive effects of a large number of fluctuations as the rate changes in response to environmental conditions. These changes probably occur on an hourly or daily basis. In contrast, the conditions under which soil is incubated in the laboratory are constant and usually optimum for mineralization and we might expect linear or curvilinear rates of net mineralization. The use of perforated containers (such as perforated plastic tubes used in this study) allows moisture in the contained soil to equilibrate fully with changing conditions in the forest (Table 2, Adams and Attiwill, 1986). The correlations between the rates of mineralization in each period of a 2-period containment and the rate of mineralization for the overlapping l-period containment (Table 3, Fig. 3) also demonstrate that the method using perforated containers is sensitive to Buctuations in environmental conditions. However, the two overlapping rates are never equal (Tables 3 and 4). Mineral-N may accumulate in, or be depleted from contained soils, but in general there is a decrease in the rate of net N-mineralization with increasing time. The discrepancy between overlapping rates is least when the period of containment is 1 week, and is greatest when the period of containment is 4 weeks. We conclude that the in situ method does affect the rate of mineralization. The effect is most probably associated with immobilization, driven by decomposition of severed roots with a relatively high C-to-N ratio. As the period of containment increases,

artifacts introduced by the containment method become more pronounced. Therefore, when the period of containment is long, both net mineralization and uptake will be underestimated. Raison et al. (1987) concluded that one of the reasons why the in situ method is satisfactory is and plant uptake of because “net mineralization N... were equivalent when the soil was moist”. It is not evident why equivalency depends on the status of soil moisture. Furthermore, it should be remembered that in the absence of leaching, measurements of mineralization and uptake are not independent. The difference between these two fluxes in any growing season can be simply calculated by the change in concentration of mineral N in the bulk soil of the first sample (B,) and that of the last sample [Br; equations (3), (4) and (S)]. We should therefore pay particular attention to replication at these critical times. The in situ method described here yields estimates of annual rates of N-mineralization which are close to rates of turnover of N in litterfall (Fig. 4) and which are highly correlated with rates of N-mineralization measured in both aerobic and anaerobic laboratory incubations (Adams and Attiwill, 1986). However, mineralization potentials measured in the laboratory are never reached in the field (Adams and Attiwill, 1986). Furthermore, the sensitivity of nitrification to changes in the chemical and physical environments may reduce the value of laboratory-based methods where the aim is to measure rates of mineralization with time or across a range of forests. The in situ method therefore offers the possibility of obtaining the best estimate of the rate of

Measuring N-mineralization 50

7

LO

.

C

429

eastern Australia. II. Indices of nitrogen mineralization. Plant and Soil 92, 341-362.

.

.

in siru

Ellenberg V. H. (1977) Stickstoff als standortsfactor, insbesondere fijr mitteleuroplische Pflanzengesellschaften. Oecologia Plantarum 12, l-22. Eno C. F. (1960) Nitrate production in the field by incubating the soil in polyethylene bags. Soil Science Society

/

of America Proceedings 24, 277-299.

Jansson S. L. (1958) Tracer studies on nitrogen transformations in soil with special attention to relationships. Kungliga mineralisation-immobilisation Lonrbrukshiigskolans Annaler 24, l&361.

1

a

0

I/, 10

I,

N !n llttorfall

I

1,

20

I kg ha.‘yoor-’

I

LO

30

I

Fig. 4. The relationship between the amount of nitrogen in annual litterfall and the annual rate of nitrogen mineralization calculated from 4-week in siru containments for nine eucalypt forests of widely different productivity in Victoria and Tasmania, south-eastern Australia.

in a forest soil. Cessation of N inputs from litterfall, and the severance of roots will always introduce some artifacts, but these are greatly limited if the period of containment is short. Our results show that the rate of accumulation of inorganic-N during containment is not linear (Fig. 3). Increasing the period of containment will therefore not necessarily produce an improvement in statistical significance. Rather, the emphasis should be on adequate replication at a time-scale which reflects the balance between mineralization and immobilization. We conclude that relatively long containment periods (~4 weeks) may be useful for comparative purposes, and their use may be dictated by the practicality of visiting distant forests more frequently. However, the best estimates of N-mineralization and uptake will be obtained if the period of containment is 2 weeks or less. Short containment periods are needed particularly where turnover of soil-N is rapid, or where the purpose is to study the effects of controlling factors and their perturbation on the rates of N-mineralization and uptake. N-mineralization

Acknowledgements-We

wish lo thank Melissa Syme and Helen Watson for technical assistance. P. .I. P. and C. J. W. were supported by Commonwealth Postgraduate Research Awards throughout this study and we acknowledge financial support from the Australian Research Grants Scheme and the Tasmanian Forest Research Council Inc.

REFERENCES Aber J. D.. Melillo J. M., Nadelhoffer K. J., McClaugherty C. A. and Pastor J. (1985) Fine root turnover in forest ecosystems in relation lo quantity and form of nitrogen availability: a comparison of two methods. Oecologiu 66, 317-321. Adams M. A. and Attiwill P. M. (1986) Nutrient cycling and nitrogen mineralization in eucalypt forests of south-

Lamb D. (1980) Soil nitrogen mineralization in a secondary rainforest succession. Oecologia 47, 257-263. Lemee G. (1967) Investigations sur la mintralisation de I’azote et son evolution annuelle dans des humus forestiers in situ. Oecologia Plantarum 2, 285-324. Matson P. A. and Boone R. D. (1984) Natural disturbance and nitrogen mineralization: wave-form dieback of Mountain Hemlock in the Oregon Cascades. Ecology 65, I51 l-1516. Nadelhoffer K. J.. Aber J. D. and Melillo J. M. (1984) Seasonal patterns of ammonium and nitrate uptake in nine forest ecosystems. Plunr and Soil 80, 321-335. Nadelhoffer K. J.. Aber J. D. and Melillo J. M. (1985) Fine roots, net primary production, and soil nitrogen availability: a new hypothesis. Ecology 66. 1377-1390. Pastor J.. Aber J. D., McClaugherty C. A. and Mellilo J. M. (1984) Aboveground production and N and P cycling along a nitrogen mineralization gradient on Blackhawk Island, Wisconsin. Ecology 65. 256-268. Paul E. A. and Juma N. G. (1981) Mineralization and immobilization of soil nitrogen by microorganisms. In Terrestrial Nitrogen Cycles. Processes, Ecosystem Strategies and ,\fanagement Impacts (F. E. Clark and T. Rosswall, Eds), pp. 179-195. Ecological Bullefins (Stockholm) 33. Swedish Natural Science Research Council, Stockholm. Polglase P. J., Attiwill P. M. and Adams M. A. (1986) Immobilization of soil nitrogen following wildfire in two eucalypt forests of south-eastern Australia. Oecologiu Planrarum 7, 261-272.

Raison R. J., Connell M. J. and Khanna P. K. (1987) Methodology for studying fluxes of soil mineral-N in situ. Soil Biology % Biochemistry 19. 521-530.

Rapp M., Le Clerc M. Cl. and Lossaint P. (1979) The nitrogen economy in a Pinus pinea L. stand. Forest Ecology and Management 2, 221-231.

Remacle J. (1977) Microbial transformation of nitrogen in forests. Oecologia Plantorum 12, 3H3. Richards B. N., Smith J. E. N., White G. J. and Charley J. L. (1985) Mineralization of soil nitrogen in three forest communities from the New England region of New South Wales. Australian Journal of Ecology 10, 42941. Stanford G. and Smith S. J. (1972) Nitrogen mineralization potentials of soils. Soil Science Society of America Proceedings 36, 465-472.

Technicon Instruments (1977a) Nitrate and nitrite in water and seawater. Industrial method No. 158-7lW/A. Technicon Industrial Systems, Tarrytown, N.Y.. U.S.A. Technicon Instruments (1977b) Individual/simultaneous determination of nitrogen andjor phosphorus in BD acid digests. Industrial method No. 329-74W/B. Technicon In&trial Systems, Tarrytown. N.Y., U.S.A. Vitousek P. M. and Denslow J. S. (1986) Nitrogen and phosphorus availability in treefall gaps of a lowland tropical rainforest. Journal of Ecology 74, 1167-l 178. Waring S. A. and Bremner J. M. (1964) Ammonium production in soil under waterlogged conditions as an index of nitrogen availability. Nurure 201, 951-952.