Biol Fertil Soils (1990) 9:273-280 Biology and Fertility

°f S o i l s © Springer-Verlag 1990

Interactions between microclimatic variables and the soil microbial biomass

D.A. Wardle 1. and D. Parkinson 1'2

1Department of Biological Sciences, and 2Kananaskis Centre for Environmental Research, University of Calgary, 2500 University Drive N.W., Calgary, Alberta, Canada T2N 1N4

Received July 5, 1989

Summary. Soil moisture, temperature, microbial sub- strate-induced respiration and basal respiration were monitored in two plots in an agricultural field from April 30 to September 25, 1987, and in a further two plots from May 26 to August 27, 1988. An attempt to relate biologi- cal variables to microclimatic variables was made through the use of correlation analysis. The microbial substrate- induced and basal respiration were both strongly positive- ly correlated with the soil moisture content, and to a less- er extent positively related to soil temperature, especially when partial correlation was used to control for variation in soil moisture. Short-term changes in substrate-induced and basal respiration were correlated with changes in soil moisture but were largely independent of soil tempera- ture. The ratio of basal to substrate-induced respiration (indicating the respiration: biomass ratio and therefore ecosystem stability or persistence) was negatively associ- ated with the soil moisture content and in some instances with soil temperature when partial correlation analysis (correcting for soil moisture variation) was used. This suggests that the climatic conditions which contributed to the lowest ecosystem stability were low temperature, low moisture conditions.

Key words: Basal respiration - Microbial biomass - Soil moisture content - Soil temperature - Substrate- induced respiration

The soil microbial biomass represents a labile pool of available nutrients (Anderson and Domsch 1980; Jenkin- son and Ladd 1981) and the turnover of this component has important consequences for nutrient flow (Rosswall and Paustian 1984; van Veen et al. 1987).

The dynamics of the soil microbial biomass have been related to both soil moisture (Kieft et al. 1987) and soil temperature (McGill et al. 1986). Wetting-drying cycles

* Present address and address f o r of fprint requests." Ruakura Agricul- tural Research Centre, Private Bag, Hamilton, New Zealand

may have considerable importance in regulating the rate of microbial biomass turnover (Sorenson 1974; Bottner 1985).

The activity of the microbial biomass is related to soil moisture (Taylor and Parkinson 1988) and temperature (Mathes and Schrifer 1985). This activity is also often substantially enhanced by rewetting dry samples (Lund and Goksoyr 1980; Orchard and Cook 1983), which is probably due, at least in part, to the decomposition of microbial biomass killed by dessication (Shields et al. 1974).

The purpose of the present study was to investigate changes in relative microbial biomass and respiration in an agricultural soil, and to relate these changes to fluxes in soil temperature and moisture. Because rapid shifts in environmental conditions are considered important in regulating the size and turnover of the microbial biomass, emphasis was placed on frequent sampling, especially during periods of rapid fluctuations in soil microclimatic conditions.

Materials and methods

Site, soil, and plot details

The site used in this study is located in a mixed cropping area in South- Central Alberta, Canada, approximately 90 km northwest of Calgary. At the time of this study it had been under hay pasture for at least 5 years. The soil is a black Chernozem, with 7.0% organic C, a sand:silt:clay ratio of 52:30: 18, and a pH of 5.9.

Within this site, two plots (each approximately 8 × 8 m) were set up in 1987, and two in 1988. Each plot was regarded as a separate entity, and data from samples collected within one plot were not incorporated with data from other plots. Thus problems associated with pseudorepli- cation (Hurlburt 1984) were avoided. The plots were tilled to 15 cm depth at least 7 days before the studies began, in 1987 and 1988. No ag- ricultural crops were grown on the plots, and weeds were removed by hand weekly.

Sample and data collection

During 1987, two random soil samples were collected from each plot ev- ery 3 -5 days, between April 30 and September 31 inclusive. Each sam-

274

ple was approximately 15 × 15 × 15 cm. The same, but a somewhat less frequent, sampling procedure was followed between May 26 and August 27, 1988. In 1988 the samples were typically collected before and after rainfall events; this approach was based on the 1987 data which suggest- ed that fluxes in soil moisture were the dominant factor influencing sub- strate-induced respiration and basal respiration, which changed rapidly following changes in soil moisture. Because of the low spatial heteroge- neity within plots, two samples were considered sufficient to detect any important trends.

Each sample was sieved (4 mm) as soon as possible after collection, and the following determinations were made:

Substrate-induced respiration, determined after the addition of 6000 ppm glucose to 30-g (dry weight) soil samples. Measurements were typically made 4 h after the glucose addition (Anderson and Domsch 1978). Results were expressed as the rate of CO2-C evolution, rather than being converted to microbial biomass figures, because comparative studies currently in progress are indicating that different methods for measuring microbial biomass often correlate poorly with each other, and therefore calibration of the substrate-induced respiration method for predicting microbial biomass is problematic.

Basal respiration, determined using CO z evolution rates from 60-g (dry weight) soil samples (Anderson 1982). Respiration measurements were made when the rate of CO 2 evolution was constant.

Basal respiration: substrate-induced respiration ratio. This was used to provide a relative measure of the respiration : biomass ratio, as proposed by Odum (1969, 1985).

Soil gravimetric moisture content, on 20 g (net weight) soil, after drying at 105 °C for 24 h.

Measurements of CO 2 release were made at 22 °C using an infrared gas analyzer and an air-flow rate of 178 ml min -a. Substrate-induced and basal respiration measurements were made on soil maintained at field moisture levels.

Throughout the experimental period, one thermograph was placed in each plot, which continuously measured soil temperature at a depth of 10 cm, recording data separately for each of three randomly located probes.

D a t a analysis

For each plot, correlation analysis was used to determine the strength and significance of relationships between the microbial and climatic variables by treating each sampling period as an independent data point. Data transformations were used to fulfil the assumptions of nor- mality for parametric analyses, if required.

The following variables were used.

Microbial variables. MSIR: Microbial substrate-induced respiration, in ~tg CO2-C. h - 1 . g soil-1. DMSIR: Percentage change in MSIR between the present (n) and previous ( n - l ) sampling period, i.e. [(MSIRn - MSIRn- I ) /MSIRn- 1] X 100. The values were then subjected to the following transformations: 1987 data: for DMSIR log e (X/100+ 1), 1988 data: V[(log e X / 1 0 0 + 1)+ 1]. BRES: Microbial basal respiration in ~tg CO2-C.h -1 .g soil -1. DBRES: Percentage change in BRES between the present (n) and previous ( n - 1) sampling period, i.e. [(BRESn-BRESn_I)/BRESn_I]XIO0. The values for DBRES were then subjected to the same transformations as the DMSIR data. RAT: Ratio of basal respiration to substrate-induced respiration. DRAT: Per- centage change in RAT between present (n) and previous ( n - 1 ) sam- pling period, i.e. [(RAT n - RAT n 0 / R A T n_ 1] x 100.

Climatic variables. WAT: Percentage of water in the soil on a dry-weight basis. DWAT: Percentage change in water between present (n) and previ- ous (n - 1) sampling periods, i.e. [(WAT n -WAT n_ 1)/WATn_ 1] × 100. The data for DWAT were then transformed by the log e (X/100+ 1) transformation.

The temperature data were divided into five variables, i.e. the per- centage of t ime in the previous 72 h with temperatures in each of five classes: <5 °C, 5 ° - 10°C, 10 ° - 15 °C, 15° -20°C, and > 2 0 ° C . Because the data for these variables were inherently non-normal , they were rank-

Table 1. Values of Pearson ' s correlation coefficient, r, between values for substrate-induced respiration and climatic variables

Independent Plot number and date (inclusive) variable(s)

1 1 2 2 3 4 May i to Sept 25 June 15 to Aug 31 May 1 to Sept 25 June 15 to Aug 31 May 26 to Aug 27 May 26 to Aug 27 1987 1987 1987 1987 1988 1988

W A T 0.484** 0.860** 0.568** 0.921 ** 0.866** 0.910"* [WAT] 0.563 ** 0.799 ** 0.533 ** 0.901 * 0.890"* 0.881 ** T (<5) - 0 . 305 - - 0 . 4 1 7 ' * - - - T ( 5 - 10) - 0 . 2 1 7 0.470* 0.023 0.372 0.408 0.554** T (10 -15 ) 0.360* 0.233 0.374* 0.317 0.185 0.293 T ( 1 5 - 2 0 ) 0. i99 - 0 . 5 1 2 " - 0 . 0 4 6 - 0 . 4 2 4 * -0 .271 - 0 . 3 8 0 T (>20) 0.134 -0 .531 * - 0 . 2 9 4 0.546** 0.363 0.350 IT (< 5)] - 0.492 ** - - 0.767 ** - - - [T ( 5 - 10)] - 0 . 570** 0.001 -0 .273 - 0 . 3 6 3 - 0 . 1 2 6 - 0 . 1 4 9 [T (10-15)] 0.454** - 0 . 1 5 9 0.549* 0.325 - 0 . 3 0 9 0.129 [T (15-20)] 0.557** 0.003 0.402 0.260 - 0 . 1 2 6 -0 .073 [T ( > 20)] 0.387 * - 0.078 0.074 0.170 0.269 0.111 PCI 0.276 0.533 * 0.558 ** 0.504" 0.182 0.457 * [PCI] 0.600** 0.068 0.521 ** 0.273 0.289 0.104 WAT, PCI 0.707 ** 0.861 ** 0.712"* 0.927"* 0.897"* 0.907"*

W A T = percentage of moisture in soil (dry weight basis); D W A T = change in W A T between present and previous sampling period; T (< 5), T ( 5 - 10), T ( 1 0 - 15), T ( 1 5 - 20), T (> 20) = rank- t ransformed variable representing percentage of time in previous 72 h that soil temperature was below 5 °C, between 5 ° - 10 °C, 10 ° - 15 °C, 15 ° - 20 °C, and > 20 °C, respectively; PCI - principal component summarizing soil temperature data. Square brackets around climatic variable names indicate that partial correlation analysis was used, after holding W A T constant (temperature vari- ables) or PCI (moisture variable) *; ** indicates that correlation coefficient is significantly different from 0 at p = 0.05 and 0.01 respectively

275

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transformed. The rank-transformed variables were then assigned the following names: T(<5) , T(5-10) , T( I0 -15) , T(15-20), and T(>20), for each of the five classes.

To summarize the entire temperature data in terms of fewer vari- ables, a principal-components analysis was performed on the sampling time × temperature class matrix. The variables representing the first and second principal axes (explaining most of the variation) were assigned the variable names PCI and PCII.

Results

Substrate-induced respiration

During 1987, substrate-induced respiration remained rel- atively constant until mid-June, then fluctuated rapidly (Fig. 1). In 1988, it fluctuated very rapidly throughout the entire season (Fig. 2).

Significant and strong correlations were found be- tween the soil moisture content and substrate-induced respiration regardless of whether or not the main temper- ature variables were held constant using partial correla- tion analysis (Table 1). These correlations were strongest during periods of rapid wetting and drying, i.e. from June 15 to August 31, 1987, for plots 1 and 2, and during the entire 1988 season for plots 3 and 4.

Relationships between substrate-induced respiration and the main temperature variables (T) were unpredict- able unless a correction was made for the soil moisture content using partial correlation. However, when partial correlation was used, all significant values for r between the respiration and T(< 5) or T(5-10) were negative; all for T(10-15), T(15-20) and T(>20) were positive.

276

Table 2. Values of Pearson's correlation coefficient, r, between values for changes in substrate-induced respiration and climatic variables

Independent Plot number and date (inclusive) variable(s)

1 1 2 2 3 4

M a y 1 to Sept 25 June 15 to Aug 31 M a y 1 to Sept 25 June 15 to Aug 3I M a y 26 to Aug 27 M a y 26 to Aug 27

1987 1987 1987 1987 1988 1988

D W A T 0 . 7 1 8 " * 0 . 8 7 8 " * 0 . 6 3 8 " * 0 . 7 1 4 " * 0 .955 ** 0 . 8 8 2 " *

[ D W A T ] 0 . 7 0 9 * * 0 . 8 6 0 * * 0 . 6 3 0 * * 0 . 7 1 2 ' * 0 . 9 4 3 * * 0 . 8 9 3 * *

T ( < 5 ) - 0 . 1 3 0 - - 0 . 0 8 4 - - -

T (5 - 10) - 0 .018 0 .220 - 0 . 0 7 4 - 0 .009 0 .309 0 .458 *

T ( 1 0 - 15) 0 .198 0 .254 0 .213 0 .317 - 0 . 1 8 0 0 . 1 4 4

T (15 - 20) 0 . 0 2 6 - 0 .284 - 0 .017 - 0 .153 - 0 .187 - 0 .358

T ( > 20) 0 . 0 3 0 - 0 .127 - 0 .057 - 0 . 0 8 2 - 0 .018 - 0 . 2 2 0

I T ( < 5 ) 1 - 0 . 2 1 4 - - 0 . 1 1 1 - - -

[ T (5 - 10)] - 0 . 1 0 9 0 .083 - 0 .092 - 0 .044 - 0 .287 0 .211

I T ( 1 0 - 15)] 0 . 2 7 0 0 .017 0 . 1 8 0 - 0 .054 - 0 . 0 2 6 0 . 0 5 9

[ T (15 - 20)] 0 . 0 7 6 - 0 . 2 8 0 0 .088 0 .027 0 . 0 4 2 - 0 . 0 6 0

[ T ( > 2 0 ) ] - 0 .063 0 .117 0 . 0 0 2 0 .131 0 . 0 9 0 - 0 . 0 3 4

P C I 0 . 0 8 0 0 .085 0 .181 0 . 1 0 6 0 . 5 1 4 " 0 .336

[ P C I ] 0 . 1 3 0 0 . 2 6 0 0 . 1 2 2 0 .076 0 .288 0 .104

D W A T , P C I 0 . 7 1 2 " * 0 . 8 7 9 * * 0 . 6 4 5 * * 0 . 7 1 6 " * 0 . 9 6 0 * * 0 . 8 9 1 ' *

See footnotes to Table 1

Significant correlations between percentage changes in substrate-induced respiration and percentage changes in soil moisture occurred regardless of whether a correc- tion was made for temperature effects (Table 2). There was no apparent relationship between the percentage changes in substrate-induced respiration and the temper- ature variables regardless of whether or not partial corre- lation was used.

Basal respiration

Soil basal respiration changed frequently throughout the experimental period in both 1987 and 1988, with changes in respiration closely following changes in the soil mois- ture content (Figs. 1, 2).

Significant correlations were usually found between basal respiration and the change in soil moisture both during 1987 and 1988, regardless of whether or not varia- tions in temperature variables (i.e. PCI) were held con- stant using partial correlation (Table 3).

Correlations between basal respiration and the tem- perature variables were unpredictable unless partial corre- lation correcting for in soil moisture was used, with those variables representing high temperatures sometimes being negatively correlated with basal respiration. However, when partial analysis was used to correct for variations in soil moisture, significant negative correlations occurred between low-temperature variables and basal respiration for plot 1, and a positive correlation also occurred be- tween T(10-15) and basal respiration. Despite this, for

Table 3. Values of Pearson's correlation coefficient, r, between values for basal respiration and climatic variables

Independent Plot number and date (inclusive) variable(s)

1 1 2 2 3 4 M a y 1 to Sept 25 June 15 to Aug 31 M a y 1 to Sept 25 June 15 to Aug 31 M a y 26 to Aug 27 M a y 26 to Aug 27

1987 1987 1987 1987 1988 1988

D W A T 0 .453 ** 0 .687 ** 0 . 3 2 9 " 0 . 6 0 7 " * 0 .715 ** 0 . 8 2 0 " *

[ D W A T ] 0 . 5 7 0 * * 0 . 6 6 4 * * 0 . 3 0 2 0 . 5 9 0 * * 0 .701 ** 0 .755 **

T ( < 5 ) - 0 . 3 4 6 * - - 0 . 4 0 2 * * - - -

T (5 - I0 ) - 0 .321 * 0 .375 - 0 .001 0 .397 - 0 . 4 5 4 " 0 .543 *

T ( 1 0 - 1 5 ) 0 . 4 3 4 * * 0 . 3 0 4 0 . 2 5 0 0 .206 0 .078 0 .255

T (15 - 20) 0 . 2 9 6 - 0 . 4 0 9 - 0 .125 - 0 .549 ** - 0 . 4 0 4 - 0 .368

T ( > 2 0 ) 0 . 1 9 9 - 0 .418 - 0 .198 - 0 .453 * - 0 .265 - 0 . 4 7 0 *

[ T ( < 5 ) ] - 0 . 3 5 0 * - - 0 . 4 2 6 * * - - -

[ T ( 5 - 1 0 ) ] - 0 . 3 6 2 * 0 .327 - 0 . 0 0 1 0 . 4 7 6 * * 0 .178 0 .265

[ T ( 1 0 - 1 5 ) ] 0 .443 ** 0 . 1 6 0 0 .226 - 0 . 1 3 0 0 .292 0 . 3 4 6

[ T ( 1 5 - 2 0 ) ] 0 .313 - 0 . 4 0 8 - 0 . 0 8 7 - 0 . 5 2 2 * - 0 . 3 2 6 - 0 . 2 5 8

[ T ( > 2 0 ) ] 0 .155 - 0 .387 0 . 1 7 9 " * - 0 . 3 9 8 " - 0 .326 - 0 .461

P C I 0 . 3 7 0 * 0 .451 * 0 .443 ** 0 . 5 2 0 * 0 .391 0 .478 *

[ P C I ] 0 .567 ** 0 .397 0 .426 ** 0 .497 * 0 .148 0 . 4 1 0

D W A T , P C I 0 . 5 6 4 * * 0 . 7 4 4 * * 0 . 5 1 9 " * 0 . 7 2 4 * * 0 . 7 3 4 * * 0 . 8 1 7 " *

See footnotes to Table 1

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277

Fig. 2. Measurements of microclimatic and microbial vari- ables for plot 3 (solid lines) and plot 4 (dash lines), during the 1988 field season. B R : S I R = basal to substrate-induced respiration ratio

plot 2 (June 15 to August 31) a significant negative par- tial correlation was detected between basal respiration and T(5-10) and a positive one between basal respira- tion and T(15-20).

Significant relationships occurred between PCI and basal respiration in plots 1, 2, and 4; these remained sig- nificant in plots 1 and 2 when variations in soil moisture change were held constant using partial correlation. Also, for plots 1 and 2, r increased significantly when both PCI and the change in soil moisture were correlated together against basal respiration rather than when each variable was used alone.

With regard to percentage changes in basal respiration significant correlations occurred with the change in soil moisture regardless of whether the variations in soil tern-

perature (i.e. PCI) were held constant (through partial correlation) or not (Table 4). With regard to temperature, the change in basal respiration was significantly correlat- ed only with T(5-10) for plot 4 and with PCI for plot 3, and only without a partial analysis correction for the change in soil moisture.

Basal respiration: Substrate-induced respiration ratio

The magnitude of the ratio of basal to substrate-induced respiration did not fluctuate as rapidly as either basal respiration or substrate-induced respiration alone.

Correlation analysis demonstrated that the ratio was typically significantly negatively correlated with soil moisture regardless of whether or not soil temperature variables were held constant (Table 5).

278

T a b l e 4. Va lues o f P e a r s o n ' s c o r r e l a t i o n coef f ic ien t , r , b e tween va lues fo r c h a n g e s in b a s a l r e s p i r a t i o n a n d c l imat ic va r i ab le s

I n d e p e n d e n t P l o t n u m b e r a n d da t e ( inclusive)

var iab le(s ) 1 1 2 2 3 4 M a y 1 to Sept 25 J u n e 15 to A u g 31 M a y 1 to Sept 25 J u n e 15 to A u g 31 M a y 26 to A u g 27 M a y 26 to A u g 27 1987 1987 1987 1987 1988 1988

D W A T 0 .438 ** 0.791 ** 0 .547 ** 0 .718 ** 0 . 7 9 2 " * 0 . 8 2 8 " * [ D W A T ] 0 .441 * 0 . 7 8 7 " * 0 .535 ** 0 . 7 0 2 ' * 0 . 7 1 9 " * 0 . 8 1 0 " *

T ( < 5 ) - 0 . 1 1 6 - - 0 . 1 4 3 - - -

T (5 - 10) - 0 . 1 3 1 0 .113 - 0 .063 0 .090 0 .287 0 .464* T ( 1 0 - 1 5 ) 0 .205 0 .120 0 .160 0 .269 - 0 . 2 3 1 0 .093 T ( 1 5 - 2 0 ) 0 .103 - 0 . 2 3 4 - 0 . 1 1 8 - 0 . 3 2 0 - 0 . 0 8 9 - 0 . 1 9 6 T ( > 20) 0 .057 - 0 .034 - 0 .134 - 0 .223 - 0 .064 - 0 .273

[T ( < 5 ) ] - 0 . 1 5 7 - - 0 . 1 7 3 - - - [T ( 5 - 1 0 ) ] - 0 . 1 5 2 - 0 . 0 8 6 - 0 . 0 7 3 - 0 . 0 9 7 - 0 . 0 1 0 0.231 [T ( 1 0 - 15)] 0.191 - 0 . 1 7 4 0 .167 - 0 . 1 3 5 - 0 . 1 4 1 - 0 . 0 2 7

[ T ( 1 5 - 2 0 ) ] 0 .102 - 0 . 1 6 1 - 0 . 0 5 5 - 0 . 2 1 8 0 .098 0 .205 [T ( > 20)] - 0 .003 0 .217 - 0 .102 - 0 .076 - 0 .043 - 0 .149

P C I 0 .150 0 .149 0 .236 0 .250 0 .493 * 0 .298

[PCI] 0 .137 0 .082 0 .195 0 .135 0 . 0 8 0 - 0 .055 D W A T , P C I 0 .525** 0 .792** 0 .572** 0 .723** 0 .789** 0 .829**

See f o o t n o t e s to T a b l e 1

There was no consistent relationship between the ratio and soil temperature unless partial correlation (correcting for soil moisture) was used. When this was done, negative correlations were found between the ratio and T(15-20) in plots 2 and 3, and between the ratio and T(> 20) in plot 4. For these three plots, PCI was significantly corre- lated with the ratio when the soil moisture variables were held constant.

Discussion

The data presented in the present study suggests that sub- strate-induced respiration is driven primarily by soil moisture. If substrate-induced respiration can be pre-

sumed to provide a relative measure of the soil microbial biomass (Anderson and Domsch 1978) then it appears that microbial biomass dynamics and turnover are largely controlled by dynamics in soil moisture. This is consistent with other studies which suggest that the size and turn- over of the microbial biomass is influenced by microclimatic factors, especially soil moisture and wet- ting/drying cycles. Bottner (1985) demonstrated that ar- tificial drying of soil samples caused reductions in micro- bial biomass. Schniarer et al. (1986) noted changes in both fungal and bacterial components following wetting and drying treatments of field plots. McGill et al. (1986) determined that between 33°70 and 69°7o of the variability in microbial biomass dynamics over two seasons could be

T a b l e 5. Va lues o f P e a r s o n ' s c o r r e l a t i o n coef f ic ien t , r , b e tween va lues fo r the b a s a l r e s p i r a t i o n : s u b s t r a t e - i n d u c e d r e s p i r a t i o n r a t io a n d c l imat ic va r i ab le s

I n d e p e n d e n t P l o t n u m b e r a n d da t e ( inclusive) var iab le(s )

1 1 2 2 3 4 M a y I to Sept 25 J u n e 15 to A u g 31 M a y 1 to Sept 25 J u n e 15 to A u g 31 M a y 26 to A u g 27 M a y 26 to A u g 27

1987 1987 1987 1987 1988 1988

W A T - 0 . 411"* - 0 .658 ** - 0 .717 ** - 0 .830 ** - 0 . 6 2 0 " * - 0 .621 ** [ W A T ] - 0 .383 * - 0 . 6 4 6 " * - 0 .703 ** - 0 . 8 6 6 ' * - 0 . 7 6 5 " * - 0 .727 ** T ( < 5) - 0 .020 - - 0 .093 - - -

T (5 - 10) - 0 .187 - 0 . 3 1 5 - 0 . 2 0 0 - 0 . 2 7 6 0.111 0 .017

T ( 1 0 - 1 5 ) 0 .048 - 0 . 0 5 0 0.031 - 0 .097 - 0 .214 - 0 .113 T (15 - 20) 0 .194 0 .347 0 .237 0 .228 - 0 .062 - 0 .025

T ( > 2 0 ) 0 .120 0 .327 0.371 * 0.381 0 .144 0 .060 [ T ( < 5 ) ] 0 .185 - 0 .205 - - -

[ T (5 - 10)] 0 .096 0 .097 0 .099 0 .462 * 0 .696 ** 0 .692 ** [ T ( 1 0 - 1 5 ) ] - 0 . 0 6 5 0 .101 - 0 . 0 9 0 0 .143 0 .106 0 .164

[T ( 1 5 - 2 0 ) ] - 0 . 0 9 2 - 0 . 1 0 8 - 0 . 2 8 9 - 0 . 4 9 5 * - 0 . 4 5 1 " - 0 . 2 8 5

[T ( > 2 0 ) ] - 0 . 0 7 6 - 0 . 1 1 8 - 0 . 1 1 4 - 0 . 3 6 7 - 0 . 3 5 5 - 0 . 4 6 4 *

P C I 0 .148 0 .325 * 0 .203 0 .306 0 .374 0 .167 [PCI] 0 .129 0.131 0 .032 0 . 5 1 8 " 0 .574* 0 .486* W A T , P C I 0 .385 ** 0 .692 ** 0 .718 ** 0 .879 ** 0 .813 ** 0 .727 **

See f o o t n o t e s to T a b l e 1

279

explained in terms of soil moisture or temperature dynamics.

The basal respiration data suggest that rewetting dry soils causes rapid increases in microbial activity, especial- ly if the soil has been maintained at low moisture levels for an extended period of time (as observed during 1988). These data suggest that microorganisms killed by drying are decomposed and that the tissue is both released as en- hanced respiration and used to build new microbial bio- mass. This is a well-known phenomenon (Adu and Oades 1978; Lund and Goksoyr 1980; Orchard and Cook 1983). That the enhanced respiration is derived from decompos- ing microbial cells has been demonstrated using 14C- studies (Shields et al. 1974; Sorenson 1974).

In the present study, the turnover of the soil microbial biomass appears to have been extremely rapid, and was probably faster than that determined by either Jenkinson and Ladd (1981) (i.e. 1.7-2.5 years) or McGill et al. (1986) (i.e. 0 .2-3 .9 years). One possible reason for this difference is that the wetting/drying cycles recorded in the present study were frequent and extreme. Also, microbial biomass determinations were made frequently, i.e. every 3 - 5 days in 1987, or before and after rainfall events in 1988. Therefore, short-term fluctuations in microbial bio- mass could be followed closely. Another possibility is that the use of different methods for measuring the mi- crobial biomass may yield different results. The chloro- form-incubation technique (Jenkinson and Powlson 1976), as used by McGill et al. (1986), assumes that the decomposition (kc) factor of the microbial biomass fol- lowing fumigation remains invariant of the soil moisture content prior to measurement, possibly influencing the results of (relative) microbial biomass determinations. Conversely, with substrate-induced respiration, it is likely, especially for extremely dry samples (such as occurred oc- casionally for the 1988 samples), much of the added glu- cose does not dissolve and thus remains unavailable to the soil microflora, thus resulting in an underestimate of the microbial biomass and the turnover time. Because the active component of the microbial biomass is more likely to respond to a glucose addition than the inactive compo- nent (van de Werf and Verstraete 1987) measurements o f substrate-induced respiration may also be biased towards the active component. If part of the inactive microbial biomass is activated upon rewetting, and is inactivated upon drying, the rapidity of turnover of the total micro- bial biomass may be overestimated.

When attempts were made to relate the biotic vari- ables directly to soil temperature, the results were incon- sistent. The data, especially for basal and substrate-in- duced respiration, suggested that the microbial biomass and activity were negatively related to temperature. These results are best explained in terms of soil-moisture rela- tionships; i.e. during the warmer periods of the summer, moisture is often limiting. Rainfall causes a rapid increase in soil moisture, and usually a drop in temperature, but increases substrate-induced or basal respiration. This is borne out further by using partial correlation analysis to correct for variations in soil moisture or the percentage change in soil moisture. When this is done, substrate-in- duced respiration is negatively associated with low

temperatures and positively associated with high ones. The same is largely true for basal respiration and temper- ature interactions. However, for plot 2 (June 15 to August 31) there appeared to be a positive effect of low tempera- ture (or a negative effect of high temperature) on basal respiration, even when partial correlation was used. There is no obvious explanation for this, but in the drier por- tions of the summer it is possible that another factor (other than soil moisture) which is associated with tem- perature, causes enhanced respiration at lower tempera- tures.

There was no apparent relationship between soil tem- perature and percentage changes in substrate-induced res- piration or in basal respiration. This indicates that short- term changes in microbial biomass and activity are con- trolled primarily by moisture dynamics, while long-term changes are also affected by soil temperature.

Part of Odum's (1969, 1985) theory of ecosystem suc- cession predicts that ecosystems which have been subject- ed to disturbance have a higher respiration: biomass ratio than those which have not. This approach has been ap- plied to soil ecosystems (Anderson and Domsch 1985; In- sam and Domsch 1988) using a microbial basal respira- t ion: microbial biomass ratio. If it can be assumed that substrate-induced respiration gives a relative measure of the microbial biomass, the present study shows that the ratio of basal respiration to substrate-induced respiration can be used to provide a relative measure of the respira- tion: biomass ratio.

The negative relationship between soil water and the basal:substrate-induced respiration ratio suggests that this ratio is enhanced by low soil moisture conditions. This is most probably because in dry soil systems, much of the microbial biomass has been killed or inactivated through the drying process, and those microbes which re- main active have a readily available supply of substrates in the form of killed microorganisms. In these circum- stances, conservation of C resources is not as important (C is not as limiting) as in a more "stable" (persistent: Richards 1987) system and a disproportionately large amount of C is released as CO2, thus increasing the basal : substrate-induced ratio. The negative relationships between this ratio and temperature indicate that in dry conditions lower temperatures create conditions of dis- turbance which increase the ratio and contribute to a less "stable" ecosystem. Therefore, the least stable combina- tion of climatic variables would be low temperature, low moisture conditions.

Acknowledgments. The authors thank John and Caroline McBain for their cooperation in the use of the land upon which this experiment was conducted. Also, we acknowledge financial support from an NSERC Operating Grant (D. Parkinson) and a Canadian Commonwealth Schol- arship (D. A. Wardle).

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