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Carbon dioxide release from decomposing wood: Effect of water content and temperature
Soil Bid. Biochem. Vol. 15, No. 5, pp. 501-510,1983 Printed in Great Britain. All rights reserved
Copyright
8
0038-0717183 $3.00 + 0.00 1983 Pergamon Press Ltd
CARBON DIOXIDE RELEASE FROM DECOMPOSING WOOD: EFFECT OF WATER CONTENT AND TEMPERATURE LYNNE BODDY* Department of Plant Biology and Microbiology, Queen Mary College, University of London,
London El 4NS, U.K. (Accepted
10 February 1983)
Summary-The use of gas chromatography to measure carbon dioxide evolution from decomposing wood was evaluated and found useful to assess the rate of wood decomposition. An increase in temperature from S-25°C was accompanied by an increase in CO: evolution. Increase in water content was accompanied by an increase in CO, evolution. When higher temperatures accompanied high moisture contents CO, evolution often levelled off or decreased. This was attributed to a decline in the rate of 0, diffusion to concentrations insufficient to meet the demand. Increase in branch length showed a similar effect.
INTRODUCTION
The aim of this study was to develop a rapid, accurate technique to assess the rate of wood decomposition in the laboratory and to quantify the effects of abiotic variables on the process. Under aerobic conditions, decomposition of wood, by micro-organisms, releases COz, H,O and heat leaving chemically altered wood (of reduced weight and strength) and the tissues of the decomposer organism. Smith (1975) suggested the possibility of using these factors or the uptake of O2 for estimating decomposition rates. Weight loss and tensile strength loss have been used (e.g. Findlay, 1940; Hartley, 1958; Bravery and Grant, 1971) but both methods suffer from the drawback that they are destructive and that long incubation is required before statistically valuable data can be obtained. Results have been obtained more rapidly using small test specimens (Allison and Murphy, 1962) but it is unlikely that such samples will accurately reflect decomposition in larger pieces of wood and intact branches. The use of water production is theoretically possible but wetting and drying during the experiment would be a confounding factor and is not practical. Measurement of heat release has been used recently in the study of microbial activity in the soil (Ljungholm et al., 1979) but as it is still in its infancy better known techniques are probably more satisfactory. The remaining factors which alter as wood decomposition proceeds are gases. Both Oz consumption and CO2 evolution have been measured to determine decomposition rates of many types of plant litter in the laboratory (e.g. Howard, 1966; Howard and Howard, 1974; Bunnell et ai., 1977). For measurement of CO, evolution, gas chromatography (GLC) compares favourably with other respirometers and gas analysers (Putman, 1976). I have here in-
*Present address: Department of Microbioiogy, University College, Newport Road, Cardiff CF2 ITA, U.K.
vestigated the effects of abiotic variables on the rate of wood decomposition in terms of CO, evolution measured by GLC. MATERIALS AND METHODS
General methods
The rate of branch decomposition was measured by GLC determination of the quantity of CO, and other gases evolved from branch samples in small chambers under various moisture and temperature regimes. Beech (Fugus syluatica L.) branches were collected from the forest floor of Blean Woods NNR (NG ref. TRlll608) an abandoned beech and oak coppiced woodland. The size of the branches varied between 1.5-2.2cm dia and 2.515cm length. Following Healey and Swift (1971) relative density (RD; g cm-j) was used as a measure of the state of branch decay. The respiration vessels were 1Ocm length glass tubes, x 2.4 cm id., with rubber bungs placed in both ends. Each bung hetd a piece of glass tubing, 7.0 x 0.4cm i.d. Butyl rubber tubing (impervious to CO,) was attached to the glass tubes and the chambers were sealed by clamping the rubber tubing with Hoffman clips (Fig. 1). The seal was tested for leaks by introducing a known concentration of CO* into the chambers which was estimated by GLC 2 h later. The chambers containing the branches were immersed in a water bath for the duration of the experiment. The temperature of the water bath was maintained constant (+O,OS°C) by a Grant cooler unit model FC 15 and pump model FH 15 or by a Churchill chiller thermocirculator model OYCTCV. Before sampling the chambers were completely flushed with 300~01 of CO,-free air, obtained by passing the air through a conical Aask containing “Carbosorb” (non-deliquescent, self-indicating, 1.74.0 mm soda lime) for 5 min. The chambers were then sealed. After at least 2 h a 0.5 ml sample of the
502
LYNNEBWDY
(where Z = volume of air in the respiration chamber, i.e. vol. chamber-vol. wood). A final expression for calculating the decomposition rate of a branch was thus given by: 5.357YZ x 6O/(W, x t)pgCh Fig. I. Controlled environment respiration chamber. fc = flask eontainin~ “Carbsorb”. ap = air pump, rc = respiration chamber containing experimental branch, wb = water bath for maintaining desired temperature.
atmosphere was extracted from the chamber by inserting the needle of 1.0 ml syringe through the butyl
rubber tubing. The gas sample was then injected into the GLC. The chromatograph A glass column packed with l5~18O~rn beads of Poropak Q (2.0 m x 4 mm i.d.) was fitted to a Varian Aerograph model 90-P gas chromatograph. At the exit of the column was a thermal conductivity detector attached to a servoscribe potentiometric recorder operating at 2 mV. Column temperature was maintained at 75’C; injection port at room temperature and the detector at 80°C. A complete gas sample was injected into He as a carrier gas (He gives a sensitivity for COz 12 times greater than that obtained with N, as carrier gas). As N, was not used as the carrier gas, the 0, peak was not dissociated from N2 in the sample and could not be measured. Separation was only attempted for CO2 and other carbon containing gases. The chromatograph was calibrated using standard gases. The injections were replicated until two values within So/:, of each other were obtained (usually twice as reproducibility was excellent). Calculation qf results To obtain the relationship between peak height and CO, concentration, 0.5 ml samples of calibration gases of known concentration were used. The intercepts (a), regression co-efficients (8) and co-efficients of determination (R2) for linear regression on the calibration curves are given below: Attenuation
i
a = -0.940 Attenuation
fr = 27.922
R* = 0.998
p = 12.769
R* = 0.998
2
!Y.= 0.980
The proportion of CO1 in a sample could thus be determined and this was then transformed to 9 g C: 0.5 ml Yo/;,CO2 contains Y x 10 x 12/(22.4 x 2) pg C (where Y is the percentage of CO2 obtained from the calibration curve). At the end of each experiment, the volume and oven dry weight of the branches were obtained along with the volume of the chambers. The quantity of C evolved by a particular branch at a particular sampting could formula:
then
be calculated
from
2.679 Y x Z/O.5 peg C
the following
= 321.429YZl(W, x r)pgC
‘g-’ (O.D. wt wood) h-‘g-’
(O.D. wt wood)
(where W, = O.D. weight of the branch; t = time from sealing the chamber to sampling in min). Multiplying this value by 2.2 gives an estimate of actual weight loss (Good and Darrah, 1967). Sensitivity and accuracy Volumes of CO, as low as 3.5 ~1 in any injected sample were measured and by increasing the volume of the sample injected from 0.5 to 1.0 ml even lower concentrations of CO, in the respiration chamber could be measured. Prolonging the period of COz accumulation in the respiration chamber further increases the possibility of measuring low respiration rates. Ten replicate measurements of 0.5 ml samples of a standard gas usually resulted in a difference of no more than 6”/d between maximum and minimum estimates and often much less indicating the accuracy of the method.
The aims were to assess the feasibility of the method, to investigate the variation between replicate samples obtained from a particular branch under a particular set of conditions and to determine whether or not 10 or 5 cm lengths from the same branch showed the same response to various moisture contents. The material used gave data on the effect of state of decay and moisture content on the rate of wood decomposition. A 1.5cm length was cut from four branches (eliminating the ends which are often more decayed than the remainder). Each branch was then cut into a 10 and a 5 cm portion. These were soaked in water for 12 h, to attain a high moisture content, and then placed in the controlled environment chambers, which were of sufficient size to allow a surrounding air space of several miliimetres. The sealed chambers were submerged in a water bath, held at 15°C for 48 h to ensure branch temperatures had equilibrated with that of the surrounding water. CO, was accumulated for at least 2 h and then sampled. Two replicate measurements of CO* evolution were made for each branch at each moisture content over immediately consecutive periods on the same day. The branches were then weighed for subsequent calculations of moisture content. The branches were replaced in the chambers in the water bath. A stream of dry air was passed through the chambers so that CO, evolution could be monitored at different moisture contents during drying of the branches. The air flow was stopped 2 h before sealing the chambers to allow the branches to equilibrate to the conditions pertaining during the sampIing period. The procedure in subsequent experiments consisted of a series of sampling periods during which two replicates measurements of CO? concentration were
Temperature, moisture and wood decomposition
branch
by
rhrough
Sfop
a,r
flow
Flush
air
2 hr
pr,or
to
I
chamber
chamber
Cop-free
503
chamber
sealing
)
pass,“g
rate
or.
for Seal
5 mln
wth
chamber l-
Weigh
Sook
branch and chamber
branch
and
replace
I”
replace
I”
chamber
I
Fig. 2. Flow chart illustrating steps involved in the first experiment.
made at each moisture content, interspersed with periods of drying to attain different moisture contents (Fig. 2). When the branches reached low moisture contents (approx. 25%) they were re-wetted and the process repeated until approximately 20 data points had been obtained. effect
qf branch length on CO1 evolution
Seven branches, most with high initial moisture contents, were cut into 10 cm lengths and placed in the respiration chambers held at 15’C. After 24 h samples were taken following the usual procedure. The branches were then sawn in half and the two halves were assessed separately. These 5 cm portions were then later sawn in half and the resultant four 2.5 cm lengths were again assessed separately. The three samplings were made on consecutive days, two replicates being made on each day. High moisture contents were used because it was considered that any differences in CO2 evolution due to length, would be more pronounced under these conditions. (To reduce the chance of water loss between samplings, sawing always took place immediately prior to the sampling.)
CO, evolution from branches maintained moisture content for 10 days
at a constant
Three pieces of branch-wood, length 5 cm, 1.8-2.2 cm dia were wetted and allowed to equilibrate for 3 days in chambers at 15°C. CO, was sampled on 5 days over a IO-day period. The branches were weighed after each sampling to check moisture contents. Equilibration
after change in temperature
The purpose of this experiment was to determine the time required for COZ evolution to stabilize after a large change in temperature. Eights branches, length 5 cm, between 1.5-2.0 cm dia, covering a range of RDs were wetted and placed in the respiration chambers. Four of the branches had no bark and four had complete bark cover. The chambers were placed in a water bath at 20°C and allowed to equilibrate for 48 h before sampling. After sampling the temperature was altered to 5°C and further samples taken after 24 and 48 h. The temperature was again altered to 20°C and samples taken after 20, 44 and 140 h. Finally, the
LY NNE BODDY
504 temperature was altered after 24 and 48 h. Efect
qf‘ temperature
to 5,‘C and samples
and moisture
taken
on CO? ewlution
Twenty-five branches, 5 cm x 1.5-2.2 cm dia. covering a range of RDs, were used. The respiration rate of each branch was obtained at three different moisture contents at temperatures ranging from 5 to 25°C. Moisture contents near the maximum possible, near the minimum and one in between were used for each branch and were altered either by wetting or drying after each set of temperature readings had been completed. The order in which the moisture readings were made varied from branch to branch and the order for the temperature readings was randomised for each set of temperatures. although not for each branch. The 25 branches were respired in two batches, thus there resulted 6 orders for temperature sets. For each batch, one of the sets of temperature readings began at 20-C and on these occasions a reading at this temperature was also obtained after the temperature readings had been completed. This was a check that the community respiration rate had not changed over this period. At least 20 h was allowed for equilibration after altering moisture contents or temperature. A flow chart of the procedure outlined above is shown in Fig. 3. In addition, 10 branches, 5 cm x 1.52.0 cm dia were selected to cover a range of RD’s. They were respired at different temperatures in a sequence of 5, 10, 15 and 20°C; the sequence was then repeated. The branches were allowed at least 40 h to equilibrate after each temperature change. Seven of the branches were maintained at a constant moisture content for both runs the other three being allowed to dry slightly before the second run. Two replicate measurements were obtained at each temperature on each run. Twelve branches were selected and cut to length 5 cm. The methods employed were those used in the experiment 1, except that each branch was respired over a range of moisture contents at 15 C, then over a range at 10’ C. followed by a range of moisture contents at 5’C. RESULTS
Experiment
1
The relationships between moisture content (% O.D. wt) and respiration rate (pg g- ’ C h - ‘) for the 5 and 1Ocm sections of each of the four branches studied are shown in Fig. 4. Each sampling is represented by a data point and those obtained on the same day, i.e. under “identical” conditions, are joined by a line to show the range in readings. All branches showed similar trends between CO, evolution and moisture content. Respiration rate increased linearly with increasing moisture content, although in some branch sections the curve flattened in the higher moisture content region and in one instance (Fig. 4c) decreased markedly under very high moisture conditions. The rate of respiration of the two branch lengths were similar in the low moisture region, although in the high moisture region CO, evolution appeared to be greater from the 5 cm than from the 1Ocm lengths. In order to obtain sufficient data the branches were re-wetted at the end of a
drying-measuring sequence on two or more occasions. No anomolous data were detected resulting from the order in which the measurements were obtained. The I$%W qf‘hranch length on carbon dioxide evolution The results have been expressed as a percentage change of CO, evolved from the original 10 cm length (Table 1). On 5 (out of 7) occasions an increase of 40% or more was found on dividing the 10 cm lengths into two 5 cm lengths. The other two sets, which were both branches having low moisture contents, yielded slightly less CO, on division. On further division, all but one of the branches evolved considerably more CO, than the original 10 cm length, and all but one of these evolved more than the equivalent 5cm lengths. COz erolution fkom branches maintained moisture content ,for 10 days
at u constant
The amount of CO, evolved from branches I and 3 did not alter appreciably during the experiment, although the initial value obtained for branch 3 was somewhat higher than later ones (Table 2). The high RH in the chambers allowed fungal mycelium to grow out from the cut ends of branch 2. The rate of CO, evolution, from this branch was constant during the first 4 days. On day 10, however, a decrease occurred (being significantly (P < 0.01) lower than that obtained on day 2) which coincided with the apparent death of the surface mycelium. Equilibration
cfter chunge in temperature
All branches showed similar patterns of CO, evolution through the cycle of temperature changes although the values obtained varied depending upon the moisture content and state of decay (see Fig. 5). There was no difference in the amount of CO, evolved at 5 ‘C after 24 and 48 h. Likewise, there was no change at 20°C after equilibration for 20. 40 and 140 h. The &ct
@‘temperature
on CO, eaolution
The data sets for each individual branch were considered separately (Fig. 6). Linear and exponential regressions were fitted to the data, at one moisture level for each branch, and most of the relationships were equally well described by either model (L. Boddy, unpublished Ph.D. thesis, University of London, 1980). Linear regressions were also fitted to the data obtained from branches which underwent two temperature sequences. No significant differences were found between the residuals for the two regressions on any one branch and further comparisons were performed (Table 3). No significant differences (P < 0.05) between slopes were found for any of the branches, but two of them (Nos 2 and 4) showed differences in the elevation of the regression lines. These two branches were ones which had had their moisture contents lowered slightly between runs. Efect
qf’moisture
on CO, ecolution
In general the data obtained at 10 C did not cover a wide enough range of moisture contents to be
Temperature,
I
moisture
wth
and wood
decomposition
505
rate
Cop-free air. Seal chamber
After 2 hr (or more depending on rote of take
CO,+
1
samples 1
11
Alrer
temperature
and allow 20 for equllibratmn
Dry branch o,r through Stop prior
I
hr
by pass,“g chamber.
o,r flow 2 hr to sealmg chamber
Sook branch and replace II? chamber
Fig. 3. Flow chart
illustrating
in an experiment investigating the effect of temperature and moisture on CO2 evolution.
steps involved
conclusive about the relationship between moisture and rate of CO, evolution. There is some evidence that at 15°C respiration rate increases with increase in moisture content up to a maximum and then tails off slightly as the maximum moisture content is reached (Fig. 7). Respiration rate was not reduced at high moisture contents at 5°C. Joint ejfect evolution
of temperature
and moisture
on CO>
By combining data sets for individual branches a picture of the joint effect of temperature and moisture on the rate of wood decomposition (i.e. C loss) can
be obtained. However, percentage moisture content values in branches of different RD are not equivalent (Boddy, 1983a) thus only data sets of branches of similar decay state were considered together. The joint effect of temperature and moisture on C loss is shown for the 0.3-0.4 g cme3 class as a threedimensional representation constructed from temperature curves of different branches held at several constant moisture contents (Fig. 8). More measurements were made at lower moisture contents and the mean response of several branches has been plotted for clarity. Regression equations for all branches are given in Table 4. The general trend of increasing
LYNNE
(a)
0
(b)
0
8 8f
s&l 1 60
I
B -I
I
50
I
100 140 180 % Moisture
likely due to between branch variation. It is important to note that only a few branches were assessed under high moisture regimes, consequently the 3-D representation is probably not as accurate in these regions as for lower moisture contents. The general effect of branch length (because of its affect on aeration) and state of decay would be to raise or lower the response surface, but not necessarily in parallel over the whole surface or for different lengths and RDs. Three dimensional plots of the data in the 0.2P0.3 g cm 3 class showed several anomalies in the general trend of upward shift in the temperature response curve with increase in moisture content. This is illustrated in Fig. 9 in two dimensions for three branches having similar RDs. The implication of this is that in this class, consisting of more decayed branches, some factors other than temperature and moisture are important determinants of decomposition rate. The factors could be type of organism present, type of decay, etc. No 3-D plots were obtained in other classes as insufficient data were available. No gases other than CO? were detected in any of the experiments.
content
Fig. 4. Relationship between percentage moisture content and rate of carbon loss for the four branches of different RD in the first experiment. (a) 0.22 g cm ‘; (b) 0.20 g cm ‘: (c) 0.30gcm-3; (d) 0.43 gem-l. (0) 5cm length; (m) 1Ocm length. N.B. The lines joining data points serve to show the range between replicates under identical conditions (not error bars). Linear regression formulae for the linear por-
tion of the curves (asterisked formulae have non-linear tails): 10 cm 5cm IOcm 5cm IOcm 5 cm 10 cm 5cm
BODDY
y = -4.0 + 0.20 x y =0.7+0.12x y = -5.7+0.24x y = -9.5+0.35x y = -18.7+0.75x _y = -23.2 + 0.77 x y = -4.4 + 0.20 x y = -8.6+0.48x
(X (X (X (X
= GlOO)*; = O-250); =Cr150)*; =0-150)*; (X =O-140)*: (.x = O-140); (X = O-80); (x =&80).
decay rate with increase in temperature and moisture can be clearly seen. There is a very sudden drop in the response surface between about 175-195% moisture which may be of biological significance but is more
Table
I. Effect of branch
DISCUSSION
The methods used were eminently suitable allowing almost “instantaneous” assessment of the effect of different variables on the rate of wood decomposition in the laboratory. Credible and consistent results were obtained between replicates for several days and also after altering temperature or moisture and then returning to the original regime. Although separate experiments were performed to investigate the e,Xect of temperature and moisture on the rate of wood decomposition (as measured by C loss) it is clear that the two are intimately associated. Further, Boddy (1983a) has discussed the interactive effect of temperature on branch moisture content and vice versa. The effects of these two factors on decay rate are thus considered together. A clear response to temperature was observed in all branches irrespective of RD or moisture content and in all cases an increase in temperature between 5 and 25°C resulted in an increase in CO? evolution (Fig. 6). In general, for individual branches, an increase in moisture content resulted in an upward shift of the temperature-CO, evolution curve although there was
length
on respmtion “,,
Branch
I 2 3A 3B 3c 4A 4B
No.
R.D. (g cm 0.20 0.49 0.20 0.35 0.30 0.49 0.35
‘)
“,, H1O 320 88 260
I10 170 32 139
change
2 x 5 cm length 119.40 -26.75 71.47+* 40.98 40.30 ~ 13.5’) 53.23*
rate
in CO, evolutmn IO cm length
over
4 x 2.5 cm length I55.42* -6.13 Il2.97** 255.13* l72.59* 50.02* 27.65
Asterisks indicate significance of difference between the CO, evolution data (not “,) of the 10 cm vs 2 x 5 cm and IO cm YS 4 x 2.5 cm pieces compared using ‘/‘-test. * = significant dilierence at P < 0.05; ** = significant dilTerencr at P < 0.01: no astmsk indicates no significant difference at P < 0.05
Temperature, Table 2. Carbon
moisture
loss m branches
and wood decomposition
held at constant
moisture
content
507
rate over a IO-day period
Days Branch no. I
C loss (pcgg-‘h-l) SEM C loss (pgg-‘h-l) SEM C loss (pgg-‘h-l)
2 3
SEM SEM
=
standard
0
2
3
4
13.95 k 1.44 37.35 31.70
12.97 f 1.10 38.45 * 0.44 22.82
15% i 0.45 35.74 23.63
14.49
~
* 0.93
f
error of the mean of two replicates
1.52
2O’C
I
5T
2OT
I
I
4G4 ~ 20.03 -
Il.82 + 0.79 28.92 f 0.76 21.62
“i, moisture
content
0.30
239
0.31
168
0.62
248
+ 1.39
(s//n).
some evidence that at very high moisture contents the curve shifted downwards. Clearly, although the temperatures used were in the range usually encountered in the field (Boddy, 1983a), they were at or below the
I
Branch RD (gcm~‘)
10
5’C
optimum for decomposition as no decrease in respiration rate occurred at higher temperatures. For most European species of wood-rotting basidiomycetes fungi the optimum temperature for growth on agar is between 2430°C (Cartwright and Findlay, 1934; Henningson, 1968; Boddy, 1983b). These types of
I
25 -
ii
b
0:
2
4
6
0
IO
12
14
16
u
s f
30-
s
:’
20
-
/ :A
‘O-
Days
Table 3. Comparisons
Branch NO. I 2 3 4 5 6 7 8 9 IO
of 2 regressions
“ID moisture content 350 310 215 195 180 171 154 160 165 193 78 340 275
/,
0
Fig. 5. The carbon loss (pg h-‘) for two representative branches is plotted over a period of 16 days through the cycle of temperature changes shown. After a temperature change, consecutive measurements showed similar values indicating that the branches had equilibrated to the temperature conditions pertaining.
Variance ratio F-value
I 5
IO
15
20
Temperature
I 25
(“Cl
Fig. 6. Typical relationships between temperature and rate of carbon loss (ngg-’ h-’ for two branches. (a) 0.398 g crne3; (b) 0.224 g cm-3 (curves fitted by eye).
(from replicate experiments) several branches
d.f.
/:A
Comparison of slopes F-value
d.f.
of C loss on temperature Comparison of elevhtions F-value
for
d.f.
1.140
6.5
0.234
1,lI
1.260
I,12
1.156
6,5
0.032
I,11
5.474*
1,12
1.005 4.569
4,5 6,5
0.058 1.427
1,9 1.11
0.299 13.717**
1,lO I,12
3.886 1.912 1.911 3.953 0.513 4.519
5,4 6,5 5.5 6.5 5,5 5.5
I.152 0.532 I.826 0.004 0.234 1.071
1.9 1,ll I,10 1,ll 1,lO 1.10
1.260 1.944 0.267 0.826 0.534 0.013
I,10 1,12 I,11 I,12 1.11 I.11
* = significant at 95%; ** = significant at 99.9% level; other values not significantly Branches 1, 2 and 4 had different moisture contents for the 2 runs.
different
LYNNE BODDY
508
;I/.fl;
Fig. 8. Joint effect of temperature and moisture on carbon loss (pg gg’ hh’ represented as a three dimensional surface for the 0.3-0.4gcmRD class constructed from temperature curves of different branches held at constant moisture
50
100 150 % Moisture content
200
Fig. 7. Typical relationships between moisture content and rate of carbon loss (pgg-‘hh’) for (a) 0.348gcm-j; (b) 0.251 gcme3; (c) 0.286gcm-‘. (0) 15°C; (0) 5°C (curves fitted by eye). data from agar plates can however, only provide a rough guide to activity in wood. Increase in respration rate with increase in moisture content occurs as a result of water becoming available not only to micro-organisms in small voids but also to those in increasingly larger voids throughout the wood. The lowering of the rate of CO,
Table 4. Regression Branch NO. I
I I 2 3 4 5 4 6 6 3 7 4 5 6 7 8 5 3 7 8
contents.
evolution at high moisture levels, at 15°C is probably due to a decrease in respiration caused by a lowering of diffusion of 0, to the organisms (or CO, from them) due to water filling the voids. Although no gases other than CO, were detected at these high moisture levels, it is possible that anaerobic conditions occurred. Products of anaerobic respiration are not necessarily volatile and would then not be recovered by the methods used here. It is unlikely that totally anaerobic conditions would prevail unless wood is totally immersed in water. However, it is likely that micro-sites often become anaerobic as it has been demonstrated that the centres of water-saturated soil crumbs become so if
equations of carbon loss on temperature for branches. in 0.3m0.4 g cm class, held at constant moisture content YC;moisture content 265 224 185 170 127 123 107 104 101 96 82 79 73 61 61 59 55 44 43 43 35
Regression equation y= r= .v = ” = b’= v= .1.= ?. = )‘= 1‘ = .I: = J’= v= :I, = 1’ = I’ = v= :r = J‘ = j‘ = 1’=
-x.5+7.04x -21.4+8.02x - 10.9 + 6.20 x -5.1 + 1.34x -2.7f 1.99.x -4.8 + 1.42.x - 2.0 + I .03 x -2.2+ 1.15.x -2.2+1.03x - 2.9 + 0.93 x -3.6+ 1.55x -5.o+ I.591 -4.3 + 1.49x -4.7 + 0.97 x -5.7 + 1.14.x -7.9+ 1.67.x ~ 3.0 + 0.93 x -3.7 + 0.63 .x -3.2 + 0.75 x - 3.3 + 0.82 .r -2.1 +0.39x
95:; confidence interval for slope 12.27 iO.68 k I .29 20.33 +0.44 i-o.39 kO.40 iO.28 kO.43 i0.38 io.17 +0.81 kO.62 + 0.33 kO.21 io.34 io.09 i-o.19 io.13 kO.25 +0.09
(x = (x = (I = (x = (x = (s = (I = (x = (x = (x = (x = (x = (x = (Y = (x = (1 = (1. = (u = (H = (.x = C.x=
10 20) 5-25) S-20) 5-20) 5 -20) llS20) 5 20) 5-25) S-20) 5-20) S-20) 5-20) 5-20) 5-25) 5-20) 10-20) 5-20) 5-25) 5-25) 5 25) S-25)
’
Temperature,
60
moisture
and wood
-
Temperature
t°C)
Fig. 9. Joint effect of temperature and moisture on carbon loss @g gg’ hh’) for three branches with very similar RD’s in the 0.2-0.3 g cm-’ class. Linear regression of temperature vs carbon loss is shown for each moisture content. (a) 0.270 g cme3; (b) 0.258 g cmm3; (c) 0.224 g cm-3.
radii are greater than about 3 mm (Greenwood, 1961; Greenwood and Goodman, 1967). It is interesting to note that respiration rate was never depressed by high moisture content at temperatures of 5°C. Flanagan and Veum (1974) studying leaf litter decomposition in Tundra, found that the moisture regimes which began to attenuate respiration rate differed for varying temperatures. The higher the temperature the lower the moisture content at which respiration rate began to attenuate. This is probably due to the fact that, for a constant moisture content, increase in temperature (up to the optimum) causes an increase in respiration rate with a consequent increase in rate of O2 uptake (and CO, production). Increased moisture content slows down the rate of diffusion and limits respiration; the effect becoming apparent sooner at higher temperatures where the demand for 0, is greater than at lower temperatures. Branch length was seen to influence CO, evolution in both the 1st experiment and in the short experiment to investigate the effect of length on respiration. In the 1st experiment CO, evolution was similar from 5 and 1Ocm portions of the same branch at low moistures. At higher moisture contents however, the 5cm portions consistently evolved more CO, than 10 cm portions in fairly well decayed (0.2-0.3 g cm - 3, branches. Also, in the experiment where branches were halved then quartered, an increase in CO, evolution with decrease in branch length was found in decayed branches although not in the less decayed (0.49 g cm ‘) branches. An obvious explanation of these observations is again based on diffusion. For a 2cm dia branch, dividing into two and four equal pieces would increase the total surface to volume ratio by 16.7 and 50% respectively. The end area to volume ratio would be increased by 100 and 300% respectively. Most gaseous exchange takes place their
decomposition
rate
509
through the cut ends of branches, although some will be via the rest of the surface area (Smith, 1964). If increased gaseous diffusion is the cause of the increase in CO, evolution with decrease in branch length, an increase of between 16.7-100% and 50.0-300% would be expected for the 5 and 2.5cm lengths respectively. Most of the results fell within these ranges making this a plausible hypothesis. Diffusion is interesting and important in connection with the size of branches from two main points of view, namely the effect that it has on the experimental results and the effect that it has on actual branches decomposing in the field. The experimental data obtained are not absolute values of respiration rate for any particular temperaturemoisture combination but must be considered with respect to length. Different values obtained from different lengths cannot be considered to be in anyway “incorrect” as they do reflect conditions which are likely to occur in the field. At optimum temperatures in particular, large branches or ones having a high water content allow slower rates of gaseous diffusion which may lead to a build up of CO2 within a branch. This has been shown in standing dead trunks where external access is limited (Thacker and Good, 1952). Thus length, and to a lesser extent diameter, may be as important as temperature, moisture and RD as regulators of wood decomposition. Rate of gaseous diffusion differs with tree species and Smith (1964) found that, out of 18 hardwoods tested, beech was the second most peremable (cm3 s-’ cm* atm cm - ‘). Thus gaseous diffusion and factors (such as temperature, moisture and length) which affect it may prove to be of even more significance, in other hardwoods, than found here for beech. In the past increased decomposition rates, with increased fragmentation of litter, have been explained by a larger surface area for microbial colonisation (e.g. van der Drift and Witkamp, 1959). This may well be the cause in general but the effect of increased gaseous diffusion should not be overlooked. In summary, it is evident that the abiotic variables, temperature, moisture, size and RD which were investigated here, influence the decomposition rate of wood. They exert their influence by having a direct effect upon the decomposer organisms and also indirectly by altering other abiotic conditions such as gaseous diffusion. Further, these abiotic variables often interact with each other. The laboratory experiments performed here in which only one factor was varied at a time has allowed some unravelling of these interactions and forms a good basis for further work. Future studies along these lines would be very profitable and allow more complete elucidation of the process of wood decomposition.
Acknowledgements-This research was supported by a grant from the Natural Environmental Research Council. I thank Professor Mike Swift for his guidance and encouragement, Mr Fred Laws for untiring technical assistance and Dr Bill Heal for keen interest in the project. I also thank Professor R. Bonnett for the use of the GLC facilities in the Department of Chemistry, Queen Mary College, London and to Mr David Ironmonger for his considerable hem with the GLC.
LYNNE BODDY
510 REFERENCES
Allison F. E. and Murphy R. M. (1962) Comparative rates of decomposition in soil of wood and bark particles of several hardwood species. Soil Science Society of America Proceedings 26, 463466. Boddy L. (1983a) Microclimate and moisture dynamics of wood decomposing in terrestrial ecosystems. Soil Biology & Biochemistry 15, 149-157. Boddy L. (1983b) The effect of temperature and water potential on the growth rate of wood-rotting basidiomycetes. Transactions of the British Mycological Society 80, 141-149. Bravery A. F. and Grant C. (1971) Preliminary investigations into the use of a thin strip tensile strength test for the rapid evaluation of wood preservatives against basidiomycete fungi. International Biodeterioration Bulletin 7, 169-173. Bunnell F. L., Tait D. E. N., Flanagan P. W. and Van Cleve K. (1977) Microbial respiration and substrate weight loss-I. A general model of the influences of abiotic variables. Soil Biology & Biochemistry 9, 33-40. Cartwright K. St. G. and Findlay W. P. K. (1934) Studies in the physiology of wood destroying fungi--II. Temperature and rate of growth. Annals qfBotany, London 48, 481495. Drift J. van der and Witkamp M. (1959) The significance of the breakdown of oak litter by Enoicyla pusilla Burm. Archives of the Netherlands Zoological Society 13, 486492. Findlay W. P. K. (1940) Studies of the physiology of wood destroying fungi-III. Progress of decay under natural and under controlled conditions. Annals af Botany. London 4, 701-712. Flanagan P. W. and Veum A. K. (1974) Relationships between respiration, weight loss, temperature and moisture in organic residues in tundra. In Soil Organisms and Decomposition in Tundra (A. J. Holding, 0. W. Heal, S. F. Maclean Jr, and P. W. Flanagan, Eds), pp. 2499277. Tundra Biome Steering Committee, Stockholm. Good H. M. and Darrah J. A. (1967) Rates of decay in
wood measured bv carbon dioxide oroduction. Annals ot ’ Applied Biology 54, 463472. Greenwood D. J. (1961) The effect of oxygen concentration on the decomposition of organic materials in soil. Plant & Soil 14, 360-376. Greenwood D. J. and Goodman D. (1967) Direct measurements of oxygen in soil aggregates and in columns of fine soil crumbs. Journal of Soil Science 18, 182-196. Hartley C. (1958) Evaluation of wood decay in experimental work. Forest Products Laboratory, Reoort. Madison. No. > 2119. Healey I. N. and Swift M. J. (1971) Aspects of the accumulation and decomposition of wood in the litter of a coppiced beech-oak woodland. In Proceedings af the 4th Colloquium of the Zoology Committee of the International Society afSoi1 Science, Dijon 1970, pp. 417-430. Institut National de la Recherche Agronomique, Paris. Henningson B. (1968) Ecology of decomposition in birch and asnen. In Biodeterioration of’Materials (A. H. Walters and J. J. Elphick, Eds), pp. 4088423. Elsevier, Amsterdam. Howard P. J. A. (1966) A method for the estimation of carbon dioxide evolved from the surface of soil in the field. Oikos 17, 261-271. Howard P. J. A. and Howard D. M. (1974) Microbial decomposition of tree and shrub leaf litter-I. Weight loss and chemical composition of decomposing litter. Oikos 25, 341-352. Ljungholm K., Noren B., Skiild R. and Wadso I. (1979) Use of micro-calorimetry for the characterization of microbial activity in soil. Oikos 33, 15-23. Putman R. J. (1976) The gas chromatograph as a respirometer. Journal of Applied Ecology 13, 445452. Smith D. N. (I 964) The permeability of wood. Proceedings of 5th World Forestry Congress 1546-1547. Smith R. S. (1975) Respiration methods to follow wood decay to evaluate toximetric potential of wood preservatives. Materials and Organism 10, 241-253. Thacker D. G. and Good H. M. (1952) The composition of air in trunks of sugar maple in relation to decay. Canadian Journal of Botany 30, 475485.
Copyright
8
0038-0717183 $3.00 + 0.00 1983 Pergamon Press Ltd
CARBON DIOXIDE RELEASE FROM DECOMPOSING WOOD: EFFECT OF WATER CONTENT AND TEMPERATURE LYNNE BODDY* Department of Plant Biology and Microbiology, Queen Mary College, University of London,
London El 4NS, U.K. (Accepted
10 February 1983)
Summary-The use of gas chromatography to measure carbon dioxide evolution from decomposing wood was evaluated and found useful to assess the rate of wood decomposition. An increase in temperature from S-25°C was accompanied by an increase in CO: evolution. Increase in water content was accompanied by an increase in CO, evolution. When higher temperatures accompanied high moisture contents CO, evolution often levelled off or decreased. This was attributed to a decline in the rate of 0, diffusion to concentrations insufficient to meet the demand. Increase in branch length showed a similar effect.
INTRODUCTION
The aim of this study was to develop a rapid, accurate technique to assess the rate of wood decomposition in the laboratory and to quantify the effects of abiotic variables on the process. Under aerobic conditions, decomposition of wood, by micro-organisms, releases COz, H,O and heat leaving chemically altered wood (of reduced weight and strength) and the tissues of the decomposer organism. Smith (1975) suggested the possibility of using these factors or the uptake of O2 for estimating decomposition rates. Weight loss and tensile strength loss have been used (e.g. Findlay, 1940; Hartley, 1958; Bravery and Grant, 1971) but both methods suffer from the drawback that they are destructive and that long incubation is required before statistically valuable data can be obtained. Results have been obtained more rapidly using small test specimens (Allison and Murphy, 1962) but it is unlikely that such samples will accurately reflect decomposition in larger pieces of wood and intact branches. The use of water production is theoretically possible but wetting and drying during the experiment would be a confounding factor and is not practical. Measurement of heat release has been used recently in the study of microbial activity in the soil (Ljungholm et al., 1979) but as it is still in its infancy better known techniques are probably more satisfactory. The remaining factors which alter as wood decomposition proceeds are gases. Both Oz consumption and CO2 evolution have been measured to determine decomposition rates of many types of plant litter in the laboratory (e.g. Howard, 1966; Howard and Howard, 1974; Bunnell et ai., 1977). For measurement of CO, evolution, gas chromatography (GLC) compares favourably with other respirometers and gas analysers (Putman, 1976). I have here in-
*Present address: Department of Microbioiogy, University College, Newport Road, Cardiff CF2 ITA, U.K.
vestigated the effects of abiotic variables on the rate of wood decomposition in terms of CO, evolution measured by GLC. MATERIALS AND METHODS
General methods
The rate of branch decomposition was measured by GLC determination of the quantity of CO, and other gases evolved from branch samples in small chambers under various moisture and temperature regimes. Beech (Fugus syluatica L.) branches were collected from the forest floor of Blean Woods NNR (NG ref. TRlll608) an abandoned beech and oak coppiced woodland. The size of the branches varied between 1.5-2.2cm dia and 2.515cm length. Following Healey and Swift (1971) relative density (RD; g cm-j) was used as a measure of the state of branch decay. The respiration vessels were 1Ocm length glass tubes, x 2.4 cm id., with rubber bungs placed in both ends. Each bung hetd a piece of glass tubing, 7.0 x 0.4cm i.d. Butyl rubber tubing (impervious to CO,) was attached to the glass tubes and the chambers were sealed by clamping the rubber tubing with Hoffman clips (Fig. 1). The seal was tested for leaks by introducing a known concentration of CO* into the chambers which was estimated by GLC 2 h later. The chambers containing the branches were immersed in a water bath for the duration of the experiment. The temperature of the water bath was maintained constant (+O,OS°C) by a Grant cooler unit model FC 15 and pump model FH 15 or by a Churchill chiller thermocirculator model OYCTCV. Before sampling the chambers were completely flushed with 300~01 of CO,-free air, obtained by passing the air through a conical Aask containing “Carbosorb” (non-deliquescent, self-indicating, 1.74.0 mm soda lime) for 5 min. The chambers were then sealed. After at least 2 h a 0.5 ml sample of the
502
LYNNEBWDY
(where Z = volume of air in the respiration chamber, i.e. vol. chamber-vol. wood). A final expression for calculating the decomposition rate of a branch was thus given by: 5.357YZ x 6O/(W, x t)pgCh Fig. I. Controlled environment respiration chamber. fc = flask eontainin~ “Carbsorb”. ap = air pump, rc = respiration chamber containing experimental branch, wb = water bath for maintaining desired temperature.
atmosphere was extracted from the chamber by inserting the needle of 1.0 ml syringe through the butyl
rubber tubing. The gas sample was then injected into the GLC. The chromatograph A glass column packed with l5~18O~rn beads of Poropak Q (2.0 m x 4 mm i.d.) was fitted to a Varian Aerograph model 90-P gas chromatograph. At the exit of the column was a thermal conductivity detector attached to a servoscribe potentiometric recorder operating at 2 mV. Column temperature was maintained at 75’C; injection port at room temperature and the detector at 80°C. A complete gas sample was injected into He as a carrier gas (He gives a sensitivity for COz 12 times greater than that obtained with N, as carrier gas). As N, was not used as the carrier gas, the 0, peak was not dissociated from N2 in the sample and could not be measured. Separation was only attempted for CO2 and other carbon containing gases. The chromatograph was calibrated using standard gases. The injections were replicated until two values within So/:, of each other were obtained (usually twice as reproducibility was excellent). Calculation qf results To obtain the relationship between peak height and CO, concentration, 0.5 ml samples of calibration gases of known concentration were used. The intercepts (a), regression co-efficients (8) and co-efficients of determination (R2) for linear regression on the calibration curves are given below: Attenuation
i
a = -0.940 Attenuation
fr = 27.922
R* = 0.998
p = 12.769
R* = 0.998
2
!Y.= 0.980
The proportion of CO1 in a sample could thus be determined and this was then transformed to 9 g C: 0.5 ml Yo/;,CO2 contains Y x 10 x 12/(22.4 x 2) pg C (where Y is the percentage of CO2 obtained from the calibration curve). At the end of each experiment, the volume and oven dry weight of the branches were obtained along with the volume of the chambers. The quantity of C evolved by a particular branch at a particular sampting could formula:
then
be calculated
from
2.679 Y x Z/O.5 peg C
the following
= 321.429YZl(W, x r)pgC
‘g-’ (O.D. wt wood) h-‘g-’
(O.D. wt wood)
(where W, = O.D. weight of the branch; t = time from sealing the chamber to sampling in min). Multiplying this value by 2.2 gives an estimate of actual weight loss (Good and Darrah, 1967). Sensitivity and accuracy Volumes of CO, as low as 3.5 ~1 in any injected sample were measured and by increasing the volume of the sample injected from 0.5 to 1.0 ml even lower concentrations of CO, in the respiration chamber could be measured. Prolonging the period of COz accumulation in the respiration chamber further increases the possibility of measuring low respiration rates. Ten replicate measurements of 0.5 ml samples of a standard gas usually resulted in a difference of no more than 6”/d between maximum and minimum estimates and often much less indicating the accuracy of the method.
The aims were to assess the feasibility of the method, to investigate the variation between replicate samples obtained from a particular branch under a particular set of conditions and to determine whether or not 10 or 5 cm lengths from the same branch showed the same response to various moisture contents. The material used gave data on the effect of state of decay and moisture content on the rate of wood decomposition. A 1.5cm length was cut from four branches (eliminating the ends which are often more decayed than the remainder). Each branch was then cut into a 10 and a 5 cm portion. These were soaked in water for 12 h, to attain a high moisture content, and then placed in the controlled environment chambers, which were of sufficient size to allow a surrounding air space of several miliimetres. The sealed chambers were submerged in a water bath, held at 15°C for 48 h to ensure branch temperatures had equilibrated with that of the surrounding water. CO, was accumulated for at least 2 h and then sampled. Two replicate measurements of CO* evolution were made for each branch at each moisture content over immediately consecutive periods on the same day. The branches were then weighed for subsequent calculations of moisture content. The branches were replaced in the chambers in the water bath. A stream of dry air was passed through the chambers so that CO, evolution could be monitored at different moisture contents during drying of the branches. The air flow was stopped 2 h before sealing the chambers to allow the branches to equilibrate to the conditions pertaining during the sampIing period. The procedure in subsequent experiments consisted of a series of sampling periods during which two replicates measurements of CO? concentration were
Temperature, moisture and wood decomposition
branch
by
rhrough
Sfop
a,r
flow
Flush
air
2 hr
pr,or
to
I
chamber
chamber
Cop-free
503
chamber
sealing
)
pass,“g
rate
or.
for Seal
5 mln
wth
chamber l-
Weigh
Sook
branch and chamber
branch
and
replace
I”
replace
I”
chamber
I
Fig. 2. Flow chart illustrating steps involved in the first experiment.
made at each moisture content, interspersed with periods of drying to attain different moisture contents (Fig. 2). When the branches reached low moisture contents (approx. 25%) they were re-wetted and the process repeated until approximately 20 data points had been obtained. effect
qf branch length on CO1 evolution
Seven branches, most with high initial moisture contents, were cut into 10 cm lengths and placed in the respiration chambers held at 15’C. After 24 h samples were taken following the usual procedure. The branches were then sawn in half and the two halves were assessed separately. These 5 cm portions were then later sawn in half and the resultant four 2.5 cm lengths were again assessed separately. The three samplings were made on consecutive days, two replicates being made on each day. High moisture contents were used because it was considered that any differences in CO2 evolution due to length, would be more pronounced under these conditions. (To reduce the chance of water loss between samplings, sawing always took place immediately prior to the sampling.)
CO, evolution from branches maintained moisture content for 10 days
at a constant
Three pieces of branch-wood, length 5 cm, 1.8-2.2 cm dia were wetted and allowed to equilibrate for 3 days in chambers at 15°C. CO, was sampled on 5 days over a IO-day period. The branches were weighed after each sampling to check moisture contents. Equilibration
after change in temperature
The purpose of this experiment was to determine the time required for COZ evolution to stabilize after a large change in temperature. Eights branches, length 5 cm, between 1.5-2.0 cm dia, covering a range of RDs were wetted and placed in the respiration chambers. Four of the branches had no bark and four had complete bark cover. The chambers were placed in a water bath at 20°C and allowed to equilibrate for 48 h before sampling. After sampling the temperature was altered to 5°C and further samples taken after 24 and 48 h. The temperature was again altered to 20°C and samples taken after 20, 44 and 140 h. Finally, the
LY NNE BODDY
504 temperature was altered after 24 and 48 h. Efect
qf‘ temperature
to 5,‘C and samples
and moisture
taken
on CO? ewlution
Twenty-five branches, 5 cm x 1.5-2.2 cm dia. covering a range of RDs, were used. The respiration rate of each branch was obtained at three different moisture contents at temperatures ranging from 5 to 25°C. Moisture contents near the maximum possible, near the minimum and one in between were used for each branch and were altered either by wetting or drying after each set of temperature readings had been completed. The order in which the moisture readings were made varied from branch to branch and the order for the temperature readings was randomised for each set of temperatures. although not for each branch. The 25 branches were respired in two batches, thus there resulted 6 orders for temperature sets. For each batch, one of the sets of temperature readings began at 20-C and on these occasions a reading at this temperature was also obtained after the temperature readings had been completed. This was a check that the community respiration rate had not changed over this period. At least 20 h was allowed for equilibration after altering moisture contents or temperature. A flow chart of the procedure outlined above is shown in Fig. 3. In addition, 10 branches, 5 cm x 1.52.0 cm dia were selected to cover a range of RD’s. They were respired at different temperatures in a sequence of 5, 10, 15 and 20°C; the sequence was then repeated. The branches were allowed at least 40 h to equilibrate after each temperature change. Seven of the branches were maintained at a constant moisture content for both runs the other three being allowed to dry slightly before the second run. Two replicate measurements were obtained at each temperature on each run. Twelve branches were selected and cut to length 5 cm. The methods employed were those used in the experiment 1, except that each branch was respired over a range of moisture contents at 15 C, then over a range at 10’ C. followed by a range of moisture contents at 5’C. RESULTS
Experiment
1
The relationships between moisture content (% O.D. wt) and respiration rate (pg g- ’ C h - ‘) for the 5 and 1Ocm sections of each of the four branches studied are shown in Fig. 4. Each sampling is represented by a data point and those obtained on the same day, i.e. under “identical” conditions, are joined by a line to show the range in readings. All branches showed similar trends between CO, evolution and moisture content. Respiration rate increased linearly with increasing moisture content, although in some branch sections the curve flattened in the higher moisture content region and in one instance (Fig. 4c) decreased markedly under very high moisture conditions. The rate of respiration of the two branch lengths were similar in the low moisture region, although in the high moisture region CO, evolution appeared to be greater from the 5 cm than from the 1Ocm lengths. In order to obtain sufficient data the branches were re-wetted at the end of a
drying-measuring sequence on two or more occasions. No anomolous data were detected resulting from the order in which the measurements were obtained. The I$%W qf‘hranch length on carbon dioxide evolution The results have been expressed as a percentage change of CO, evolved from the original 10 cm length (Table 1). On 5 (out of 7) occasions an increase of 40% or more was found on dividing the 10 cm lengths into two 5 cm lengths. The other two sets, which were both branches having low moisture contents, yielded slightly less CO, on division. On further division, all but one of the branches evolved considerably more CO, than the original 10 cm length, and all but one of these evolved more than the equivalent 5cm lengths. COz erolution fkom branches maintained moisture content ,for 10 days
at u constant
The amount of CO, evolved from branches I and 3 did not alter appreciably during the experiment, although the initial value obtained for branch 3 was somewhat higher than later ones (Table 2). The high RH in the chambers allowed fungal mycelium to grow out from the cut ends of branch 2. The rate of CO, evolution, from this branch was constant during the first 4 days. On day 10, however, a decrease occurred (being significantly (P < 0.01) lower than that obtained on day 2) which coincided with the apparent death of the surface mycelium. Equilibration
cfter chunge in temperature
All branches showed similar patterns of CO, evolution through the cycle of temperature changes although the values obtained varied depending upon the moisture content and state of decay (see Fig. 5). There was no difference in the amount of CO, evolved at 5 ‘C after 24 and 48 h. Likewise, there was no change at 20°C after equilibration for 20. 40 and 140 h. The &ct
@‘temperature
on CO, eaolution
The data sets for each individual branch were considered separately (Fig. 6). Linear and exponential regressions were fitted to the data, at one moisture level for each branch, and most of the relationships were equally well described by either model (L. Boddy, unpublished Ph.D. thesis, University of London, 1980). Linear regressions were also fitted to the data obtained from branches which underwent two temperature sequences. No significant differences were found between the residuals for the two regressions on any one branch and further comparisons were performed (Table 3). No significant differences (P < 0.05) between slopes were found for any of the branches, but two of them (Nos 2 and 4) showed differences in the elevation of the regression lines. These two branches were ones which had had their moisture contents lowered slightly between runs. Efect
qf’moisture
on CO, ecolution
In general the data obtained at 10 C did not cover a wide enough range of moisture contents to be
Temperature,
I
moisture
wth
and wood
decomposition
505
rate
Cop-free air. Seal chamber
After 2 hr (or more depending on rote of take
CO,+
1
samples 1
11
Alrer
temperature
and allow 20 for equllibratmn
Dry branch o,r through Stop prior
I
hr
by pass,“g chamber.
o,r flow 2 hr to sealmg chamber
Sook branch and replace II? chamber
Fig. 3. Flow chart
illustrating
in an experiment investigating the effect of temperature and moisture on CO2 evolution.
steps involved
conclusive about the relationship between moisture and rate of CO, evolution. There is some evidence that at 15°C respiration rate increases with increase in moisture content up to a maximum and then tails off slightly as the maximum moisture content is reached (Fig. 7). Respiration rate was not reduced at high moisture contents at 5°C. Joint ejfect evolution
of temperature
and moisture
on CO>
By combining data sets for individual branches a picture of the joint effect of temperature and moisture on the rate of wood decomposition (i.e. C loss) can
be obtained. However, percentage moisture content values in branches of different RD are not equivalent (Boddy, 1983a) thus only data sets of branches of similar decay state were considered together. The joint effect of temperature and moisture on C loss is shown for the 0.3-0.4 g cme3 class as a threedimensional representation constructed from temperature curves of different branches held at several constant moisture contents (Fig. 8). More measurements were made at lower moisture contents and the mean response of several branches has been plotted for clarity. Regression equations for all branches are given in Table 4. The general trend of increasing
LYNNE
(a)
0
(b)
0
8 8f
s&l 1 60
I
B -I
I
50
I
100 140 180 % Moisture
likely due to between branch variation. It is important to note that only a few branches were assessed under high moisture regimes, consequently the 3-D representation is probably not as accurate in these regions as for lower moisture contents. The general effect of branch length (because of its affect on aeration) and state of decay would be to raise or lower the response surface, but not necessarily in parallel over the whole surface or for different lengths and RDs. Three dimensional plots of the data in the 0.2P0.3 g cm 3 class showed several anomalies in the general trend of upward shift in the temperature response curve with increase in moisture content. This is illustrated in Fig. 9 in two dimensions for three branches having similar RDs. The implication of this is that in this class, consisting of more decayed branches, some factors other than temperature and moisture are important determinants of decomposition rate. The factors could be type of organism present, type of decay, etc. No 3-D plots were obtained in other classes as insufficient data were available. No gases other than CO? were detected in any of the experiments.
content
Fig. 4. Relationship between percentage moisture content and rate of carbon loss for the four branches of different RD in the first experiment. (a) 0.22 g cm ‘; (b) 0.20 g cm ‘: (c) 0.30gcm-3; (d) 0.43 gem-l. (0) 5cm length; (m) 1Ocm length. N.B. The lines joining data points serve to show the range between replicates under identical conditions (not error bars). Linear regression formulae for the linear por-
tion of the curves (asterisked formulae have non-linear tails): 10 cm 5cm IOcm 5cm IOcm 5 cm 10 cm 5cm
BODDY
y = -4.0 + 0.20 x y =0.7+0.12x y = -5.7+0.24x y = -9.5+0.35x y = -18.7+0.75x _y = -23.2 + 0.77 x y = -4.4 + 0.20 x y = -8.6+0.48x
(X (X (X (X
= GlOO)*; = O-250); =Cr150)*; =0-150)*; (X =O-140)*: (.x = O-140); (X = O-80); (x =&80).
decay rate with increase in temperature and moisture can be clearly seen. There is a very sudden drop in the response surface between about 175-195% moisture which may be of biological significance but is more
Table
I. Effect of branch
DISCUSSION
The methods used were eminently suitable allowing almost “instantaneous” assessment of the effect of different variables on the rate of wood decomposition in the laboratory. Credible and consistent results were obtained between replicates for several days and also after altering temperature or moisture and then returning to the original regime. Although separate experiments were performed to investigate the e,Xect of temperature and moisture on the rate of wood decomposition (as measured by C loss) it is clear that the two are intimately associated. Further, Boddy (1983a) has discussed the interactive effect of temperature on branch moisture content and vice versa. The effects of these two factors on decay rate are thus considered together. A clear response to temperature was observed in all branches irrespective of RD or moisture content and in all cases an increase in temperature between 5 and 25°C resulted in an increase in CO? evolution (Fig. 6). In general, for individual branches, an increase in moisture content resulted in an upward shift of the temperature-CO, evolution curve although there was
length
on respmtion “,,
Branch
I 2 3A 3B 3c 4A 4B
No.
R.D. (g cm 0.20 0.49 0.20 0.35 0.30 0.49 0.35
‘)
“,, H1O 320 88 260
I10 170 32 139
change
2 x 5 cm length 119.40 -26.75 71.47+* 40.98 40.30 ~ 13.5’) 53.23*
rate
in CO, evolutmn IO cm length
over
4 x 2.5 cm length I55.42* -6.13 Il2.97** 255.13* l72.59* 50.02* 27.65
Asterisks indicate significance of difference between the CO, evolution data (not “,) of the 10 cm vs 2 x 5 cm and IO cm YS 4 x 2.5 cm pieces compared using ‘/‘-test. * = significant dilierence at P < 0.05; ** = significant dilTerencr at P < 0.01: no astmsk indicates no significant difference at P < 0.05
Temperature, Table 2. Carbon
moisture
loss m branches
and wood decomposition
held at constant
moisture
content
507
rate over a IO-day period
Days Branch no. I
C loss (pcgg-‘h-l) SEM C loss (pgg-‘h-l) SEM C loss (pgg-‘h-l)
2 3
SEM SEM
=
standard
0
2
3
4
13.95 k 1.44 37.35 31.70
12.97 f 1.10 38.45 * 0.44 22.82
15% i 0.45 35.74 23.63
14.49
~
* 0.93
f
error of the mean of two replicates
1.52
2O’C
I
5T
2OT
I
I
4G4 ~ 20.03 -
Il.82 + 0.79 28.92 f 0.76 21.62
“i, moisture
content
0.30
239
0.31
168
0.62
248
+ 1.39
(s//n).
some evidence that at very high moisture contents the curve shifted downwards. Clearly, although the temperatures used were in the range usually encountered in the field (Boddy, 1983a), they were at or below the
I
Branch RD (gcm~‘)
10
5’C
optimum for decomposition as no decrease in respiration rate occurred at higher temperatures. For most European species of wood-rotting basidiomycetes fungi the optimum temperature for growth on agar is between 2430°C (Cartwright and Findlay, 1934; Henningson, 1968; Boddy, 1983b). These types of
I
25 -
ii
b
0:
2
4
6
0
IO
12
14
16
u
s f
30-
s
:’
20
-
/ :A
‘O-
Days
Table 3. Comparisons
Branch NO. I 2 3 4 5 6 7 8 9 IO
of 2 regressions
“ID moisture content 350 310 215 195 180 171 154 160 165 193 78 340 275
/,
0
Fig. 5. The carbon loss (pg h-‘) for two representative branches is plotted over a period of 16 days through the cycle of temperature changes shown. After a temperature change, consecutive measurements showed similar values indicating that the branches had equilibrated to the temperature conditions pertaining.
Variance ratio F-value
I 5
IO
15
20
Temperature
I 25
(“Cl
Fig. 6. Typical relationships between temperature and rate of carbon loss (ngg-’ h-’ for two branches. (a) 0.398 g crne3; (b) 0.224 g cm-3 (curves fitted by eye).
(from replicate experiments) several branches
d.f.
/:A
Comparison of slopes F-value
d.f.
of C loss on temperature Comparison of elevhtions F-value
for
d.f.
1.140
6.5
0.234
1,lI
1.260
I,12
1.156
6,5
0.032
I,11
5.474*
1,12
1.005 4.569
4,5 6,5
0.058 1.427
1,9 1.11
0.299 13.717**
1,lO I,12
3.886 1.912 1.911 3.953 0.513 4.519
5,4 6,5 5.5 6.5 5,5 5.5
I.152 0.532 I.826 0.004 0.234 1.071
1.9 1,ll I,10 1,ll 1,lO 1.10
1.260 1.944 0.267 0.826 0.534 0.013
I,10 1,12 I,11 I,12 1.11 I.11
* = significant at 95%; ** = significant at 99.9% level; other values not significantly Branches 1, 2 and 4 had different moisture contents for the 2 runs.
different
LYNNE BODDY
508
;I/.fl;
Fig. 8. Joint effect of temperature and moisture on carbon loss (pg gg’ hh’ represented as a three dimensional surface for the 0.3-0.4gcmRD class constructed from temperature curves of different branches held at constant moisture
50
100 150 % Moisture content
200
Fig. 7. Typical relationships between moisture content and rate of carbon loss (pgg-‘hh’) for (a) 0.348gcm-j; (b) 0.251 gcme3; (c) 0.286gcm-‘. (0) 15°C; (0) 5°C (curves fitted by eye). data from agar plates can however, only provide a rough guide to activity in wood. Increase in respration rate with increase in moisture content occurs as a result of water becoming available not only to micro-organisms in small voids but also to those in increasingly larger voids throughout the wood. The lowering of the rate of CO,
Table 4. Regression Branch NO. I
I I 2 3 4 5 4 6 6 3 7 4 5 6 7 8 5 3 7 8
contents.
evolution at high moisture levels, at 15°C is probably due to a decrease in respiration caused by a lowering of diffusion of 0, to the organisms (or CO, from them) due to water filling the voids. Although no gases other than CO, were detected at these high moisture levels, it is possible that anaerobic conditions occurred. Products of anaerobic respiration are not necessarily volatile and would then not be recovered by the methods used here. It is unlikely that totally anaerobic conditions would prevail unless wood is totally immersed in water. However, it is likely that micro-sites often become anaerobic as it has been demonstrated that the centres of water-saturated soil crumbs become so if
equations of carbon loss on temperature for branches. in 0.3m0.4 g cm class, held at constant moisture content YC;moisture content 265 224 185 170 127 123 107 104 101 96 82 79 73 61 61 59 55 44 43 43 35
Regression equation y= r= .v = ” = b’= v= .1.= ?. = )‘= 1‘ = .I: = J’= v= :I, = 1’ = I’ = v= :r = J‘ = j‘ = 1’=
-x.5+7.04x -21.4+8.02x - 10.9 + 6.20 x -5.1 + 1.34x -2.7f 1.99.x -4.8 + 1.42.x - 2.0 + I .03 x -2.2+ 1.15.x -2.2+1.03x - 2.9 + 0.93 x -3.6+ 1.55x -5.o+ I.591 -4.3 + 1.49x -4.7 + 0.97 x -5.7 + 1.14.x -7.9+ 1.67.x ~ 3.0 + 0.93 x -3.7 + 0.63 .x -3.2 + 0.75 x - 3.3 + 0.82 .r -2.1 +0.39x
95:; confidence interval for slope 12.27 iO.68 k I .29 20.33 +0.44 i-o.39 kO.40 iO.28 kO.43 i0.38 io.17 +0.81 kO.62 + 0.33 kO.21 io.34 io.09 i-o.19 io.13 kO.25 +0.09
(x = (x = (I = (x = (x = (s = (I = (x = (x = (x = (x = (x = (x = (Y = (x = (1 = (1. = (u = (H = (.x = C.x=
10 20) 5-25) S-20) 5-20) 5 -20) llS20) 5 20) 5-25) S-20) 5-20) S-20) 5-20) 5-20) 5-25) 5-20) 10-20) 5-20) 5-25) 5-25) 5 25) S-25)
’
Temperature,
60
moisture
and wood
-
Temperature
t°C)
Fig. 9. Joint effect of temperature and moisture on carbon loss @g gg’ hh’) for three branches with very similar RD’s in the 0.2-0.3 g cm-’ class. Linear regression of temperature vs carbon loss is shown for each moisture content. (a) 0.270 g cme3; (b) 0.258 g cmm3; (c) 0.224 g cm-3.
radii are greater than about 3 mm (Greenwood, 1961; Greenwood and Goodman, 1967). It is interesting to note that respiration rate was never depressed by high moisture content at temperatures of 5°C. Flanagan and Veum (1974) studying leaf litter decomposition in Tundra, found that the moisture regimes which began to attenuate respiration rate differed for varying temperatures. The higher the temperature the lower the moisture content at which respiration rate began to attenuate. This is probably due to the fact that, for a constant moisture content, increase in temperature (up to the optimum) causes an increase in respiration rate with a consequent increase in rate of O2 uptake (and CO, production). Increased moisture content slows down the rate of diffusion and limits respiration; the effect becoming apparent sooner at higher temperatures where the demand for 0, is greater than at lower temperatures. Branch length was seen to influence CO, evolution in both the 1st experiment and in the short experiment to investigate the effect of length on respiration. In the 1st experiment CO, evolution was similar from 5 and 1Ocm portions of the same branch at low moistures. At higher moisture contents however, the 5cm portions consistently evolved more CO, than 10 cm portions in fairly well decayed (0.2-0.3 g cm - 3, branches. Also, in the experiment where branches were halved then quartered, an increase in CO, evolution with decrease in branch length was found in decayed branches although not in the less decayed (0.49 g cm ‘) branches. An obvious explanation of these observations is again based on diffusion. For a 2cm dia branch, dividing into two and four equal pieces would increase the total surface to volume ratio by 16.7 and 50% respectively. The end area to volume ratio would be increased by 100 and 300% respectively. Most gaseous exchange takes place their
decomposition
rate
509
through the cut ends of branches, although some will be via the rest of the surface area (Smith, 1964). If increased gaseous diffusion is the cause of the increase in CO, evolution with decrease in branch length, an increase of between 16.7-100% and 50.0-300% would be expected for the 5 and 2.5cm lengths respectively. Most of the results fell within these ranges making this a plausible hypothesis. Diffusion is interesting and important in connection with the size of branches from two main points of view, namely the effect that it has on the experimental results and the effect that it has on actual branches decomposing in the field. The experimental data obtained are not absolute values of respiration rate for any particular temperaturemoisture combination but must be considered with respect to length. Different values obtained from different lengths cannot be considered to be in anyway “incorrect” as they do reflect conditions which are likely to occur in the field. At optimum temperatures in particular, large branches or ones having a high water content allow slower rates of gaseous diffusion which may lead to a build up of CO2 within a branch. This has been shown in standing dead trunks where external access is limited (Thacker and Good, 1952). Thus length, and to a lesser extent diameter, may be as important as temperature, moisture and RD as regulators of wood decomposition. Rate of gaseous diffusion differs with tree species and Smith (1964) found that, out of 18 hardwoods tested, beech was the second most peremable (cm3 s-’ cm* atm cm - ‘). Thus gaseous diffusion and factors (such as temperature, moisture and length) which affect it may prove to be of even more significance, in other hardwoods, than found here for beech. In the past increased decomposition rates, with increased fragmentation of litter, have been explained by a larger surface area for microbial colonisation (e.g. van der Drift and Witkamp, 1959). This may well be the cause in general but the effect of increased gaseous diffusion should not be overlooked. In summary, it is evident that the abiotic variables, temperature, moisture, size and RD which were investigated here, influence the decomposition rate of wood. They exert their influence by having a direct effect upon the decomposer organisms and also indirectly by altering other abiotic conditions such as gaseous diffusion. Further, these abiotic variables often interact with each other. The laboratory experiments performed here in which only one factor was varied at a time has allowed some unravelling of these interactions and forms a good basis for further work. Future studies along these lines would be very profitable and allow more complete elucidation of the process of wood decomposition.
Acknowledgements-This research was supported by a grant from the Natural Environmental Research Council. I thank Professor Mike Swift for his guidance and encouragement, Mr Fred Laws for untiring technical assistance and Dr Bill Heal for keen interest in the project. I also thank Professor R. Bonnett for the use of the GLC facilities in the Department of Chemistry, Queen Mary College, London and to Mr David Ironmonger for his considerable hem with the GLC.
LYNNE BODDY
510 REFERENCES
Allison F. E. and Murphy R. M. (1962) Comparative rates of decomposition in soil of wood and bark particles of several hardwood species. Soil Science Society of America Proceedings 26, 463466. Boddy L. (1983a) Microclimate and moisture dynamics of wood decomposing in terrestrial ecosystems. Soil Biology & Biochemistry 15, 149-157. Boddy L. (1983b) The effect of temperature and water potential on the growth rate of wood-rotting basidiomycetes. Transactions of the British Mycological Society 80, 141-149. Bravery A. F. and Grant C. (1971) Preliminary investigations into the use of a thin strip tensile strength test for the rapid evaluation of wood preservatives against basidiomycete fungi. International Biodeterioration Bulletin 7, 169-173. Bunnell F. L., Tait D. E. N., Flanagan P. W. and Van Cleve K. (1977) Microbial respiration and substrate weight loss-I. A general model of the influences of abiotic variables. Soil Biology & Biochemistry 9, 33-40. Cartwright K. St. G. and Findlay W. P. K. (1934) Studies in the physiology of wood destroying fungi--II. Temperature and rate of growth. Annals qfBotany, London 48, 481495. Drift J. van der and Witkamp M. (1959) The significance of the breakdown of oak litter by Enoicyla pusilla Burm. Archives of the Netherlands Zoological Society 13, 486492. Findlay W. P. K. (1940) Studies of the physiology of wood destroying fungi-III. Progress of decay under natural and under controlled conditions. Annals af Botany. London 4, 701-712. Flanagan P. W. and Veum A. K. (1974) Relationships between respiration, weight loss, temperature and moisture in organic residues in tundra. In Soil Organisms and Decomposition in Tundra (A. J. Holding, 0. W. Heal, S. F. Maclean Jr, and P. W. Flanagan, Eds), pp. 2499277. Tundra Biome Steering Committee, Stockholm. Good H. M. and Darrah J. A. (1967) Rates of decay in
wood measured bv carbon dioxide oroduction. Annals ot ’ Applied Biology 54, 463472. Greenwood D. J. (1961) The effect of oxygen concentration on the decomposition of organic materials in soil. Plant & Soil 14, 360-376. Greenwood D. J. and Goodman D. (1967) Direct measurements of oxygen in soil aggregates and in columns of fine soil crumbs. Journal of Soil Science 18, 182-196. Hartley C. (1958) Evaluation of wood decay in experimental work. Forest Products Laboratory, Reoort. Madison. No. > 2119. Healey I. N. and Swift M. J. (1971) Aspects of the accumulation and decomposition of wood in the litter of a coppiced beech-oak woodland. In Proceedings af the 4th Colloquium of the Zoology Committee of the International Society afSoi1 Science, Dijon 1970, pp. 417-430. Institut National de la Recherche Agronomique, Paris. Henningson B. (1968) Ecology of decomposition in birch and asnen. In Biodeterioration of’Materials (A. H. Walters and J. J. Elphick, Eds), pp. 4088423. Elsevier, Amsterdam. Howard P. J. A. (1966) A method for the estimation of carbon dioxide evolved from the surface of soil in the field. Oikos 17, 261-271. Howard P. J. A. and Howard D. M. (1974) Microbial decomposition of tree and shrub leaf litter-I. Weight loss and chemical composition of decomposing litter. Oikos 25, 341-352. Ljungholm K., Noren B., Skiild R. and Wadso I. (1979) Use of micro-calorimetry for the characterization of microbial activity in soil. Oikos 33, 15-23. Putman R. J. (1976) The gas chromatograph as a respirometer. Journal of Applied Ecology 13, 445452. Smith D. N. (I 964) The permeability of wood. Proceedings of 5th World Forestry Congress 1546-1547. Smith R. S. (1975) Respiration methods to follow wood decay to evaluate toximetric potential of wood preservatives. Materials and Organism 10, 241-253. Thacker D. G. and Good H. M. (1952) The composition of air in trunks of sugar maple in relation to decay. Canadian Journal of Botany 30, 475485.