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The redox characteristics of four peat profiles
Soif Bid. Biochem. Vol. 5, pp. 659-672. Pergamoo Press 1973. Printed in Great Britain
THE REDOX
CHARACTERISTICS
OF FOUR
PEAT PROFILES
C. URQUHART*and A. J. P. GORE Natural Environment Research Council, The Nature Conservancy, Grange-over-Sands,
Lancashire,
Merlewood Research Station, England
(Accepted22 December 1972) Summary-In several previous observations on peat soils, redox potentials appeared to vary with season and depth. To clarify this variation, a systematic study was made over 1 yr at four peat sites, including wet and dry areas at high and low altitude in northern England. The coefficients of linear regressions of redox potential on seasonal temperatures became progressively more negative with increasing depth at all four sites. These observations support an hypothesis that increased aeration accompanying increased temperature played a role at the surface, but was of less importance in the deeper layers, where the reducing conditions that resuh from microbial activity were dominant. By using orthogonal polynomials, it was possible to demonstrate the existence of redox minima in the profiles at all sites. There were rather more minima present using this method at the lowland sites that at the upland ones where trends of redox with depth were otherwise linear or non-significant. Although the mean redox potentials over whole profiles were lower in the wetter sites than the drier, the mean depths of the redox minima were similar. These mean depths were poorly defined however having standard deviations of 30-50 per cent of the means calculated from the year’s observations. Finally, it was shown that although the potentials were measured 60 s after closing the circuit, very similar conclusions would have been reached if the readings had been taken after 10s.
C~RA~ERISTI~ changes in redox potential with depth in the surface layers of soils have been reported by several workers (McKenzie and Erickson, 1954; Blagidov, Rabinovich and Sell’Bekman, 1957; Sell’-Bekman, Rabinovich and Kurovskaya, 1960; Malmer, 1962) indicating a layer of minimum values between 10 and 20 cm. Variations with season have been recorded by Blagidov et al. (1957) and by McKenzie, Whiteside and Erickson (1960). In a study of peat soils, Armstrong (1967) has shown that relations between redox potential and oxygen diffusion rates are complex. Below certain rates of oxygen diffusion, redox varied independently of oxygen diffusion. Armstrong and Boatman (1967) have illustrated that although the upper few centimetres of peat profiles are aerobic and have appreciable amounts of oxygen present, the intermediate and lower horizons, where oxygen is too low to detect, can still be characterized by redox potentials. In his recent review, Bohn (1971) has emphasized the fact, e.g. Jeffery (1961), that redox measurements are more stable and more reproducible in reducing soil systems. Redox is essentially qualitative but its relative behaviour can give an indication of microbiological activity and also of the depths to which the roots of higher plants can penetrate. Gore and Urquhart (1966) found that root penetration of Molinia caerulea and Eriophorum vaginaturn was related to the redox potentials developed in pots of waterlogged Sphagnum. * Present address: Yorkshire River Authority,
Olympia House, Gelderd Lane, Leeds 12. 659
660
C. URQUHART
AND A. J. P. GORE
McKenzie and Erickson (1954) pointed out that, because of the small area sampled in redox measurements in the field, variation between measurements made at the same time on adjacent profiles was sometimes large. Consequently, seasonal effects could possibly be obscured by this source of variation. Previous observations (Urquhart, 1969) made over a wide range of conditions had proved difficult to compare in an objective way. A systematic study was therefore made to test for seasonal change and to define the layer of minimum values for each of four peat sites by intensive measurements confined to small sampling areas. These four sites included waterlogged and drier sites on sea-level peat and on highlevel blanket peat in Northern England.
MATERIALS AND METHODS
Sites Four sites (Table 1) were selected on the basis of previous work. They represent a wide range of peat soils under differing conditions of aeration and seasonal temperature. TABLE1. NAMES,LOCATION AND Site
DESCRIPTION OF SITES
Grid. ref.
Bog type
Deer Dike Moss (D.D.M.)
SD335826
Striber’s Moss (S.M.) Bog End (B.E.) Burnt Hill (B.H.)
SD34681 1 NY765335 NY752328
Drained estuarine bog (at a point midway between drainage channels) Undrained estuarine bog (edge of pool) Undrained blanket bog (convex type) Undrained blanket bog (pool and hummock complex, edge of pool)
The first two sites are near sea-level on the North Lonsdale Mosses and the last two are at about 560 m on the Moor House National Nature Reserve. The two pairs of sites are separated by about 70 km and there is about 2 km between the individual sites at low and high altitudes respectively.
Deer Dike Moss was drained at least 70, and probably more than 100 yr ago. The channels are 11 m apart and about 1 m deep. The vegetation at the site selected is predominantly tall Calluna vulgaris with Erica tetralix, Eriophorum vaginatum and Eriophorum angustifolium commonly occurring, Narthecium ossifvagum, Andromeda polifolia and isolated patches of Sphagna are also present. Osvald (1949) who visited Deer Dike Moss in 1921 also recorded these species. The surface of the peat is generally flat and covered with a layer of dry heather litter. The drains appear to have been effective over the whole cross-section between channels, although very wet conditions occur beneath 15 cm at the point where the redox measurements were made, midway between channels. The total depth of peat at this point is 4.45 m. Striber’s Moss is in a neighbouring part of the Levens Estuary and forms part of the same series of Lonsdale Mosses as Deer Dike Moss. The site selected is thought to be near the one described by Gorham (1949) who reported on the gross redox profile down to the peat base (5.5 m). His species list includes abundant Sphagnum papillosum but is otherwise similar to the one given above for Deer Dike Moss. The whole of the “least disturbed part” of the moss is very wet and quaking. Pools are ptesent containing Sphagnum cuspidatum but there is no obvious hummock and hollow micro-topography. Measurements were made at the edge of such a pool.
REDOX
CHARACTERISTICS
OF PEAT
661
The peat at Moor House is typical of north Pennine blanket peat where the variable land form strongly influences the depth of peat. At Burnt Hill the peat is an intact remnant of the type which has formed on saddles and on large flat hill tops but much of which has now suffered erosion. Clymo (1965) described the site studied here in connection with his comparative work on decomposition. The microtopography consists of large irregular hummocks (2 m across) surrounded by elongated pools, both appear to be relatively long lived and are distinct from the idealized hummock and hollow topography of raised bogs described by Tansley (1949). The total peat depth is 3 m. Measurements were made at the pool edge in a Sphagnum papillosum and Sphagnum magellanicum carpet. In this position it is possible to take a series of profiles in similar conditions without re-use of the same sampling location and consequent risk of disturbance effects. Care was exercised, to avoid these errors at all sites, to keep the reference surface constant and at the same time to limit other variation due to any large change of location. At Bog End, redox profiles were sited within patches of Sphagnum acutifolium. The above requirements were less easy to meet at this site however and the measuring site was moved 7 m to a similar one in May 1965 to avoid damage by the previously sampling activity. Generally, this site is typical of peat on the flanks and summits of the small morainic hills at the eastern end of the Moor House Nature Reserve, having depth of l-2 m at the point of measurement. Calluna vulgaris and Eriophorum vaginatum are the co-dominant higher plants. This site and the one at Deer Dike Moss are close to the sites of field experiments and other study areas. (e.g. Gore, 1963; Gore and Olson, 1967). Measurement of Redox (E) and pH Redox (E) and pH measurements were made in the field using an Electronic Instruments Ltd., Richmond, Surrey, Model 30C portable pH meter. Platinum-in-glass electrodes were used to measure E and glass electrodes to measure pH. The circuit wascompletedfor both measurements with a saturated KCllcalomel half-cell and a saturated KCI-agar bridge. The platinum electrodes were constructed by fusing about 1 cm of O-508 mm diameter platinum wire to the end of a length of brass brazing rod. A piece of soft glass tubing was then sealed to leave about 0.5 cm of wire extending beyond the glass and enough brazing rod exposed at the open end to act as a terminal. Electrodes of several standard lengths were constructed in this way so that readings could be made down profiles of peat by insertion from the surface. This method involves least disturbance of the peat and essentially similar results had been obtained in previous observations by using electrodes inserted horizontally into a pit face. Readings were taken down to 30 cm depth at 2 cm intervals. Duplicate profiles a few centimetres apart were measured at the same locality, within a metre or so, on each sampling occasion. As explained above, it became necessary to move the sampling position at Bog End in May 1965. Electrodes were cleaned by a method essentially similar to that described by Bradfield, Batjer and Oskamp (1934). After an initial washing, electrodes were placed in boiling chromic acid for 3-4 min then cooled for 15 min. After this, they’were rinsed in distilled water, wiped and stood in industrial spirit. Cleaning was completed by heating the tip of the wire to redness in an alcohol flame. Small cracks or otherwise damaged electrodes were detected by measuring the redox potential in tap water before use (Quispel, 1947). As it had been found that re-use of an uncleaned electrode sometimes produced errors, each reading was made with a clean electrode. Drifting of potentials is a well-known phenomenon and an arbitrary, but standard, time of insertion of 60 s before taking readings
S.1i.H.
‘.?
bl
662
C. URQUHART
AND
A. J. P. GORE
was adopted. However, readings were also noted after 10 s to enable a test of the effect of drift to be made. Correction of readings to the hydrogen scale was made using the formula:
E,, = E + 241.5 - 0.76 (t -25)
mV
(1)
241.5 is the potential of a saturated KCl-calomel half-cell at 25°C (Glasstone 1951 p. 941). The average temperature of 10°C was the value adopted for t in formula (1). The use of an average temperature introduces a negligible error of about f8 mV between 0 and 20°C. Correction to pH 4.0 was made using formula (2)
Eh4 = E,, + 56.2 (pH -4) All references otherwise.
to redox potentials
in the Results section mean Eh4 unless explicitly
(2) stated
Temperature and water table A thermistor was mounted in a polythene tube so that the bead protruded a few millimetres from the end. In this way, it was possible to measure temperature at equivalent depths to those of redox. A mercury-in-glass thermometer was used to check the thermistor bridge down to 20 cm at depth intervals of 5 cm. Water table variation was assessed by reading the heights on a marked stake placed in the open water of the pools at the two waterlogged sites. No attempt was made to measure the water table at the drier sites. At Deer Dike Moss the water level in the nearby drainage channel was usually near the channel floor. At Bog End the colloidal nature of the peat makes the measurement of water table difficult.
Sulphide The silver-plate method described varied between 1 and 2 h.
by Urquhart
(1966) was used; the time of exposure
Statistical analyses Owing to snow and frozen ground it was not possible to obtain sets of readings for each month of the 12-month period selected. Eleven complete sets of data were obtained in which duplicate profiles were measured at each of four sites. Emphasis has been placed on these eleven sets in the statistical analyses. Orthogonal polynomials have been fitted to the data down to 30 cm depth with the object of assessing how far sites had characteristic features, such as well defined minima. Polynomial terms up to quintic were tested for the increase in significance resulting from the successive fittings of terms of higher and higher degree (Snedecor, 1956 p. 461). Estimates of the minimum redox potential and its depth in each profile were obtained by taking the highest significant degree and finding the minimum value, either by differentiation in the, case of quadratic and cubic equations or by inspection of curves plotted by a computer in the quartic and quintic equations. Where more than one minimum occurred in a fitted polynomial the depth and value of both were recorded. In addition to this method the lowest single observed value and its depth were recorded for each profile. Three separate sets were obtained in this way based on: (i) the minimum first occurring when going down the profile; (ii) the minimum having the lower redox value where more than one minimum occurred; and (iii) the lowest observed single value.
REDOX
CHA~~ERISTI~
OF PEAT
663
RESULTS
Mean values over year of observation
Average redox values down to 30 cm temperatures over the same depth range for the two waterlogged sites. pH values with negligible variations over time, and
500
z $
~,~
*.
are plotted in Fig. 1 for the four sites, Average are also shown. Water table levels are recorded at the four sites were similar (see Figs, 4 and 5), have therefore been omitted.
x,~%--++----------
--.__..
__“, :
300
f
x+
-+
100 ~~~
-
___---
+
+
+
+-
MAMJJASONDJFM
Fig. 1. Mean Eh4(60s)and mean temperature values for the upper 30 cm at each of four sites, during 1965-6. The water-table fluctuation at the two wet sites is also shown.
First ins~ction of the mean redox values indicated a cyclic variation at the two waterlogged sites with time. The results from the two drier sites were more erratic and showed no obvious variation with season. In a more detailed examination of these points, the regressions of redox on temperature were examined (Table 2). The main points demonstrated by Table 2 are, first, that significant linear regressions of redox on temperature exist and, second, that the coefficients of these regressions become progressively more negative with increasing depth. The coefficients of the regressions for the Bog End data are all positive, but they too show a reduction in value with increasing depth. TABLE~.LINEAR
Number of observations 15 11 15 13 * Si~ificant
REGR~ION~QUA~ONSOFRE~XON~MPERAT~E
Site 2-10 D.D.M. B.E. S.M. B.H.
mV=4661.3t mV = 359 + 15.7 t* mV = 322 - 4.0 t mV = 245 - 0.3 t
at 0.01 probability or less.
("C)
Depth (cm) 12-20 mV=407-3.6t mV=3OOf9.9t mV = 182 - 5.3 t* mV = 187 - 50t*
22-30 mV mV mV mV
= = = =
451 291 210 238
-t -
9.4 8.3 84 7.4
t* t t* t*
664
C. URQUHART AND A. J. P. GORE
Fewer observations (11) were made at Bog End, so a comparison was made of regressions based on observations taken on equivalent dates at the other sites. The resulting equations are shown in Table 3. However, with the exception of the 12-20 cm layer at Burnt Hill the coefficients were more negative in Table 3 than in Table 2. This apparent anomaly is due to the strong influence on the regressions of the high values obtained at the first observation (March 1965, see Fig. 1) in the reduced sets of 11 pairs. This influence is reduced by the presence of the more variable groups of observations also obtained under conditions of low temperature in the winter and spring 1965/66 cited in Table 2. Clearly, the better distribution of seasonal temperatures represented in Table 2 is to be regarded as more satisfactory. Table 3 gives useful confirmation but emphasizes the deficiencies due to the missing winter time values in the Bog End results. TABLE 3. LINEAR REGRESSION EQUATIONSBASEDON OBSERVA~ONSEQUIVALENTTO THOSEMADE AT BOG END Depth (cm) Number of observations 11 11 11
Site
2-10
D.D.M.
mV = 492 - 2.9 t mV = 337 - 5.0 t mV=244-0.2t
SM. B.H.
12-20
22-30
mV = 451 - 6.9 tt mV --z 205 - 7.1 tt mV = 186 - 5.0 t*
mV = 520 mV = 231 mV = 262 -
14.3 tt 10.0 tt 9.9 tt
* Significant at between 0.05 and 0.01 probability. t Significant at 0.01 probability or less.
In Table 4, the correlation coefficients of the relationships between Eh4 (60 s) and temperature and E (60 s) and temperature are compared for the different depths at the different sites. The effect of making corrections for pH was only very small, and although the correlation was improved by the correction at D.D.M., S.M. and B.H., at B.E. the uncorrected values gave the better correlation. TABLE 4. CORRELATION COEFFICIENTS (r) DERIVEDFROM RELATIONSBETWEENTEMPERATURE(“C) AND AND E RESPECTIVELY
Eh4
Depth (cm) Number of observations
Site
2-10 E h4
15 11 15 13
D.D.M. B.E. S.M. B.H.
-0.10 0.57t -0.32 -0.03
12-20 E -0.07 0.58t -0.33 -0.03
E h4 -0.30 040 -0.54t -0.517
22-30 E -0.25 0.47* -0.52t -@48*
E h‘s -0.69t 0.36 -0.59t -0.56t
E -0.65t 0.40 -0.57t -0.52t
* Significant at between 0.05 and 0.01 probability. t Significant at 0.01 probability or less.
Projiles A more detailed examination of the individual site results was now carried out. Table 5 illustrates the number of times the Ist-5th order polynomial terms were significant at each
REDOX
CHARACTERISTICS
OF PEAT
665
site out of a possible 22 times for 11 sets. For example, eight replicates, of 22 at D.D.M. were fitted by a significant quintic term, one replicate by a significant quartic term, eight replicates by a significant cubic term and so on. TABLE
5. NUMBERS
OF POLYNOMIALS SIGNIFICANT AT THE OF PROBABILITYORLESS
0.05
LEVEL
Order of Polynomial Site
1
2
3
D.D.M. B.E.
1 I
:.::
:
2 3 6 8
: 8 2
4
5
1
8 2 3 3
4 0
At Deer Dike Moss a high proportion of replicates (19) had a significant term above linear and therefore had identifiable minima. At the upland drier site (Bog End), there were only 12 in this category, with seven replicates for which only the linear term was significant. The lowland waterlogged site (Striber’s Moss) also had a high number (19) of profiles with significant minima. The upland waterlogged site (Burnt Hill) had 13 profiles with significant minima. Figure 2 shows representative cases of redox profiles with the associated significant highest order curve. Examination of photocopies of the silver plates indicated that the two wet sites invariably developed characteristic heavy deposits of sulphide in a period of exposure, about 1 hr, whereas the drier sites were variable in this respect. At Bog End, a lighter, but clearly detectable deposit of sulphide was formed in a similar period. The Deer Redox,inV
lO-
x 20-
30
Fig. 2. Representative profiles of observed values & (60 s) with the highest significant order polynomial fitted in each case. Deer Dike Moss, August; Bog End, August; Striber’s Moss, May and Burnt Hill, June.
666
C. URQUHART
AND
A. J. P. GORE
Dike Moss plates had traces of deposit similar to those already reported by Urquhart (1966). Figure 3 illustrates the silver plate results corresponding to the redox profiles of Fig. 2. Although the silver plate measurements were made at the same time as the redox profiles, they were separated by several metres of horizontal distance and would not be expected to show any close agreement. Correspondence between the low redox values developed near the surface and the heavy deposits of sulphide is clearly shown in the measurements made at the waterlogged sites. The photocopy of the silver-plate from Burnt Hill shows the sloping peat surface at the point where the hummock met the pool. The redox probe was normally inserted near this point. The existence of well established redox minima at Deer Dike Moss was not associated with corresponding regular deposits of sulphide, at least not after 2 h exposure. However much longer periods of exposure produced heavy deposits of sulphide below 10 cm at all sites, including Deer Dike Moss. Table 6 shows the set of minimum redox values and their depths derived from fitted polynomials. Where quartic or quintic terms were significant, the minimum occurring nearest the surface in the profile was used in Table 6. In Table 7 the means, standard deviations and numbers of observations of the values given in Table 6 are presented together with two further sets i.e. those based on fitted polynomials using secondary minima occurring lower in the profile if the redox values of these were least, and those based on the least observed single redox value in each profile. Inspection of the redox values for SM. and B.H. sites in Table 6 shows the low values occurring in the summer months, confirming that the minima behave in a similar way to the average redox values at similar depths (Table 2). However, there was no corresponding regular variation in the depths at which the minima occurred. In Table 7 the differences caused by including either the upper or lower minima are small, usually only three or four values were affected. The means based on least single values were in most cases more variable than those based on fitted polynomials, and as might be expected, the redox values were all much lower. The mean depth of the minima at Deer Dike Moss was much less variable using Table 6 data and nearer the surface by all three estimates than the corresponding values at Striber’s Moss. Minima at B.E. and B.H. occurled at about the same depth. Although Bog End data were often best described by fitting linear regressions, this does not mean that minima were absent. In fact, in many profiles there were single values with a low redox potential at the lo-15 cm level. The fact that only a single low value was recorded was probably related to the 2 cm spacing of the redox probe. The curve fitting procedure would only be relatively little influenced by a single low value, or even a pair of low values, lying outside the generally downward linear trend. The equation for this trend at Bog End, calculated from the mean linear characteristics of 22 values was: mV = 510.2 - 6.98 (cm depth). The &ect
of the correction for pH and of duration of electrode insertion
To examine the effect of these factors, sets of data uncorrected for pH and sets obtained after 10 s were tested for significant polynomial terms. The first of two replicates was selected in each case. The orders of significant terms were the same as those found for the data corrected to pH 4.0 after 60 s insertion (Eh4). The highest order cases which were significant have been plotted for comparison. Correspondence between the curves to three sets of data from four sites is close and is illustrated in Fig. 4. The 200 mV difference between the E and the Eh4 values is of course mainly attributable to the constant correction for the calomel
3 ,’
C3D.M.
B.E.
10, IO
20,
i 20
‘! E ,o
Fig. 3. Photocopies of sulphide deposited on silver plates inserted at time of observations recorded in Fig. 2. At Deer Dike Moss the plate was left in the peat for 2 h: at Bog End for 1 h 45 min: at Striber’s Moss for 1 h and at Burnt Hill for 1 h 5 min.
SBB
f.p. 6661
(L) (U) (L) (U) (L) (U)
10 Feb 3 Feb 10 Mar 3 Mar 7 Apr 31 Mar
341 316
366
361
365 333
“it:: +310 248 520 242 250 290 360 326 325
456 1390 350 +330
N.S. N.S. N.S. N.S. 13 N.S. 12 12
18 13 12 12 12 11 11 21 7 11 8 14 17 9 10 15 20 N.S. 9
N.S.
Depth
3 3
3
3
287 340 -
350
:
1 2
-300 280 480 t470 -
225 362 334
382 403
&
Min.
: 2 5 3 5 5 4 :
2 5 5 5 5
polynomial
of
Order
6
N.S.
Missing Missing Missing Missing 21 18
18 N.S. Missing Missing Missing Missing
d
i4: 17 -
8 12 N.S. 11 23
Depth
B.E.,
1
2 4
1 1 4
78 97
82 50
t:;
100 136 101 96 155 +2so
+1z:
+:z
102
-
t: 70 38 90
: 39
3 2 3 21 4 1 1 1 :
223 228
Eh4
Min.
3
4
polynomial
of
Order
8 11 9
g
9 15 14
:I: 23 13 N.S. 14 18 15 14 6
:: 20 22 17 16 8 -
11
N.S.
Depth
S.M.,
: 4
: 2 2
4
: 3 5
3
:: 3
: 1 4
; 3
:
3
2 2
polynomial
of
Order
83
+120
t160 160 135
151
tloo
108
-
94 75
65 70
188 150 103 69 -
Eh4
Min.
Missing Missing 8 18 16 N.S. N.S. 4
17 12 17 18 N.S. 19 19 N.S. 21 19 24 N.S. 12 7 Missing Missing N.S. 17
Depth
B.H.
5
:
4
2
1 3 1 5
:
i:
;
; 2 -
2
polynomial
of
Order
* The dates shown as(L) refer to observations made at Deer Dike Moss and Striber’s Moss and those shown as(U) to observations made at Bog End and Burnt Hill. + Indicates polynomials which have a secondary minimum having a lower value than the one given.
(L)
(L) (U)
16 Dee 23 Dee 1966 13 Jan
(L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L)
E h4
Observation
196.5 11 Mar 18 Mar 8 Apr 1.5 Apr 6 May 13 May 3 Jun 10 Jun 1 Jul 6 Jul 29 Jul 6 Ang 26 Aug 2 Sep 23 Sep 30 Sep 21 Ott 28 Ott 18 Nov
Min.
Date of
D.D.M.,
TABLE 6.MINIMUM REDOX POTENTIALS,THEIRDEPTHAND THE ORDEROF POLYNOMIALFROMWHICH THEY WERE DERIVED
Least observed sinde values
Fitted polynomial (least values)
L.
Fitted polynomial kxwermost values) \
Method
Mean S.D. N Mean SD. N
N
Mean S.D.
Parameter
244 91
326 14
343 64
Min. Eh4
17.7 8.5
1.58 7.1
12.7 3.7
Depth of Min.
D.D.M.
REDOX
30
21
21
No. of obs.
TABLE7, MEAN MINIMUM
233 94
336 69
351 85
Min. Eh‘s
B.E.
18.0 6.5
16.7 6.1
15.5 6.7
DEPTHS
22
12
12
No. of obs.
AND THEIR
Depth of Min.
POTENTIALS
45 47
87 57
96 61
E h4
Min.
USING
21.3 65
16.9 5.3
14.8 4.9
Depth of Min.
30
27
27
No. of obs.
METHODS
S.M.
THREE
49 46
112 38
114 39
Min. E h4
17.6 7.0
18.4 4.0
15.5 5.4
Depth of Min.
B.H.
26
16
16
No. of obs.
REDOX
CHARACTERISTICS
669
OF PEAT
PH
Redox, mV
536
Fig. 4. A comparison of Eh4 (60 s) with I?,,_,(10 s) and E(60 s) profiles at the four sites in June; pH and temperature profiles observed at the same time are also shown.
Redox,
d
Fig. 5. A comparison
Ehd
-E
200 0
mV
400 200
Temp,%
PH 35
1 I
45
,
0
I
5
10
1 ,
of redox profiles at Striber’s Moss in January with associated pH and temperature measurements. Symbols as in Fig. 4.
670
C. URQUHART
AND
A. J. P. GORE
electrode. The corresponding pH and temperature profiles are also shown in Fig. 4. The observations illustrated in this Figure were made in June. There was no complete set of winter values for all four sites, but the available results for Striber’s Moss for January (Fig. 5) showed a similar close correspondence between the three forms of data. It is appreciated that here the range of observed pH values is small and for a wider range of values a correction for pH would be expected to have a useful effect (Heintze, 1934). On the other hand, the time of insertion is an arbitrary one and if 10 s provides an essentially similar result to 60 s, the time saved on large numbers of measurements may be an important consideration. DISCUSSION
Variation in redox potentials is large within all four sites but it has been possible to show significant underlying trends in both season and depth. Regressions of redox against seasonal temperature were positive or non-significant in the surface layers but with the exception of Bog End, were significantly negative lower in the profiles. The observations from Bog End are subject to some doubt, firstly because the site of measurement was transferred several metres in May 1965 and secondly because no winter-time values were obtained at the new site. Therefore the positive regressions at Bog End may be misleading although they did behave consistently with profiles at the other sites becoming more negative (less positive) with increasing depth. From the water-table fluctuation, it is likely that some drying out occurred with the increase in temperature in the spring of 1965 so that the redox/temperature regressions seem to reflect the result of two aspects of increasing temperatures which have opposing effects, atmospheric aeration and anaerobic microbiological activity. In the lower layers of the profile, atmospheric aeration has less influence, and increasing temperature results in decreasing redox potentials. The seasonal variation observed by McKenzie and Erickson (1954), by Sell’-Bekman et al. (1960) and by McKenzie et al. (1960) on predominantly mineral soils were also thought to be compounded effects of differences in soil moisture and temperature. Leaf fall may also have played a part in some cases. Two observations particularly relevant to the present work were made by McKenzie et al. (1960); first, they found increasing redox potentials with increasing resistance using nylon blocks, and second, that the most negative potentials occurred on a poorly drained site when the temperatures were high and the soil saturated. The present study amplifies these observations and suggests they can occur not only in different soils but also in the same soil at different depths. Because they represent mixed events the regression coefficients and constants themselves have no biologically meaningful dimensions. It is the trends and their relationships between horizons which are useful in helping to indicate the processes present. It is worth stressing that these trends were by no means obvious from an inspection of the profiles of individual redox values plotted against depth. As far as the profiles are concerned there were features in common to the upland sites and to the lowland sites as distinct for example, from redox values common to the wet sites compared with the drier sites. At the two lowland sites there were more minima detectable using the polynomial fitting method. The upland sites had a greater number of linear trends of redox with depth or were not significant. The depths of minimum redox potentials were similar at Moor House but tended to be greater at Striber’s Moss than at Deer Dike Moss. These differences were not significant but may be associated with effects of draining if it is assumed that Deer Dike Moss was similar to Striber’s Moss before draining. It is of general interest that Gorham’s (1949)
REDOX CHARACTERISTICS
OF PEAT
671
measurements of redox at Striber’s Moss suggested that the lowest values in the whole profile, 5.5 m deep, also occurred in the upper 50 cm. Although the minimum redox potentials varied with season in a similar way to the average potentials for the corresponding depth, there was no correlation between the depths at which the minima occurred and the minima values themselves. Dr. Vera Collins has been interested in the connection between these observations on redox minima and the microbial activity and has kindly measured the numbers of sulphate reducing bacteria in profiles at Bog End and Striber’s Moss. She found the greatest number of these organisms in the IO-20 cm horizon in both cases. Clymo (1965) found that the rate of breakdown of Sphagnum in these horizons was intermediate between the high rate of the surface horizons and the relatively slow rates of greater depth. are especially grateful to Miss JOYCECHARNOCKwho was concerned with the data preparation and analysis and to Mrs GILLIANHOWSONwho helped with much of the field work. Dr VERA COLLINShas been responsible for detailed analyses of sulphate reducing bacteria in the Striber’s Moss and the Bog End profiles and it is a pleasure to acknowledge her active participation. Miss LINDA GOLDSMITH adapted an existing computer programme for calculating the orthogonal polynomials; to her, to Mr DAVID LINDLEYand Miss PAMELALATTERwe also express our thanks. Mr P. RHODESmade many of the electrodes, constructed a field kit box and helped in many practical ways. Acknowledgements-We
REFERENCES ARMSTRONG W. (1967) The relation between “oxidation-reduction’ potentials and oxygen diffusion levels in some waterlogged organic soils. J. Soil 5% 18, 27-34. ARMSTRONGW. and BOATMAND. J. (1967) Some field observations relating the growth of bog plants to conditions of soil aeration. J. Ecui. 55, 101-110. BLAGIDOVN. L., RABINOVICHV. A. and SELL’-BEKMAN I. YA. (1957) Characteristics of the alterations of the oxidation potential along the profile of some soils of the Leningrad region. Pochvouedenie 6, 81-85. BOHN H. L. (1971) Redox potentials Soil Sci. 112, 39-45. BRADFIELDR., BATJERL. P. and OSKAMPJ. (1934) Soils in relation to fruit growing in New York. IV. The significance of the oxidation-reduction potential in evaluating soils for orchard purposes. Bull. Cornell Univ. agric. Exp. Stn. 592, l--29. CLYMOR. S. (1965) Experiments on the breakdown of Sphagnum in two bogs J. Eeol. 53,747-758. GLAZ.~TONE S. (1951) Textbook of Physical Chemistry, Macmillan, London. GORE A. J. P. (1963) Factors limiting plant growth on high-level blanket peat III. An analysis of growth of MoZinia caerulea (L.) Moench in the second year. J. Ecol. 51, 481-491. GORE A. J. P. and OLSONJ. S. (1967) Preliminary models for the accumulation of organic matter in an Eriophorum/Calluna ecosystem. Aquilo, Ser. Botanica 6, 297-3 13. GORE A. J. P. and URQUHARTC. (1966) The effects of waterlogging on the growth of Molinia caerulea and Eriophorum vaginafum. J. Eeol. 54, 617-634. GORHAME. (1949) Some chemical aspects of a peat profile. J. Erol. 37,24-27. HEINTZE S. G. (1934) The use of the glass electrode in soil reaction and oxidation reduction potential measurements. J. Agric. Sci. 24, 28-37. JEFF~RYJ. W. 0. (1961) Measuring the state of reduction of a waterlogged soil. J. Soil Sci. 12, 317-325. MALMERN. (1962) Studies on mire vegetation in the Archaean area of Southwestern Giitaland (South Sweden). 1. Vegetation and habitat conditions on the Akhult mire, Opera Botanica 7, l-309. MCKENZIE L. J. and ERICKSONA. E. (1954) The use of redox potentials in studies of soil genesis. Proc. Soil Sci. Sot.
Am. 18, 481-485.
MCKENZIEL. J., WHITESIDEE. P. and ERICKSONA. E. (1960) Oxidation-reduction studies on the mechanism of B horizon formation in podzols. Proc. Soil Sri. Sot. Am. 24, 300-305. OSVALDH. (1949) Notes on the vegetation of British and Irish mosses. Acta phyfogeogr. Suet. 26, l-62. QUISPEL A. (1947) Measurement of the oxidation-reduction potentials of normal and inundated soils. Soil Sci. 63, 265-75.
SELL’-BBKMANI. YA, RABINOVICHV. A. and KUROVSKAYA0. V. (1960) Oxidation-reduction potential profile curves in relation to the conditions of soil formation. Pochuocedenie, 6, 630-633. SNEDECORG. W. (1956) Statistical ~et~u.is Applied to Experimenfs in Agriculture and Biology. 5th Ed. Iowa State University Press. TANSLEYA. G. (1949) The British Islands and their Vegetation Cambridge.
672
C. URQUHART
URQUHART C. (1966) An improved
AND A. J. P. GORE
method of demonstrating the distribution of sulphide in peat soils. Nature, Lond. 211, 550. URQUHARTC. (1969) The e&&s of waterlogging on the growth and mineral nutrition of some moorlandplant species with special reference to the soil redox ~otentjaf. M.Sc. Thesis, University of Newcastle-uponTyne.
THE REDOX
CHARACTERISTICS
OF FOUR
PEAT PROFILES
C. URQUHART*and A. J. P. GORE Natural Environment Research Council, The Nature Conservancy, Grange-over-Sands,
Lancashire,
Merlewood Research Station, England
(Accepted22 December 1972) Summary-In several previous observations on peat soils, redox potentials appeared to vary with season and depth. To clarify this variation, a systematic study was made over 1 yr at four peat sites, including wet and dry areas at high and low altitude in northern England. The coefficients of linear regressions of redox potential on seasonal temperatures became progressively more negative with increasing depth at all four sites. These observations support an hypothesis that increased aeration accompanying increased temperature played a role at the surface, but was of less importance in the deeper layers, where the reducing conditions that resuh from microbial activity were dominant. By using orthogonal polynomials, it was possible to demonstrate the existence of redox minima in the profiles at all sites. There were rather more minima present using this method at the lowland sites that at the upland ones where trends of redox with depth were otherwise linear or non-significant. Although the mean redox potentials over whole profiles were lower in the wetter sites than the drier, the mean depths of the redox minima were similar. These mean depths were poorly defined however having standard deviations of 30-50 per cent of the means calculated from the year’s observations. Finally, it was shown that although the potentials were measured 60 s after closing the circuit, very similar conclusions would have been reached if the readings had been taken after 10s.
C~RA~ERISTI~ changes in redox potential with depth in the surface layers of soils have been reported by several workers (McKenzie and Erickson, 1954; Blagidov, Rabinovich and Sell’Bekman, 1957; Sell’-Bekman, Rabinovich and Kurovskaya, 1960; Malmer, 1962) indicating a layer of minimum values between 10 and 20 cm. Variations with season have been recorded by Blagidov et al. (1957) and by McKenzie, Whiteside and Erickson (1960). In a study of peat soils, Armstrong (1967) has shown that relations between redox potential and oxygen diffusion rates are complex. Below certain rates of oxygen diffusion, redox varied independently of oxygen diffusion. Armstrong and Boatman (1967) have illustrated that although the upper few centimetres of peat profiles are aerobic and have appreciable amounts of oxygen present, the intermediate and lower horizons, where oxygen is too low to detect, can still be characterized by redox potentials. In his recent review, Bohn (1971) has emphasized the fact, e.g. Jeffery (1961), that redox measurements are more stable and more reproducible in reducing soil systems. Redox is essentially qualitative but its relative behaviour can give an indication of microbiological activity and also of the depths to which the roots of higher plants can penetrate. Gore and Urquhart (1966) found that root penetration of Molinia caerulea and Eriophorum vaginaturn was related to the redox potentials developed in pots of waterlogged Sphagnum. * Present address: Yorkshire River Authority,
Olympia House, Gelderd Lane, Leeds 12. 659
660
C. URQUHART
AND A. J. P. GORE
McKenzie and Erickson (1954) pointed out that, because of the small area sampled in redox measurements in the field, variation between measurements made at the same time on adjacent profiles was sometimes large. Consequently, seasonal effects could possibly be obscured by this source of variation. Previous observations (Urquhart, 1969) made over a wide range of conditions had proved difficult to compare in an objective way. A systematic study was therefore made to test for seasonal change and to define the layer of minimum values for each of four peat sites by intensive measurements confined to small sampling areas. These four sites included waterlogged and drier sites on sea-level peat and on highlevel blanket peat in Northern England.
MATERIALS AND METHODS
Sites Four sites (Table 1) were selected on the basis of previous work. They represent a wide range of peat soils under differing conditions of aeration and seasonal temperature. TABLE1. NAMES,LOCATION AND Site
DESCRIPTION OF SITES
Grid. ref.
Bog type
Deer Dike Moss (D.D.M.)
SD335826
Striber’s Moss (S.M.) Bog End (B.E.) Burnt Hill (B.H.)
SD34681 1 NY765335 NY752328
Drained estuarine bog (at a point midway between drainage channels) Undrained estuarine bog (edge of pool) Undrained blanket bog (convex type) Undrained blanket bog (pool and hummock complex, edge of pool)
The first two sites are near sea-level on the North Lonsdale Mosses and the last two are at about 560 m on the Moor House National Nature Reserve. The two pairs of sites are separated by about 70 km and there is about 2 km between the individual sites at low and high altitudes respectively.
Deer Dike Moss was drained at least 70, and probably more than 100 yr ago. The channels are 11 m apart and about 1 m deep. The vegetation at the site selected is predominantly tall Calluna vulgaris with Erica tetralix, Eriophorum vaginatum and Eriophorum angustifolium commonly occurring, Narthecium ossifvagum, Andromeda polifolia and isolated patches of Sphagna are also present. Osvald (1949) who visited Deer Dike Moss in 1921 also recorded these species. The surface of the peat is generally flat and covered with a layer of dry heather litter. The drains appear to have been effective over the whole cross-section between channels, although very wet conditions occur beneath 15 cm at the point where the redox measurements were made, midway between channels. The total depth of peat at this point is 4.45 m. Striber’s Moss is in a neighbouring part of the Levens Estuary and forms part of the same series of Lonsdale Mosses as Deer Dike Moss. The site selected is thought to be near the one described by Gorham (1949) who reported on the gross redox profile down to the peat base (5.5 m). His species list includes abundant Sphagnum papillosum but is otherwise similar to the one given above for Deer Dike Moss. The whole of the “least disturbed part” of the moss is very wet and quaking. Pools are ptesent containing Sphagnum cuspidatum but there is no obvious hummock and hollow micro-topography. Measurements were made at the edge of such a pool.
REDOX
CHARACTERISTICS
OF PEAT
661
The peat at Moor House is typical of north Pennine blanket peat where the variable land form strongly influences the depth of peat. At Burnt Hill the peat is an intact remnant of the type which has formed on saddles and on large flat hill tops but much of which has now suffered erosion. Clymo (1965) described the site studied here in connection with his comparative work on decomposition. The microtopography consists of large irregular hummocks (2 m across) surrounded by elongated pools, both appear to be relatively long lived and are distinct from the idealized hummock and hollow topography of raised bogs described by Tansley (1949). The total peat depth is 3 m. Measurements were made at the pool edge in a Sphagnum papillosum and Sphagnum magellanicum carpet. In this position it is possible to take a series of profiles in similar conditions without re-use of the same sampling location and consequent risk of disturbance effects. Care was exercised, to avoid these errors at all sites, to keep the reference surface constant and at the same time to limit other variation due to any large change of location. At Bog End, redox profiles were sited within patches of Sphagnum acutifolium. The above requirements were less easy to meet at this site however and the measuring site was moved 7 m to a similar one in May 1965 to avoid damage by the previously sampling activity. Generally, this site is typical of peat on the flanks and summits of the small morainic hills at the eastern end of the Moor House Nature Reserve, having depth of l-2 m at the point of measurement. Calluna vulgaris and Eriophorum vaginatum are the co-dominant higher plants. This site and the one at Deer Dike Moss are close to the sites of field experiments and other study areas. (e.g. Gore, 1963; Gore and Olson, 1967). Measurement of Redox (E) and pH Redox (E) and pH measurements were made in the field using an Electronic Instruments Ltd., Richmond, Surrey, Model 30C portable pH meter. Platinum-in-glass electrodes were used to measure E and glass electrodes to measure pH. The circuit wascompletedfor both measurements with a saturated KCllcalomel half-cell and a saturated KCI-agar bridge. The platinum electrodes were constructed by fusing about 1 cm of O-508 mm diameter platinum wire to the end of a length of brass brazing rod. A piece of soft glass tubing was then sealed to leave about 0.5 cm of wire extending beyond the glass and enough brazing rod exposed at the open end to act as a terminal. Electrodes of several standard lengths were constructed in this way so that readings could be made down profiles of peat by insertion from the surface. This method involves least disturbance of the peat and essentially similar results had been obtained in previous observations by using electrodes inserted horizontally into a pit face. Readings were taken down to 30 cm depth at 2 cm intervals. Duplicate profiles a few centimetres apart were measured at the same locality, within a metre or so, on each sampling occasion. As explained above, it became necessary to move the sampling position at Bog End in May 1965. Electrodes were cleaned by a method essentially similar to that described by Bradfield, Batjer and Oskamp (1934). After an initial washing, electrodes were placed in boiling chromic acid for 3-4 min then cooled for 15 min. After this, they’were rinsed in distilled water, wiped and stood in industrial spirit. Cleaning was completed by heating the tip of the wire to redness in an alcohol flame. Small cracks or otherwise damaged electrodes were detected by measuring the redox potential in tap water before use (Quispel, 1947). As it had been found that re-use of an uncleaned electrode sometimes produced errors, each reading was made with a clean electrode. Drifting of potentials is a well-known phenomenon and an arbitrary, but standard, time of insertion of 60 s before taking readings
S.1i.H.
‘.?
bl
662
C. URQUHART
AND
A. J. P. GORE
was adopted. However, readings were also noted after 10 s to enable a test of the effect of drift to be made. Correction of readings to the hydrogen scale was made using the formula:
E,, = E + 241.5 - 0.76 (t -25)
mV
(1)
241.5 is the potential of a saturated KCl-calomel half-cell at 25°C (Glasstone 1951 p. 941). The average temperature of 10°C was the value adopted for t in formula (1). The use of an average temperature introduces a negligible error of about f8 mV between 0 and 20°C. Correction to pH 4.0 was made using formula (2)
Eh4 = E,, + 56.2 (pH -4) All references otherwise.
to redox potentials
in the Results section mean Eh4 unless explicitly
(2) stated
Temperature and water table A thermistor was mounted in a polythene tube so that the bead protruded a few millimetres from the end. In this way, it was possible to measure temperature at equivalent depths to those of redox. A mercury-in-glass thermometer was used to check the thermistor bridge down to 20 cm at depth intervals of 5 cm. Water table variation was assessed by reading the heights on a marked stake placed in the open water of the pools at the two waterlogged sites. No attempt was made to measure the water table at the drier sites. At Deer Dike Moss the water level in the nearby drainage channel was usually near the channel floor. At Bog End the colloidal nature of the peat makes the measurement of water table difficult.
Sulphide The silver-plate method described varied between 1 and 2 h.
by Urquhart
(1966) was used; the time of exposure
Statistical analyses Owing to snow and frozen ground it was not possible to obtain sets of readings for each month of the 12-month period selected. Eleven complete sets of data were obtained in which duplicate profiles were measured at each of four sites. Emphasis has been placed on these eleven sets in the statistical analyses. Orthogonal polynomials have been fitted to the data down to 30 cm depth with the object of assessing how far sites had characteristic features, such as well defined minima. Polynomial terms up to quintic were tested for the increase in significance resulting from the successive fittings of terms of higher and higher degree (Snedecor, 1956 p. 461). Estimates of the minimum redox potential and its depth in each profile were obtained by taking the highest significant degree and finding the minimum value, either by differentiation in the, case of quadratic and cubic equations or by inspection of curves plotted by a computer in the quartic and quintic equations. Where more than one minimum occurred in a fitted polynomial the depth and value of both were recorded. In addition to this method the lowest single observed value and its depth were recorded for each profile. Three separate sets were obtained in this way based on: (i) the minimum first occurring when going down the profile; (ii) the minimum having the lower redox value where more than one minimum occurred; and (iii) the lowest observed single value.
REDOX
CHA~~ERISTI~
OF PEAT
663
RESULTS
Mean values over year of observation
Average redox values down to 30 cm temperatures over the same depth range for the two waterlogged sites. pH values with negligible variations over time, and
500
z $
~,~
*.
are plotted in Fig. 1 for the four sites, Average are also shown. Water table levels are recorded at the four sites were similar (see Figs, 4 and 5), have therefore been omitted.
x,~%--++----------
--.__..
__“, :
300
f
x+
-+
100 ~~~
-
___---
+
+
+
+-
MAMJJASONDJFM
Fig. 1. Mean Eh4(60s)and mean temperature values for the upper 30 cm at each of four sites, during 1965-6. The water-table fluctuation at the two wet sites is also shown.
First ins~ction of the mean redox values indicated a cyclic variation at the two waterlogged sites with time. The results from the two drier sites were more erratic and showed no obvious variation with season. In a more detailed examination of these points, the regressions of redox on temperature were examined (Table 2). The main points demonstrated by Table 2 are, first, that significant linear regressions of redox on temperature exist and, second, that the coefficients of these regressions become progressively more negative with increasing depth. The coefficients of the regressions for the Bog End data are all positive, but they too show a reduction in value with increasing depth. TABLE~.LINEAR
Number of observations 15 11 15 13 * Si~ificant
REGR~ION~QUA~ONSOFRE~XON~MPERAT~E
Site 2-10 D.D.M. B.E. S.M. B.H.
mV=4661.3t mV = 359 + 15.7 t* mV = 322 - 4.0 t mV = 245 - 0.3 t
at 0.01 probability or less.
("C)
Depth (cm) 12-20 mV=407-3.6t mV=3OOf9.9t mV = 182 - 5.3 t* mV = 187 - 50t*
22-30 mV mV mV mV
= = = =
451 291 210 238
-t -
9.4 8.3 84 7.4
t* t t* t*
664
C. URQUHART AND A. J. P. GORE
Fewer observations (11) were made at Bog End, so a comparison was made of regressions based on observations taken on equivalent dates at the other sites. The resulting equations are shown in Table 3. However, with the exception of the 12-20 cm layer at Burnt Hill the coefficients were more negative in Table 3 than in Table 2. This apparent anomaly is due to the strong influence on the regressions of the high values obtained at the first observation (March 1965, see Fig. 1) in the reduced sets of 11 pairs. This influence is reduced by the presence of the more variable groups of observations also obtained under conditions of low temperature in the winter and spring 1965/66 cited in Table 2. Clearly, the better distribution of seasonal temperatures represented in Table 2 is to be regarded as more satisfactory. Table 3 gives useful confirmation but emphasizes the deficiencies due to the missing winter time values in the Bog End results. TABLE 3. LINEAR REGRESSION EQUATIONSBASEDON OBSERVA~ONSEQUIVALENTTO THOSEMADE AT BOG END Depth (cm) Number of observations 11 11 11
Site
2-10
D.D.M.
mV = 492 - 2.9 t mV = 337 - 5.0 t mV=244-0.2t
SM. B.H.
12-20
22-30
mV = 451 - 6.9 tt mV --z 205 - 7.1 tt mV = 186 - 5.0 t*
mV = 520 mV = 231 mV = 262 -
14.3 tt 10.0 tt 9.9 tt
* Significant at between 0.05 and 0.01 probability. t Significant at 0.01 probability or less.
In Table 4, the correlation coefficients of the relationships between Eh4 (60 s) and temperature and E (60 s) and temperature are compared for the different depths at the different sites. The effect of making corrections for pH was only very small, and although the correlation was improved by the correction at D.D.M., S.M. and B.H., at B.E. the uncorrected values gave the better correlation. TABLE 4. CORRELATION COEFFICIENTS (r) DERIVEDFROM RELATIONSBETWEENTEMPERATURE(“C) AND AND E RESPECTIVELY
Eh4
Depth (cm) Number of observations
Site
2-10 E h4
15 11 15 13
D.D.M. B.E. S.M. B.H.
-0.10 0.57t -0.32 -0.03
12-20 E -0.07 0.58t -0.33 -0.03
E h4 -0.30 040 -0.54t -0.517
22-30 E -0.25 0.47* -0.52t -@48*
E h‘s -0.69t 0.36 -0.59t -0.56t
E -0.65t 0.40 -0.57t -0.52t
* Significant at between 0.05 and 0.01 probability. t Significant at 0.01 probability or less.
Projiles A more detailed examination of the individual site results was now carried out. Table 5 illustrates the number of times the Ist-5th order polynomial terms were significant at each
REDOX
CHARACTERISTICS
OF PEAT
665
site out of a possible 22 times for 11 sets. For example, eight replicates, of 22 at D.D.M. were fitted by a significant quintic term, one replicate by a significant quartic term, eight replicates by a significant cubic term and so on. TABLE
5. NUMBERS
OF POLYNOMIALS SIGNIFICANT AT THE OF PROBABILITYORLESS
0.05
LEVEL
Order of Polynomial Site
1
2
3
D.D.M. B.E.
1 I
:.::
:
2 3 6 8
: 8 2
4
5
1
8 2 3 3
4 0
At Deer Dike Moss a high proportion of replicates (19) had a significant term above linear and therefore had identifiable minima. At the upland drier site (Bog End), there were only 12 in this category, with seven replicates for which only the linear term was significant. The lowland waterlogged site (Striber’s Moss) also had a high number (19) of profiles with significant minima. The upland waterlogged site (Burnt Hill) had 13 profiles with significant minima. Figure 2 shows representative cases of redox profiles with the associated significant highest order curve. Examination of photocopies of the silver plates indicated that the two wet sites invariably developed characteristic heavy deposits of sulphide in a period of exposure, about 1 hr, whereas the drier sites were variable in this respect. At Bog End, a lighter, but clearly detectable deposit of sulphide was formed in a similar period. The Deer Redox,inV
lO-
x 20-
30
Fig. 2. Representative profiles of observed values & (60 s) with the highest significant order polynomial fitted in each case. Deer Dike Moss, August; Bog End, August; Striber’s Moss, May and Burnt Hill, June.
666
C. URQUHART
AND
A. J. P. GORE
Dike Moss plates had traces of deposit similar to those already reported by Urquhart (1966). Figure 3 illustrates the silver plate results corresponding to the redox profiles of Fig. 2. Although the silver plate measurements were made at the same time as the redox profiles, they were separated by several metres of horizontal distance and would not be expected to show any close agreement. Correspondence between the low redox values developed near the surface and the heavy deposits of sulphide is clearly shown in the measurements made at the waterlogged sites. The photocopy of the silver-plate from Burnt Hill shows the sloping peat surface at the point where the hummock met the pool. The redox probe was normally inserted near this point. The existence of well established redox minima at Deer Dike Moss was not associated with corresponding regular deposits of sulphide, at least not after 2 h exposure. However much longer periods of exposure produced heavy deposits of sulphide below 10 cm at all sites, including Deer Dike Moss. Table 6 shows the set of minimum redox values and their depths derived from fitted polynomials. Where quartic or quintic terms were significant, the minimum occurring nearest the surface in the profile was used in Table 6. In Table 7 the means, standard deviations and numbers of observations of the values given in Table 6 are presented together with two further sets i.e. those based on fitted polynomials using secondary minima occurring lower in the profile if the redox values of these were least, and those based on the least observed single redox value in each profile. Inspection of the redox values for SM. and B.H. sites in Table 6 shows the low values occurring in the summer months, confirming that the minima behave in a similar way to the average redox values at similar depths (Table 2). However, there was no corresponding regular variation in the depths at which the minima occurred. In Table 7 the differences caused by including either the upper or lower minima are small, usually only three or four values were affected. The means based on least single values were in most cases more variable than those based on fitted polynomials, and as might be expected, the redox values were all much lower. The mean depth of the minima at Deer Dike Moss was much less variable using Table 6 data and nearer the surface by all three estimates than the corresponding values at Striber’s Moss. Minima at B.E. and B.H. occurled at about the same depth. Although Bog End data were often best described by fitting linear regressions, this does not mean that minima were absent. In fact, in many profiles there were single values with a low redox potential at the lo-15 cm level. The fact that only a single low value was recorded was probably related to the 2 cm spacing of the redox probe. The curve fitting procedure would only be relatively little influenced by a single low value, or even a pair of low values, lying outside the generally downward linear trend. The equation for this trend at Bog End, calculated from the mean linear characteristics of 22 values was: mV = 510.2 - 6.98 (cm depth). The &ect
of the correction for pH and of duration of electrode insertion
To examine the effect of these factors, sets of data uncorrected for pH and sets obtained after 10 s were tested for significant polynomial terms. The first of two replicates was selected in each case. The orders of significant terms were the same as those found for the data corrected to pH 4.0 after 60 s insertion (Eh4). The highest order cases which were significant have been plotted for comparison. Correspondence between the curves to three sets of data from four sites is close and is illustrated in Fig. 4. The 200 mV difference between the E and the Eh4 values is of course mainly attributable to the constant correction for the calomel
3 ,’
C3D.M.
B.E.
10, IO
20,
i 20
‘! E ,o
Fig. 3. Photocopies of sulphide deposited on silver plates inserted at time of observations recorded in Fig. 2. At Deer Dike Moss the plate was left in the peat for 2 h: at Bog End for 1 h 45 min: at Striber’s Moss for 1 h and at Burnt Hill for 1 h 5 min.
SBB
f.p. 6661
(L) (U) (L) (U) (L) (U)
10 Feb 3 Feb 10 Mar 3 Mar 7 Apr 31 Mar
341 316
366
361
365 333
“it:: +310 248 520 242 250 290 360 326 325
456 1390 350 +330
N.S. N.S. N.S. N.S. 13 N.S. 12 12
18 13 12 12 12 11 11 21 7 11 8 14 17 9 10 15 20 N.S. 9
N.S.
Depth
3 3
3
3
287 340 -
350
:
1 2
-300 280 480 t470 -
225 362 334
382 403
&
Min.
: 2 5 3 5 5 4 :
2 5 5 5 5
polynomial
of
Order
6
N.S.
Missing Missing Missing Missing 21 18
18 N.S. Missing Missing Missing Missing
d
i4: 17 -
8 12 N.S. 11 23
Depth
B.E.,
1
2 4
1 1 4
78 97
82 50
t:;
100 136 101 96 155 +2so
+1z:
+:z
102
-
t: 70 38 90
: 39
3 2 3 21 4 1 1 1 :
223 228
Eh4
Min.
3
4
polynomial
of
Order
8 11 9
g
9 15 14
:I: 23 13 N.S. 14 18 15 14 6
:: 20 22 17 16 8 -
11
N.S.
Depth
S.M.,
: 4
: 2 2
4
: 3 5
3
:: 3
: 1 4
; 3
:
3
2 2
polynomial
of
Order
83
+120
t160 160 135
151
tloo
108
-
94 75
65 70
188 150 103 69 -
Eh4
Min.
Missing Missing 8 18 16 N.S. N.S. 4
17 12 17 18 N.S. 19 19 N.S. 21 19 24 N.S. 12 7 Missing Missing N.S. 17
Depth
B.H.
5
:
4
2
1 3 1 5
:
i:
;
; 2 -
2
polynomial
of
Order
* The dates shown as(L) refer to observations made at Deer Dike Moss and Striber’s Moss and those shown as(U) to observations made at Bog End and Burnt Hill. + Indicates polynomials which have a secondary minimum having a lower value than the one given.
(L)
(L) (U)
16 Dee 23 Dee 1966 13 Jan
(L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L) (U) (L)
E h4
Observation
196.5 11 Mar 18 Mar 8 Apr 1.5 Apr 6 May 13 May 3 Jun 10 Jun 1 Jul 6 Jul 29 Jul 6 Ang 26 Aug 2 Sep 23 Sep 30 Sep 21 Ott 28 Ott 18 Nov
Min.
Date of
D.D.M.,
TABLE 6.MINIMUM REDOX POTENTIALS,THEIRDEPTHAND THE ORDEROF POLYNOMIALFROMWHICH THEY WERE DERIVED
Least observed sinde values
Fitted polynomial (least values)
L.
Fitted polynomial kxwermost values) \
Method
Mean S.D. N Mean SD. N
N
Mean S.D.
Parameter
244 91
326 14
343 64
Min. Eh4
17.7 8.5
1.58 7.1
12.7 3.7
Depth of Min.
D.D.M.
REDOX
30
21
21
No. of obs.
TABLE7, MEAN MINIMUM
233 94
336 69
351 85
Min. Eh‘s
B.E.
18.0 6.5
16.7 6.1
15.5 6.7
DEPTHS
22
12
12
No. of obs.
AND THEIR
Depth of Min.
POTENTIALS
45 47
87 57
96 61
E h4
Min.
USING
21.3 65
16.9 5.3
14.8 4.9
Depth of Min.
30
27
27
No. of obs.
METHODS
S.M.
THREE
49 46
112 38
114 39
Min. E h4
17.6 7.0
18.4 4.0
15.5 5.4
Depth of Min.
B.H.
26
16
16
No. of obs.
REDOX
CHARACTERISTICS
669
OF PEAT
PH
Redox, mV
536
Fig. 4. A comparison of Eh4 (60 s) with I?,,_,(10 s) and E(60 s) profiles at the four sites in June; pH and temperature profiles observed at the same time are also shown.
Redox,
d
Fig. 5. A comparison
Ehd
-E
200 0
mV
400 200
Temp,%
PH 35
1 I
45
,
0
I
5
10
1 ,
of redox profiles at Striber’s Moss in January with associated pH and temperature measurements. Symbols as in Fig. 4.
670
C. URQUHART
AND
A. J. P. GORE
electrode. The corresponding pH and temperature profiles are also shown in Fig. 4. The observations illustrated in this Figure were made in June. There was no complete set of winter values for all four sites, but the available results for Striber’s Moss for January (Fig. 5) showed a similar close correspondence between the three forms of data. It is appreciated that here the range of observed pH values is small and for a wider range of values a correction for pH would be expected to have a useful effect (Heintze, 1934). On the other hand, the time of insertion is an arbitrary one and if 10 s provides an essentially similar result to 60 s, the time saved on large numbers of measurements may be an important consideration. DISCUSSION
Variation in redox potentials is large within all four sites but it has been possible to show significant underlying trends in both season and depth. Regressions of redox against seasonal temperature were positive or non-significant in the surface layers but with the exception of Bog End, were significantly negative lower in the profiles. The observations from Bog End are subject to some doubt, firstly because the site of measurement was transferred several metres in May 1965 and secondly because no winter-time values were obtained at the new site. Therefore the positive regressions at Bog End may be misleading although they did behave consistently with profiles at the other sites becoming more negative (less positive) with increasing depth. From the water-table fluctuation, it is likely that some drying out occurred with the increase in temperature in the spring of 1965 so that the redox/temperature regressions seem to reflect the result of two aspects of increasing temperatures which have opposing effects, atmospheric aeration and anaerobic microbiological activity. In the lower layers of the profile, atmospheric aeration has less influence, and increasing temperature results in decreasing redox potentials. The seasonal variation observed by McKenzie and Erickson (1954), by Sell’-Bekman et al. (1960) and by McKenzie et al. (1960) on predominantly mineral soils were also thought to be compounded effects of differences in soil moisture and temperature. Leaf fall may also have played a part in some cases. Two observations particularly relevant to the present work were made by McKenzie et al. (1960); first, they found increasing redox potentials with increasing resistance using nylon blocks, and second, that the most negative potentials occurred on a poorly drained site when the temperatures were high and the soil saturated. The present study amplifies these observations and suggests they can occur not only in different soils but also in the same soil at different depths. Because they represent mixed events the regression coefficients and constants themselves have no biologically meaningful dimensions. It is the trends and their relationships between horizons which are useful in helping to indicate the processes present. It is worth stressing that these trends were by no means obvious from an inspection of the profiles of individual redox values plotted against depth. As far as the profiles are concerned there were features in common to the upland sites and to the lowland sites as distinct for example, from redox values common to the wet sites compared with the drier sites. At the two lowland sites there were more minima detectable using the polynomial fitting method. The upland sites had a greater number of linear trends of redox with depth or were not significant. The depths of minimum redox potentials were similar at Moor House but tended to be greater at Striber’s Moss than at Deer Dike Moss. These differences were not significant but may be associated with effects of draining if it is assumed that Deer Dike Moss was similar to Striber’s Moss before draining. It is of general interest that Gorham’s (1949)
REDOX CHARACTERISTICS
OF PEAT
671
measurements of redox at Striber’s Moss suggested that the lowest values in the whole profile, 5.5 m deep, also occurred in the upper 50 cm. Although the minimum redox potentials varied with season in a similar way to the average potentials for the corresponding depth, there was no correlation between the depths at which the minima occurred and the minima values themselves. Dr. Vera Collins has been interested in the connection between these observations on redox minima and the microbial activity and has kindly measured the numbers of sulphate reducing bacteria in profiles at Bog End and Striber’s Moss. She found the greatest number of these organisms in the IO-20 cm horizon in both cases. Clymo (1965) found that the rate of breakdown of Sphagnum in these horizons was intermediate between the high rate of the surface horizons and the relatively slow rates of greater depth. are especially grateful to Miss JOYCECHARNOCKwho was concerned with the data preparation and analysis and to Mrs GILLIANHOWSONwho helped with much of the field work. Dr VERA COLLINShas been responsible for detailed analyses of sulphate reducing bacteria in the Striber’s Moss and the Bog End profiles and it is a pleasure to acknowledge her active participation. Miss LINDA GOLDSMITH adapted an existing computer programme for calculating the orthogonal polynomials; to her, to Mr DAVID LINDLEYand Miss PAMELALATTERwe also express our thanks. Mr P. RHODESmade many of the electrodes, constructed a field kit box and helped in many practical ways. Acknowledgements-We
REFERENCES ARMSTRONG W. (1967) The relation between “oxidation-reduction’ potentials and oxygen diffusion levels in some waterlogged organic soils. J. Soil 5% 18, 27-34. ARMSTRONGW. and BOATMAND. J. (1967) Some field observations relating the growth of bog plants to conditions of soil aeration. J. Ecui. 55, 101-110. BLAGIDOVN. L., RABINOVICHV. A. and SELL’-BEKMAN I. YA. (1957) Characteristics of the alterations of the oxidation potential along the profile of some soils of the Leningrad region. Pochvouedenie 6, 81-85. BOHN H. L. (1971) Redox potentials Soil Sci. 112, 39-45. BRADFIELDR., BATJERL. P. and OSKAMPJ. (1934) Soils in relation to fruit growing in New York. IV. The significance of the oxidation-reduction potential in evaluating soils for orchard purposes. Bull. Cornell Univ. agric. Exp. Stn. 592, l--29. CLYMOR. S. (1965) Experiments on the breakdown of Sphagnum in two bogs J. Eeol. 53,747-758. GLAZ.~TONE S. (1951) Textbook of Physical Chemistry, Macmillan, London. GORE A. J. P. (1963) Factors limiting plant growth on high-level blanket peat III. An analysis of growth of MoZinia caerulea (L.) Moench in the second year. J. Ecol. 51, 481-491. GORE A. J. P. and OLSONJ. S. (1967) Preliminary models for the accumulation of organic matter in an Eriophorum/Calluna ecosystem. Aquilo, Ser. Botanica 6, 297-3 13. GORE A. J. P. and URQUHARTC. (1966) The effects of waterlogging on the growth of Molinia caerulea and Eriophorum vaginafum. J. Eeol. 54, 617-634. GORHAME. (1949) Some chemical aspects of a peat profile. J. Erol. 37,24-27. HEINTZE S. G. (1934) The use of the glass electrode in soil reaction and oxidation reduction potential measurements. J. Agric. Sci. 24, 28-37. JEFF~RYJ. W. 0. (1961) Measuring the state of reduction of a waterlogged soil. J. Soil Sci. 12, 317-325. MALMERN. (1962) Studies on mire vegetation in the Archaean area of Southwestern Giitaland (South Sweden). 1. Vegetation and habitat conditions on the Akhult mire, Opera Botanica 7, l-309. MCKENZIE L. J. and ERICKSONA. E. (1954) The use of redox potentials in studies of soil genesis. Proc. Soil Sci. Sot.
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MCKENZIEL. J., WHITESIDEE. P. and ERICKSONA. E. (1960) Oxidation-reduction studies on the mechanism of B horizon formation in podzols. Proc. Soil Sri. Sot. Am. 24, 300-305. OSVALDH. (1949) Notes on the vegetation of British and Irish mosses. Acta phyfogeogr. Suet. 26, l-62. QUISPEL A. (1947) Measurement of the oxidation-reduction potentials of normal and inundated soils. Soil Sci. 63, 265-75.
SELL’-BBKMANI. YA, RABINOVICHV. A. and KUROVSKAYA0. V. (1960) Oxidation-reduction potential profile curves in relation to the conditions of soil formation. Pochuocedenie, 6, 630-633. SNEDECORG. W. (1956) Statistical ~et~u.is Applied to Experimenfs in Agriculture and Biology. 5th Ed. Iowa State University Press. TANSLEYA. G. (1949) The British Islands and their Vegetation Cambridge.
672
C. URQUHART
URQUHART C. (1966) An improved
AND A. J. P. GORE
method of demonstrating the distribution of sulphide in peat soils. Nature, Lond. 211, 550. URQUHARTC. (1969) The e&&s of waterlogging on the growth and mineral nutrition of some moorlandplant species with special reference to the soil redox ~otentjaf. M.Sc. Thesis, University of Newcastle-uponTyne.