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Microbial sulphur in some scottish soils
003843717i87 $3.00+ 0.00 Prrspmoa Journals Ltd
Soil Bio!. &c/rem. Vol. 19, No. 3. pp. 301-305. 1987 Printed in Great Britain
MICROBIAL SULPHUR IN SOME SCOTTISH SOILS S.
J.
CHAPMAN
Department of Microbiology, The Macauiay Institute for Soil Research, Craigiebuckbr, Aberdeen AB9 2QJ, Scotland (Accepted
20 i&sober
1986)
Sunnnary-Sulphur in the microbial biomass of nine soils from North East Scotland was estimated from the flush following chloroform fumigation. The sulphur was extracted from fumigated and non-fumigated soils with either 10 mt+rCaCl,. 16 mbt NaH,PO, or 100 mM NaHCOs. These three extractants gave similar biomass S values, but those obtained using CaCI, were the most precise. Biomass S, calculated assuming a ks of 0.35, comprised 0%2.6% of the total soil organic S and was strongly correlated with biomass C. The C:S ratios ranged from 31 to 70. In soils of very low sulphur status and known to respond to sulphur fertilizers, biomass S exceeded “plant-avaiIabie” S extracted with phosphate and may thus be an important pool of sulphur in these soils.
of organic sulphur to mineral forms in soil indicate that a relatively small fraction of the total soil S is involved at any one time. McLaren ef al. (1985) estimated the active pool in both unamended and glucose-amended soils to be about 3-6% of the total S and of a similar magnitude to the inorganic S pool. Following the application of “S-labeiled gypsum, Goh and Gregg (1982) found that 80-90% of the total soil S was not involved in cycling and part of the remaining S was inorganic sulphate. In three of the five sites examined, the quantity of organic S being cycled was 4-8% of the total S while in the other two sites large quantities of subsoil inorganic S interfered with the estimates. An important fraction of the soil organic S is microbial biomass S. Microbial biomass S in unamended soils accounted for about 2.3% of the total soil S (Saggar et al., 198la). &rick and Nakas (1984) reported similar values of 1.2 and 2.2% for two acid organic soils. Together these results suggest that microbial biomass S can form a significant proportion of the organic S pool which is involved in cycling and is potentially available to plants. A number of soils in Scotland have been shown to be low in S and to respond to S fertilizers (McLaren, 1975; Scott and Munro, 1979). In England and Wales some areas may show deficiencies when atmospheric S levels are low (Jones et al., 1972). Sulphur held in the biomass will assume greater importance in soils which are low in inorganic sulphate. My main aim was to measure biomass S in a range of soils, including some known to respond to S, and to compare biomass S concentrations with other soil s pools. Biomass S has been measured using the flush of extractable S immediately following chloroform fumigation in a way analogous to the m~sur~ment of biomass C (Jenkinson and Pow&on, 1976). Saggar ef Studies on the transformations
0 The Macaulay institute for Soil Research, 1987.
al. (198la) used IOrnM CaCI: and tOOmk( NaHCO, to extract the flush of S. while Strick and Nakas (1984) found these extractants unsatisfactory for their soils and used I6 rnM NaH2P0,. A subsidiary aim was to determine the most suitable ext~ctant for biomass S for some of the soils in North East Scotland. &lA+ERIAl.S AND WXHODS
Soils were collected fresh from the field and kept not more than 4days at 5-IO’C, except for soil I which was stored in bulk for 5 weeks. Before analysis, the mineral soils (arable, O-10 cm depth) were sieved to <2 mm and the organic soil (soil 6; F-H horizon of a Scats pine and Sitka spruce stand) to < 5 mm. Some relevant soil characteristics are given in Table I. Further details of some of the soil types are given by Scott and Anderson (1976) and the soil associations have been described by Glentworth and Muir (1963). Total soil organic S was measured by subtracting phosphate-extractable S (see below) from total soil S determined by the method of Butters and Chenery (1959) with the modification of Massoumi and Cornfield (1963). Total soil carbon and nitrogen were determined using a Carlo Erba model II06 elemental analyser. Biomass C was estimated using the chloroform fumigation technique (Jenkinson and Powlson, 1976) with the CO, released being measured by gas chromatography (Sparling, 1981). A conversion factor (kc) of 0.45 was adopted (Jen~nson and Ladd, 1981). Biomass S was estimated from the flush of S following chloroform fumigation using the procedure of Strick and Nakas (1984) on 20g (fresh wt) samples. Both control and fumigated soils were extracted with either 10 mM Cat&, 16 KtM NaH,PO, or 100 mM NaHCO, (I:5 soil:solution ratio) using a I h shaking period. The extracts were centrifuged and filtered through a 0.3 pm pore membrane filter. TotaI S in the extracts was measured by X-ray tl uorescence spectroscopy: to 80 ml of the extract 8 ml of I M NaHCO, was added to increase the bulk and the
301
302
S.
J. CHAPUAS whole then freeze-dried. Samples were prepared by compressing 0.4 g of freeze-dried extract, backed by boric acid, into pellets. Analyses were performed under vacuum in a Philips PW 1404/00 automated wavelength-dispersive spectrophotometer using a germanium crystal, flow-proportional detector and appropriate pulse-height discriminator settings. The S Kx intensity was measured for 40 s, corrected for background and converted to concentration of S by calibration with standards prepared in the same way as the samples using appropriate proportions of K,SO, in NaHCO,. The conversion factors (ks values) used to convert the S flush to biomass S were 0.35 with CaCI, and 0.41 with NaHCOJ extracts (Saggar et al., 1981a). Strick and Nakas (1984) fount ks values of 0.25 and 0.37 for two forest soils using NaH:PO,. As an extractant for S, NaHIPO, is inter mediate in strength between CaCI, and NaHCO, therefore the value of 0.37 was applied to NaH,PO, extracts. These values are approximate and furthel calibration would be desirable. All results were ex, pressed on an oven dry-weight basis. Extraction of the control soils and measurement o the total S in the extracts gave estimates of the following S pools. CaCI, extracts “free” (readil! soluble) sulphate (Barrow, 1961). NaH,PO, extract: both “free” and adsorbed sulphate with some lab& organic S (Ensminger, 1954). NaHCO, extract “free” plus adsorbed sulphate with greater amount of organic S (Tabatabai, 1982). Bartlett tests for homogeneity of variances wer carried out on the extractable S, biomass S ant biomass C data and gave non-significant results Thus variances were pooled to give a more concis presentation of results. Correlations among the vat-i ous soil properties were examined using linear re gression analysis. RESULTS AND DlSCUSSlON
For each of the soils, I6 mM NaH,PO, general1 extracted more sulphur than IO mM CaCI, (Table 2; The CaCI, extracts were completely clear while onl the NaH,PO, extract of soil 6 showed any appre ciable colour. The difference between the twl extractants then indicates mainly adsorbed sulphat (Tabatabai. 1982). More deeply coloured extract indicated that the weakly alkaline 100 mM NaHCC extracted some organic matter and that the ad ditional S extracted by this extractant was probabl organic. Fumigation increased the concentration of removed by each extractant, giving a two-fold in crease for CaCI, extractions but less for NaH,PC and NaHCO,. The S flushes for the three extractant were similar (data not shown) and, except for th NaHCO, extract of soil 4, not significantly differen This similarity suggests that NaH,PO, and NaHCC are not extracting much microbial biomass S fror unfumigated soil, i.e. the organic S in the NaHCC extract is not biomass S. As the NaH,PO, an NaHCO, flushes are the difference of two large numbers, compared to the CaCI, flush, the error involved are larger. The calculated biomass S value (Table 3) are correspondingly most precise when th CaCI, flush is used, the pooled variance bein significantly less than that for either the NaH,PO, c
303
Microbial sulphur in some Scottish soils Table 2. Concentrations of total sulphur cx~racted by IO my CaC&, 16mu. NaH$O, and 100 mb( NaHCO,. from nine Scottish soils. Mean of three replicates; Pooled SE (standard error) = 0.41 Soil
CaC&-cxtractable S (~8 8-l soil)
I 2 3 4 5 6 7 8 9
1.1’ 0.9 1.3 2.3’ 2.6 9.0 4.6’ 4.3 3.4
NaH,PO,-extfactablc (ae g-’ soil)
S
NaHCO,txtractablc @g g-’ soil)
S
6.6’ 3.7 7.3 19.6 54.6 45.5’ 26.5’ 26.2 44.5
1.0 2.2 3.1 4.7 12.2 12.8’ 13.1 19.0 26.5
‘SE = 0.50 (due to lost replicate).
NaHCO, data. Only the CaClz values are used in further consideration of biomass S. The biomass S varied from 0.9-2.6% of the total organic S (Table 1) and agrees closely with the reports of 2.3% (Saggar et al., 1981a). 1.2 and 2.2% (Strick and Nakas, 1984) and 0.5-X3% (Maynard et al., 1983). The microbial biomass C values (I’able 3) varied from 0.5 to 1.6% of the total C (Table I). These are low compared with the values normally reported for arable soils (e.g. l.7-2.0%; Jenkinson and Powlson, 1976) and may reflect the higher organic matter contents associated with a cooler climate. There was a significant correlation between biomass C and biomass S for the nine soils (Table 4) which is reflected in the narrow range of biomass C:S ratios (31-70; Table 3). These have large standard errors which are compounded from the variability in both biomass C and S determinations. Saggar et al. (198 I b) found biomass C:S ratios between 47 and I2 I for two incubated soils with and without sulphate or cellulose amendments. The mean C:S ratios for their unamended control incubated soils were 60 and 101. As they note, C:S ratios greater by a factor of 2 or 3 have been found for solution-grown microorganisms (Saggar et al., 198 I a) or from calculations based on structural composition (Coughenour et al., 1980). It should be noted that the biomass C:S ratios are dependent on the values used for kc and k,. The biomass C:S ratios did not correlate significantly with any other soil property but, on average, were 60% of the C:S ratios of the total soil organic matter indicating a higher concentration of S in the biomass than in the dead organic matter. The biomass C:S ratio was not significantly greater in the low S soils
(soils 14) and this suggests that the biomass was not S-limited in these soils. Regressions between the various soil properties showed that biomass S, biomass C, total soil C, CaCIz-extractable S and total soil organic S were significantly correlated with each other (Table 4). In contrast NaHCO,-extractable S and NaH,POdextractable S showed little correlation except with total soil organic S. The correlation between biomass S and CaClr-extractable S was unexpected since “free” sulphate. being mobile, would also be influenced by rainfall and leaching. The correlation might be due to the biomass S values being based on CaCI, extraction. However, similar comparisons of the biomass S values and extractable S levels using both NaH,PO, and NaHCO, showed no correlation (data not shown). On the basis of these correlations it is difficult to say whether “free” sulphate (CaCI,-extractable S) in these soils depends directly on the microbial biomass or indirectly on the biomass through their common relationship with the soil organic matter. Phosphate-extractable S is generally considered to be “plant-available” (Jones ef al., 1972; Scott, 1981) and soils having below 8pg S g-’ soil would be expected to give a crop response to S fertilization. It is usually measured on soil which has been air-dried so the values in Table 2, measured on fresh soil, might be depressed although these estimates include any traces of organic S, both carbon-bonded S and HI-reducible S. On soils l-4, crops have been shown to respond to S fertilizers (Scott et al., 1983, 1984; Millard er al., 1985). In these soils biomass S was similar to or greater than phosphate-extractable S. In
Table 3. Biomass S, dewmined using three different soil extractants. and biomass C in nine Scottish soils
Soil (exwactant) I
IO mM CaCI, I .7’
: 4 5 6 7 8 9
4.3 7.0 3.8’ 6.4 29.1 10.5’ 9.3 6.7 I.31
(SE)
Biomass C (fig g-’ soil)
Biomass S (pg g-’ soil) I6 mM NaH,PO, 4.51 2.8 5.5 3.9 1.7 29.9’ 7.6 7.1 ND I ss
IO0 mM NaHCO, 4.2’ 2.9 7.1 II.5 1.0 35.2’ 4.94 10.3 ND 3.07
Biomass C:S ratio IOmM CaCI,
%’ 282 395 II7 350 1724’ 435 409 468’ 12.9
‘SE P 1.55. ‘SE = 1.72. ‘SE - 1.73. ‘SE = 3.15. ‘SE = 15.8. 6sE = 14.1. ND = NOI determined.
58 63 57 31 55 59 41 44 70
Biomau S as % of Total Organic S IO my CaCI, 0.9 I.5 1.8 I.1 I.3 2.6 2.6 I.9 I.2
304
S. J. CH~PZMN Table 4. Correlation
w&cienls between microbial biomass S and
Biomass C Total c CaCI,-extractabk S Total organic S NaH,PO,-cxtractable S NaHCQ-cxtractable S
Biomass C’
Biomass C
0.99*** 0.9s*** 0.94*** 0.94”’ 0.60~ 0.39*
0.97”’ osW** 0.96*‘* 0.72= 0.47b
other soil parameters for the nine soils studied
Total C
CaClr extractable S
0.95*** 0.9P’ 0.10 0.64
0.9O*** 0.50
Total Organic S
NaH,PO,extractable S
0.87@.’ O.74b’
0.73’
0.62
Wing CaCI, derived values. bRegrcssionomitted the orgattic soil (soil 61 which was a significant ourtier. ***/J < 0.001; l*p < 0.01; *fJ < 0.05.
soils 5 and 7-9,.where no crop response to S has been shown, biomass S was smaller than phosphateextractable S. In soil 6, biomass S was greater than phosphate-extractable S, This is attributed to the much greater biomass content of the organic soil. In the very low S soils the biomass could be an important pool of S relative to phosphate-extractable SO,-S. The availability of S from the biomass S pool would depend on the rate of turnover of the biomass S. This is not well understood although throughout the growing season biomass C can vary by a factor of at least three (Lynch and Panting, 1980, 1982) while periods of soil drying can lead to a 30% loss in biomass (Bottner, 1985). In my study most of the soils were sampled in late autumn or winter when the biomass might be expected to be at its lowest. Those sampled in April and September (Soils I, 7 and 8) may be higher. During a period of decrease in biomass, biomass S might become available to plants or, conversely, during an increase in biomass. soil sulphate may be taken up and become unavailable to plants. This aspect requires further study. Acknow~ledgemennr-1
thank fluorescence measurements, Hepburn for the analysis for R. H. E. lnkson for statistical for technical assistance.
Dr D. C. Bain for the X-ray Messrs C. S. Sharp and A. total S. N and C in soils, Mr advice and Mrs J. A. Leighton
pools and their significance. New Ze&nfi
Journal
i
Science 25, 135439.
Jenkinson D. S. and Ladd J. N. (1981) Microbial biomar in soil: measurement and turnover. in Soil Biochemislq Vat. 5 (E. A. Paul and J. N. Ladd. Eds), pp. 415-47 Dekker, New York. Jenkinson D. S. and Powlson D. S. (1976) The effects c biocidal treatments on metabolism in soil-v. A metho of measuring soil biomass. Soil Biology & Biochemistry I 209-213. Jones L. H. P., Cowling D. W. and Lockyer D. R. (197: Plant-available and extractable sulphur in some soils c England and Wales. Soil Science 114, 104-114. Lynch J. M. and Panting L. M. (1980) Cultivation and tt soil biomass. Soil Sioiogy & ~~ochem~~ry 12, 29-33. Lynch J. M. and Panting L. M. (1982) Effects of seasol cuitivation and nitrogen fertilizer on the soil biomas Journal of the Science of Food and Agriculture 3. 249-252.
Massoumi A. and Cornfield A. H. A. (1963) A rapid methc for determining sulphate in water extracts of soils. Analy 88, 321-322. Maynard D. C., Stewart J. W. B. and Bettany J. R. (198. Sulfur and nitrogen mineralization in soils compan using two techniques. Soil Biology % 3iochemistry I
251--255. McLaren R. G. (1975) Marginal sulphur supplies for gras land herbage in south-east Scotland. Journaf of Agricu rural Science, Cambridge 85, 571-573. McLaren R. G., Keer J. I. and Swift R. S. (1985) Sulpht transformations in soils using sulphur-35 labelling. SC Biology & Biochem~siry 17, 73-19.
REFERENCFS 1. (1961) Studies on mineralisation of sulphur from soil organic’matter. Australian Journal of Agricul-
Barrow
N.
turn1 Research 12. 306-319.
Bottner P. (1985) Response of microbial biomass to alternate moist and dry conditions in a soil incubated with %- and LJN-labelled plant material. Soil Siology & Biochemistry 17, 329-337. Butters 3. and Chenery E. M. (1959) A rapid method for the dete~ination of total sulphur in s&Is and plants. Analyst 84, 239-245. Coughenour M. E., Parton W. J., Lauenroth W. K.. Dodd J. L. and Woodmansee R. G. (1980) Simulation of a nrassland sulfur cycle. Ecological Modelling 19, 179-213. Eniminger L. E. (i954) Some factors afT&ting the adsortttion of suifate by Alabama soils. Soil Science Society of America Proceed&gs 18, 259-264.
R. and Muir 3. W. (1963) The Soils of rhe Counrrv round Aberdeen, Inuerurie and Fraserburgh. HMSd, Edinburgh. Goh K. M. and Gregg P. E. H. (1982) Field studies with Glentworth
radioactive sulphur-labelted gypsum fertilizer III. Sulphur
Millard P., Sharp G. S. and Scott N. M. (1985) The effe of sulphur deficiency on the uptake and incorporation 1 nitrogen in tyegrass. Journul of Agricuituro~ Scienc Cambridge IO5, 50 I-504. Saggar S., Bettany 1. R. and Stewart J. W. B. (i981 Measurement of microbial sulfur in soil. Soil Biology Biochemistry 13, 493-498.
Sanaar S.. Bettanv J. R. and Stewart J. W. B. (1981 ??ansformations of sulphur in relation to carbon at nitrogen in incubated soils. Soil Biology & ~~ochern~~ 13.499-51 I. Scott N. M. (1981) Evaluation of strIphate status of soils / plant and soil tests. Journal of the Science of Food a kgriculfure 32, 193-199. Scott N. M. and Anderson G. (1976) Organic sulph fractions in Scottish soils. Journal of the Science of FOI and Agriculrure 27, 358-366.
Scott N. M. and Munro J. (1979) The sulphate status of so from North Scotland. Journal of the Science of Food at Agriculture 30. 1S-20.
Scott N. M., Dyson P. W., Ross J. and Sharp G. S. (198 The effect of sulnhur on the yield and chemical car position of winter-barley. Journbl of Agricufturoi Scient Cambridge
103, 699-702.
Scott N. M., Watson M, E., Caldwell K. S. and Inks R. H. E. (1983) Response of grassland to the applicatic
Microbial sulphur in some Scottish soils of sulphur at two sites in North-east Scotland. Journal o/ the Science of Food ad Agrictdttue34, 357-361. Sparling G. P. (1981) Microcalorimetry and other methods to assess biomass and activity in soil. Soil Biology & Biochemiury 13, 93-98. Strick J. E. and Nakas J. P. (1984) Calibration of a
305
microbial sulfur technique for use in forest soils. Soil Biology & Biochemistry 16, 289-292. Tabatabai M. A. (1982) Sulfur. In Methoa3 of Soil Anaf.vsb, Part 2. Chemical and Microbiological Properties. Agronomy Monograph No. 9 (C. A. Black, Ed.), 2nd cdn, pp. 501-538. ASA-SSSA, Madison.
Soil Bio!. &c/rem. Vol. 19, No. 3. pp. 301-305. 1987 Printed in Great Britain
MICROBIAL SULPHUR IN SOME SCOTTISH SOILS S.
J.
CHAPMAN
Department of Microbiology, The Macauiay Institute for Soil Research, Craigiebuckbr, Aberdeen AB9 2QJ, Scotland (Accepted
20 i&sober
1986)
Sunnnary-Sulphur in the microbial biomass of nine soils from North East Scotland was estimated from the flush following chloroform fumigation. The sulphur was extracted from fumigated and non-fumigated soils with either 10 mt+rCaCl,. 16 mbt NaH,PO, or 100 mM NaHCOs. These three extractants gave similar biomass S values, but those obtained using CaCI, were the most precise. Biomass S, calculated assuming a ks of 0.35, comprised 0%2.6% of the total soil organic S and was strongly correlated with biomass C. The C:S ratios ranged from 31 to 70. In soils of very low sulphur status and known to respond to sulphur fertilizers, biomass S exceeded “plant-avaiIabie” S extracted with phosphate and may thus be an important pool of sulphur in these soils.
of organic sulphur to mineral forms in soil indicate that a relatively small fraction of the total soil S is involved at any one time. McLaren ef al. (1985) estimated the active pool in both unamended and glucose-amended soils to be about 3-6% of the total S and of a similar magnitude to the inorganic S pool. Following the application of “S-labeiled gypsum, Goh and Gregg (1982) found that 80-90% of the total soil S was not involved in cycling and part of the remaining S was inorganic sulphate. In three of the five sites examined, the quantity of organic S being cycled was 4-8% of the total S while in the other two sites large quantities of subsoil inorganic S interfered with the estimates. An important fraction of the soil organic S is microbial biomass S. Microbial biomass S in unamended soils accounted for about 2.3% of the total soil S (Saggar et al., 198la). &rick and Nakas (1984) reported similar values of 1.2 and 2.2% for two acid organic soils. Together these results suggest that microbial biomass S can form a significant proportion of the organic S pool which is involved in cycling and is potentially available to plants. A number of soils in Scotland have been shown to be low in S and to respond to S fertilizers (McLaren, 1975; Scott and Munro, 1979). In England and Wales some areas may show deficiencies when atmospheric S levels are low (Jones et al., 1972). Sulphur held in the biomass will assume greater importance in soils which are low in inorganic sulphate. My main aim was to measure biomass S in a range of soils, including some known to respond to S, and to compare biomass S concentrations with other soil s pools. Biomass S has been measured using the flush of extractable S immediately following chloroform fumigation in a way analogous to the m~sur~ment of biomass C (Jenkinson and Pow&on, 1976). Saggar ef Studies on the transformations
0 The Macaulay institute for Soil Research, 1987.
al. (198la) used IOrnM CaCI: and tOOmk( NaHCO, to extract the flush of S. while Strick and Nakas (1984) found these extractants unsatisfactory for their soils and used I6 rnM NaH2P0,. A subsidiary aim was to determine the most suitable ext~ctant for biomass S for some of the soils in North East Scotland. &lA+ERIAl.S AND WXHODS
Soils were collected fresh from the field and kept not more than 4days at 5-IO’C, except for soil I which was stored in bulk for 5 weeks. Before analysis, the mineral soils (arable, O-10 cm depth) were sieved to <2 mm and the organic soil (soil 6; F-H horizon of a Scats pine and Sitka spruce stand) to < 5 mm. Some relevant soil characteristics are given in Table I. Further details of some of the soil types are given by Scott and Anderson (1976) and the soil associations have been described by Glentworth and Muir (1963). Total soil organic S was measured by subtracting phosphate-extractable S (see below) from total soil S determined by the method of Butters and Chenery (1959) with the modification of Massoumi and Cornfield (1963). Total soil carbon and nitrogen were determined using a Carlo Erba model II06 elemental analyser. Biomass C was estimated using the chloroform fumigation technique (Jenkinson and Powlson, 1976) with the CO, released being measured by gas chromatography (Sparling, 1981). A conversion factor (kc) of 0.45 was adopted (Jen~nson and Ladd, 1981). Biomass S was estimated from the flush of S following chloroform fumigation using the procedure of Strick and Nakas (1984) on 20g (fresh wt) samples. Both control and fumigated soils were extracted with either 10 mM Cat&, 16 KtM NaH,PO, or 100 mM NaHCO, (I:5 soil:solution ratio) using a I h shaking period. The extracts were centrifuged and filtered through a 0.3 pm pore membrane filter. TotaI S in the extracts was measured by X-ray tl uorescence spectroscopy: to 80 ml of the extract 8 ml of I M NaHCO, was added to increase the bulk and the
301
302
S.
J. CHAPUAS whole then freeze-dried. Samples were prepared by compressing 0.4 g of freeze-dried extract, backed by boric acid, into pellets. Analyses were performed under vacuum in a Philips PW 1404/00 automated wavelength-dispersive spectrophotometer using a germanium crystal, flow-proportional detector and appropriate pulse-height discriminator settings. The S Kx intensity was measured for 40 s, corrected for background and converted to concentration of S by calibration with standards prepared in the same way as the samples using appropriate proportions of K,SO, in NaHCO,. The conversion factors (ks values) used to convert the S flush to biomass S were 0.35 with CaCI, and 0.41 with NaHCOJ extracts (Saggar et al., 1981a). Strick and Nakas (1984) fount ks values of 0.25 and 0.37 for two forest soils using NaH:PO,. As an extractant for S, NaHIPO, is inter mediate in strength between CaCI, and NaHCO, therefore the value of 0.37 was applied to NaH,PO, extracts. These values are approximate and furthel calibration would be desirable. All results were ex, pressed on an oven dry-weight basis. Extraction of the control soils and measurement o the total S in the extracts gave estimates of the following S pools. CaCI, extracts “free” (readil! soluble) sulphate (Barrow, 1961). NaH,PO, extract: both “free” and adsorbed sulphate with some lab& organic S (Ensminger, 1954). NaHCO, extract “free” plus adsorbed sulphate with greater amount of organic S (Tabatabai, 1982). Bartlett tests for homogeneity of variances wer carried out on the extractable S, biomass S ant biomass C data and gave non-significant results Thus variances were pooled to give a more concis presentation of results. Correlations among the vat-i ous soil properties were examined using linear re gression analysis. RESULTS AND DlSCUSSlON
For each of the soils, I6 mM NaH,PO, general1 extracted more sulphur than IO mM CaCI, (Table 2; The CaCI, extracts were completely clear while onl the NaH,PO, extract of soil 6 showed any appre ciable colour. The difference between the twl extractants then indicates mainly adsorbed sulphat (Tabatabai. 1982). More deeply coloured extract indicated that the weakly alkaline 100 mM NaHCC extracted some organic matter and that the ad ditional S extracted by this extractant was probabl organic. Fumigation increased the concentration of removed by each extractant, giving a two-fold in crease for CaCI, extractions but less for NaH,PC and NaHCO,. The S flushes for the three extractant were similar (data not shown) and, except for th NaHCO, extract of soil 4, not significantly differen This similarity suggests that NaH,PO, and NaHCC are not extracting much microbial biomass S fror unfumigated soil, i.e. the organic S in the NaHCC extract is not biomass S. As the NaH,PO, an NaHCO, flushes are the difference of two large numbers, compared to the CaCI, flush, the error involved are larger. The calculated biomass S value (Table 3) are correspondingly most precise when th CaCI, flush is used, the pooled variance bein significantly less than that for either the NaH,PO, c
303
Microbial sulphur in some Scottish soils Table 2. Concentrations of total sulphur cx~racted by IO my CaC&, 16mu. NaH$O, and 100 mb( NaHCO,. from nine Scottish soils. Mean of three replicates; Pooled SE (standard error) = 0.41 Soil
CaC&-cxtractable S (~8 8-l soil)
I 2 3 4 5 6 7 8 9
1.1’ 0.9 1.3 2.3’ 2.6 9.0 4.6’ 4.3 3.4
NaH,PO,-extfactablc (ae g-’ soil)
S
NaHCO,txtractablc @g g-’ soil)
S
6.6’ 3.7 7.3 19.6 54.6 45.5’ 26.5’ 26.2 44.5
1.0 2.2 3.1 4.7 12.2 12.8’ 13.1 19.0 26.5
‘SE = 0.50 (due to lost replicate).
NaHCO, data. Only the CaClz values are used in further consideration of biomass S. The biomass S varied from 0.9-2.6% of the total organic S (Table 1) and agrees closely with the reports of 2.3% (Saggar et al., 1981a). 1.2 and 2.2% (Strick and Nakas, 1984) and 0.5-X3% (Maynard et al., 1983). The microbial biomass C values (I’able 3) varied from 0.5 to 1.6% of the total C (Table I). These are low compared with the values normally reported for arable soils (e.g. l.7-2.0%; Jenkinson and Powlson, 1976) and may reflect the higher organic matter contents associated with a cooler climate. There was a significant correlation between biomass C and biomass S for the nine soils (Table 4) which is reflected in the narrow range of biomass C:S ratios (31-70; Table 3). These have large standard errors which are compounded from the variability in both biomass C and S determinations. Saggar et al. (198 I b) found biomass C:S ratios between 47 and I2 I for two incubated soils with and without sulphate or cellulose amendments. The mean C:S ratios for their unamended control incubated soils were 60 and 101. As they note, C:S ratios greater by a factor of 2 or 3 have been found for solution-grown microorganisms (Saggar et al., 198 I a) or from calculations based on structural composition (Coughenour et al., 1980). It should be noted that the biomass C:S ratios are dependent on the values used for kc and k,. The biomass C:S ratios did not correlate significantly with any other soil property but, on average, were 60% of the C:S ratios of the total soil organic matter indicating a higher concentration of S in the biomass than in the dead organic matter. The biomass C:S ratio was not significantly greater in the low S soils
(soils 14) and this suggests that the biomass was not S-limited in these soils. Regressions between the various soil properties showed that biomass S, biomass C, total soil C, CaCIz-extractable S and total soil organic S were significantly correlated with each other (Table 4). In contrast NaHCO,-extractable S and NaH,POdextractable S showed little correlation except with total soil organic S. The correlation between biomass S and CaClr-extractable S was unexpected since “free” sulphate. being mobile, would also be influenced by rainfall and leaching. The correlation might be due to the biomass S values being based on CaCI, extraction. However, similar comparisons of the biomass S values and extractable S levels using both NaH,PO, and NaHCO, showed no correlation (data not shown). On the basis of these correlations it is difficult to say whether “free” sulphate (CaCI,-extractable S) in these soils depends directly on the microbial biomass or indirectly on the biomass through their common relationship with the soil organic matter. Phosphate-extractable S is generally considered to be “plant-available” (Jones ef al., 1972; Scott, 1981) and soils having below 8pg S g-’ soil would be expected to give a crop response to S fertilization. It is usually measured on soil which has been air-dried so the values in Table 2, measured on fresh soil, might be depressed although these estimates include any traces of organic S, both carbon-bonded S and HI-reducible S. On soils l-4, crops have been shown to respond to S fertilizers (Scott et al., 1983, 1984; Millard er al., 1985). In these soils biomass S was similar to or greater than phosphate-extractable S. In
Table 3. Biomass S, dewmined using three different soil extractants. and biomass C in nine Scottish soils
Soil (exwactant) I
IO mM CaCI, I .7’
: 4 5 6 7 8 9
4.3 7.0 3.8’ 6.4 29.1 10.5’ 9.3 6.7 I.31
(SE)
Biomass C (fig g-’ soil)
Biomass S (pg g-’ soil) I6 mM NaH,PO, 4.51 2.8 5.5 3.9 1.7 29.9’ 7.6 7.1 ND I ss
IO0 mM NaHCO, 4.2’ 2.9 7.1 II.5 1.0 35.2’ 4.94 10.3 ND 3.07
Biomass C:S ratio IOmM CaCI,
%’ 282 395 II7 350 1724’ 435 409 468’ 12.9
‘SE P 1.55. ‘SE = 1.72. ‘SE - 1.73. ‘SE = 3.15. ‘SE = 15.8. 6sE = 14.1. ND = NOI determined.
58 63 57 31 55 59 41 44 70
Biomau S as % of Total Organic S IO my CaCI, 0.9 I.5 1.8 I.1 I.3 2.6 2.6 I.9 I.2
304
S. J. CH~PZMN Table 4. Correlation
w&cienls between microbial biomass S and
Biomass C Total c CaCI,-extractabk S Total organic S NaH,PO,-cxtractable S NaHCQ-cxtractable S
Biomass C’
Biomass C
0.99*** 0.9s*** 0.94*** 0.94”’ 0.60~ 0.39*
0.97”’ osW** 0.96*‘* 0.72= 0.47b
other soil parameters for the nine soils studied
Total C
CaClr extractable S
0.95*** 0.9P’ 0.10 0.64
0.9O*** 0.50
Total Organic S
NaH,PO,extractable S
0.87@.’ O.74b’
0.73’
0.62
Wing CaCI, derived values. bRegrcssionomitted the orgattic soil (soil 61 which was a significant ourtier. ***/J < 0.001; l*p < 0.01; *fJ < 0.05.
soils 5 and 7-9,.where no crop response to S has been shown, biomass S was smaller than phosphateextractable S. In soil 6, biomass S was greater than phosphate-extractable S, This is attributed to the much greater biomass content of the organic soil. In the very low S soils the biomass could be an important pool of S relative to phosphate-extractable SO,-S. The availability of S from the biomass S pool would depend on the rate of turnover of the biomass S. This is not well understood although throughout the growing season biomass C can vary by a factor of at least three (Lynch and Panting, 1980, 1982) while periods of soil drying can lead to a 30% loss in biomass (Bottner, 1985). In my study most of the soils were sampled in late autumn or winter when the biomass might be expected to be at its lowest. Those sampled in April and September (Soils I, 7 and 8) may be higher. During a period of decrease in biomass, biomass S might become available to plants or, conversely, during an increase in biomass. soil sulphate may be taken up and become unavailable to plants. This aspect requires further study. Acknow~ledgemennr-1
thank fluorescence measurements, Hepburn for the analysis for R. H. E. lnkson for statistical for technical assistance.
Dr D. C. Bain for the X-ray Messrs C. S. Sharp and A. total S. N and C in soils, Mr advice and Mrs J. A. Leighton
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