Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an australian vertisol

0038-0717i89 S3.00 + 0.00 Copyright 0 1989 Pergamon Press plc

Soil Bid. Biochem. Vol. 21. No. 6, pp. 759-763. 1989 Printed in Great Britain. All rights reserved

INFLUENCE OF SORGHUM RESIDUES AND TILLAGE ON SOIL ORGANIC MATTER AND SOIL MICROBIAL BIOMASS IN AN AUSTRALIAN VERTISOL P. G. SAFFIGNA,* D. S. POWLSON, P. C. BR~~KESand G. A. THOMAS? Soils and Crop Production Division, AFRC Institute of Arable Crops Research, Rothamsted Experimental Station, Harpenden, Herts AL5 UQ. England (Accepted 28 February 1989) Summary-Shortand medium-term changes in soil organic matter content following a change in soil management or land use are oRen difficult to measure because they occur slowly against a large background of soil organic matter which can have considerable spatial variability. Results from an experiment with grain sorghum (Sorghum bicolor L.) on a vertisol in sub-tropical Australia demonstrate the usefulness of two techniques for detecting trends in surface soil organic matter before they can be assessed by conventional methods. Firstly, using soil microbial biomass C as a sensitive indicator of changes in soil organic matter. Secondly, by using initial values of soil organic C or total N. measured before imposition of treatments, as a covariate in an analysis of variance. The combination of these techniques provided the most sensitive approach for detecting changes. The above-ground residues of sorghum (4 t dry matter ha-’ ) were either retained or removed from plots that received conventional or zero tillage for 6 yr. Averaged over tillage treatments, soil organic C in the surface O-IOcm layer was 8% greater in the residue-retained than in the residue-removed treatment, a difference equivalent to 16% of the C added as residues. The trend to increased soil total N was not significant. Residue retention caused larger percentage increases in microbial biomass C, measured by the chloroform fumigation-incubation method, than in total organic C and total N. The increase in biomass C was 12%. biomass N 23% and biomass P 45%. quivalent to 0.7% of the C, 7% of the N and 32% of the P added in residues. Residue retention decreased the biomass C-to-P ratio from 48 to 35, but these values were still much wider than those previously measured in U.K. soils. Residue retention increased respiration by about 45% (measured by CO2 evolution during a 30-day incubation) but had little effect on biomass C-to-N ratio or mineralization of N. Averaged over the two residue management treatments, soil organic C in the surface 10 cm layer was 7% greater under zero tillage than under conventional tillage. The corresponding increase in biomass C was 14-21%. but there were no differences in biomass N or biomass P. CO, evolution and specific respiration by the biomass bg CO& evolved g-’ biomass C day-‘) were less in zero-tilled than in conventionally tilled soils. The combined effects of residue retention and zero tillage caused increases of 15% in surface soil organic C, 18% in soil total N and 31% in biomass C.

IMRODUtXlON

There is worldwide interest in the retention of crop residues in soil. In many areas residues are retained in order to reduce soil erosion, e.g. in Australia (Freebaim and Wockner, 1986). North America (Unger and McCalla, 1980) and West Africa (Lat. 1984). In Britain, incorporation of cereal straw is

being considered as an alternative to burning because of concern over the adverse environmental impact of burning (Tinker, 1985). In Australia, there has been a marked reduction in the burning of sugar cane prior to harvest with a concomitant increase in the retention of sugar cane residues on the soil surface *Present address: Division of Australian Environmental Studies, Griffith University, Brisbane, Queensland 41 I I, Australia. tPresent address: Queensland Wheat Research Institute. I3 Holbcrton Street, Towoomba, Queensland 4350, Australia.

(Prammanee et al., 1988). Retention of crop residues is an integral part of organic-biological-alternative farming systems (Doran et nl., 1987; Arden-Clarke and Hodges, 1988). Residue retention alters soil properties, mainly by causing a gradual increase in soil organic matter content (Jenkinson et al., 1987). However, such changes are slow and therefore difficult to measure accurately against the large background of organic matter present in most soils (Powlson and Jenkinson, 1981). The soil microbial biomass comprises only l-3% of the total organic C in soil (Jenkinson and Ladd, 1981) but is a relatively labile fraction of soil organic matter. Jenkinson and Rayner’s (1977) model for the turnover of C in soil predicts that the soil microbial biomass will respond much more rapidly than soil organic matter as a whole to changes in agricultural practice that alter the annual input of organic matter into soil. This has been demonstrated experimentally in situations where soil organic matter is declining under both temperate (Powlson and 759

P. G.

760

SamGNA

Jenkinson, 1976) and tropical (Ayanaba et al., 1976) conditions. Biomass measurements can therefore give advance warning of the direction of long-term changes in soil organic matter content long before they can be detected by conventional techniques. Knowledge of such trends in biomass are important because of their implications for the availability of plant nutrients in soil. For example, incorporation of residues of high C-to-N ratio can cause immobilization of N for a short period (Saffigna et al., 1982), but longcontinued incorporation will eventually lead to increased mineralization of N (Powlson er ol., 1987). Zero tillage also results in higher soil microbial biomass and increased potentially mineralizable N (Doran, 1987), but changes in soil organic matter content can be difficult to detect (Staley, 1988). Changes in the size of the biomass could well affect the cycling of N and P and their availability to plants because the biomass is a dynamic pool containing appreciable reserves of these elements. Under temperate conditions, the retention of cereal straw over a period of years caused a much larger proportional increase in soil microbial biomass than in total soil organic matter (Powlson et al., 1987; Schniirer er al., 1985). Mulongoy (1986) reported greater soil microbial biomass and organic matter content in an alfisol after 4 yr of “live”-mulch than where soil was kept bare. However, there is little comparable information in the semi-arid sub-tropics. Our aim was to measure differences in soil organic matter and microbial biomass in an Australian vertisol. cropped to sorghum, where different tillage treatments and residue management practices had been applied for 6 yr. MATERIALS AND METHODS

Site

and field treatments

Soils were taken from an experiment 8 km from Biloela in central Queensland, Australia (latitude 24’21’s. longitude 150’3O’E). The mean annual temperature is 20.3’C, the mean annual open pan evaporation is 1868 mm. and the mean annual rainfall is 698 mm, most of which falls between October and March. The soil is a vertisol, classified as an Entic Pellustert having the following properties: 41% clay, 12% silt, 47% sand, pH 8, CEC 33 m-equiv. 100 g-l. Soil from the conventional tillage treatment with residues removed (see below) contained 1.24% C, 0.102% N, 0.034% P, 5.0 pg g-’ of 0.5 M NaHCO,extractable P. An experiment on residue management was started in June 1978 on a site which had been cleared of natural vegetation (Brigalow scrub, Acacia harpophyllu) and cultivated for about 20yr. Grain sorghum (Sorghum bicolor L.) has been grown from about December to June in four replicate plots (each I.5 x 25 m) of each treatment every year. Four contrasting treatments were sampled: 1. Conventional tillage (disced twice, cultivated with a tined implement three times to a depth of IO cm), crop residues removed. 2. Conventional tillage (as above), crop residues retained.

el

a/.

3. Zero tillage (no cultivation other than the disturbance caused by the planting tines), crop residues removed. 4. Zero tillage (as above), crop residues retained. The treatments were cumulative, the same tillage and residue management being applied to a given plot each year. No fertilizers were applied. Annual grain yields ranged from 1.4 to 3.6 t ha-’ (mean 2.6 t ha-r). Crop residue (straw, chaff and stubble) yields were between 2.5 and 5.6 t ha-’ (mean 3.7 t ha-‘). After removal of grain, straw is not chopped but left standing to a height of 30cm. The average element composition of the crop residue was 41% C, 0.70% N (0.59X1.93%) and 0.05% P (0.03-0.07%). Hence, the cumulative additional quantities of C, N and P returned to the residue-retained plots amounted to 8532, 153 and 12.6 kg ha-‘.

Soil sampling

and storage

The plots were sampled to a depth of IO cm in June 1978, before the treatments were first applied, to assess spatial variability. The four replicate plots of each treatment were sampled on 27 October 1983, 5 months after harvesting the fifth sorghum crop since the start of the experiment and the sixth retention of residues. (Residues from the sorghum crop grown immediately prior to the start of the experiment were used to initiate the experiment.) Soils were sampled to a depth of 1Ocm on the zero-till plots and l2cm on the cultivated plots, to sample the same weight of soil per unit area for each treatment (bulk densities ranged from II30 to 1260 kg m-‘). Forty cores were taken from each plot with a 5 cm dia. tube auger and bulked. Large pieces of organic matter were removed by hand. Subsamples of field-moist soil (about 7OOg) were then sent to England by air. About IO days later they were sieved (~2 mm) and then held for 7 days under moist aerobic conditions in large air-tight drums (75 1. vol) at 25°C in the presence of soda lime and free water. This allowed metabolism to stabilize after sampling, transport and sieving. Portions of moist soil containing either IO or 50 g oven-dry soil were then weighed into glass vials (45 or 60 ml volume) in preparation for biomass measurements. Distilled water was added to bring the soil to about 48% WHC and storage continued for a further 4 days. Thus, biomass measurements were done within 4 wk of field sampling.

Laboratory

measurements

Biomass C was measured by the chloroform fumigation-incubation method (Jenkinson and Powlson, 1976) with some small modifications as described by Powlson et al. (1987) on duplicate portions of moist soil, each containing 50 g oven-dry soil. Before incubation all portions of soil were inoculated with I ml of a soil suspension prepared by shaking 15 g of Biloela soil with 200 ml distilled water. The addition of inoculum brought the water content to 50% WHC. Biomass C was calculated from the expression Bc = F,/k,, where F, = (CO& evolved by fumigated

Sorghum

residues, tillage and soil microbial

soil during &IO days) minus (C02-C evolved by non-fumigated soil during &IO days) and kc = 0.45 (Jenkinson and Ladd, 1981). Because the soils had already been incubated, a IO-20 day incubation of the control soil was considered unnecessary. However, a further two portions of unfumigated soil were incubated for 30 days and COz evolution measured. Biomass N was measured as described by Shen et al. (1984) except that a k, of 0.57 was used (Jenkinson, 1988a); biomass C and N were measured on the same portions of soil. Biomass P was measured in triplicate portions of moist soil containing log oven-dry soil (Brookes er al., 1982). Even though the quantities of inorganic P released by CHCI, fumigation were small (2-3 pg g-l), they were readily detectable against the low background of inorganic P in the soils (5-7/rg g-l). Organic P was measured in the extracts of unfumigated soil (Brookes er al., 1982). Portions of each soil were air-dried and ground in a Tema disc mill. Total N was measured by the Kjeldahl method, modified to include nitrate-N (Pruden et al., 1985) and total organic C by dichromate oxidation (Kalembasa and Jenkinson, 1973). Soil pH and CEC were measured (Bruce and Rayment. 1982). Plant C was determined by the method of Dalal (1979). plant N and P by micro-Kjeldahl digestion, and plant S by X-ray fluorescence (Rayment, personal communication).

RESULTS

AND

DISCUSSION

Spatial cariabi[ity in soil properties

There was considerable variation in total soil N content between plots at the beginning of the experiment before treatments were imposed, values ranging from 0.103 to 0.138% N with a coefficient of variation (CV) of 9%. Five years later there was a similar range and CV, and most of the plots remained in the same order, those with highest total N in 1978 still being highest in 1983. Thus it was possible to adjust the individual plot values measured in 1983 to take account of this inherent soil variability at the site. This adjustment was done by using Table

total N as measured in 1978 as the covariate in an analysis of variance. Covariance analysis halved the variability in soil total N measurements between replicate plots. For example, the CV for total N was decreased from 10 to 5% (see Table I). A similar result was obtained using soil organic C as measured in 1978 as the covariate (data not presented). However, with this statistical procedure. the changes in means were all less than 4% of the original value. Because soil organic C and microbial biomass content are closely related to total N, the 1978 total N values were also used as the covariate to adjust these other measurements. Again, this adjustment caused little change in the mean values, but decreased the variability between measurements from replicate plots for organic C, biomass C and biomass N (see Tables 1 and 2). However, the adjustment had little effect on the variability of biomass P values (Table 2) probably because P cycling in soils is much more dependent on inorganic fractions.

Soil organic C and total N

On average, total N values declined by II% between 1978 and 1983 but here we are only concerned with the differences between treatments that existed in 1983. Residue retention and zero tillage tended to increase soil organic C and total N but none of the differences were significant (Table I). Covariance analysis decreased variability between replicate plots (see CVs in Table I). Thus the trends in surface soil organic C became significant at the 5% level for the individual effects of crop residue retention and zero tillage and at the 1% level for the combined effects (see. lower part of Table 1). However, for soil total N, sorghum residue retention did not have any significant effect. Zero tillage only increased total N significantly in the absence of sorghum residues. In addition to the C and N returned to soil in the residue-retained treatment, other factors may also

I. Effect of crop residue retention and zero tillage on soil organic C and total N

6 yr after imposition of treatments Total N (%)

Organic C (%) Crop residue management treatment

Conventional tillage

Zero tillage

1.24 1.34

1.24 1.38

As measured Residue removed Residue retained Increase (%)b cv (%)

9

II

Increase’ (%) 0 3 II’

Conventional tillage

2x0 tillage

0.102 0.111

0.111 0.115

9

9

Increase’ (%) 9 4

4

l3c

0.115 0.117

16’. 8

I

18-C

IO

Adjusted for inherent spatial variability Residue removed I.21 Residue retained 1.30 Increase (%)b cv (%)

761

biomass

I.29 I .39

8’

8’ 4

1.

0.099

7’

0.108

IS**’

9 5

‘Increase in zero tillage treatment as a percentage of measurement in the conventional tillage treatment, i.e. across the table. blncreax in crop residue retained treatment as a percentage of measurement in the crop residue removed treatment, i.e. down the table. Clncreasc in zero tillage. crop residue retained treatment as a percentage of measurement in conventional tillage, crop residue removed treatment. i.e. diagonally across and down the table. *Indicates

l*lndicatcs

a significant a significant

increase increase

at 5% at I%

level. level.

762

P. G.

SAFF~GNA PI al.

Tabk 2. E&c! of crup residue retention and zero tillage on microbial biomass, C. N and P 6yr after im~itioa Biomass C fjig g-‘1 Crop r&due management treatment

Conventional tillaxe

7.kO

dllaae

Biomass N @g g-‘1

Incd

(%)

Conventional tillaxe

zero tillane

of treatments

Biomass P (fig g-‘)

Iacrcarc’ (%)

Conventional tillaxe

tillape

6.9 9.9

6.1 9.5

7&O

1nCFSX.V‘

wo)

As measured Residue removed Residue retained Incmaxc (%)b cv (%)

273 315

313 347

IS’

II

39 48

27’*’

229.

37 46 278.

8

8*

Is**

-6 -3 18.’

44.

9

Adjusted for inherent spatial variability Residue removed 267 323 Residue retained 308 3.50 Incrcasc f%)b cv f%f

14 10

21’8 14.8

38 47

31**=

22.9

4

55’

-I1 -5 3P

23 38 47 23””

4

8 23=*

6.6 9.6

6.6 9.6 4s

4s

0 0 45=

20

Set Tabk I for explanation of footnotes.

have contributed to the extra C and N found in these soils. For example:

Retention of N- and C-rich fractions of surface soil against water erosion because of the protective effect of crop residues on the soil surface (Rose and Dalal, 1988). Addition of sorghum residues of high C-to-N ratio (about 60) would immobilize some inorganic N that otherwise might be lost from soil (White et al., 1986). Biological fixation of N may be greater in the presence of cereal straw (Gibson ef al., 1988). In contrast, biological denitrification may be enhanced in the presence of straw in vertisols (Satiigna ef (II., 1982).

Averaged over cultivation treatments, the surface soil of the residue-retained treatments contained 1.2 t ha-’ more organic C than where residues were removed. The additional C added as sorghum tops was 8.5 t ha-‘, so 14% of this was retained in soil. This is a much higher percentage retention than the 4-7% recorded in soils in Denmark where 38 t C ha-’ was added as barley straw over I8 yr (Powlson et al., 1987). One explanation for this difference is that the higher clay content (41%) in the Biloela soil would be expected to have a greater protective effect (Jenkinson et al., 1987; van Veen et al., 1987) on added organic matter than in the Danish soils which contained only 614% clay. In addition the Danish soils were probably closer to an equilibrium value for soil organic matter content, so the percentage retention of added C would be expected to be lower. The extra organic C and total N in the surface soil of zero tilled plots could be attributed to: A slower rate of decomposition of organic matter (Baeumer and Bakermans, 1973). A different distribution of organic matter in the soil profile, there being a greater concentration in the surface layers (House et al., 1984). A decrease in the loss of surface soil by erosion under zero tillage (Freebain and Wockner, 1986).

Quantities

ofC,N and P in the soil microbial biomass

Using the unadjusted data, there was a trend to increased (IO-15%) microbial biomass C where crop residues were retained or where zero tillage was practised (see upper part of Table 2). When the data were adjusted using the covariance analysis technique (lower part of Table 2), the variability was halved, the CV decreasing from 8 to 4%. As a result, the trends described for the unadjusted data in the upper part of Table 2 all become significant. The additional biomass C in the residue-retained plots amounted to 58 kg C ha-‘, equivalent to 0.7% of the C added in residues and 5% of the additional C retained in soil. The combined effects of residue retention and zero tillage caused a proportionately larger increase (3 1%) in microbial biomass C than in total soil organic C (15%). Since the CVs were equal for both measurements, microbial biomass C provides a more sensitive method for measuring difference in soil organic matter. A similar result was found with barley straw added to sandy soils in Denmark (Powlson et al., 1987). Biomass C measurements by the fumigationincubation method may be erroneously low if made within a few weeks of adding fresh crop residues to soil because of difficulties in measuring the control (Jenkinson, 1988a). It is unlikely that this occurred in the soils from Biloela which were sampled 6 months after sorghum residues were added. If, however, the biomass C values for residue retained plots were underestimated, the difference between residue treatments would, in fact, be even greater than shown in Table 2. The retention of crop residues resulted in a significant (P c 0.01) increase (22-27%) in microbial biomass N even with unadjusted measurements (Table 2). These increases were pro~rt~onately larger than with microbial biomass C. The additional 11 kg N ha-’ as biomass N was equivalent to 7% of the N added in residues. In contrast to microbial biomass C. zero tillage had no significant effect on microbial biomass N, even with adjusted data. This also differs from the results reported by Doran (1987) for a range of soils in the U.S.A. The retention of crop residues resulted in significant increases in microbial biomass P (Table 2). The most likely source of this additional 4 kg P ha-’ in the

Sorghum residues, tillage and soil microbial biomass Table 3.

Ratiosof

C. N ud

P in the biomass I& N~to~P~~o~

organic C

Crop roidue

Conventional Conventional &X0

Zero LSD (P * 0.05)

Removal Retained Removed Retained

N/P

(W

w

W)

7.0 6.6 8.6 7.5 0.7

43 33 52 38 9.1

5.7 4.9

2.2 2.4

2.0 2.8

:2 0.9

;: 0:1

3.8 4.3 3.3 4.0 0.3

Mineralization of C and N in unfumigated soil Retention of crop residues significantly increased CO1 evolution from soil during the O-10 and IO-20 day laboratory incubations, but had little effect on mineralization of N (Table 5). With conventional tillage, the ratio (CO& evolved)/(N mineralized) was wider (24.2) where residues were retained than where they were removed (12.7). This is consistent with decomposition of a residue having a high C-to-N ratio and causing some immobiIi~tion. In contrast, there was no such effect in the zero tillage treatment, suggesting that high C-to-N residues were not being decomposed, probably because they were mainly on and above the soil surface. Zero tillage resulted in less CO* evolution than in conventional tillage throughout incubation, the difference becoming more marked as incubation proceeded. Evolution of CO, in conventionally tilled soil decreased by about 48% between the first and second IO-day incubation periods but then stabilized. In zero tilled soil it continued to decrease throughout the whole 30-day period (Table 5). Table 5 also shows the specific respiration of biomass (clg CO& g-r biomass C day-‘). Specific respiration of the biomass was greater with conventional than with zero tillage throughout incubation. In the conventional tillage treatment, it reached a stable value of about 13 after the first 10 days. In zero tillage treatments, specific respiration continued to decrease throughout incubation; retention of crop residues had much less effect on this variable. The reasons for these differences are not known but they presumably reflect differences in soil organic matter

inorganic P (Pi) or organic P (PO) in unfumigatcd and fumigated soil 0.5 M NaH~O,~t~c~bie in soil (jig g-‘)

Conventional Conventional Zero Zero LSD (P = 0.05)

Crop msiducs Removed Rctainai Removed Rctaincd

::: 0.5

treatments (zero tillage, residue removed) contained a larger amount of NaHCO~~xt~c~ble organic P (7.7 pg g-r) than in the other treatments (Table 4). The siguiflcance of this organic P to plant nutrition is not known.

Retention of crop residues had little effect on the C-to-N ratio of the soil microbial biomass, but zero tillage caused a slight increase (Table 3). Residue retention tended to increase the proportion of soil organic N present in the microbial biomass. The percentages of total soil organic C and N present in the microbial biomass of this subtropical vertisol were 2.2-2.5 and 3.3-4.3%, respectively, very similar to values measured in soils from the temperate zone (Brookes et al., 1985; Jenkinson and Powlson. 1976) and in the humid tropics (Ayanaba et al., 1976). However, the 1.8-2.8% of total P as biomass in the Biloela soil was higher than the 0.9% from a plot of the Broadbalk wheat experiment that also receives no fertilizer and has a low total P content (Brookes et al., 1984). The biomass C-to-P ratios were very wide in these soils (33-52, Table 3) representing a P concentration in the microbial biomass between 1 and I.#!!, assuming that dry cells contain 50% C. These biomass C-to-P ratios were much wider than the mean value of 14 (range 1l-36) found by Brookes et al. (1984) for a range of U.K. soils, presumably a reflection of the low P status of the Australian soil. NaHCO,extractable inorganic P in the moist Biloefa soil averaged only 6 pg g-r (Table 4). Soil from one of the

Tillage

total P

CIP

Relationships between C, N and P in the soil microbial biomass and total soil C, N and P

Treatment

total N

C/N

biomass is from the added residues which contained 12.6 kg P ha-‘, suggesting that 32% of the residue P was incorporated into the biomass. As with microbial biomass N, and in contrast to microbial biomass C, zero tillage had no significant effect on microbial biomass P content. The variability in microbial biomass P measurements was greater than for any other parameter in this study. For example, with the unadjusted data, the CV was 23% for microbial biomass P compared with 9% for microbial biomass N.

Table 4. 0.5 M NaHCO,sxtractable

proportionsof soil oqsnic C, total Biomass as a percentage of soil:

Biomass

Treatment Tillage

the

763

P

Unfumigated soil Moist soil Pi

Moist soil PO

Fumigated moist soil Pi

5.0 5.7 7.4 5.6 2.9

0.7 0.7 7.7 2.4 0.6

7.1 9.4 9.3 8.7 3.2

Recovery of Pi (25 JIB g-’ added to unfumigated soil) W) 79 80 78 82 4

764

P. G. S.4sriGEi~et al. Table 5. CO&

evolved and N mineralized by unfumi~t~ CO&

Treatment

Tillage

evolved

&tg

Conventional Removed Conventional Retained ZWO Removed Zero Retained LSD (95%)

52 70 38 58 I4

32 45 22 37 I2

and fumigated soil and the specific respiration of biomass

N mineral&d (rg g“

soil)

20-30 days

CHCl,fumigated O-IO days

Unfumigatcd O-10 days

CHCI,fumigated O-IO days

(CO,-c ev0lved)i (N mheralized) for unfumipted soil 610 days

36 45 I5 22 II

175 211 I79 214 29

4.1 3.1 4.3 6.4 2.2

26.4 30.2 2s. I 32.8 5.2

12.7 24.2 9.2 9.7 5.8

Unfumigatcd

crop O-10 IO-20 residues days days

g-’ soif)

Spcciflc respiration of btomass (fig CO,-C g-’ biomass C day”‘) O-10 da) s

la-20 days

20-30 days

19.0 22.0 12.1 16.6 2.7

11.6 14.4 7.1 10.5 2.3

13.1 14.4 5.0 8.3 3.3

(P = 0.05)

decom~~bility or in biomass activity. Specific respiration of the biomass in the Biloela soils was generally similar to values for a range of ploughed and direct-drilled soils in the U.K. (calculated from data in Table 3 of Powbon and Jenkinson, 1981). However, the difference between tillage treatments was not apparent in the U.K. soils. Detecting

trends

in soil organic

matter

content

Differences in soil organic matter content. resulting from alterations in management or land use, can have profound effects on soil physical properties and fertility (Unger and McCalla, 1980). However, such differences between treatments are often difficult to measure in short- or medium-tee studies for several reasons: 1. Total organic matter content generally changes slowly. 2. Such changes have to be measured against the large background of organic matter present in soil. 3. Spatial variability of soil can result in considerable differences between measurements made in replicate field plots. This study has demonstated two approaches that can be used to detect trends in soil organic matter content before they can be measured by conventional methods. The first is by measuring soil microbial biomass C, as this is a sensitive indicator of changes in soil organic matter. The microbial biomass is a relatively small pool containing only l-3% of the organic C in soil, but it responds more rapidly to changes in organic matter input or rate of decomposition than the soil organic matter as a whole (Powlson and Jenkinson, 1981; Powlson ef af., 1987; Schniirer ef al., 1985). If the change in management involves the addition of crop residues, biomass measurements can be used in two different ways to examine the effect on the turnover of organic matter in soil. Sequential measurements can be made over fairly short periods following addition to measure the effect of one particular residue input (see Brookes and O&o, 1989); in this case the fumigation~xtraction method must be used rather than fumigationincubation in order to avoid errors due to the control value as discussed above (Ocio and Brookes, 1989). Alternatively, as in our work, biomass measurements can be used to give an indication of long-term differences in soil organic matter between contrasted treat-

ments. Inthiscasethemeasurementsshouldbemadeas long as possible after the most recent residue addition to minimize the contribution from short-term effects.

The second approach to detecting changes in soil organic matter content more readily is to adjust measured values using covariance analysis to take account of inherent spatial variability in soil properties. This requires samples to be taken from individual plots at some earlier time, ideally before the different treatments are imposed. In the present work it was not possible to detect any significant differences in total soil organic C between treatments unless the covariance adjustment was made (Table 1). This technique also decreased the variability between biomass measurements made on replicate plots. Thus, a combination of soil microbial biomass C measurements and covariance analysis provided the most sensitive approach for detecting differences in soil organic matter management.

content

resulting

from differences

in

Acknowledgements-We thank Mr A. D. Todd for statistical analyses and Mr N. D. Sills, Mrs M. Kragt-Cottaar and Mr J. Standley for technical assistance.We are indebted to the late Mr G. Pruden for the total N analyses and to Mr J. A. Ckio and Mr J. Standley for useful discussion. We also thank an anonymous referee for his thoughtful review of the manuscript. P. G. Saffigna thanks The Royal Society, the Wheat Industry Research Council and Griffith University for financial assistance with his visit to Rothamsted Experimental Station.

REFERENCES

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