Dynamics of soil C and microbial biomass in whole soil and aggregates in two cropping systems

Dynamics of soil C and microbial biomass in whole soil and aggregates in two cropping systems

Applied Soil Ecology Applied Soil Ecology 2 (1995) 253-261 Dynamics of soil C and microbial biomass in whole soil and aggregates in two cropping syst...

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Applied Soil Ecology Applied Soil Ecology 2 (1995) 253-261

Dynamics of soil C and microbial biomass in whole soil and aggregates in two cropping systems Morten Miller a71,Richard P. Dick b7* aMicrobiology Section, The Royal Veterinary and Agricultural [email protected], Rolighedsvej 21. DK-1958 Frederiksberg C. Copenhagen, Denmark h Department of Crop and Soil Science, Agriculture and Life Sciences Building, Room 3067, Oregon State University, Corvallis, OR 97331- 7306, USA

Accepted 18 January 1995

Abstract Soil samples were collected to a depth of 10 cm in 1991 and 1993 in a vegetable crop rotation experiment initiated in 1989. The two cropping treatments (under either 0 or 280 kg N ha- ‘), were the traditional vegetable rotation (TVR) currently being practiced and an alternative rotation (LVR) in which a vegetable crop alternated with a legume, red clover (Trifolium prutense L.) , that was incorporated as green manure the following spring. Chemical and microbiological parameters were determined on whole soil and five soil aggregate size fractions: l.OO-2.00,0.50-1.d0,0.25-0.50,0.10-0.25, and less than 0.1 mm. Within a 2 year period, there was a major shift in the natural fabric aggregate size distribution in the LVR with a 35% decrease in microaggregates ( < 0.25 mm) and a similar percentage increase in macroaggregates ( > 0.25 mm). On the whole soil, the shift was accompanied by large increases in the microbial biomass to soil C ratio (C,,: C,,) and microbial biomass (C,,). Furthermore, labile organic matter pools (particulate organic matter and dissolved organic C) were significantly (P < 0.05) higher in the LVR than TVR soils in both sampling years. Large aggregates had a high C,, Ctic : Corgand low qC02 (mg CO*C mg - ’C,,) . There was a negative correlation (r = 0.73 ***) between qCO* and aggregate size, and a positive correlation of C,,, or C,,, : C,, with aggregate size (r= 0.76*** and r = 0.74** respectively). Nitrogen fertilization caused a general increase in qCOz, CO1-C and Cmic.The results documented a significant improvement of soil aggregation and maintenance of organic C pools with a soil management system that provides greater root activity and C input. Further, the results indicated that there was a qualitative difference in microbial communities between macroaggregates and microaggregates. Keywords: Carbon; Soil aggregation; Green manure; Metabolic quotient; Microbial biomass; Microbial ecology; Sustainable agriculture

1. Introduction Dramatic decreases in soil organic matter in cultivated soils in North America over the past 100 years have raised concerns about the long-term productivity * Corresponding author. Tel: + I-503-7375718; Fax: + 1-5037375725. ’Tel: +45-35282640. 0929-1393/95/$09.50 0 1995 Elsevier Science B.V. All rights reserved SSDIO929-1393(95)00060-7

of these soils. Restoration of soils depends on a better understanding of how soil management affects organic matter stabilization, soil biology and aggregation. Long-term experimental sites can be valuable in elucidating mechanisms of biologically mediated processes that are important to sustainable agricultural systems (Dick, 1992). The small-scale spatial distribution of organic substrates and microbial biomass, and microbial activity in soils, is a major determinant in


M. Miller, R.P. Dick /Applied Soil Ecology 2 (1995) 253-261

nutrient cycling and organic matter stabilization (Ladd et al., 1993). In addition, soil biota (plant roots, bacterial and fungal biomass) are important factors in aggregation processes and in the maintenance and stabilization of soil structure (Oades, 1993). Investigations into the processes of biological activity and soil aggregation are needed for the development of management practices that decrease erosion and loss of soil organic C ( Corg). A number of soil measurements have been proposed that use measurements of microbial biomass (C,,) as potential indicators of management effects on biotic processes in agroecosystems. The Ctii, to C,, ratio ( Ctic : Corg) and the microbial respiration (CO,-C) to Ctic ratio (qC0, ) (also known as metabolic quotient) have been shown to be sensitive indicators of quantitative and qualitative changes in microbial communities caused by various management systems (Anderson and Domsch, 1989, 1990; Insam et al., 1991). The fractionation of soils into different aggregate size classes enables investigation into spatial interactions among fractions with different biological, physical and chemical characteristics. Temporal sampling and aggregate fractionation may reveal shifts in aggregate size distribution and microbiological spatial dynamics as affected by soil management. We hypothesized that green manure applications and the maintenance of plant cover in all seasons would have important long-term impacts on soil C dynamics and aggregation processes, and soil biological parameters. The objective of this study was to determine the impact of two management systems that differed in C input and root activity, on whole soil Corg pools and the distribution of Corg, Cmic, Ctic : Corg and qC02 as a function of aggregate size.

2. Materials and methods 2.1. Sample collection and preparation Soil samples were collected from two treatments in a vegetable crop rotation experiment initiated in 1989 at North Willamette Research and Extension Center, Aurora, Oregon. For 10 years prior to initiation of the experiment, the site had been under conventional winter wheat production. The two treatments represent the traditional vegetable rotation (TVR) currently being

practiced with a winter fallow and an alternative vegetable rotation that includes a legume crop (LVR) . In the LVR, a vegetable crop alternated with red clover which was harvested for seeds in the summer and allowed to regrow over the winter followed by spring plow-down as a green manure (Table 1). To determine residual effects of clover on biological and biochemical parameters, the two cropping treatments were subdivided into two summer fertilizer treatments, 0 kg N ha-’ (No) or 280 kg N ha-’ (N2s0) (red clover was not fertilized). The design of the experiment is a randomized complete block split plot (four replications) with cropping system as the main plot and N rate as the subplot. The soil is a Willamette silt loam (Pachic Ultic Argixeroll) . The climate is Mediterranean characterized by cool winters and hot dry summers. The mean annual temperature is 1 1.l”C and the mean annual precipitation is 1040 mm, with 70% occurring during the winter months (November to April). The average soil temperature in the winter and summer at a depth of 5 cm is 7.2”C and 19.3”C respectively. The plots were sampled the first week of September 1991 and 1993 because seasonal effects due to spring plow-down would be minimal and because this period follows a late summer drought period. Thus this time of year would best represent the effect of soil management on the long-term trajectory of soil properties. Two of the four replications were sampled by taking 30 cores (2.54 cm diameter) from each rep to a depth of 10 cm. These 30 cores were thoroughly homogenized prior to removing a 500 g subsample. The soil samples were Table 1 Traditional

and alternative crop a rotations

Year Fall 89/winter Spring 90 Fall 90/winter Spring 91 Fall 91 /winter Spring 92 Fall 92/winter Spring 33 Fall 93

90 91 92 93



Red clover Red clover Red clover Broccoli ’ Fallow Red clover Red clover Broccoli b Fallow

Fallow Sweet corn Fallow Broccoli Wheat Wheat Fallow Broccoli Fallow

a Scientific crop names are Trifolium pratense, .&a mays, Brassica oleracea and Triticum aestiuum for red clover, sweet corn, broccoli, and wheat, respectively. b Red clover was plowed under as green manure prior to establishment of broccoli crop.

M. Miller, R.P. Dick/Applied Soil Ecology 2 (1995) 253-261

stored at 4°C and analyzed within 4 days. Field-moist soil samples with a volumetric water content of approximately 1O%, were carefully sieved by hand through a series of sieves in order to obtain five soil aggregate size fractions: l.OO-2.00,0.5-1.00,0.25-0.5,0.1-0.25, and less than 0.1 mm. To avoid disruption of natural fabric aggregates and minimize mechanical stress during sieving, soil aggregates that did not readily pass a 2 mm sieve were excluded from this study. These were primarily small rock debris and resulted in minimal losses compared with whole soil. This was also done to avoid massive losses of microbial biomass as reported in earlier studies, where high energy inputs and suspensions of soil in water were used in order to achieve dispersion of soil samples into various aggregate size classes (Amato and Ladd, 1980; Ahmed and Oades, 1984). To minimize moisture loss, the samples were sieved in a cool room at 4°C. Microbial biomass C and N flush of both whole soil and aggregates were measured on field-moist soil samples. Subsamples were air dried for chemical and biochemical analyses. All results are expressed on a per g oven dry ( 105”C, 24 h) weight basis. The data were analyzed with standard ANOVA procedure for RCB split plot design (Statistical Analysis Systems Institute Inc., Cary, NC). Each analysis of a field replication sample was done at least in duplicate with coefficients of variation of 5%. 2.2. Chemical analyses The pH of the soil samples was measured in a 1: 2 soil to water ratio. Total organic C of soil samples was determined by dry combustion with a Dohrman DC-80 total carbon analyzer. Total N was determined by the Kjeldahl digestion method (Bremner, 1960). The original procedure of Dick and Tabatabai (1979) for the ion chromatographic determination of N03-N was modified using 0.0017 M Ca( H,PO,) *. On the same extract NH, + -N was determined by steam distillation. Particulate organic matter (POM) was isolated and quantified as proposed by Cambardella and Elliott (1992). Dissolved organic carbon (DOC) was determined as follows. Soil samples of 10 g were shaken overnight with 30 ml deionized water. The soil slurry was centrifuged and the supernatant filtered through a 0.2 pm filter. The DOC was then determined on a Dohrman DC-80 carbon analyzer.


2.3. Microbial biomass analyses Microbial biomass C (C,,) and N flush (N,) were determined by the chloroform-fumigation-incubation method described by Jenkinson and Powlson ( 1976) on a 12 g (fresh weight) soil sample. After fumigation, inoculum was added and the soil was adjusted to 65% water holding capacity. The amount of CO1 evolved during the 10 day incubation following fumigation was determined on a thermal conductivity gas chromatograph. After CO1 sampling, NH4+-N was extracted with 50 ml 2 M KC1 and quantified by steam distillation. The Ctic and N, were calculated with the following formulas: cnlic = ( co*-cf-



and Domsch,

/0.4 1



Nfl = (NH, + -N,- NH, + -NU&


Specific respiratory activity ( qC02) was calculated as pg CO,-C evolved per pg biomass C (where pg CO,-C was from unfumigated samples incubated for 10 days at 24’C.) (Schntirer et al., 1985). 2.4. Physical analysis Water-stable aggregates (WSA) were measured using a wet sieving procedure (SCS testing lab, Lincoln, Nebraska; R. Burt, personal communication, 1994) modified as follows. A 0.5 mm stainless steel sieve was submerged in distilled water so that the water level was at 20 mm above the base of the screen. A 3.0 g aggregate sample ( l-2 mm) was distributed on the surface of the water in the sieve. The sample was allowed to sit for 6 h. The sample was agitated by raising and lowering the sieve below the water 20 times per 30 s. The sieve was then removed from the water placed on an aluminum plate and dried in a oven at 105°C for 2 h. WSA is reported as the percentage weight of aggregates retained on the sieve plus the aggregates that dropped through the sieve during drying.

3. Results 3.1. General soil properties General soil properties for the two treatments are given in Table 2. At both N-fertilization levels the LVR


M. Miller, R.P. Dick /Applied Soil Ecology 2 (1995) 253-261

had a higher Cmic, which at the N2so was nearly two times greater than the TVR. In addition qC0, was 50% lower and the Ctic : C,, was nearly two times greater in the LVR at the high N fertilization level relative to TVR. At the No in the LVR there was a 30% decrease in qC0, (P < 0. lo), an increase in total N and a higher microbial respiration (CO,-C) relative to TVR. Comparison of data between 199 1 and 1993 (Table 3) shows a high redistribution of soil in the LVR with a significant (P < 0.05) 35% decrease in the less than 0.10 mm aggregates and a similar percentage increase in the 1.00-2.00 mm aggregates. In comparison the TVR showed no change in aggregate distribution. 3.2. Chemical characteristics of aggregates and whole soil Table 4 presents distribution of C fractions in whole soil and aggregates at N2so. Similar results were found at N,, rates (data not shown). In general, regardless of cropping treatment, the macroaggregates contained higher concentrations of C,, (Table 4) and total N (data not shown). No significant differences were found between C/N ratios of various aggregate sizes (data not shown). In both rotations, POM and DOC on whole soil decreased between 1991 and 1993 (Table 4). However, only in the TVR was this reflected in significant (P <0.05) decreases of C,,. There was a rotation effect in both 1991 and 1993 on the POM fraction

which was significantly higher (P < 0.05) in the LVR as compared with the TVR regardless of N rate. In 199 1 we observed a significantly higher (P <0.05) DOC concentration in the LVR, but in 1993 there were no significant rotation effects. We observed similar decreases in soil C among aggregates size classes (Table 4). On average, regardless of crop rotation and N rate, the macroaggregates ( >0.25 mm) decreased 8-9% and the microaggregates ( CO.25 mm) decreased l-5% in C, (data not shown).

3.3. Microbiological properties of aggregates

Macroaggregates contained higher amounts of C,, than microaggregates. When the relative distribution of Ctic among different aggregate size classes was calculated, the macroaggregates ( >0.25 mm) contributed 93% of the total Ctic (data not shown). For both Ctic and the Ctii, : Co_ ratio there was a peak in the 0.50-1.00 mm aggregates regardless of rotation or N rate (Fig. 1) . In contrast, the respiration data shows a reverse trend, evidenced by a one- to five-fold decrease in the physiological constants qC02 and CO;?-C with increasing aggregate size regardless of rotation or N rate (Fig. 1). A general increase in qC02, CO,-C, C,, and N, were observed with increased N fertilization (data not shown).

Table 2 Soil propertiesin September 1993 as affected by crop rotation and N- fertilizer level a Characteristics

PH TotalN (mgg-‘) NitrateN (pg-I) Ammonium N ( pg g - ’) WSA (%) C/N ratio Cnl,, (I.LgC g-‘) N, (cLgNC) coz-c (pgCOz-Cg-‘) 9coz (PgCo?-CCg-‘Cmic) C,,, :Carp( mgGi, g- ’Co,)


280 kg N ha-’





6.00 1.31 5.24 2.63 20.80 1O:l 156.19 10.69 2.94 0.45 11.91

5.83 0.99 3.62 4.70 19.20 15:1* 135.69 * 10.40 1.98 0.66 9.05

5.31 1.13 53.10 25.92 28.90 13: 1 245.28 17.27 3.28 0.37 17.61

5.45 1.20 41.80 19.16 26.0 13:l 138.29 * 7.66 ** 2.97 * 0.73 * 9.42 **

a All measurements had CV less than 5%. * ** Indicates significance level between crop rotations within each N rate at P < 0.05 and P < 0.01, respectively.

M. Miller, R.P. Dick /Applied Soil Ecology 2 (1995) 253-26I Table 3 Effect of green manure and N fertilizer level on the percentage N level



of aggregate


size classes

1991 (baseline)


(mm) LVR (%)

TVR (%)

LVR (%)

TVR (%)

1.00-2.00 0.50-1.00 0.25-0.50 0.10-0.25
25.8 17.9 15.5 12.5 28.3

31.6 19.5 15.3 10.8 22.8

33.3 21.9 15.2 11.2 18.5

31.0 21.3 15.7 13.1 19.0

I .oo-2.00 0.50-l .oo 0.25-0.50 0.10-0.25 co.10

24.7 18.7 16.2 12.4 27.5

28.3 17.2 15.7 12.1 26.7

34.7 22.7 15.5 10.4 17.1

27.2 21.9 16.5 14.2


280 kg N ha-

Table 4 Changes in C,,,, POM and DOC among aggregates


and whole soil between 1991 and 1993 at the N2s0 as affected by crop rotation TVR

LVR 1991









16.89 18.36 17.77 16.23 12.93

14.77 15.55 16.23 14.75 12.67

- 12.5 - 15.3 ** - 8.6 -9.1 -2.0

16.79 20.30 19.28 16.92 15.04

15.72 17.22 15.62 16.55 13.25

-6.3 - 15.1 - 18.9 ** -2.1 -11.9

17.43 0.562 0.131

16.30 0.474 0.070

-6.5 - 14.7 * -46.5 *

20.86 0.479 0.102

17.38 0.377 0.077

-11.1 * -21.3 * - 24.0 *




C kg-‘)


Aggregates C “‘F

1.oo-2.00 0.50-I .oo 0.25-0.50 0.10-0.25
Whole soil C orp POM DOC * ** at P

4. Discussion 4. I. Redistribution of aggregates In the present study a major shift in the natural fabric aggregate size distribution was observed in the LVR, i.e. a 35% redistribution between the microaggregates and macroaggregates. The shift was unaffected by N fertilizer level and accompanied by a large increase in the Cmic : C,, and C,,,ic. Furthermore the labile organic matter pools POM and DOC were significantly higher (P < 0.05) in the LVR.

Positive effects of leguminous crops on aggregate stabilization and formation have been attributed to enhanced root activity and incorporation of legumes as green manure (Dufey et al., 1986; Latif et al., 1992). Carter and Kunelius (1993) found a six- to 1 l-fold increase in root biomass when clover or ryegrass were undersown compared with barley alone. Stabilization of macroaggregates in particular has been linked to plant roots and associated root hairs acting as adhesive structures, enmeshing soil particles in larger aggregates (Oades, 1993). Beare et al. ( 1994) also found higher POM concentrations in no-till systems where plant res-

M. Miller, R. P. Dick /Applied Soil Ecology 2 (1995) 253-261






40 cm, : co, 5 0


N 1.5 Ip 20

s e 1.0 0.5

0” .. .; 10 0




I. Release of COz-C and distribution of C,,, qC02 and C&C,,

idue remained on the soil surface than conventional tillage systems (moldboard plowing). Cambardella and Elliott (1992) showed with direct microscopic inspection that the POM fraction was composed largely of root fractions in various stages of decomposition. The significantly higher POM concentrations in the LVR may thus reflect a higher root biomass due to the inclusion of red clover in the rotation. The importance of Cmic in the formation and stabilization of macroaggregates was emphasized by Gupta and Germida ( 1988) and they suggested that C,, was the primary source of C and nutrients released upon cultivation and subsequent degradation of aggregation. This was consistent with earlier studies implicating labile organic matter and Ctic in the formation and stabilization of soil macroaggregates (Tisdall and Oades, 1982; Elliott, 1986). The relatively high decomposition rate of clover tissue due to a low C/N ratio may explain the high Ctic in the LVR. In corroboration, Angers et al. ( 1993) found that rotations with red clover were associated with an enrichment of labile organic matter and increased C,,,ic and Ctii, : Corg. This

among aggregate

sizes (averaged

across treatment and N-rate).

suggests that a qualitative difference in the C input in the two cropping systems had a role in the changes we have observed in this study. Insamet al., ( 1991) found that crop yields are an important quantitative determinant for the Ctic : Co_. Higher crop yields will increase the C input into the soil and be accompanied by higher Ctic and an increased Ctic : Corg ratio. In the present study, however, no significant differences were observed between broccoli yields in the two cropping systems when adequate N was available (data not shown). This strongly suggests that plowing under red clover as green manure was the major factor contributing to the increase in the Ctic : C, ratio, Cmicr labile organic matter, and aggregate formation in the LVR. 4.2. Carbon dynamics of whole soil and aggregates In the present study, we observed a significant decrease in Co_ concentrations ranging from 3 to 16% on the whole soil and up to 18% in individual aggregate size classes compared with baseline data from 1991 (Table 4). When the relative contribution among

M. Miller, R.P. Dick /Applied Soil Ecology 2 (1995) 253-261

aggregate size classes was calculated (based on aggregate size distribution in 1993), 75% of the decrease was associated with macroaggregates. The high mineralization potential of macroaggregates has been reported in a number of earlier studies (Elliott, 1986; Gupta and Germida, 1988). Our study was also consistent with these studies in that levels of Corg, total N and Ctic were highest in the macroaggregates. Insam et al. ( 199 1) pointed out that soil C equilibrium has to be understood as a long-term steady state that allows annual fluctuations or fluctuations corresponding to the crop rotation cycle. Further, these fluctuations may be attributed to drying and rewetting and short-term changes of temperature. The increased mineralization after drying and rewetting has been related to biological availability of organic substrates from dead microorganisms and, to a larger extent, from other non-living soil organic matter which has been exposed or relocated in the soil, making it more available for decomposition (Van Gestel et al., 1991). The pattern of precipitation in the Willamette Valley, with hot, dry summers and warm, wet fall seasons (with wetting/drying cycles) increases the potential for large fluctuations in C,, and the rate of mineralization. The size and availability of different soil C,, pools will play an important role on the impact of such seasonal fluctuations. Transient binding agents in the soil are organic materials which are rapidly decomposed by microorganisms (Tisdall and Oades, 1982). It has been suggested that these compounds are the primary source of nutrients released when organic matter is lost on cultivation (Elliott, 1986). In the TVR treatment, we observed no such concurrent change in weight distribution of aggregates between 1991 and 1993. That there was no degradation of macroaggregates, indicates that the decrease in C,, apparently did not come from C pools involved in stabilization of aggregates. We observed significant decreases in soil Corg only in the traditional cropping system, suggesting that the application of green manure enhanced the buffering capacity of soil CorEpools in the LVR, decreasing the impact of a climate highly conducive to mineralization processes and seasonal fluctuations in soil Corg. 4.3. Microbiological


of aggregates

A negative correlation (r = - 0.73**) between the physiological constant qC0, and aggregate size was


observed concurrently with a positive correlation between Cmicr the Ctic : C,, and aggregate size ( r = 0.76*** and r = 0.74** , respectively). If the basal respiration of the individual aggregate size classes reflects the overall activity of the microbial community in these aggregates and the qC0, their metabolic efficiency, then it appears that there is a qualitative difference between microbial communities in the macroaggregates and microaggregates. We have observed that aggregates characterized by a high C,, : Corg also have a low qC0,. Thus the microbial population in macroaggregates displays a higher metabolic efficiency, meaning that relatively more C from organic substrates is channeled into anabolic processes. These qualitative differences in microbial communities between microaggregates and macroaggregates are likely related to substrate availability and pore size distribution. The major determinants of C and nutrient turnover in soils are thought to be physical protection of substrates and the spatial distribution of microorganisms (primary decomposers) and microfaunal predators (secondary decomposers) among pore sizes (Ladd et al., 1993). The distribution of pore size among aggregates and the associated microbial community provides a theoretical basis (Hattori, 1988) for the change in distribution of biological properties among aggregate sizes shown in our study. The porosity exclusion principle dictates that the microaggregates have the lowest porosity (Dexter, 1988), and thus the smallest proportions of voids that can be occupied by air and water. In a soil under cultivation, Gupta and Germida ( 1988) reported dramatically higher bacterial counts in microaggregates (average 12.7 X lo6 colony forming units on 0.3% tryptic soy agar) (cfu g - ’) as compared with macroaggregates (average 2.5 X lo6 cfu g- ’) , a surprising finding given that the total microbial biomass was actually higher in the macroaggregates which was due to greater fungal biomass in macroaggregates. In effect this qualitative change between microaggregates and macroaggregates may also reelect a change in the fungi/bacteria ratio where a drastic shift in favour of bacterial biomass could change qC0, (Anderson, 1994). Our results support this construct where macroaggregates had a low qC0, and a high Ctic : Corg as compared with microaggregates which had a high qC0, and low C,, : Corp. Another avenue of evidence for changes in the microbial community is provided from


M. Miller, R. P. Dick /Applied Soil Ecology 2 (I 995) 253-261

our investigations of enzymes on the same treatments of this study where L-asparaginase activity increased with increasing aggregate size fraction but amidase decreased with increasing aggregate size fraction (Miller and Dick, 1995). The main location of microorganisms on the surface of microaggregates may also have played a role in the redistribution of microaggregates into macroaggregates in the LVR treatment. Tisdall and Oades (1982) indicate that these surface organisms play a major role in initial cementation and stabilization of smaller aggregates into larger aggregates, due to their production of exocellular mucilage and gums. 4.4. Conclusion The results document the restoration of soil aggregation and maintenance of organic C pools with a soil management system with greater root activity and C inputs. They also provide evidence that alternative systems that maintain plant root activity most of the year and have biennial legume-green manure incorporations can improve soil aggregation within a relatively short time of 2 years. Further the results indicate that a qualitative difference can exist between microbial communities in macroaggregates and microaggregates.

Acknowledgments The authors wish to acknowledge Dr. Del Hemphill for his assistance with the research and management of long-term plots and the Danish Agricultural and Veterinary Research Council and Knud Hoejgaards Fond for support. This is Oregon Ag. Experiment Station Technical Paper No. 10 495.

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Latif, M.A., Meheus, G.R., Mackenzie, A.F., Alli, I. and Faris, M.A., 1992.Effects of legumes on soil physical quality in a maize crop. Plant Soil, 140: 15-23. Miller, M. and Dick, R.P. 1995. Thermal stability and activities of soil enzymes in two cropping systems. Soil Biol. Biochem., in press. Oades, S., 1993. The role of soil biology in the formation, stabilization and degradation of soil structure. Geoderma, 56: 377-400.

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Schniirer, J., Clarholm, M. and Rosswail, T., 1985. Microbial biomass and activity in an agricultural soil with different organic matter contents Soil Biol. B&hem., 17: 6 I 1-6IS. Tisdall, J. and Oades, J.M., 1982. Organic matter and water-stable aggregates in soil. J. Soil Sci., 33: 141-163. Van Gestel, M.. Ladd, J.N. and Amato, M., I99 I Carbon and nitrogen mineralization from two soils of contrasting texture and microaggregate stability: influence of sequential fumigation, drying and storage. Soil. Biochem., 23: 313-322.