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DIVISION S-6-SOIL & WATER MANAGEMENT & CONSERVATION

Method to Quantify Short-Term Dynamics in Carbon Dioxide Emission Following Controlled Soil Deformation

^a Dep. of Plant Sciences, Univ. of Cambridge, Downing Street, Cambridge CB2 3EA, UK
^b Silsoe Research Institute, Soil Science Group, Wrest Park, Silsoe, Bedford MK 45 4 HS, UK

wo200{at}cus.cam.ac.uk

	ABSTRACT

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Tillage of soils often decreases soil organic matter content and increases CO₂ emission. Enhanced CO₂ emission induced by tillage may provide an early indication of the likely consequences of soil management on soil organic C. This study was conducted to investigate a link between the mechanical properties of soil and the short-term (24 h) dynamics in CO₂ emission induced by soil deformations in controlled laboratory experiments. The degree to which soil is deformed depends on the normal and shear stresses acting upon the sample and on the stress history experienced by the soil. Respirometers were designed in which a range of combined shear and compressive forces could be applied to a soil sample while simultaneously measuring CO₂ emission. Mechanical characteristics of repacked sieved aggregates of soil were first established. Following the critical state concept, we identified changes in bulk density that resulted from different combinations of normal and shear stresses, and for different stress histories of the soil. We selected treatments that resulted in permanent deformation (compression) and expansion or consolidation upon shear. The CO₂ emission increased by 18%, even for samples that were severely compressed with a normal load of 430 kPa. Shearing of soil samples also resulted in greater CO₂ emission, in particular when combined with a normal load. The increased CO₂ emission following compression was maintained for a period longer than 7 d. As the increase did not correlate with the physical changes in pore volume, we concluded that an increase in availability of organic matter was the main mechanism responsible for enhanced CO₂ emission. We discuss these findings from experiments under controlled conditions in the context of the mechanical deformations that can be expected during tillage and assess the implications for biological activity in the field.

Abbreviations: PVC, polyvinyl chloride

	INTRODUCTION

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
AGRONOMIC PRACTICES such as cultivation, fertilization, and irrigation management affect the conservation and enhancement of soil organic matter (Kern and Johnson, 1993; Franzluebbers et al., 1994; Grant, 1997). Numerous field studies have shown changes in microbial activity resulting from different tillage operations (Franzluebbers et al., 1994; Angers et al., 1995; Kessavalou et al., 1998a). Most research focuses on long-term effects over years, but recent field studies have shown that enhanced CO₂ emission occurs immediately, within hours after tillage. It has been suggested that the short-term dynamics in CO₂ emission may provide an early estimate of the long-term effect of tillage (Fortin et al., 1996). Until now, however, these dynamics have received surprisingly little attention in more controlled experiments that can identify the processes leading to enhanced CO₂ emission. We describe a technique to quantify the effects of defined soil deformations under controlled laboratory conditions upon the emission of CO₂, and we discuss the implications of such small-scale studies in the context of tillage operations and biological activity in a field.

Among the factors that determine the way soil structure is altered by tillage are the stress history of the soil, the water content, and the combination of compressive and shear forces that act upon the soil. Generally, compressive forces lead to permanent reduction in bulk pore volume, whereas shear stresses can lead to either an increase or reduction (Kirby, 1989, 1991a, 1991b). Different mechanisms have been suggested for the way these structural changes can affect biological activity and CO₂ emission: (i) CO₂ may be released directly by opening large voids in soil or may be forced from the soil upon compression (Kessavalou et al., 1998b; Watts et al., 1999); (ii) physical properties (such as volumetric water content, air permeability, and diffusivity) are changed by tillage, which in turn affect the biological activity (O'Sullivan et al., 1999; Reicosky, 1997); (iii) the mechanical forces can disrupt aggregates, thereby freeing organic matter that was previously protected from biological breakdown (Rovira and Greacen, 1957; Watts et al., 1999); (iv) clusters of microorganisms are redistributed, bringing them in contact with new substrate (Rovira and Greacen, 1957); and (v) redistribution of pore water containing soluble organic matter can occur (Watts et al., 1999). These processes may be favorable (enhanced substrate availability or enhanced exchange of gasses) or unfavorable (reduced pore-space and exchange of gasses) for biological activity and can be expected to result in a short flush, involving release of CO₂ from large voids, or a more sustained change in CO₂ emission caused by improved conditions for biological activity. It is the combination of these, sometimes counteracting, processes that mediates the response of biological activity to tillage operations.

The consequences of tillage operations on short-term CO₂ emissions are evident from recent field studies. Reicosky (1997) showed that the magnitude of CO₂ loss from a wheat (Triticum aestivum L.) field was greatest within 5 h after tillage and attributed the changes to improved soil porosity. Rochette and Angers (1999) showed that the short-term impact of tillage on soil CO₂ emission depended on the time of year when the soil is tilled, as conditions for decomposition following tillage were more favorable in fall and summer then in the spring. Kessavalou et al. (1998b) found a 69% increase in CO₂ production within 30 min following tillage, which declined to background levels 8 h after tillage. They suggested that the increase in CO₂ emission immediately following tillage did result from a combination of enhanced microbial respiration and a flush of CO₂ released from large voids generated by tillage. However, it is difficult to infer from field studies what the processes are that lead to enhanced CO₂ emission, as these processes operate at a microscopic scale. Such information may be better obtained from complementary experiments under more controlled conditions.

Effects of soil deformation on physical characteristics such as air permeability and diffusivity have been the subject of many recent laboratory studies (Kirby, 1991a, 1991b; Kirby and Blunden, 1991; O'Sullivan et al., 1999), but only a few studies have addressed the consequences for biological activity and CO₂ emission under controlled conditions (Rovira and Greacen, 1957; Powlson, 1980; Watts et al., 1999). Rovira and Graecen (1957) showed that respiration increased following "laboratory tillage" and related this to the degree of disruption of aggregates. Watts et al. (1999) found increased respiration following compression or shearing of undisturbed samples and concluded that soil mechanics and biology are highly correlated as mechanical processes modify the soil pore space. None of these studies accounted for the stress history of the soil, which affects the response of soil to forces, or used combinations of shear and compressive forces as would be experienced in the field. In addition, manual handling of samples was required to place them inside the respirometers after shear forces had been applied. This may have resulted in a further uncontrolled deformation. Moreover, it excludes quantification of immediate responses at the moment the deformation is performed, thereby missing, for example, the flush of CO₂ that is released from large voids generated by deformation (Reicosky, 1997; Kessavalou et al., 1998).

We describe a technique that enables quantification of the short-term (24 h) dynamics in CO₂ emission induced by soil deformation in controlled and repeatable laboratory experiments. We designed a series of respirometers in which direct shear and compressive (normal) stresses, either independently or combined, can be applied to soil samples while we simultaneously monitored the dynamics of the CO₂ emission. Shear and compression measurements were performed with samples from repacked sieved aggregates within the respirometers, using a wide range of combinations of forces. Following an experimental protocol described by Kirby (1991a, 1991b), we used the mechanical behavior of the soil to predict consistently whether the samples expand or compress upon shearing. Specific treatments were then selected to quantify the relationship between changes in pore volume and CO₂ emission. Specifically we asked the following questions:

1. How rapidly does CO₂ emission respond to soil deformation, and how long do the changes persist? We concentrate on the CO₂ emission rate in the first 24 h following deformation, while we assess the longer-term effect by comparing this with the emission after 7 d relative to an untreated sample.
   
2. Does the magnitude of a compressive force affect the dynamics of the CO₂ emission? Here we quantify the change in CO₂ emission for several levels of compression.
   
3. Does the effect of shearing on CO₂ emission relate to the changes in bulk volume? Here we use the mechanical behavior of the soil to select treatments in either the expanding or compressing regime following shearing, and we ask if the reduction in bulk pore volume leads to a reduction in CO₂ emission, for example, due to reduced gas diffusion.
   

We discuss how the results obtained under controlled conditions can be placed in the context of tillage operations in the field, and how the technique described here can be used further to distinguish between processes that lead to enhanced CO₂ emission following tillage operations.

	Materials and methods

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

 
Respirometer System
Twelve respirometers were designed so that compressive and shear forces could be applied to soil samples while continuously measuring the CO₂ emission (Fig. 1) . Each respirometer consists of three separate units: a closed respirometer chamber containing the soil sample (Fig. 1a), a measuring cell with an alkali solution to trap and quantify CO₂ (Fig. 1b), and a pump to circulate air through the chamber and measuring cell (Fig. 1c). The chamber is connected to the measuring cell and pump by two 0.5-m lengths of flexible rubber tube (3-mm bore and 3-mm wall thickness). This allowed all 12 chambers to be placed in turn on a standard soil mechanics, direct-shear apparatus (Wykeham- Farrance, model 2500, Slough, UK) without interrupting the pump or the trapping of CO₂ in the alkali solution.

View larger version (39K): [in this window] [in a new window]   Fig. 1 The respirometer comprising a chamber in which a soil sample is contained (a) , an electro-conductive cell (b), and a pump to circulate air through the cell and chamber (c). The respirometer chamber can be placed in standard direct shear equipment to apply controlled mechanical disturbances of soil structure with simultaneous monitoring of the CO2 emission

Normal loads are applied by hanging weights and the force is transferred to the soil sample via a bridge hanger to the rod and piston, which pass through the lid. Changes in the volume of the sample were inferred from the vertical displacement of the weight hanger relative to the base plate of the direct shear apparatus. Details of the applied shear and normal stresses are specified below. During respiration measurements, the normal load was removed and the platen was raised (Fig. 1) to allow for unrestrained gas exchange between the soil sample and the atmosphere in the chamber.

Measuring Cell
Each cell consists of a glass cylinder (14-mm diam. and 170-mm height), which was mounted on a plexiglass stand. The cylinder was filled with 20 mL of 0.05 M KOH. The OH converts the CO₂ evolved from the soil into CO^2-₃, which has an ionic mobility that is 0.368 times that of hydroxide ions (Watts et al., 1999). Therefore, the reaction between CO₂ and OH increases the cell resistance. Cell conductivity was measured using platinum electrodes connected to a measuring circuit with an AC excitation voltage V_E (2.0 V, peak to peak, 125 Hz square wave) and a standard resistor R_S (10 k) provided by a Delta-T, DL2 logger (Delta-T Devices Ltd., Burwell, Cambridge, UK). The cell resistance, R_X, was determined as , where V_x is the voltage measured across the cell. All 12 cells were connected to the data logger and readings (V_x) were taken every 10 min.

Aeration Pump
Twelve, 10 L h^-1, aquarium, diaphragm aeration pumps (Tetra Whisper model 100, Eastleigh, UK) were adapted so that they could be used to circulate air from the respiration chambers through the measuring cells. The air inlet of each measuring cell was just above the electrodes so that the air was bubbled through the KOH solution without affecting the conductivity measurements. The CO₂-free air was then returned to the respirometer chamber.

Calibration and Data Analysis
Each measuring cell was calibrated using successive 40-mL volumes of a standard (6%, w/w) CO₂ gas injected into the respirometer chamber. Following each injection, V_X increased rapidly, reaching a steady value after 2 h. This was then followed by further injections of gas. A monomolecular curve was fitted to the time series following each injection to obtain an accurate estimate of the final level. The estimated final level after each injection followed a sigmoidal shaped relationship with the cumulative amount of CO₂ injected, given by , in which x equals the cumulative amount of CO₂ absorbed in the cell and y₀, a, b, and x₀ are parameters.

The rate of CO₂ emission from the soil sample was estimated from V_X measurements at 10-min intervals. The cumulative amount of CO₂ evolved during each measuring time was estimated by inverse predictions from the sigmoidal curve fitted to calibration data for each of the cells. Subsequently, the average respiration rate was estimated at 100-min intervals from the amount of CO₂ plotted vs. time, by linear interpolation over 20 measurements (taken at 10-min intervals). Finally, the obtained rate was converted from milliliters CO₂ per hour to milligrams CO₂ per cubic meter per second, taking into account the sample size of 114 cm³. Soil samples were left in the sealed chamber for a sufficient time so that an accurate estimation could be made of the CO₂ emission rate before disturbance.

Compression and Shear Characteristics
Sieved aggregates (>50 and <2000 µm) were obtained from an air-dried loamy sand (Dystrochrepts; Kingsmead, UK) with an organic matter content of 1.7% and a C/N ratio of 9.5. The particle-size distribution of the Kingsmead soil is 9.4% clay (<2 µm), 10.8% silt (2–63 µm), 79.8% sand (63–2000 µm). Prior to use, the air-dried soil had been stored for 2 yr. In stainless steel rings that fitted in the respirometer, the air-dried aggregates were packed at a density of 1.27 g cm^-3, then slowly saturated from below before they were allowed to equilibrate at -5.5 kPa on a tension table for 24 h at 23°C. After equilibration, the soil samples had a gravimetric water content of 14.6 g g^-1. Assuming a particle density of 2.6 g cm^-3, the volumetric air and water content are 0.332 and 0.188, respectively. Equilibrated samples were compressed at preidentified levels of 0, 79, or 430 kPa to provide samples with different stress histories. Samples were stored for 1 wk at 23°C to allow for equilibration of the biological activity prior to measuring the CO₂ evolution rate and prior to shearing.

The compression and shear characteristics of the soil samples were determined within the respirometer. The normal stress was controlled by putting successive weights (up to 98 kg) onto the hanger that rested on the platen (Fig. 1). The maximum stress that was applied on the samples within the respirometer was 430 kPa. Once the maximum stress was reached, the process was reversed. The volume strain was then calculated as the ratio of the change in volume (inferred from vertical displacement) to the original volume. All measurements were carried out in triplicate.

The shear characteristics were summarized by the shear stress that was required for a horizontal displacement of the upper half of a soil sample relative to the bottom half (shear strain) and the resulting changes in volume (volume strain). Again, these measurements were all performed within the respirometers. A displacement rate of 1.18 mm min^-1 (gears 60/30, position A) was selected for a period of up to 5 min. This resulted in a shear strain of 0.12 (corresponding with a 5.9-mm horizontal displacement of the upper ring). At maximum displacement a new surface area is exposed, which is 22% of the original surface area before shearing. To minimize the gas exchange through the newly exposed surface area, a 50-mm-wide flexible seal (PM-992 laboratory film, Parafilm "M", American National Can, Chicago, IL) was put around the rings during the respiration measurements. The normal stresses that were applied during shearing were either equal to or less than the normal load applied before shearing, and expressed as a ratio (normal load during shearing/ normal load prior to shearing). Shear tests were performed for five stress ratios ranging from 0.05 (normal load 1/20th of the load prior to shearing) to 1 (normal load equal to load prior to shearing). Volume changes were recorded for shearing at each of the stress ratios. From this relationship treatments were identified that resulted in either volume expansion or consolidation upon shearing.

Soil Mechanics and Carbon Dioxide Emission
The first set of experiments aimed to quantify the immediate (hours) and longer-term (after 1 wk) effects of compression upon CO₂ emission. For the long-term effects, six increasing levels of compression ranging from 0 to 430 kPa were selected. After compression, the samples were stored at 23°C for 7 d and subsequently placed into the respirometers. The respiration rate was determined as the average rate for 8 h. There were two replicates at each compression level within each measurement series, and the series was repeated three times.

Short-term responses were investigated by compressing samples while monitoring the change in CO₂ emission. Carbon dioxide emission was measured for 15 h prior to compression. The samples were subsequently compressed within the respirometer while we monitored the CO₂ emission continuously. The samples were either moderately (79 kPa) or severely (430 kPa) compressed. There were five replicates at each compression level, and the entire series was repeated, leading to a total of 10 replicates at each compression level.

The effect of shearing on short-term dynamics in CO₂ emission was also monitored for samples with different stress histories. Soil samples had been moderately (79 kPa) or severely (430 kPa) compressed 1 wk prior to the shearing. Samples were sheared at a constant rate of 1.18 mm min^-1 up to a horizontal displacement of 5.9 mm (shear strain 0.12). Various normal loads were applied during shearing to generate stress ratios (normal load during shearing/normal load prior to shearing) of 0.05, which resulted in volume expansion, or 1, which resulted in further consolidation. Treatment combinations and the resulting volume changes are listed in Table 1 . There were five replicates for each treatment within one series, and each series was repeated. Untreated (i.e., not sheared or compressed) samples were used as a reference.

	Results

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View larger version (22K): [in this window] [in a new window]   Fig. 2 The ratio between volume strain and stress for a sample of repacked sieved aggregates. The dots along the continuous line are measured data for replicated samples with successive increase in normal load up to 430 kPa, followed by a successive decrease. The arrows indicate the pathways followed. The numbers (1–6) along the line are the treatments (for normal load) that were selected to asses the effect upon biological activity. The dashed line represents the critical state line that separates conditions that lead to consolidation from those that lead to volume expansion upon shearing (see text for further details). Letters (A–D) in the graph refer to four conditions that were selected to asses the effect of shearing upon biological activity, as defined in the text and summarized in Table 1

View larger version (14K): [in this window] [in a new window]   Fig. 3 The relationship between volume strain (change in volume/original volume) and stress ratio (normal load during shearing/maximum normal load experienced by the sample before shearing). The stress ratio at which the sample behavior switched from expansion to consolidation upon shearing is indicated by the dotted line and determines the location of the critical state line in Fig. 2

View larger version (28K): [in this window] [in a new window]   Fig. 4 Dynamics in CO2 emission rate before and after controlled shear and compressive soil deformations . All treatments resulted in enhanced CO2 emission of the soil upon deformation (see Table 1). The time refers to the hours before (negative values) or following (positive values) the soil deformations performed within the respirometer. (a) Compression alone with normal loads of 430 kPa, or (b) 79 kPa (6 and 4, respectively, in Fig. 2). Combined shear and compressive stresses of samples with different (precompression) stress histories: (c) samples precompressed with 430 kPa were sheared with the same load during shearing (Stress ratio 1, Point B in Fig. 2) or (d) reduced load (Stress ratio 0.05, Point D in Fig. 2); (e) samples precompressed with 79 kPa were sheared with the same load during shearing (Stress ratio 1, Point A in Fig. 2) or (f) reduced load (Stress ratio 0.05, Point C in Fig. 2). (g) The effect of the mechanical disturbance as summarized in Table 1 was assessed relative to untreated soil samples

View larger version (14K): [in this window] [in a new window]   Fig. 5 The relationship between the volume strain and the relative CO2 emission rate (rate for compressed sample/rate for uncompressed sample) measured 1 wk after compression of a sample of repacked sieved aggregates equilibrated at -5.5 kPa

	Discussion

TOP ABSTRACT INTRODUCTION Materials and methods Results Discussion REFERENCES

Selection and Preparation of Samples
In this study we used repacked, sieved soil that was rewetted under controlled conditions to minimize variability between soil samples. The mechanical characteristics were highly reproducible; this enabled selection of treatments that would lead to either an increase or decrease in soil volume upon deformation. It was more difficult to reduce the variability in biological activity, even for samples prepared under such controlled conditions. There was high variability between replicates within a series, and also between the mean basal respiration rate of different series before deformation (Table 1, Fig. 4). The variability in respiration within each treatment was dealt with by including a sufficient number of replicates (10 for each treatment). The nature of the difference in basal respiration rate between two series was not clear, but may be due to small differences in sample preparation (sieving, packing, and wetting), which was done separately for each series. Overall, the samples used in this study had a basal respiration rate lower than for field samples (Watts et al., 1999), probably due to the absence of roots and the low (1.7%) soil organic matter content. In addition, much of the labile organic matter pool may have been lost in the field or may have been used in the period between wetting and the initiation of the measurements.

Carbon Dioxide Emission and Soil Deformation
The sustained increase in CO₂ emission following soil compression was striking. Despite a considerable reduction in soil volume and concomitantly air-filled pore space, we found an increase in CO₂ emission (Fig. 5, Table 1). An initial increase or flush might be expected as CO₂ is forced from the pores upon compression. Alternatively, a reduction in air-filled pore space will reduce the diffusion of gases and therefore reduce the C mineralization, which is the main source of the CO₂ emission in soil (Zibilske, 1994). An increase in CO₂ emission is consistent with the observations in many field studies that enhanced tillage is associated with increased C mineralization (Kern and Johnson, 1993; Franzluebbers et al., 1994; Fortin et al., 1996). Watts et al. (1999) also found similar increases in respiration. Our results differ from Watts et al. (1999), who found a peak value in CO₂ emission at an applied stress approximately equal to the precompressive stress. They concluded that compression had a threefold effect on respiration: (i) some new pore space is created by stresses, which may expose previously protected organic matter; (ii) pore water becomes redistributed, which may provide a flush or translocation of dissolved organic matter; and (iii) air-filled pore space and hence gas diffusion is reduced. The first two would result in an increase in respiration, whereas the latter would result in a reduction. Apparently, in our experiment, the gas-diffusion did not diminish sufficiently to reduce biological activity, or at least not to an extent that would compensate for the increased availability of organic matter. We expect that if the compression is performed under wetter conditions, a decrease or optimum in relation to normal load could be found. We suggest that a peak value in biological response upon compression as found by Watts et al. (1999) results from a reduction in diffusion that counteracts the increase in availability of organic matter. Such a tradeoff between altered physical conditions and enhanced availability of soil organic matter requires further investigation, for which the equipment designed in this study can form the basis.

The biological responses to shear forces were comparable with those for compression. The respiration rate increased immediately upon shearing of the samples. This increase occurred for samples that both increased or decreased in volume. Although a reduction in CO₂ emission was hypothesized for conditions under which the soil became more compressed upon shearing, the results are consistent with field studies where enhanced CO₂ emissions are found immediately following tillage (Kessavalou et al., 1998b; Reicosky, 1997). This suggests that the enhanced biological activity in our loamy sand was not the result of improved physical conditions, but more likely caused by enhanced availability of organic matter, which counteracted the changes in physical conditions such as a reduction in pore volume. It is not possible to distinguish directly between exposure of protected organic matter through disruption of aggregates and redistribution of pore water with dissolved organic matter or bacterial colonies. As the center of aggregates can be anaerobic, disruption of aggregates may improve aeration to areas that previously were anaerobic. With the current shear equipment, disruption of aggregates would be expected to occur in a thin shear plane (Kirby, 1991b). However, redistribution of pore water and dissolved organic matter upon compression is likely to occur throughout the soil sample and seems therefore the more plausible explanation for the enhanced biological activity following soil deformations. Disruption of aggregation and redistribution of pore water upon soil deformations warrant further investigation, as they have a considerable impact on the biological activity in disturbed soil samples. The mechanisms involved can have important consequences for other processes involving biological activity, such as breakdown of contaminants either directly (through enhanced bioavailability) or indirectly (through enhanced overall biological activity).

The CO₂ emission changed immediately (within an hour) upon soil deformation. We summarized the effect by averaging the CO₂ emission rate for the first 24 h following the deformation, thereby ignoring short-term dynamics in the rate. However, such dynamics could provide crucial information on underlying processes. Specifically, a short flush in CO₂ emission would be expected if CO₂ enclosed in the aggregates was exposed by opening up voids (Reicosky et al., 1997). Despite that such a flush of CO₂ has been reported from field studies (Reicosky et al., 1997; Rochette and Angers, 1999), it was not evident in our samples. Additional research is required into the conditions under which a flush in CO₂ would occur, with selection of treatments to test specific hypothesis towards the occurrence of such a flush together with the development of nonlinear models for the analysis of the short-term dynamics.

Implication for Biological Activity in the Field
The main conclusion to be drawn from this study is that there is a marked increase in CO₂ release from soil samples following mechanical disturbance of its structure, and that this increase results from enhanced availability of organic matter, either by breaking of aggregates or redistribution of soluble organic matter and bacterial colonies. Altering the physical conditions may then further mediate the biological activity as compacted soils are likely to be more anaerobic or CO₂ is released from previously enclosed voids and forced into the atmosphere. To assess the relevance of our results for organic matter sequestration and biological activity in the field two questions spring to mind: Are the changes significant enough to have an overall effect on the C dynamics, and are the mechanical deformations related to what can be expected in the field?

We have shown that mechanical disturbance of soil gave up to 20% increase in CO₂ emission in the first 24 h following shear at 23°C. This enhanced emission was sustained for 7 d following compression. Although these time scales are relatively short, it would be expected to persist at a lower overall rate for longer periods at the temperatures normally experienced in the field. Successive tillage operations are common practice, and will further enhance the release of organic matter and biological activity (Rovira and Greacen, 1957). The soil type and agricultural history also affect the magnitude of increase in biological activity. As shown by Watts et al. (1999), the effect of shear on respiration was higher for grassland soil than for previously cultivated soil. The use of sieved aggregates in our study may not reflect the level of protection of organic matter as can be experienced in the field. Loss of C as a result of cultivation was found to be higher in larger aggregates (4.5–5 mm) than in smaller aggregates (<50 µm) (Conteh and Blair, 1998). The spatial hierarchical organization of soil aggregates is believed to be important in the degree of protection from decomposition of organic matter (Jastrow et al., 1996). Certainly, larger effects were found at initial trials on undisturbed samples obtained from a field, and increases up to 69% have been reported in field studies (Kessavalou et al., 1998b).

The technique described here enabled us to investigate the effect of combined shear and compressive forces on biological activity. Although the general objective of tillage in most cases will be to improve the porosity, compression will occur at the advancing edges of a plough or at greater depths in the soil. Compaction of soil by vehicles is also often substantial and undesirable. Substantial knowledge of how mechanical forces affect soil physical conditions is required before the likely consequences on biological properties can be understood. It is important that these studies include combinations of shear and normal stresses. Accurate assessment of soil mechanical behavior combined with quantification of the short-term dynamics in CO₂ emission will further enhance our understanding of the relationship between disruption of soil structure and likely biological consequences and may help to optimize tillage operations for sustainable agriculture systems.O'Sullivan Robertson Henshall 1998

	ACKNOWLEDGMENTS

 
This work is part of a linked research grant between Silsoe Research Institute and Rothamsted Experimental Station, funded by the Biotechnology and Biological Research Council, which we gratefully acknowledge. We also thank Professor Chris Gilligan (University of Cambridge) for valuable discussion and comments on the manuscript.

Received for publication October 8, 1999.

	REFERENCES

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