Directory UMM :Data Elmu:jurnal:S:Soil & Tillage Research:Vol53.Issue3-4.Feb2000:
Soil & Tillage Research 53 (2000) 201±213
Tillage, habitat space and function of soil microbes
I.M. Young*, K. Ritz
Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Cellular and Environmental Physiology,
Dundee, Scotland DD2 5DA, UK
Accepted 16 July 1999
Abstract
This review examines the effect of tillage on microbial habitat space, and the roles of microbes in in¯uencing Ntransformation processes within a heterogeneous soil environment. Literature relating tillage to microbial processes is assessed
critically focusing on (a) degrees of physical disruption and N-processes, (b) interactions between organisms and the soil pore
network, and (c) the role of soil structure in mediating oxygen movement to sites of microbial activity in soil. Spatial
heterogeneity is shown to be a key characteristic of soil structure and N-transformation processes, impacting on predator:prey
relations, microbial habitable pore space, and the modelling of the soil system with respect to denitri®cation. The latter area is
discussed with respect to the notion of how a functional appraisal of soil structure may be approached theoretically, at the
aggregate and soil pro®le scale. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil structure; Soil pore network; Spatial heterogeneity; Microbial activity; Nitrogen transformations; Tillage
1. Introduction
A large body of literature exists relating tillage
practices to microbial activity and microbially
mediated processes (e.g., Haban, 1986; Bowman
et al., 1990; Shtina and Kirov, 1992; Franzluebbers
et al., 1994, 1995; McGarty et al., 1995). Manipulation
of soil structure is one of the principal means by which
microbial dynamics can be controlled both at the
small- and ®eld-scale (Elliott and Coleman, 1988).
*
Corresponding author. Present address: Statistical and Informatics Modelling of Biological Systems, Abertay University,
Kydel Building, Bell Street, Dundee, DD1 1HG. Tel.: 44-1382308646; fax: 44-1382-562426.
E-mail address: i.m.young@tay.ac.uk (I.M. Young).
This control arises through alterations in habitat space,
water and substrate distribution, and the spatial
arrangement of pore pathways. The primary effect
of tillage is to physically disturb the soil pro®le.
Microbial inhabitants of the soil will react differently
to such disturbance. The speci®c effect will depend
largely on the disturbance that occurs, or is `sensed', at
the spatial scale to which the organisms are sensitive.
For instance, an extensive network of fungal hyphae
ramifying through the soil pro®le may be affected
dramatically by a plough tearing apart hyphal connections, and disrupting ¯ow paths within the mycelium. Bacterial colonies living in the centre of
aggregates may, on the other hand, remain initially
unaffected, as long as the zone of soil in which they
inhabit remains largely intact. These intuitive consid-
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 1 0 6 - 3
202
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
erations focus on a number of important issues for the
farmer; ranging from the scale of habitat space within
which organisms live, to the ®nal effect tillage has on
processes within the soil system.
Different tillage systems will disturb the physical
framework of the soil to different degrees (Gantzer
and Blake, 1978), affecting changes in organic matter
levels, mineral concentrations and physical parameters (Bowman et al., 1990). In this review we
examine the role of soil structure, and changes in
structure through tillage operations, in affecting some
microbially mediated processes.
2. No-tillage versus conventional tillage
The two extremes of the physical disruption spectrum are represented by comparisons of soil parameters associated with conventional and reduced
tillage systems. The latter is commonly referred to
as no-tillage, no-till, or minimum tillage in the literature, and connections have been made between no-till
systems and `bene®cial' effects on soil micro-organisms (Elliott and Coleman, 1988).
Doran (1980, 1987) and Linn and Doran (1984a, b)
present convincing scenarios of the effects of largescale disruption of soil on the small-scale behaviour,
existence and function of soil micro-organisms. In this
work, these relations centre on the changes in soil
structure, and associated moisture regimes, in each
tillage system. A general picture of the relation
between microbial activity and soil moisture is
described by Linn and Doran (1984a) and summarised
in Fig. 1, and illustrates that the wetter, denser and
cooler conditions typically associated with reduced
tillage systems result in higher amounts of organic
matter and greater microbial activity/biomass, principally in the upper layers of the soil (Lynch and
Panting, 1980; Blevins et al., 1983, 1984; Arshad
et al., 1990; Dalal et al., 1991). Some knock-on effects
are greater leaching of mineral-N (Carter and Rennie,
1982) and greater denitri®cation rates (Aulak et al.,
1984). For instance, Doran (1987) found that, in seven
no-tilled soils, microbial biomass and potentially
mineralisable nitrogen averaged 54 and 37% higher,
respectively, than those in the surface layer of
ploughed soils. Deeper in the soil pro®les (7.5±
30 cm) differences between tillage treatments were
negligible.
The body of work carried out in comparing no-till
and conventional tillage, relating N-transformation
processes to tillage systems, is compelling. At one
scale, differences in physical parameters (bulk density,
volumetric moisture content, etc.) are related to the
presence of speci®c micro-organisms (Linn and
Doran, 1984a), enzyme activity (Sequi et al., 1985;
Doran, 1987; Pagliai and De Nobili, 1993), through to
differences in CO2 and N2O production (Linn and
Fig. 1. Schematic diagram of the relation between microbial activity and soil moisture (adapted from Linn and Doran, 1984a).
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
Doran, 1984b). A clear connection exists, and emphasis is often placed on, the role of water and associated
bulk density differences between tillage systems.
However, there are interesting departures from the
proposed scenarios, which only become apparent by
comparing the body of work which Doran and coworkers have produced.
Linn and Doran (1984a) state that ``Populations of
aerobic and anaerobic micro-organisms in the surface
(0±75 mm) of no-till soils, were generally greater than
those from conventionally tilled soil''. This is connected by the authors to differences in bulk densities,
volumetric water contents and water-®lled pore space.
The latter term actually refers to the degree of saturation of the pore space. Data from two separate years
(1980 and 1981), at ®ve or more different locations in
the United States, were examined. Despite the statement of Linn and Doran's (1984a) it is clear from the
data presented that no signi®cant differences were
observed in 1980, for total aerobic and anaerobic
organisms, whilst large and signi®cant differences
are seen in the physical parameters. This is supported
by data from the same year, from the same locations
(Doran, 1987), where signi®cant differences were
observed for microbial biomass measurements. Several points are worth attention. Firstly, in 1980,
although no differences were observed in total aerobes
there were large and signi®cant differences in microbial biomass. Secondly, the observed link between soil
physical parameters and microbial parameters, which
is seen in 1981, is not seen in 1980. The question is
why? It is clear that substrate quantity and quality play
a leading role in modulating microbial activity. The
quality of organic matter plays an important role in
mediating the presence and functioning of microorganisms over and above any observed physical
differences. Arshad et al. (1990) found that no-till
soil had greater amounts of carbohydrates, amino
acids, aliphatic C and less aromatic C. Schulten
et al. (1990) observed similar differences in organic
matter quality between two such tillage systems.
Given that no-till systems also have greater amounts
of organic-C than conventional systems (Doran,
1987), it is probable that the quality of the organic
matter as well as the quantity in no-till may have been
suf®cient to provide the observed differences in
microbial populations and activity, despite the similarity in microbial numbers seen in both tillage
203
systems. Another reason may be that the bulk physical
parameters were not measured at a high enough
resolution to account for the link between moisture
and microbial activity. There is a growing awareness
that the measurement of many soil parameters is
scale-dependent (Starr et al., 1995). Whilst the overall
moisture contents between tillage treatments may
have been dissimilar, the distribution of moisture in
the soil pro®les, at scales relevant to microbial populations, may have been the same. We discuss this
further below, in relation to modelling N-transformation events.
3. Organism interactions
Elliott and Coleman (1988) point out that control of
soil structure is one of the most effective ways to
manipulate soil biota. In this section we will examine
the predator:prey dynamics which may interact with
N-transformation processes, brie¯y outlining the principles of predator±prey interactions and the importance of microbial community structure.
3.1. Effect of tillage on soil structure
The habitat space in soils is essentially de®ned by
the architecture of the soil pore network. The topography of this pore space in¯uences strongly the
nature and extent of interactions between the myriad
of organisms inhabiting the soil. In no-till soils biological activity is characteristically higher than in
tilled soils, resulting in the formation of more pores
of biological origin, e.g., macropores due to earthworm activity (Hendrix et al., 1987; Shipitalo and
Protz, 1987; Drees et al., 1994).
Soil pore networks can be characterised indirectly
by means of moisture release curves, bulk density,
porosity measurements and aggregate size distributions, or directly by soil thin sections and Xor gamma-ray computer-assisted tomography (see
Fig. 2, and Joschko et al., 1993; Aylmore, 1993).
The major problem with the indirect approaches is
that they are gross parameters averaging across many
spatial scales. Aggregate size distributions based on
sieving techniques provide little information as to how
the aggregates were connected in the original ped, i.e.,
their context in relation to the overall soil matrix is
204
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
Fig. 2. X-ray computer-assisted tomography images of macropore
network (>1 mm) in soil cores (10 cm diameter) taken from (a) notilled and (b) conventional tilled ®eld plots. Top of these cores is
the soil surface. Continuity of macropores is signi®cantly increased
in the pro®le of the no-till system. Courtesy of H. Rogasik, M.
Joschko and J. Brunotte.
lost, and thus they do not provide information about
the global pore architecture. Nonetheless some information can be gleaned from such data in that there are
obviously relationships between mean aggregate sizes
and the way such structures are likely to pack together.
Aggregate sizes have generally been found to be
greater in no-till compared to tilled soils (Drees et
al., 1994; Lal et al., 1994), although Vyn and Raim-
bault (1993) found no-till resulted in a lower proportion of aggregates
Tillage, habitat space and function of soil microbes
I.M. Young*, K. Ritz
Soil-Plant Dynamics Unit, Scottish Crop Research Institute, Cellular and Environmental Physiology,
Dundee, Scotland DD2 5DA, UK
Accepted 16 July 1999
Abstract
This review examines the effect of tillage on microbial habitat space, and the roles of microbes in in¯uencing Ntransformation processes within a heterogeneous soil environment. Literature relating tillage to microbial processes is assessed
critically focusing on (a) degrees of physical disruption and N-processes, (b) interactions between organisms and the soil pore
network, and (c) the role of soil structure in mediating oxygen movement to sites of microbial activity in soil. Spatial
heterogeneity is shown to be a key characteristic of soil structure and N-transformation processes, impacting on predator:prey
relations, microbial habitable pore space, and the modelling of the soil system with respect to denitri®cation. The latter area is
discussed with respect to the notion of how a functional appraisal of soil structure may be approached theoretically, at the
aggregate and soil pro®le scale. # 2000 Elsevier Science B.V. All rights reserved.
Keywords: Soil structure; Soil pore network; Spatial heterogeneity; Microbial activity; Nitrogen transformations; Tillage
1. Introduction
A large body of literature exists relating tillage
practices to microbial activity and microbially
mediated processes (e.g., Haban, 1986; Bowman
et al., 1990; Shtina and Kirov, 1992; Franzluebbers
et al., 1994, 1995; McGarty et al., 1995). Manipulation
of soil structure is one of the principal means by which
microbial dynamics can be controlled both at the
small- and ®eld-scale (Elliott and Coleman, 1988).
*
Corresponding author. Present address: Statistical and Informatics Modelling of Biological Systems, Abertay University,
Kydel Building, Bell Street, Dundee, DD1 1HG. Tel.: 44-1382308646; fax: 44-1382-562426.
E-mail address: i.m.young@tay.ac.uk (I.M. Young).
This control arises through alterations in habitat space,
water and substrate distribution, and the spatial
arrangement of pore pathways. The primary effect
of tillage is to physically disturb the soil pro®le.
Microbial inhabitants of the soil will react differently
to such disturbance. The speci®c effect will depend
largely on the disturbance that occurs, or is `sensed', at
the spatial scale to which the organisms are sensitive.
For instance, an extensive network of fungal hyphae
ramifying through the soil pro®le may be affected
dramatically by a plough tearing apart hyphal connections, and disrupting ¯ow paths within the mycelium. Bacterial colonies living in the centre of
aggregates may, on the other hand, remain initially
unaffected, as long as the zone of soil in which they
inhabit remains largely intact. These intuitive consid-
0167-1987/00/$ ± see front matter # 2000 Elsevier Science B.V. All rights reserved.
PII: S 0 1 6 7 - 1 9 8 7 ( 9 9 ) 0 0 1 0 6 - 3
202
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
erations focus on a number of important issues for the
farmer; ranging from the scale of habitat space within
which organisms live, to the ®nal effect tillage has on
processes within the soil system.
Different tillage systems will disturb the physical
framework of the soil to different degrees (Gantzer
and Blake, 1978), affecting changes in organic matter
levels, mineral concentrations and physical parameters (Bowman et al., 1990). In this review we
examine the role of soil structure, and changes in
structure through tillage operations, in affecting some
microbially mediated processes.
2. No-tillage versus conventional tillage
The two extremes of the physical disruption spectrum are represented by comparisons of soil parameters associated with conventional and reduced
tillage systems. The latter is commonly referred to
as no-tillage, no-till, or minimum tillage in the literature, and connections have been made between no-till
systems and `bene®cial' effects on soil micro-organisms (Elliott and Coleman, 1988).
Doran (1980, 1987) and Linn and Doran (1984a, b)
present convincing scenarios of the effects of largescale disruption of soil on the small-scale behaviour,
existence and function of soil micro-organisms. In this
work, these relations centre on the changes in soil
structure, and associated moisture regimes, in each
tillage system. A general picture of the relation
between microbial activity and soil moisture is
described by Linn and Doran (1984a) and summarised
in Fig. 1, and illustrates that the wetter, denser and
cooler conditions typically associated with reduced
tillage systems result in higher amounts of organic
matter and greater microbial activity/biomass, principally in the upper layers of the soil (Lynch and
Panting, 1980; Blevins et al., 1983, 1984; Arshad
et al., 1990; Dalal et al., 1991). Some knock-on effects
are greater leaching of mineral-N (Carter and Rennie,
1982) and greater denitri®cation rates (Aulak et al.,
1984). For instance, Doran (1987) found that, in seven
no-tilled soils, microbial biomass and potentially
mineralisable nitrogen averaged 54 and 37% higher,
respectively, than those in the surface layer of
ploughed soils. Deeper in the soil pro®les (7.5±
30 cm) differences between tillage treatments were
negligible.
The body of work carried out in comparing no-till
and conventional tillage, relating N-transformation
processes to tillage systems, is compelling. At one
scale, differences in physical parameters (bulk density,
volumetric moisture content, etc.) are related to the
presence of speci®c micro-organisms (Linn and
Doran, 1984a), enzyme activity (Sequi et al., 1985;
Doran, 1987; Pagliai and De Nobili, 1993), through to
differences in CO2 and N2O production (Linn and
Fig. 1. Schematic diagram of the relation between microbial activity and soil moisture (adapted from Linn and Doran, 1984a).
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
Doran, 1984b). A clear connection exists, and emphasis is often placed on, the role of water and associated
bulk density differences between tillage systems.
However, there are interesting departures from the
proposed scenarios, which only become apparent by
comparing the body of work which Doran and coworkers have produced.
Linn and Doran (1984a) state that ``Populations of
aerobic and anaerobic micro-organisms in the surface
(0±75 mm) of no-till soils, were generally greater than
those from conventionally tilled soil''. This is connected by the authors to differences in bulk densities,
volumetric water contents and water-®lled pore space.
The latter term actually refers to the degree of saturation of the pore space. Data from two separate years
(1980 and 1981), at ®ve or more different locations in
the United States, were examined. Despite the statement of Linn and Doran's (1984a) it is clear from the
data presented that no signi®cant differences were
observed in 1980, for total aerobic and anaerobic
organisms, whilst large and signi®cant differences
are seen in the physical parameters. This is supported
by data from the same year, from the same locations
(Doran, 1987), where signi®cant differences were
observed for microbial biomass measurements. Several points are worth attention. Firstly, in 1980,
although no differences were observed in total aerobes
there were large and signi®cant differences in microbial biomass. Secondly, the observed link between soil
physical parameters and microbial parameters, which
is seen in 1981, is not seen in 1980. The question is
why? It is clear that substrate quantity and quality play
a leading role in modulating microbial activity. The
quality of organic matter plays an important role in
mediating the presence and functioning of microorganisms over and above any observed physical
differences. Arshad et al. (1990) found that no-till
soil had greater amounts of carbohydrates, amino
acids, aliphatic C and less aromatic C. Schulten
et al. (1990) observed similar differences in organic
matter quality between two such tillage systems.
Given that no-till systems also have greater amounts
of organic-C than conventional systems (Doran,
1987), it is probable that the quality of the organic
matter as well as the quantity in no-till may have been
suf®cient to provide the observed differences in
microbial populations and activity, despite the similarity in microbial numbers seen in both tillage
203
systems. Another reason may be that the bulk physical
parameters were not measured at a high enough
resolution to account for the link between moisture
and microbial activity. There is a growing awareness
that the measurement of many soil parameters is
scale-dependent (Starr et al., 1995). Whilst the overall
moisture contents between tillage treatments may
have been dissimilar, the distribution of moisture in
the soil pro®les, at scales relevant to microbial populations, may have been the same. We discuss this
further below, in relation to modelling N-transformation events.
3. Organism interactions
Elliott and Coleman (1988) point out that control of
soil structure is one of the most effective ways to
manipulate soil biota. In this section we will examine
the predator:prey dynamics which may interact with
N-transformation processes, brie¯y outlining the principles of predator±prey interactions and the importance of microbial community structure.
3.1. Effect of tillage on soil structure
The habitat space in soils is essentially de®ned by
the architecture of the soil pore network. The topography of this pore space in¯uences strongly the
nature and extent of interactions between the myriad
of organisms inhabiting the soil. In no-till soils biological activity is characteristically higher than in
tilled soils, resulting in the formation of more pores
of biological origin, e.g., macropores due to earthworm activity (Hendrix et al., 1987; Shipitalo and
Protz, 1987; Drees et al., 1994).
Soil pore networks can be characterised indirectly
by means of moisture release curves, bulk density,
porosity measurements and aggregate size distributions, or directly by soil thin sections and Xor gamma-ray computer-assisted tomography (see
Fig. 2, and Joschko et al., 1993; Aylmore, 1993).
The major problem with the indirect approaches is
that they are gross parameters averaging across many
spatial scales. Aggregate size distributions based on
sieving techniques provide little information as to how
the aggregates were connected in the original ped, i.e.,
their context in relation to the overall soil matrix is
204
I.M. Young, K. Ritz / Soil & Tillage Research 53 (2000) 201±213
Fig. 2. X-ray computer-assisted tomography images of macropore
network (>1 mm) in soil cores (10 cm diameter) taken from (a) notilled and (b) conventional tilled ®eld plots. Top of these cores is
the soil surface. Continuity of macropores is signi®cantly increased
in the pro®le of the no-till system. Courtesy of H. Rogasik, M.
Joschko and J. Brunotte.
lost, and thus they do not provide information about
the global pore architecture. Nonetheless some information can be gleaned from such data in that there are
obviously relationships between mean aggregate sizes
and the way such structures are likely to pack together.
Aggregate sizes have generally been found to be
greater in no-till compared to tilled soils (Drees et
al., 1994; Lal et al., 1994), although Vyn and Raim-
bault (1993) found no-till resulted in a lower proportion of aggregates