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Round table on water movement, oxygen supply, nutrient supply, and biological pr processes on the micro-scale
Geoderma, 57 (1993) 167-170 Elsevier Science Publishers B.V., Amsterdam
167
Round Table on water movement, oxygen supply, nutrient supply, and biological processes on the micro-scale Chair: P.A. Leffelaar Dept. of Theoretical Production Ecology, WageningenAgricultural University, P.O. Box 430, 6700 AK, Wageningen, The Netherlands Rapporteur: P.A.C. Raats Institute of Soil Fertility, P.O. Box 30003, RA 9750 Haren, The Netherlands In this Round Table three themes were discussed. Theme l When explanatory models on aeration in homogeneous soil are developed, the dynamics of soil micro-organisms is usually included. Therefore physiological parameters, such as relative growth rates and maintenance coefficients, are needed, Unfortunately, these are difficult to separate from the influence of the soil physical and chemical characteristics on micro-organisms. This raises the question whether it is meaningful to measure physiological parameters and soil physical and chemical parameters separately or whether all parameters can be measured in the soil system.
Anderson, Germany: Separate studies of physical and physiological parameters are not very useful. If you want to understand processes at the microbial community level or if you want to compare microbial communities in different soils, you need the habitat of the soil for physiological parameters. Ingham, USA: She agrees with Anderson. In laboratory cultures availability of C, N, and other nutrients tends to be high and the resulting growth rates are meaningless relative to soil conditions. Soil scientists should give detailed descriptions of the habitats for various organisms in the soil. The biologists can then tell which transformations can occur, possibly causing changes of the habitats. Soil scientists and biologists have to interact. Chairman: You seem to agree that physiological parameters have to be studied in situ. Smiles, Australia: Biologists have to provide information to the physicists. Fluxes and concentrations cannot be described, unless the boundary conditions at interfaces of activily respiring organisms and the soil system are specified. This is a challenge for the biologists. Van Breemen, The Netherlands:The clustering of cells on agar plates and in soils is very different. So ideally physiological parameters should be determined in the soil habitat. But it is doubtful that this can be done. Physicists and biologists keep throwing the ball to each other, but it should stick long enough to be worthwhile. Chairman: Are you thinking of adhesion of organisms?
0016-7061/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.
168
ROUNDTABLEREPORTSSESSIONIII
Anderson, Germany: Physiological parameters should be determined at the microbial community level. Comparison of maintenance coefficients in vitro and in situ shows that there is luxury consumption in vitro. Chairman: Rijnaarts and his colleagues of the Department of Microbiology of Wageningen Agricultural University have compared the functioning of free organisms and organisms adhered to teflon spheres, using parameters in the Monod equation as the basis for comparison. The specific growth rate of the adhered organisms first followed the Monod equation with a small half saturation value, but later slowed down. The free organisms followed the Monod equation initially, with an order of magnitude larger saturation value, and were poisoned at higher concentrations. Theme 2 Field soils may contain large biopores and cracks. Preferential flow in macropores may contribute to heterogeneity of the distribution of water and nutrients. Decomposable organic matter is usually also heterogeneously distributed. Soil physical properties generally have widely different values at different scales and as a result the inherent characteristic lengths and times of processes are also scale dependent. This raises the question, how should heterogeneity be dealt with in connection with soil aeration and water movement.
Bouma, The Netherlands: One should not lump ( 1 ) macropores and cracks in structured soils, (2) distribution of the decomposable organic matter content, and (3) distribution of water and nutrients resulting from preferential flow paths. Rather these aspects should be treated in a sequential manner. Roots often tend to follow cracks and other large continuous pores and this, apart from the influence of soil tillage, determines strongly the distribution of decomposable organic matter. Macropores and cracks in structured soils may lead to preferential flow. Such flow has its characteristic dynamics and techniques are available now to monitor it.In his lecture Altemueiler pointed out that we should watch out with regarding aggregates as discrete entities. The size distribution resulting from sieving depends on the procedure followed, longer sieving gives more smaller aggregates. If you look at an undisturbed soil in a thin section you often (not always) see a matrix with larger pores in it. It may then be more meaningful to look at characteristic patterns near larger pores.
Chairman: It is not our intention to lump separate causes but to indicate various aspects of the causes of heterogeneity. Box, USA: In the nineteen sixties the soil physicist Grable used maize seed germination to demonstrate at various soil bulk densities that soil aeration becomes a problem at the air entry value (A.R. Grable and E.G. Siemer, 1968. Effect of bulk density, aggregate size and soil water suction on oxygen diffusion, redox potentials and elongation of corn roots. Soil Sci. Soc. Am. Proc., 32: 180-186). Later Fliihler considered some probabilistic aspects of this problem and showed that for germination aeration becomes a problem in the range of suctions of 70 to 100 mb (H. Fliihler, L.H. Stolzy and M.S. Ardakani, 1976. A statistical approach to define soil aeration in respect to denitrification. Soil Sci. 122:115-123). This would not be very different for microbes. Therefore an evaluation of the air content of aggregates under various circumstances may provide a simple and direct approach to the aeration problem. Crawford, UK: Avoid making distinction between macro and micro structures. Rather concentrate on trying to understand the scaling relationships, that is on the functions relating phenomena at different scales. This is the approach used in the poster on gas movement in structured soil.
ROUNDTABLEREPORTSSESSIONIII
169
Rappoldt, The Netherlands: If you have an experimental aggregate with a simple geometry and if the biologists can give all the necessary parameters you can try to describe the situation in full detail (see paper by Leffelaar). Such studies may clarify the nature of the processes. If you have soil heterogeneity at different scales and if you have heterogeneous aggregates you have to give up on a completely deterministic approach, if not for fundamental reasons then definitely for practical reasons, considering the data needed to describe the structure. Homogeneous aggregates may have anoxic centers, heterogeneous aggregates may have scattered anoxic regions, depending on the location of the decomposable organic matter. What we need is a clever choice of macroscopic parameters and establishment of empirical relationships among such parameters.
Chairman: An example of such an approach is the description of biodecomposition of a soil pollutant as a first order decay process, requiring just a decay constant. Of course, for microbiologists such an approach may not be very exciting. Raats, The Netherlands: Detailed studies at smaller scales may be important. Crawford rightly suggests that one always should keep the adjoining scales in mind. Sometimes the study of phenomena at a certain scale yields information about phenomena at smaller scales. A prime example is the information about pore size distribution that can be obtained from studying retention and movement of water at the Darcy-Richards scale. Often one studies processes at a small scale and next wishes to integrate to a larger scale. The work of Rappoldt is a good example. It has been remarked that large scale phenomena are rather dull. However, large scale phenomena become more interesting if they have built-in information from smaller scales. Along a growing root you get quite a succession of organisms. Root growth has been successfully modelled by considering a coordinate system attached to the tip of the growing root. If the root is growing at a constant rate, then in terms of such a coordinate system the process is steady. Silk, among others, has done a lot of work describing root growth in this fashion and she also has analyzed a variety of biochemical processes in roots (see e.g.W.K. Silk, 1984. Quantitative descriptions of development. Ann. Rev. Plant Physiol., 35:479-518). It appears that this approach could also be used to formulate a population dynamics model for succession organisms along roots. Has that been done?
Dighton, UK reacts and says he has not actually developed such a sequential model. But he has studied ecto-mycorrhiza colonization of roots. He looked at interaction between species and colonization patterns in space and time, considering relative growth rates of both roots and the mycorrhizal fungi. To understand the processes in the ecto-mycorrhizal sphere, the bacterial populations have to be considered also. The difficulty is again to determine the necessary physiological parameters at the small scale of interest. The impasse is that the biologists do not yet have the necessary technologies.
Van Noordwijk, The Netherlands: Darrah (1991) (Plant and Soil, 133: 187-191, 1991 and 138:147-158 ) studied the microbial population dynamics along a growing root producing exudates. It was found that if the root grows slow enough, the microbes might respond before the root arrives, whereas if the root grows fast, the microbes might respond after the root tip passes. Crawford, UK: The group of Lynch at Queens College (London) is also looking at microbial dynamics near an exuding root tip. Their theory suggests that the dynamics of the population are chaotically unstable. So far there appears to be not yet any experimental backup for that prediction.
170
ROUNDTABLEREPORTSSESSIONIII
Theme 3 Soil animals have a certain power to change soil structure and with that soil characteristics, e.g., parameters governing transports of water and gases, and the soil penetrability for roots. The time scale at which these processes take place is long (years). Traffic and soil tillage will partially destroy these structures. How can management techniques, such as soil tillage, be implemented so that existing positive effects from soil animals are conserved or promoted? What is the time period that animal-made holes and structures will exist?
McKenzie, Australia Some answer to the second question is contained in the poster on earthworms. At 10 cm depth they last only a few days, whereas at 20 cm depth they may last for up to 2 years.
Marinissen, The Netherlands: The questions posed are difficult and very wide. The answers depend on which species one is talking about. One has to know what animals do. Only when that is known can one look at persistence of holes and possibilities for managing the situation.
Chairman: Can the action of animals render soil tillage superfluous? How do crop yields on tilled and not tilled soils compare? Koehler, Germany: The question only considers the direct effects of the animals. Soil structure is also promoted by indirect effects of soil animals. It is difficult to distinguish between animal made holes and structures which are influenced by activity of animals at a wide range of spatial and temporal scales, down to the scale of microbes.
Chairman: The question arises from two physicists and that may explain why it is rather wide. Anonymous: Regarding the first question, one should ask not only biologists, but also farmers. Without tillage, a lot of money may have to be spend on herbicides. Under dry conditions, drought may be more severe without tillage. In general yields are the same without tillage.
Rappoldt, The Netherlands: The presence of various species may imply a range of characteristic times for the persistence of biopores. However, one can also focus on overall response times. If one has tilled a long time and stops, it is of most interest to know how long it takes to reach a new steady situation.
Box, USA: In studies over a period of 15 to 17 years comparing tillage and non-tillage or directdrill in the southeast of the USA, yields of soybeans and grain sorghum after a few years of nontillage were just as high as with conventual tillage. In this region, similar results are expected for cotton and maize.
Marinissen, The Netherlands: Results will depend on the climate and the crop rotation. In the rather wet climate of the Netherlands and with a narrow rotation including sugar beets and potatoes, soil tillage cannot be missed.
167
Round Table on water movement, oxygen supply, nutrient supply, and biological processes on the micro-scale Chair: P.A. Leffelaar Dept. of Theoretical Production Ecology, WageningenAgricultural University, P.O. Box 430, 6700 AK, Wageningen, The Netherlands Rapporteur: P.A.C. Raats Institute of Soil Fertility, P.O. Box 30003, RA 9750 Haren, The Netherlands In this Round Table three themes were discussed. Theme l When explanatory models on aeration in homogeneous soil are developed, the dynamics of soil micro-organisms is usually included. Therefore physiological parameters, such as relative growth rates and maintenance coefficients, are needed, Unfortunately, these are difficult to separate from the influence of the soil physical and chemical characteristics on micro-organisms. This raises the question whether it is meaningful to measure physiological parameters and soil physical and chemical parameters separately or whether all parameters can be measured in the soil system.
Anderson, Germany: Separate studies of physical and physiological parameters are not very useful. If you want to understand processes at the microbial community level or if you want to compare microbial communities in different soils, you need the habitat of the soil for physiological parameters. Ingham, USA: She agrees with Anderson. In laboratory cultures availability of C, N, and other nutrients tends to be high and the resulting growth rates are meaningless relative to soil conditions. Soil scientists should give detailed descriptions of the habitats for various organisms in the soil. The biologists can then tell which transformations can occur, possibly causing changes of the habitats. Soil scientists and biologists have to interact. Chairman: You seem to agree that physiological parameters have to be studied in situ. Smiles, Australia: Biologists have to provide information to the physicists. Fluxes and concentrations cannot be described, unless the boundary conditions at interfaces of activily respiring organisms and the soil system are specified. This is a challenge for the biologists. Van Breemen, The Netherlands:The clustering of cells on agar plates and in soils is very different. So ideally physiological parameters should be determined in the soil habitat. But it is doubtful that this can be done. Physicists and biologists keep throwing the ball to each other, but it should stick long enough to be worthwhile. Chairman: Are you thinking of adhesion of organisms?
0016-7061/93/$06.00 © 1993 Elsevier Science Publishers B.V. All fights reserved.
168
ROUNDTABLEREPORTSSESSIONIII
Anderson, Germany: Physiological parameters should be determined at the microbial community level. Comparison of maintenance coefficients in vitro and in situ shows that there is luxury consumption in vitro. Chairman: Rijnaarts and his colleagues of the Department of Microbiology of Wageningen Agricultural University have compared the functioning of free organisms and organisms adhered to teflon spheres, using parameters in the Monod equation as the basis for comparison. The specific growth rate of the adhered organisms first followed the Monod equation with a small half saturation value, but later slowed down. The free organisms followed the Monod equation initially, with an order of magnitude larger saturation value, and were poisoned at higher concentrations. Theme 2 Field soils may contain large biopores and cracks. Preferential flow in macropores may contribute to heterogeneity of the distribution of water and nutrients. Decomposable organic matter is usually also heterogeneously distributed. Soil physical properties generally have widely different values at different scales and as a result the inherent characteristic lengths and times of processes are also scale dependent. This raises the question, how should heterogeneity be dealt with in connection with soil aeration and water movement.
Bouma, The Netherlands: One should not lump ( 1 ) macropores and cracks in structured soils, (2) distribution of the decomposable organic matter content, and (3) distribution of water and nutrients resulting from preferential flow paths. Rather these aspects should be treated in a sequential manner. Roots often tend to follow cracks and other large continuous pores and this, apart from the influence of soil tillage, determines strongly the distribution of decomposable organic matter. Macropores and cracks in structured soils may lead to preferential flow. Such flow has its characteristic dynamics and techniques are available now to monitor it.In his lecture Altemueiler pointed out that we should watch out with regarding aggregates as discrete entities. The size distribution resulting from sieving depends on the procedure followed, longer sieving gives more smaller aggregates. If you look at an undisturbed soil in a thin section you often (not always) see a matrix with larger pores in it. It may then be more meaningful to look at characteristic patterns near larger pores.
Chairman: It is not our intention to lump separate causes but to indicate various aspects of the causes of heterogeneity. Box, USA: In the nineteen sixties the soil physicist Grable used maize seed germination to demonstrate at various soil bulk densities that soil aeration becomes a problem at the air entry value (A.R. Grable and E.G. Siemer, 1968. Effect of bulk density, aggregate size and soil water suction on oxygen diffusion, redox potentials and elongation of corn roots. Soil Sci. Soc. Am. Proc., 32: 180-186). Later Fliihler considered some probabilistic aspects of this problem and showed that for germination aeration becomes a problem in the range of suctions of 70 to 100 mb (H. Fliihler, L.H. Stolzy and M.S. Ardakani, 1976. A statistical approach to define soil aeration in respect to denitrification. Soil Sci. 122:115-123). This would not be very different for microbes. Therefore an evaluation of the air content of aggregates under various circumstances may provide a simple and direct approach to the aeration problem. Crawford, UK: Avoid making distinction between macro and micro structures. Rather concentrate on trying to understand the scaling relationships, that is on the functions relating phenomena at different scales. This is the approach used in the poster on gas movement in structured soil.
ROUNDTABLEREPORTSSESSIONIII
169
Rappoldt, The Netherlands: If you have an experimental aggregate with a simple geometry and if the biologists can give all the necessary parameters you can try to describe the situation in full detail (see paper by Leffelaar). Such studies may clarify the nature of the processes. If you have soil heterogeneity at different scales and if you have heterogeneous aggregates you have to give up on a completely deterministic approach, if not for fundamental reasons then definitely for practical reasons, considering the data needed to describe the structure. Homogeneous aggregates may have anoxic centers, heterogeneous aggregates may have scattered anoxic regions, depending on the location of the decomposable organic matter. What we need is a clever choice of macroscopic parameters and establishment of empirical relationships among such parameters.
Chairman: An example of such an approach is the description of biodecomposition of a soil pollutant as a first order decay process, requiring just a decay constant. Of course, for microbiologists such an approach may not be very exciting. Raats, The Netherlands: Detailed studies at smaller scales may be important. Crawford rightly suggests that one always should keep the adjoining scales in mind. Sometimes the study of phenomena at a certain scale yields information about phenomena at smaller scales. A prime example is the information about pore size distribution that can be obtained from studying retention and movement of water at the Darcy-Richards scale. Often one studies processes at a small scale and next wishes to integrate to a larger scale. The work of Rappoldt is a good example. It has been remarked that large scale phenomena are rather dull. However, large scale phenomena become more interesting if they have built-in information from smaller scales. Along a growing root you get quite a succession of organisms. Root growth has been successfully modelled by considering a coordinate system attached to the tip of the growing root. If the root is growing at a constant rate, then in terms of such a coordinate system the process is steady. Silk, among others, has done a lot of work describing root growth in this fashion and she also has analyzed a variety of biochemical processes in roots (see e.g.W.K. Silk, 1984. Quantitative descriptions of development. Ann. Rev. Plant Physiol., 35:479-518). It appears that this approach could also be used to formulate a population dynamics model for succession organisms along roots. Has that been done?
Dighton, UK reacts and says he has not actually developed such a sequential model. But he has studied ecto-mycorrhiza colonization of roots. He looked at interaction between species and colonization patterns in space and time, considering relative growth rates of both roots and the mycorrhizal fungi. To understand the processes in the ecto-mycorrhizal sphere, the bacterial populations have to be considered also. The difficulty is again to determine the necessary physiological parameters at the small scale of interest. The impasse is that the biologists do not yet have the necessary technologies.
Van Noordwijk, The Netherlands: Darrah (1991) (Plant and Soil, 133: 187-191, 1991 and 138:147-158 ) studied the microbial population dynamics along a growing root producing exudates. It was found that if the root grows slow enough, the microbes might respond before the root arrives, whereas if the root grows fast, the microbes might respond after the root tip passes. Crawford, UK: The group of Lynch at Queens College (London) is also looking at microbial dynamics near an exuding root tip. Their theory suggests that the dynamics of the population are chaotically unstable. So far there appears to be not yet any experimental backup for that prediction.
170
ROUNDTABLEREPORTSSESSIONIII
Theme 3 Soil animals have a certain power to change soil structure and with that soil characteristics, e.g., parameters governing transports of water and gases, and the soil penetrability for roots. The time scale at which these processes take place is long (years). Traffic and soil tillage will partially destroy these structures. How can management techniques, such as soil tillage, be implemented so that existing positive effects from soil animals are conserved or promoted? What is the time period that animal-made holes and structures will exist?
McKenzie, Australia Some answer to the second question is contained in the poster on earthworms. At 10 cm depth they last only a few days, whereas at 20 cm depth they may last for up to 2 years.
Marinissen, The Netherlands: The questions posed are difficult and very wide. The answers depend on which species one is talking about. One has to know what animals do. Only when that is known can one look at persistence of holes and possibilities for managing the situation.
Chairman: Can the action of animals render soil tillage superfluous? How do crop yields on tilled and not tilled soils compare? Koehler, Germany: The question only considers the direct effects of the animals. Soil structure is also promoted by indirect effects of soil animals. It is difficult to distinguish between animal made holes and structures which are influenced by activity of animals at a wide range of spatial and temporal scales, down to the scale of microbes.
Chairman: The question arises from two physicists and that may explain why it is rather wide. Anonymous: Regarding the first question, one should ask not only biologists, but also farmers. Without tillage, a lot of money may have to be spend on herbicides. Under dry conditions, drought may be more severe without tillage. In general yields are the same without tillage.
Rappoldt, The Netherlands: The presence of various species may imply a range of characteristic times for the persistence of biopores. However, one can also focus on overall response times. If one has tilled a long time and stops, it is of most interest to know how long it takes to reach a new steady situation.
Box, USA: In studies over a period of 15 to 17 years comparing tillage and non-tillage or directdrill in the southeast of the USA, yields of soybeans and grain sorghum after a few years of nontillage were just as high as with conventual tillage. In this region, similar results are expected for cotton and maize.
Marinissen, The Netherlands: Results will depend on the climate and the crop rotation. In the rather wet climate of the Netherlands and with a narrow rotation including sugar beets and potatoes, soil tillage cannot be missed.