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Chapter 12 Biological Properties
Chapter 12 BIOLOGICAL PROPERTIES
INTRODUCTION
Soil biological activity ranges from a high level at the humid fringe of the arid region to nearly zero in the most arid sections. Soils of the driest part of the Sahara, the dry valleys of the Antarctic, and the center of the Atacama Desert of Chile may be nearly abiotic because of the inhospitality of the climate. In fact, the startling report that no bacteria were found t o a depth of 1m in a soil of the dry Taylor Valley in Antarctica led Horowitz et al. (1972) to make an exhaustive study of a Victoria Valley subsoil in an attempt t o determine whether a sterile soil - never before reported on earth in nontoxic soils - actually did exist in Antarctica. They concluded that their soil was truly sterile and, significantly, nontoxic to introduced organisms. Lack of water was the limiting factor. The likelihood that a barren, mobile sand dune in the most arid part of the Sahara-probably the least hospitable environment in the hot deserts -would be sterile appears t o be remote since wind-borne dust can bring in bacteria and other soil microorganisms from more hospitable areas. Killian and Fehkr (1936) reported a wide variety of microflora in Saharan soils. Microbial studies of uncultivated soils in arid regions are limited; attention has been centered upon croplands. Consequently, data are scarce on the kinds and numbers of microorganisms present and their distribution, although some progress was made in Desert Biome studies conducted under the International Biological Programme. Cameron (1969) has summarized his extensive investigation of the microorganisms of hot and cold deserts in a diagram (Fig.lZ.l) of the diversity of flora as influenced by environmental conditions. As the diversity increases, numbers also increase, with bacteria being the most numerous in favorable environments. The dominant microflora include Bacillus, Schizothrix, Streptomyces, Penicillium, and Aspergillus. As a first approximation, it seems likely that nearly all of the major microbial genera will be present, to a greater or lesser degree, in hot-desert soils if the annual precipitation is 100 mm or more. ALGAE
Soil algae are among the most important microorganisms in arid-region soils because of their drought resistance, growth habit, and effect on soil
BIOLOGICAL PROPERTIES
220
-
Protozoa
I
Algae
1 Unfavorable E n v i r o n m e n t a l conditions
=
1
Favoroble
Fig.12.1. Influence of environmental conditions on diversity of microorganisms (adapted from Cameron, 1969).
organic matter and nitrogen content. They grow best in soils having a neutral reaction and they frequently dominate the surface microflora in dry regions. Forming mats on the soil surface as they do, algae serve to bind soil particles together and to shed water; the first effect is an advantage, the second becomes a disadvantage when it increases runoff. Some blue-green algae have the ability to fix atmospheric nitrogen, and their contribution to the nitrogen status of soils is significant in the absence of fertilizer applications.
Nitrogen and organic matter An incubation study of three Arizona algal soil crusts by Cameron and Fuller (1960) showed that nitrogen content of the crusts increased by 25-42’36 in four weeks (Table 12.1). The Gila sandy loam crust contained lichens as well as algae and had twice as many genera of algae (16) as the other two. Azotobacter were absent from all three crusts. In pure cultures, nitrogenfixing algae fixed much more nitrogen than did the crusts of Table 12.1, which were mixtures of algae, soil, and other matter. One calculation of the nitrogen-supplying power of blue-green algae indicated that a soil surface covered by actively growing algae would be capable of fixing 800 kg of nitrogen per hectare per month. In the field, nitrogen fixation rates are much lower because, among other things, only a fraction of the surface has an algal cover. MacGregor and Johnson (1971) in Arizona measured an increase of
221
ALGAE TABLE 12.1
Nitrogen fixation by algal soil crusts in moist chambers (from Cameron and Fuller, 1960) soil
Tucson sandy loam
Incubation time (weeks)
Nitrogen
0
0.081 0.107
4
Gila fine sandy loam
0 4
0.095 0.119
Gila sandy loam
0 4
0.096 0.136
3-4 g of nitrogen per hour per hectare following a rain. The algal crust covered about 4% of the land area. Algae add organic matter to soils along with nitrogen, and leaching of organic compounds and nitrates increases the amount of those substances in subsurface layers. Carbon/nitrogen ratios of 14 crusts in the Cameron and Fuller (1960)study ranged from 5 to 18, with an average of 8.5. Nitrate nitrogen was detected in nearly all surface and subsurface soil samples. Annual precipitation is about 280 mm in the study area. Cameron (1963,1964) identified a rather bewildering variety of algae in and around the Sonoran Desert in Arizona where the rainfall average ranged from 75 to more than 300 mm. Blue-green algae were represented by 7 families, 32 genera, and 86 species and were by far the most numerous. Other algae (mostly green) belonged to 27 families, 47 genera, and 66 species. Some algae are nitrogen fixers, others are not. Algal growth
The principal factors controlling algal growth are moisture and sunlight. After a rain, renewed growth of algae begins in less than 30 minutes, under favorable conditions. By far the greatest algal concentration is in the surface few centimeters of soil in small surface depressions where water accumulates after rains. The undersides of partially embedded wood and stones and of translucent rocks also are favored habitats because sunlight is present yet water loss is less rapid than it is from more exposed surfaces. Blue-green algae growth is profuse in irrigated fields of hot and regions after a rain or irrigation if the vegetative cover is fairly dense. The combination of adequate sunlight, high air temperatures, and reduced evaporation from the shaded soil promote rapid growth of algae. Fine-textured takyr soils of closed basins have a profuse cover of algae and
222
BIOLOGICAL PROPERTIES
lichens -but no vascular plants - following the few periods when runoff water collects after rains. However, Lobova (1960) believes that the effect of algae and lichens on takyr formation and humus content is small, contrary to the views of some other Soviet scientists. Water relations Microbial crusts have been said to reduce water infiltration and increase runoff in arid regions. Fletcher and Martin (1948) concluded that the opposite was true. They noted that the crusts curled when dry, allowing water t o infiltrate while at the same time protecting the soil from the direct impact of raindrops. An additional beneficial effect of the curled crusts is t o capture grass seeds when the soil is dry. Upon subsequent wetting, the crusts uncurl and cover the wetted seed, thereby helping germination by reducing evaporation. Microbial crusts had organic carbon and nitrogen contents as much as 300% (for carbon) and 400% (for nitrogen) higher than the underlying soil. The commonly observed phenomenon of better growth of grass seedlings when they are surrounded by microbial crusts indicates that a more favorable plant environment exists where algal, mold, and lichen crusts are present. AZOTOBACTER
Nitrogen f ixa tion Nitrogen fixation by free-living (nonsymbiotic) microorganisms is a property shared by both algae and Azotobacter. Rhizobia, by contrast, are symbiotic nitrogen fixers inhabiting the roots of cultivated and native leguminous plants. There is evidence that some nitrogen fixing organisms have a symbiotic relation with nonleguminous plants. Azotobacter are responsive t o pH conditions: the most favorable pH for nitrogen fixation is that from 6 t o 8, which includes the majority of aridregion soils. Moisture is the limiting environmental factor. Sandy soils have fewer azotobacter than finer-textured soils, judging from the data of Peterson and Goodding (1941). These authors speculated that most of the soils of Nebraska (semiarid t o subhumid climate) have been inoculated with azotobacter during major dust storms but that survival has depended upon local environmental conditions. Azotobacter activity frequently is limited by the availability of organic carbon for energy. Growth In Arizona, Martin (1940) concluded that uncultivated soils in areas having an annual rainfall between 75 and 375 mm were generally lacking in
SOIL AGGREGATION
223
azotobacter. Only about 23% of the soils contained any of the organism and they were usually inactive in fixing nitrogen. Eighty seven percent of the irrigated soils in the same rainfall zones contained azotobacter, and they were generally active. The scarcity and inactivity of azotobacter in the uncultivated soils appeared to be the result of high temperatures, low humidity, dry soil for most of the year, and low soil organic-matter content. Similar results were obtained by Vandecaveye and Moodie (1942)in climatic zones of central Washington receiving 150-375 mm of precipitation. They concluded that azotobacter populations in irrigated and nonimgated soils of the region probably are too small to be a significant factor in nitrogen fixation.
SOIL AGGREGATION
Aggrega te format ion
Microorganisms play an important, if not dominant, role in the formation and stability of soil aggregates. Secretion of microbial gums was long thought to be the principal mechanisms by which individual soil grains were bound together. Significant though that effect is, Hubbell and Chapman (1946) concluded that it is not the main cause of aggregate stability. Their experiments with two calcareous soils showed that aggregates formed only in the presence of living organisms. Bacteria were capable of forming small and fragile water-stable aggregates which were easily crushed, whereas larger aggregates having the filaments of fungi and actinomycetes in and around them were resistant t o crushing and were persistent. Microbial secretions, in the absence of living organisms, were incapable of forming water-stable aggregates. Roots contributed t o aggregate formation only after it had been initiated microbially. Bond and Harris (1964)confirmed the results for fungi and found that algae have a similar effect. These authors also noted that water repellence of dry sands was associated with the growth of microorganisms. Erosion control
In arid regions of Australia, many plant communities have a relatively open canopy exposing bare areas of erodible sand. Where the vegetation consists of low shrubs, crusts dominated by algae in association with bacteria and fungi promote aggregate formation and are good soil stabilizers (Bond and Harris, 1964). The crusts usually are about 3 mm thick, and sometimes twice that thick. Undisturbed crusts effectively resisted sand movement by wind and water but once the crust was broken by animals, wind scouring occurred.
BIOLOGICAL PROPERTIES
224 ORGANIC MATTER
Equilibrium levels
Average organic-matter levels in arid-region soils generally range from less than 0.5% in the drier areas to 2-496 in the semiarid zone. Equilibrium levels are dependent upon the rate of organic-matter additions by roots and plant debris and the rate of decomposition by microorganisms. Each climatic zone has a characteristic equilibrium level, under natural conditions, where plant growth and decomposition over a period of years become equal. When irrigation is introduced, soil organic-matter content usually will increase as a result of the increased plant growth and a greater return of plant residues to the soil. Dryland farming, on the other hand, generally leads to lower organicmatter levels than were present in the original soil. Textural effects Among the factors controlling organic-matter levels, soil texture is of major importance. Within any one climatic zone, all other conditions being equal, fine-textured soils will contain more organic matter than coarsetextured soils. Data from Dregne and Maker (1955) demonstrated the effect of soil texture on organic-matter content in an irrigated area of southern New Mexico (Table 12.2). The absolute amount of organic matter would be less in nonirrigated soils in the same climatic zone but the textural relations would be similar. Organic matter adsorbed on soil particles is more difficultly decomposable than when it is loose in the soil. Since fine-textured soils have a much greater surface area and are, therefore, capable of absorbing more TABLE 1 2 . 2 Percentage of organic matter in irrigated soils having different textures (from Dregne and Maker, 1955)
Soil texture
Organic matter
Clay, silty clay
1.8
Clay loam, silty clay loam, sandy clay
2.0
Loam, sandy clay loam, silt loam
1.6
Sandy loam
1.o
Loamy sand, sand
0.3
("/.I
P LANT-SO IL RELATIONS
225
organic matter than coarse-textured soils, the rate of decomposition is less in clays than in sands. Temperature and moisture differences also play a part. Distribution in shrublands In heavily grazed desert grasslands where shrubs are present, soil organicmatter content commonly is higher under the shrubs than between them. Tiedemann and Klemmedson (1973) found that not only was the surface organic-matter content three times greater under mesquite (Prosopisjuliflora) trees than between the mesquite, the availability of soil nitrogen to native perennial grasses was up t o 15 times higher under the trees. Phosphorus availability also was significantly higher under the mesquite. Low nutrient availability in the open areas is likely, then, t o be responsible for at least part of the forage-production problem in desert grasslands. Carbonhaitrogen ratios Carbon/nitrogen ratios of and-region soils, on the average, increase from the more arid to the less arid areas. Ratios as low as 6 or 7 have been reported for surface soils in rainfall zones of less than 150 mm where vascular plants are absent. As precipitation increases, the ratio rises to 1 0 or 11 in the wet end of the semiarid zone. Low C/N ratios indicate a greater degree of decomposition of organic residues than do high ratios. Hot regions tend to have lower ratios than cold regions. PLANT-SOIL RELATIONS
Plants as soil indicators The fact that plants modify the soil environment in which they are growing and also are affected by soil conditions has long been known. In the arid regions, attempts have been made t o relate the presence or absence - or the general state of health - of single shrub species t o soil characteristics. Thus, the indicator significance of Prosopis, Larrea, Atriplex, Artemisia, and other shrubs for the selection of land suitable for irrigation has been investigated, with varying degrees of success. Current thinking is that the composition of the plant community is more significant than the status of a single species in evaluating soil conditions. Soil-vegetation maps have demonstrated the relation of plant communities to soil types in the arid regions, particularly with respect t o soil physical properties (texture, depth to rock, etc.). Despite the importance of communities, individual shrubs do have a profound effect on the soil under their canopy. The data of Tiedemann and Klemmedson (1973) are just one example of the notable accumulation of
226
BIOLOGICAL PROPERTIES
organic matter and soluble salts under Prosopis as compared t o soils outside the shrub canopy. A classical study by Fireman and Hayward (1952) of soilplant relations in the Escalante Desert of Utah pointed out that Artemisia tridentata had no detectable effect, whereas Sarcobatus vermiculatus had a major effect on soil salinity, pH, and exchangeable-sodiumpercentage.
Toxic exudates Some shrubs are said to release an exudate or to contain within their leaves a toxin which inhibits germination of other seeds and accounts for the poor growth of competing plants in the vicinity. A desert grassland study of the effect of Larrea divaricata (creosotebush) plant parts on seed germination showed that water extracts from all parts of the plant (stems, leaves, roots) inhibited germination of some, but not all, grasses (Knipe and Herbel, 1966). Furthermore, the carpels of the shrub contain a substance that inhibits germination of the creosotebush seed, itself.
REFERENCES Bond, R.D. and Harris, J.R., 1964.The influence of the microflora on physical properties of soils. I. Effects associated with filamentous algae and fungi. Aust. Soil Res., 2: 11 1-123. Cameron, R.E., 1963. Algae of southern Arizona. Part I. Introduction - blue-green algae. Rev. Agrol., 7: 282-318. Cameron, R.E., 1964. Algae of southern Arizona. Part 11. Algal flora (exclusive of bluegreen algae). Rev. Agrol., 7: 151-177. Cameron, R.E., 1969. Abundance of microflora in soils of desert regions. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif., Technical Report
32-1378.
Cameron, R.E. and Fuller, W.H., 1960. Nitrogen fixation by some algae in Arizona soils. Soil Sci. SOC.A m . , Proc., 24: 353-356. Dregne, H.E. and Maker, H.J., 1955. Fertility levels of New Mexico soils. N.M. Agric. Exp. Stn., Bull., No. 396. Fireman, M. and Hayward, H.E., 1952. Indicator significance of some shrubs in the Escalante Desert, Utah. Bot. Gaz., 114: 143-155. Fletcher, J.E. and Martin, W.P., 1948. Some effects of algae and molds in the rain crust of desert soils. Ecology, 29: 95-100. Horowitz, N.H., Cameron, R.E. and Hubbard, J.S., 1972.Microbiology of the dry valleys of Antarctica. Science, 176:242-245. Hubbell, D.S. and Chapman, J.E., 1946. The genesis of structure in two calcareous soils. Soil Sci., 62: 271-281. Killian, C. and Feher, D., 1936. La fertilit6 des sols du Sahara. Genie Rural, 108(2790): 1 1 4-1 24. Knipe, D. and Herbel, C.H., 1966. Germination and growth of some semidesert grassland species treated with aqueous extract from creosotebush. Ecology, 47 : 775-781. Lobova, E.V., 1960. Pochvy Pustynnoi Zony SSSR. (Soils of the Desert Zone of the U.S.S.R. Issued in translation by the Israel Program for Scientific Translations, Jerusalem, 1967,405 pp.; also cited as TT 67-61279.)
REFERENCES
227
MacGregor, A.N. and Johnson, D.E., 1971. Capacity of desert algal crusts to fix atmospheric nitrogen. Soil Sci. SOC. Am., Proc., 35: 843-844. Martin, W.P., 1940. Distribution and activity of azotobacter in the range and cultivated soils of Arizona. A r k . Agric. Exp. Stn., Tech. Bull., No. 83. Peterson, H.B. and Goodding, T.H., 1941. The geographic distribution of Azotobacter and Rhizobium meliloti in Nebraska soils in relation to environmental factors. Nebr. Agric. Exp. Stn., Res. Bull., No. 121. Tiedemann, A.R. and Klemmedson, J.O., 1973. Nutrient availability in desert grassland soils under mesquite (Prosopis juliflora) trees and adjacent open areas. Soil Sci. Soc. A m . , Proc., 37: 107-111. Vandecaveye, S.C. and Moodie, C.D., 1942. Occurrence and activity of Azotobacter in A m . , Proc., 7 : 229-236. semiarid soils in Washington. Soil Sci. SOC.
INTRODUCTION
Soil biological activity ranges from a high level at the humid fringe of the arid region to nearly zero in the most arid sections. Soils of the driest part of the Sahara, the dry valleys of the Antarctic, and the center of the Atacama Desert of Chile may be nearly abiotic because of the inhospitality of the climate. In fact, the startling report that no bacteria were found t o a depth of 1m in a soil of the dry Taylor Valley in Antarctica led Horowitz et al. (1972) to make an exhaustive study of a Victoria Valley subsoil in an attempt t o determine whether a sterile soil - never before reported on earth in nontoxic soils - actually did exist in Antarctica. They concluded that their soil was truly sterile and, significantly, nontoxic to introduced organisms. Lack of water was the limiting factor. The likelihood that a barren, mobile sand dune in the most arid part of the Sahara-probably the least hospitable environment in the hot deserts -would be sterile appears t o be remote since wind-borne dust can bring in bacteria and other soil microorganisms from more hospitable areas. Killian and Fehkr (1936) reported a wide variety of microflora in Saharan soils. Microbial studies of uncultivated soils in arid regions are limited; attention has been centered upon croplands. Consequently, data are scarce on the kinds and numbers of microorganisms present and their distribution, although some progress was made in Desert Biome studies conducted under the International Biological Programme. Cameron (1969) has summarized his extensive investigation of the microorganisms of hot and cold deserts in a diagram (Fig.lZ.l) of the diversity of flora as influenced by environmental conditions. As the diversity increases, numbers also increase, with bacteria being the most numerous in favorable environments. The dominant microflora include Bacillus, Schizothrix, Streptomyces, Penicillium, and Aspergillus. As a first approximation, it seems likely that nearly all of the major microbial genera will be present, to a greater or lesser degree, in hot-desert soils if the annual precipitation is 100 mm or more. ALGAE
Soil algae are among the most important microorganisms in arid-region soils because of their drought resistance, growth habit, and effect on soil
BIOLOGICAL PROPERTIES
220
-
Protozoa
I
Algae
1 Unfavorable E n v i r o n m e n t a l conditions
=
1
Favoroble
Fig.12.1. Influence of environmental conditions on diversity of microorganisms (adapted from Cameron, 1969).
organic matter and nitrogen content. They grow best in soils having a neutral reaction and they frequently dominate the surface microflora in dry regions. Forming mats on the soil surface as they do, algae serve to bind soil particles together and to shed water; the first effect is an advantage, the second becomes a disadvantage when it increases runoff. Some blue-green algae have the ability to fix atmospheric nitrogen, and their contribution to the nitrogen status of soils is significant in the absence of fertilizer applications.
Nitrogen and organic matter An incubation study of three Arizona algal soil crusts by Cameron and Fuller (1960) showed that nitrogen content of the crusts increased by 25-42’36 in four weeks (Table 12.1). The Gila sandy loam crust contained lichens as well as algae and had twice as many genera of algae (16) as the other two. Azotobacter were absent from all three crusts. In pure cultures, nitrogenfixing algae fixed much more nitrogen than did the crusts of Table 12.1, which were mixtures of algae, soil, and other matter. One calculation of the nitrogen-supplying power of blue-green algae indicated that a soil surface covered by actively growing algae would be capable of fixing 800 kg of nitrogen per hectare per month. In the field, nitrogen fixation rates are much lower because, among other things, only a fraction of the surface has an algal cover. MacGregor and Johnson (1971) in Arizona measured an increase of
221
ALGAE TABLE 12.1
Nitrogen fixation by algal soil crusts in moist chambers (from Cameron and Fuller, 1960) soil
Tucson sandy loam
Incubation time (weeks)
Nitrogen
0
0.081 0.107
4
Gila fine sandy loam
0 4
0.095 0.119
Gila sandy loam
0 4
0.096 0.136
3-4 g of nitrogen per hour per hectare following a rain. The algal crust covered about 4% of the land area. Algae add organic matter to soils along with nitrogen, and leaching of organic compounds and nitrates increases the amount of those substances in subsurface layers. Carbon/nitrogen ratios of 14 crusts in the Cameron and Fuller (1960)study ranged from 5 to 18, with an average of 8.5. Nitrate nitrogen was detected in nearly all surface and subsurface soil samples. Annual precipitation is about 280 mm in the study area. Cameron (1963,1964) identified a rather bewildering variety of algae in and around the Sonoran Desert in Arizona where the rainfall average ranged from 75 to more than 300 mm. Blue-green algae were represented by 7 families, 32 genera, and 86 species and were by far the most numerous. Other algae (mostly green) belonged to 27 families, 47 genera, and 66 species. Some algae are nitrogen fixers, others are not. Algal growth
The principal factors controlling algal growth are moisture and sunlight. After a rain, renewed growth of algae begins in less than 30 minutes, under favorable conditions. By far the greatest algal concentration is in the surface few centimeters of soil in small surface depressions where water accumulates after rains. The undersides of partially embedded wood and stones and of translucent rocks also are favored habitats because sunlight is present yet water loss is less rapid than it is from more exposed surfaces. Blue-green algae growth is profuse in irrigated fields of hot and regions after a rain or irrigation if the vegetative cover is fairly dense. The combination of adequate sunlight, high air temperatures, and reduced evaporation from the shaded soil promote rapid growth of algae. Fine-textured takyr soils of closed basins have a profuse cover of algae and
222
BIOLOGICAL PROPERTIES
lichens -but no vascular plants - following the few periods when runoff water collects after rains. However, Lobova (1960) believes that the effect of algae and lichens on takyr formation and humus content is small, contrary to the views of some other Soviet scientists. Water relations Microbial crusts have been said to reduce water infiltration and increase runoff in arid regions. Fletcher and Martin (1948) concluded that the opposite was true. They noted that the crusts curled when dry, allowing water t o infiltrate while at the same time protecting the soil from the direct impact of raindrops. An additional beneficial effect of the curled crusts is t o capture grass seeds when the soil is dry. Upon subsequent wetting, the crusts uncurl and cover the wetted seed, thereby helping germination by reducing evaporation. Microbial crusts had organic carbon and nitrogen contents as much as 300% (for carbon) and 400% (for nitrogen) higher than the underlying soil. The commonly observed phenomenon of better growth of grass seedlings when they are surrounded by microbial crusts indicates that a more favorable plant environment exists where algal, mold, and lichen crusts are present. AZOTOBACTER
Nitrogen f ixa tion Nitrogen fixation by free-living (nonsymbiotic) microorganisms is a property shared by both algae and Azotobacter. Rhizobia, by contrast, are symbiotic nitrogen fixers inhabiting the roots of cultivated and native leguminous plants. There is evidence that some nitrogen fixing organisms have a symbiotic relation with nonleguminous plants. Azotobacter are responsive t o pH conditions: the most favorable pH for nitrogen fixation is that from 6 t o 8, which includes the majority of aridregion soils. Moisture is the limiting environmental factor. Sandy soils have fewer azotobacter than finer-textured soils, judging from the data of Peterson and Goodding (1941). These authors speculated that most of the soils of Nebraska (semiarid t o subhumid climate) have been inoculated with azotobacter during major dust storms but that survival has depended upon local environmental conditions. Azotobacter activity frequently is limited by the availability of organic carbon for energy. Growth In Arizona, Martin (1940) concluded that uncultivated soils in areas having an annual rainfall between 75 and 375 mm were generally lacking in
SOIL AGGREGATION
223
azotobacter. Only about 23% of the soils contained any of the organism and they were usually inactive in fixing nitrogen. Eighty seven percent of the irrigated soils in the same rainfall zones contained azotobacter, and they were generally active. The scarcity and inactivity of azotobacter in the uncultivated soils appeared to be the result of high temperatures, low humidity, dry soil for most of the year, and low soil organic-matter content. Similar results were obtained by Vandecaveye and Moodie (1942)in climatic zones of central Washington receiving 150-375 mm of precipitation. They concluded that azotobacter populations in irrigated and nonimgated soils of the region probably are too small to be a significant factor in nitrogen fixation.
SOIL AGGREGATION
Aggrega te format ion
Microorganisms play an important, if not dominant, role in the formation and stability of soil aggregates. Secretion of microbial gums was long thought to be the principal mechanisms by which individual soil grains were bound together. Significant though that effect is, Hubbell and Chapman (1946) concluded that it is not the main cause of aggregate stability. Their experiments with two calcareous soils showed that aggregates formed only in the presence of living organisms. Bacteria were capable of forming small and fragile water-stable aggregates which were easily crushed, whereas larger aggregates having the filaments of fungi and actinomycetes in and around them were resistant t o crushing and were persistent. Microbial secretions, in the absence of living organisms, were incapable of forming water-stable aggregates. Roots contributed t o aggregate formation only after it had been initiated microbially. Bond and Harris (1964)confirmed the results for fungi and found that algae have a similar effect. These authors also noted that water repellence of dry sands was associated with the growth of microorganisms. Erosion control
In arid regions of Australia, many plant communities have a relatively open canopy exposing bare areas of erodible sand. Where the vegetation consists of low shrubs, crusts dominated by algae in association with bacteria and fungi promote aggregate formation and are good soil stabilizers (Bond and Harris, 1964). The crusts usually are about 3 mm thick, and sometimes twice that thick. Undisturbed crusts effectively resisted sand movement by wind and water but once the crust was broken by animals, wind scouring occurred.
BIOLOGICAL PROPERTIES
224 ORGANIC MATTER
Equilibrium levels
Average organic-matter levels in arid-region soils generally range from less than 0.5% in the drier areas to 2-496 in the semiarid zone. Equilibrium levels are dependent upon the rate of organic-matter additions by roots and plant debris and the rate of decomposition by microorganisms. Each climatic zone has a characteristic equilibrium level, under natural conditions, where plant growth and decomposition over a period of years become equal. When irrigation is introduced, soil organic-matter content usually will increase as a result of the increased plant growth and a greater return of plant residues to the soil. Dryland farming, on the other hand, generally leads to lower organicmatter levels than were present in the original soil. Textural effects Among the factors controlling organic-matter levels, soil texture is of major importance. Within any one climatic zone, all other conditions being equal, fine-textured soils will contain more organic matter than coarsetextured soils. Data from Dregne and Maker (1955) demonstrated the effect of soil texture on organic-matter content in an irrigated area of southern New Mexico (Table 12.2). The absolute amount of organic matter would be less in nonirrigated soils in the same climatic zone but the textural relations would be similar. Organic matter adsorbed on soil particles is more difficultly decomposable than when it is loose in the soil. Since fine-textured soils have a much greater surface area and are, therefore, capable of absorbing more TABLE 1 2 . 2 Percentage of organic matter in irrigated soils having different textures (from Dregne and Maker, 1955)
Soil texture
Organic matter
Clay, silty clay
1.8
Clay loam, silty clay loam, sandy clay
2.0
Loam, sandy clay loam, silt loam
1.6
Sandy loam
1.o
Loamy sand, sand
0.3
("/.I
P LANT-SO IL RELATIONS
225
organic matter than coarse-textured soils, the rate of decomposition is less in clays than in sands. Temperature and moisture differences also play a part. Distribution in shrublands In heavily grazed desert grasslands where shrubs are present, soil organicmatter content commonly is higher under the shrubs than between them. Tiedemann and Klemmedson (1973) found that not only was the surface organic-matter content three times greater under mesquite (Prosopisjuliflora) trees than between the mesquite, the availability of soil nitrogen to native perennial grasses was up t o 15 times higher under the trees. Phosphorus availability also was significantly higher under the mesquite. Low nutrient availability in the open areas is likely, then, t o be responsible for at least part of the forage-production problem in desert grasslands. Carbonhaitrogen ratios Carbon/nitrogen ratios of and-region soils, on the average, increase from the more arid to the less arid areas. Ratios as low as 6 or 7 have been reported for surface soils in rainfall zones of less than 150 mm where vascular plants are absent. As precipitation increases, the ratio rises to 1 0 or 11 in the wet end of the semiarid zone. Low C/N ratios indicate a greater degree of decomposition of organic residues than do high ratios. Hot regions tend to have lower ratios than cold regions. PLANT-SOIL RELATIONS
Plants as soil indicators The fact that plants modify the soil environment in which they are growing and also are affected by soil conditions has long been known. In the arid regions, attempts have been made t o relate the presence or absence - or the general state of health - of single shrub species t o soil characteristics. Thus, the indicator significance of Prosopis, Larrea, Atriplex, Artemisia, and other shrubs for the selection of land suitable for irrigation has been investigated, with varying degrees of success. Current thinking is that the composition of the plant community is more significant than the status of a single species in evaluating soil conditions. Soil-vegetation maps have demonstrated the relation of plant communities to soil types in the arid regions, particularly with respect t o soil physical properties (texture, depth to rock, etc.). Despite the importance of communities, individual shrubs do have a profound effect on the soil under their canopy. The data of Tiedemann and Klemmedson (1973) are just one example of the notable accumulation of
226
BIOLOGICAL PROPERTIES
organic matter and soluble salts under Prosopis as compared t o soils outside the shrub canopy. A classical study by Fireman and Hayward (1952) of soilplant relations in the Escalante Desert of Utah pointed out that Artemisia tridentata had no detectable effect, whereas Sarcobatus vermiculatus had a major effect on soil salinity, pH, and exchangeable-sodiumpercentage.
Toxic exudates Some shrubs are said to release an exudate or to contain within their leaves a toxin which inhibits germination of other seeds and accounts for the poor growth of competing plants in the vicinity. A desert grassland study of the effect of Larrea divaricata (creosotebush) plant parts on seed germination showed that water extracts from all parts of the plant (stems, leaves, roots) inhibited germination of some, but not all, grasses (Knipe and Herbel, 1966). Furthermore, the carpels of the shrub contain a substance that inhibits germination of the creosotebush seed, itself.
REFERENCES Bond, R.D. and Harris, J.R., 1964.The influence of the microflora on physical properties of soils. I. Effects associated with filamentous algae and fungi. Aust. Soil Res., 2: 11 1-123. Cameron, R.E., 1963. Algae of southern Arizona. Part I. Introduction - blue-green algae. Rev. Agrol., 7: 282-318. Cameron, R.E., 1964. Algae of southern Arizona. Part 11. Algal flora (exclusive of bluegreen algae). Rev. Agrol., 7: 151-177. Cameron, R.E., 1969. Abundance of microflora in soils of desert regions. Jet Propulsion Laboratory, California Institute of Technology, Pasadena, Calif., Technical Report
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Cameron, R.E. and Fuller, W.H., 1960. Nitrogen fixation by some algae in Arizona soils. Soil Sci. SOC.A m . , Proc., 24: 353-356. Dregne, H.E. and Maker, H.J., 1955. Fertility levels of New Mexico soils. N.M. Agric. Exp. Stn., Bull., No. 396. Fireman, M. and Hayward, H.E., 1952. Indicator significance of some shrubs in the Escalante Desert, Utah. Bot. Gaz., 114: 143-155. Fletcher, J.E. and Martin, W.P., 1948. Some effects of algae and molds in the rain crust of desert soils. Ecology, 29: 95-100. Horowitz, N.H., Cameron, R.E. and Hubbard, J.S., 1972.Microbiology of the dry valleys of Antarctica. Science, 176:242-245. Hubbell, D.S. and Chapman, J.E., 1946. The genesis of structure in two calcareous soils. Soil Sci., 62: 271-281. Killian, C. and Feher, D., 1936. La fertilit6 des sols du Sahara. Genie Rural, 108(2790): 1 1 4-1 24. Knipe, D. and Herbel, C.H., 1966. Germination and growth of some semidesert grassland species treated with aqueous extract from creosotebush. Ecology, 47 : 775-781. Lobova, E.V., 1960. Pochvy Pustynnoi Zony SSSR. (Soils of the Desert Zone of the U.S.S.R. Issued in translation by the Israel Program for Scientific Translations, Jerusalem, 1967,405 pp.; also cited as TT 67-61279.)
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MacGregor, A.N. and Johnson, D.E., 1971. Capacity of desert algal crusts to fix atmospheric nitrogen. Soil Sci. SOC. Am., Proc., 35: 843-844. Martin, W.P., 1940. Distribution and activity of azotobacter in the range and cultivated soils of Arizona. A r k . Agric. Exp. Stn., Tech. Bull., No. 83. Peterson, H.B. and Goodding, T.H., 1941. The geographic distribution of Azotobacter and Rhizobium meliloti in Nebraska soils in relation to environmental factors. Nebr. Agric. Exp. Stn., Res. Bull., No. 121. Tiedemann, A.R. and Klemmedson, J.O., 1973. Nutrient availability in desert grassland soils under mesquite (Prosopis juliflora) trees and adjacent open areas. Soil Sci. Soc. A m . , Proc., 37: 107-111. Vandecaveye, S.C. and Moodie, C.D., 1942. Occurrence and activity of Azotobacter in A m . , Proc., 7 : 229-236. semiarid soils in Washington. Soil Sci. SOC.