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Cellulolytic population dynamics in a vertic soil under three tillage systems in the Humid Pampa of Argentina
Soil & Tillage Research 49 (1998) 79±83
Cellulolytic population dynamics in a vertic soil under three tillage systems in the Humid Pampa of Argentina S. Toresani, E. Gomez*, B. Bonel, V. Bisaro, S. Montico Facultad de Ciencias Agrarias, Universidad Nacional de Rosario Campo Experimental J. Villarino, (2123) Zavalla, Santa Fe, Argentina Received 12 May 1997; accepted 17 April 1998
Abstract Tillage systems impact soil biota involved in the decomposition of stubble and the biota, in turn, is responsible for the transformation of nutrients that are essential to fertility. Modi®cations in soil microbial populations could be indicative of alterations in the soil health caused by different management practices. The purpose of this work was to quantify the aerobic cellulolytic population as a sensitive indicator of the changes caused by stubble placement in three tillage systems: chisel (ChT), no-till (NT) and fall plow followed by NT (FP±NT), and at two depths (0±5 cm and 5±10 cm). The study was conducted on a Chernozemic clay loam soil (Vertic Argiudoll) during 1994±1996, in a corn (Zea mays L.)-double crop wheat (Triticum aestivum L.)±soybean (Glycine max (L.) Merr.)-soybean rotation sequence. Samples were taken after wheat harvest. Dilution series of soil suspensions were inoculated in selective medium for cellulolytics and counts were done by most probable number (MPN). Crop residue was separated from soil by sieving and dry weight determined. No-till showed the largest number of aerobic cellulolytics in 1995 and 1996 compared to the other tillages (p0.05). In respect to soil depth, NT showed a greater concentration of cellulolytics in the 0±5 cm depth. We did not ®nd differences between tillages in the total amount of residue, but there were differences in its distribution, with residue concentrations in the NT soil, being 2.5 times higher at the 0±5 cm than at 5±10 cm soil layer. The dynamics of cellulolytic population indicated sensitivity to stubble placement. It may be possible to use it together with other properties as an indicator of changes due to management practices. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Cellulolytic population; Residue management; Soil micro-organisms; Tillage systems; Soil health
1. Introduction Tillage and crop residue management are major factors in¯uencing sustainability of continual agricultural systems. Moldboard plowing tillage practices have resulted in the deterioration of many soils,
*Corresponding author. Tel: +54 41567548; fax: +54 41970624.
increased erosion and reduced organic matter content (Dalal and Mayer, 1986). These problems are easily detected in areas of Argentina, and consequently there is a growing emphasis on soil conservation. Information from AAPRESID (AsociacioÂn Argentina de Productores en Siembra Directa, 1995) indicates notillage and direct drilling have been increasing up to 1990 and the area under conservation practices was 2 440 000 ha in 1994±1995.
0167-1987/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0167-1987(98)00157-3
80
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Tillage systems have a signi®cant impact on the soil biota involved in the decomposition of stubble. Several changes in the soil ecosystem are associated with residue placement under reduced or no-tillage as compared with plow tillage. The management of the stubble has an important role in the formation of microsites with suitable conditions for the activity of the micro-organisms (Roper and Gupta, 1995). Plant residue provides energy and nutrients for heterotrophic micro-organisms and directly in¯uences the size and activity of the soil microbial population (Voroney et al., 1989). Biederbeck et al. (1984) observed an increase in the population of heterotrophic micro-organisms in response to surface retention of wheat residue. The retention of residues also affects the distribution of microbial populations in the soil pro®le. Doran (1980) and Gupta et al. (1994) found that micro-organisms were concentrated in the surface layer of soil (0±7.5 cm) in no-tillage systems whereas they were more evenly distributed throughout the plowed layer in a plowed tillage system. While research in the Humid Pampa of Argentina has emphasized the relationship between tillage and physical and chemical properties of soils, less is known about the dynamics of microbial population associated with stubble degradation. However, changes in these soil micro-organisms could provide an early signal of soil alterations due to different management practices, and could be useful to characterize soil health. The purpose of this work was to evaluate the cellulolytic population as a sensitive indicator of changes caused by differences in stubble deposition under three tillage systems at two depths. 2. Materials and methods The ®eld study was carried out on a Chernozemic clay loam soil (Vertic Argiudoll) located in Zavalla, Argentina (328430 2700 S latitude; 608550 1800 W longitude). The soil's main characteristics in the 0±15 cm layer (horizon A11) are: clay, 26%; sand, 6%; organic matter, 3.27%; total N, 0.223% and pH in water (1:2.5), 5.9. The soil has a good water storage capacity and is moderately well drained. The experiment was initiated in 1993, on plots that had been previously cropped six years with plowed tillage. The crop rotation established was corn-double
crop wheat±soybean -soybean, commonly used in the region. This crop sequence was repeated so that wheat planting occurred each year. Treatments were chisel tillage (ChT), no tillage (NT) and fall plowing prior to wheat planting followed by no-till for summer crops (FP±NT), and two depths (0±5 cm and 5±10 cm). Plots under ChT were twice chisel plowed at a depth of 25 cm before planting and the seedbed was prepared with a cultivator at a depth of 10 cm. In the FP±NT treatment, crop residue was incorporated into the soil by moldboard plowing at a depth of 18 cm and plots were disked prior to wheat planting. The experimental design was a randomized complete block with split plots and three replications, with tillages as main plots (50 m2) and depths as subplots. Samples composed of ten sub-samples which were taken at random from plots under the three tillages in December 1994, 1995 and 1996 after wheat harvest. Samples were collected with a corer of 5 cm i.d. and 5 cm height. After air drying, crop residue was separated from soil by a 2 mm mesh sieve (Andriulo et al., 1991) and dry weight of the retained residue determined. For microbiological analysis, soil suspensions were shaken 10 minutes in a Bouyoucos shaker (soil 10 g, sodium silicate 1 ml, sodium oxalate 1 ml, distilled water 198 ml). Tenfold dilution series were prepared, inoculated in selective medium for cellulolytic micro-organisms (K2HPO4, 1 g; NaNO3, 0.5 g; MgSO4, 0.5 g; FeSO4, 0.01 g; minor elements solution 1 ml; water to 1000 ml; a band of ®lter paper as source of cellulose), and incubated at 288C for 21 days. After this period counts of cellulolytics were done by the MPN procedure, using McCrady tables and results were expressed as number of cellulolytic micro-organisms per gram of soil on a dry-weight basis (Frioni, 1990). Before statistical analysis, data of number of cellulolytics were log-transformed and dry weight of residue transformed by square root. The mean coef®cients of variation were 7% and 30%, respectively. The ANOVA technique was used for testing all mean effects and their interactions and Duncan's test was used for mean separations. 3. Results The total number of cellulolytic micro-organisms in soil, in the 0±10 layer, was signi®cantly different
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
81
Fig. 1. Cellulolytic population in three tillage systems: Chisel tillage (ChT), No tillage (NT) and Fall plow-NO tillage (FP/NT) in the 0±10 cm soil layer. Estimate of error0.5797. LSD0.5954. indicates significance of tillage at P0.05.
Fig. 2. Distribution of cellulolytic population in three tillage systems: ChT, NT and FP±NT at 0±5 cm and 5±10 cm of depth. Estimate of error0.6261. LSD1.1135. indicates significance of depth at p0.05.
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S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Fig. 3. Dry weight of residue in three tillage systems: ChT, NT and FP±NT at 0±5 cm and 5±10 cm of depth. Estimate of error0.195. LSD0.7852. indicates significance of depth at p0.01.
(p0.05) among the tillage systems for 1995 and 1996, whereas no difference was observed for 1994 (Fig. 1). In 1995, the cellulolytic population in soil of the NT was 18% and 25% larger than in FP±NT and in ChT treatments, respectively. In 1996, cellulolytic population in soil under NT treatment only differed by 3% from that of FP±NT but was 26% greater than in ChT. The distribution of cellulolytic population was signi®cantly but differently affected by soil depth in NT and FP±NT as compared with ChT (Fig. 2). In the NT treatment, differences were found in the three years, with values being 15% to 40% higher in the 0±5 cm soil layer as compared to the 5±10 cm soil layer. In FP±NT the cellulolytic population only differed between the two soil depths in 1996, the difference being 19%. Residue weight analyzed without taking depth into account, was found not to differ signi®cantly (p0.01) among the three tillage systems (Fig. 3). However, there were signi®cant differences when residue distribution in the soil pro®le was considered. In NT the amount of stubble in the 0±5 cm soil layer was on average 2.5 times higher than in the 5±10 cm soil layer.
4. Discussion The lack of signi®cant differences among tillages in the total cellulolytic population, in the 0±10 cm soil layer in 1994, may have been due to the fact that it was on December of that year when residue was left on soil surface for the ®rst time in the three tillage systems. As MartõÂnez et al. (1991) have stated for cellulose degrading micro-organisms, speci®c populations need a period of time to colonize the substrate. In 1995 and 1996, NT presented the highest number of cellulolytic micro-organisms in the 0±10 cm soil layer as compared to the other tillage treatments. Similar ®ndings were supported by Linn and Doran (1984) who found that populations of aerobic heterotrophic micro-organisms were 35% higher in NT compared to plowed soils. Tillage treatments by soil depth effects on cellulolytic population showed that in NT treatment its number was always higher in the 0± 5 cm soil layer. In respect to the amount of residue, previous work had shown more stubble in NT soil than in a plowed soil (Alvarez et al., 1995). We did not ®nd differences in the total amount of residue, but we did in its distribution in the soil pro®le.
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Residue retained on the soil surface contributes to a more moist soil in the surface centimeters of soil. Thus the larger amount of stubble found in NT treatment in the 0±5 cm soil layer could have generated a more suitable environment for cellulolytic micro-organisms. Future studies should focus on the relationship between the size of the stubble degrading population, the cellulolytic activity and the decomposition rate of residue in different tillage systems. Cellulolytic population was sensitive to stubble placement by different tillage systems. It may be used, together with other indicators, to characterize changes in the soil due to the management practices. Acknowledgements We thank M. Emilia Sanchez and Gladys Urraco for their laboratory assistance and Marta Constanzo for her collaboration in the ®eld experiment. References AsociacioÂn Argentina de Productores en Siembra Directa, 1995, Gacetilla Informativa, AnÄo 6, Set-Oct. Alvarez, R., DõÂaz, R.A., Barbero, N., Santanatoglia, O., Blotta, L., 1995. Soil organic carbon, microbial biomass and CO2±C production from three tillage systems. Soil Tillage Res. 33, 17±28.
83
Andriulo, A., Galantini, J., Pecorari, C., Torioni, E., 1991. Materia orgaÂnica del suelo en la RegioÂn Pampeana Argentina: I. Un meÂtodo de fraccionamiento por tamizado. INTA, Pergamino, Argentina, Informe TeÂcnico No 250: 1±16. Biederbeck, V.A., Campbell, C., Zentner, R.P., 1984. Effect of crop rotations and fertilization on some biological properties of a loam in south western Saskatchewan. Can. J. Soil Sci. 64, 355±367. Dalal, R.C., Mayer, R.J., 1986. Long-term trends in fertility of soils under contiuous cultivation and cereal cropping in southern Queensland. II: Total organic carbon and its rate of loss from the soil profile. Aust. J. Soil Res. 24, 281±292. Doran, J., 1980. Soil microbial and biochemical changes associated with reduced tillage. Soil Sci. Soc. Am. J. 44, 765±771. Frioni, L., 1990. EcologõÂa microbiana del suelo. 1st ed. Universidad de la RepuÂblica, Montevideo, Uruguay, 519 pp. Gupta, V.V.C.R., Roper, M.M., Kinkegaard, J.A., Angus, J.F., 1994. Changes in microbial biomass and organic matter levels during the first year of modified tillage and stubble management practices on a red earth. Aust. J. Soil Res. 32, 1339±1354. Linn, D.M., Doran, J.D., 1984. Aerobic and anaerobic microbial populations in no-till and plowed soils. Soil Sci. Soc. Am. J. 48, 794±799. MartõÂnez, A.E., Godeas, A.M., Mc Allister, C., 1991. DescomposicioÂn de celulosa en un suelo de la Provincia de Buenos Aires. Revista Ciencia del Suelo, 9 N812: 33-40. Roper, M.M., Gupta, V.V.S.R., 1995. Management practices and soil biota. Aust. J. Soil Res. 33, 321±339. Voroney, R.P., Paul, E.A., Anderson, D.W., 1989. Decomposition of wheat straw and stabilization of microbial products. Can. J. Soil Sci. 69, 63±77.
Cellulolytic population dynamics in a vertic soil under three tillage systems in the Humid Pampa of Argentina S. Toresani, E. Gomez*, B. Bonel, V. Bisaro, S. Montico Facultad de Ciencias Agrarias, Universidad Nacional de Rosario Campo Experimental J. Villarino, (2123) Zavalla, Santa Fe, Argentina Received 12 May 1997; accepted 17 April 1998
Abstract Tillage systems impact soil biota involved in the decomposition of stubble and the biota, in turn, is responsible for the transformation of nutrients that are essential to fertility. Modi®cations in soil microbial populations could be indicative of alterations in the soil health caused by different management practices. The purpose of this work was to quantify the aerobic cellulolytic population as a sensitive indicator of the changes caused by stubble placement in three tillage systems: chisel (ChT), no-till (NT) and fall plow followed by NT (FP±NT), and at two depths (0±5 cm and 5±10 cm). The study was conducted on a Chernozemic clay loam soil (Vertic Argiudoll) during 1994±1996, in a corn (Zea mays L.)-double crop wheat (Triticum aestivum L.)±soybean (Glycine max (L.) Merr.)-soybean rotation sequence. Samples were taken after wheat harvest. Dilution series of soil suspensions were inoculated in selective medium for cellulolytics and counts were done by most probable number (MPN). Crop residue was separated from soil by sieving and dry weight determined. No-till showed the largest number of aerobic cellulolytics in 1995 and 1996 compared to the other tillages (p0.05). In respect to soil depth, NT showed a greater concentration of cellulolytics in the 0±5 cm depth. We did not ®nd differences between tillages in the total amount of residue, but there were differences in its distribution, with residue concentrations in the NT soil, being 2.5 times higher at the 0±5 cm than at 5±10 cm soil layer. The dynamics of cellulolytic population indicated sensitivity to stubble placement. It may be possible to use it together with other properties as an indicator of changes due to management practices. # 1998 Elsevier Science B.V. All rights reserved. Keywords: Cellulolytic population; Residue management; Soil micro-organisms; Tillage systems; Soil health
1. Introduction Tillage and crop residue management are major factors in¯uencing sustainability of continual agricultural systems. Moldboard plowing tillage practices have resulted in the deterioration of many soils,
*Corresponding author. Tel: +54 41567548; fax: +54 41970624.
increased erosion and reduced organic matter content (Dalal and Mayer, 1986). These problems are easily detected in areas of Argentina, and consequently there is a growing emphasis on soil conservation. Information from AAPRESID (AsociacioÂn Argentina de Productores en Siembra Directa, 1995) indicates notillage and direct drilling have been increasing up to 1990 and the area under conservation practices was 2 440 000 ha in 1994±1995.
0167-1987/98/$ ± see front matter # 1998 Elsevier Science B.V. All rights reserved. PII: S0167-1987(98)00157-3
80
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Tillage systems have a signi®cant impact on the soil biota involved in the decomposition of stubble. Several changes in the soil ecosystem are associated with residue placement under reduced or no-tillage as compared with plow tillage. The management of the stubble has an important role in the formation of microsites with suitable conditions for the activity of the micro-organisms (Roper and Gupta, 1995). Plant residue provides energy and nutrients for heterotrophic micro-organisms and directly in¯uences the size and activity of the soil microbial population (Voroney et al., 1989). Biederbeck et al. (1984) observed an increase in the population of heterotrophic micro-organisms in response to surface retention of wheat residue. The retention of residues also affects the distribution of microbial populations in the soil pro®le. Doran (1980) and Gupta et al. (1994) found that micro-organisms were concentrated in the surface layer of soil (0±7.5 cm) in no-tillage systems whereas they were more evenly distributed throughout the plowed layer in a plowed tillage system. While research in the Humid Pampa of Argentina has emphasized the relationship between tillage and physical and chemical properties of soils, less is known about the dynamics of microbial population associated with stubble degradation. However, changes in these soil micro-organisms could provide an early signal of soil alterations due to different management practices, and could be useful to characterize soil health. The purpose of this work was to evaluate the cellulolytic population as a sensitive indicator of changes caused by differences in stubble deposition under three tillage systems at two depths. 2. Materials and methods The ®eld study was carried out on a Chernozemic clay loam soil (Vertic Argiudoll) located in Zavalla, Argentina (328430 2700 S latitude; 608550 1800 W longitude). The soil's main characteristics in the 0±15 cm layer (horizon A11) are: clay, 26%; sand, 6%; organic matter, 3.27%; total N, 0.223% and pH in water (1:2.5), 5.9. The soil has a good water storage capacity and is moderately well drained. The experiment was initiated in 1993, on plots that had been previously cropped six years with plowed tillage. The crop rotation established was corn-double
crop wheat±soybean -soybean, commonly used in the region. This crop sequence was repeated so that wheat planting occurred each year. Treatments were chisel tillage (ChT), no tillage (NT) and fall plowing prior to wheat planting followed by no-till for summer crops (FP±NT), and two depths (0±5 cm and 5±10 cm). Plots under ChT were twice chisel plowed at a depth of 25 cm before planting and the seedbed was prepared with a cultivator at a depth of 10 cm. In the FP±NT treatment, crop residue was incorporated into the soil by moldboard plowing at a depth of 18 cm and plots were disked prior to wheat planting. The experimental design was a randomized complete block with split plots and three replications, with tillages as main plots (50 m2) and depths as subplots. Samples composed of ten sub-samples which were taken at random from plots under the three tillages in December 1994, 1995 and 1996 after wheat harvest. Samples were collected with a corer of 5 cm i.d. and 5 cm height. After air drying, crop residue was separated from soil by a 2 mm mesh sieve (Andriulo et al., 1991) and dry weight of the retained residue determined. For microbiological analysis, soil suspensions were shaken 10 minutes in a Bouyoucos shaker (soil 10 g, sodium silicate 1 ml, sodium oxalate 1 ml, distilled water 198 ml). Tenfold dilution series were prepared, inoculated in selective medium for cellulolytic micro-organisms (K2HPO4, 1 g; NaNO3, 0.5 g; MgSO4, 0.5 g; FeSO4, 0.01 g; minor elements solution 1 ml; water to 1000 ml; a band of ®lter paper as source of cellulose), and incubated at 288C for 21 days. After this period counts of cellulolytics were done by the MPN procedure, using McCrady tables and results were expressed as number of cellulolytic micro-organisms per gram of soil on a dry-weight basis (Frioni, 1990). Before statistical analysis, data of number of cellulolytics were log-transformed and dry weight of residue transformed by square root. The mean coef®cients of variation were 7% and 30%, respectively. The ANOVA technique was used for testing all mean effects and their interactions and Duncan's test was used for mean separations. 3. Results The total number of cellulolytic micro-organisms in soil, in the 0±10 layer, was signi®cantly different
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
81
Fig. 1. Cellulolytic population in three tillage systems: Chisel tillage (ChT), No tillage (NT) and Fall plow-NO tillage (FP/NT) in the 0±10 cm soil layer. Estimate of error0.5797. LSD0.5954. indicates significance of tillage at P0.05.
Fig. 2. Distribution of cellulolytic population in three tillage systems: ChT, NT and FP±NT at 0±5 cm and 5±10 cm of depth. Estimate of error0.6261. LSD1.1135. indicates significance of depth at p0.05.
82
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Fig. 3. Dry weight of residue in three tillage systems: ChT, NT and FP±NT at 0±5 cm and 5±10 cm of depth. Estimate of error0.195. LSD0.7852. indicates significance of depth at p0.01.
(p0.05) among the tillage systems for 1995 and 1996, whereas no difference was observed for 1994 (Fig. 1). In 1995, the cellulolytic population in soil of the NT was 18% and 25% larger than in FP±NT and in ChT treatments, respectively. In 1996, cellulolytic population in soil under NT treatment only differed by 3% from that of FP±NT but was 26% greater than in ChT. The distribution of cellulolytic population was signi®cantly but differently affected by soil depth in NT and FP±NT as compared with ChT (Fig. 2). In the NT treatment, differences were found in the three years, with values being 15% to 40% higher in the 0±5 cm soil layer as compared to the 5±10 cm soil layer. In FP±NT the cellulolytic population only differed between the two soil depths in 1996, the difference being 19%. Residue weight analyzed without taking depth into account, was found not to differ signi®cantly (p0.01) among the three tillage systems (Fig. 3). However, there were signi®cant differences when residue distribution in the soil pro®le was considered. In NT the amount of stubble in the 0±5 cm soil layer was on average 2.5 times higher than in the 5±10 cm soil layer.
4. Discussion The lack of signi®cant differences among tillages in the total cellulolytic population, in the 0±10 cm soil layer in 1994, may have been due to the fact that it was on December of that year when residue was left on soil surface for the ®rst time in the three tillage systems. As MartõÂnez et al. (1991) have stated for cellulose degrading micro-organisms, speci®c populations need a period of time to colonize the substrate. In 1995 and 1996, NT presented the highest number of cellulolytic micro-organisms in the 0±10 cm soil layer as compared to the other tillage treatments. Similar ®ndings were supported by Linn and Doran (1984) who found that populations of aerobic heterotrophic micro-organisms were 35% higher in NT compared to plowed soils. Tillage treatments by soil depth effects on cellulolytic population showed that in NT treatment its number was always higher in the 0± 5 cm soil layer. In respect to the amount of residue, previous work had shown more stubble in NT soil than in a plowed soil (Alvarez et al., 1995). We did not ®nd differences in the total amount of residue, but we did in its distribution in the soil pro®le.
S. Toresani et al. / Soil & Tillage Research 49 (1998) 79±83
Residue retained on the soil surface contributes to a more moist soil in the surface centimeters of soil. Thus the larger amount of stubble found in NT treatment in the 0±5 cm soil layer could have generated a more suitable environment for cellulolytic micro-organisms. Future studies should focus on the relationship between the size of the stubble degrading population, the cellulolytic activity and the decomposition rate of residue in different tillage systems. Cellulolytic population was sensitive to stubble placement by different tillage systems. It may be used, together with other indicators, to characterize changes in the soil due to the management practices. Acknowledgements We thank M. Emilia Sanchez and Gladys Urraco for their laboratory assistance and Marta Constanzo for her collaboration in the ®eld experiment. References AsociacioÂn Argentina de Productores en Siembra Directa, 1995, Gacetilla Informativa, AnÄo 6, Set-Oct. Alvarez, R., DõÂaz, R.A., Barbero, N., Santanatoglia, O., Blotta, L., 1995. Soil organic carbon, microbial biomass and CO2±C production from three tillage systems. Soil Tillage Res. 33, 17±28.
83
Andriulo, A., Galantini, J., Pecorari, C., Torioni, E., 1991. Materia orgaÂnica del suelo en la RegioÂn Pampeana Argentina: I. Un meÂtodo de fraccionamiento por tamizado. INTA, Pergamino, Argentina, Informe TeÂcnico No 250: 1±16. Biederbeck, V.A., Campbell, C., Zentner, R.P., 1984. Effect of crop rotations and fertilization on some biological properties of a loam in south western Saskatchewan. Can. J. Soil Sci. 64, 355±367. Dalal, R.C., Mayer, R.J., 1986. Long-term trends in fertility of soils under contiuous cultivation and cereal cropping in southern Queensland. II: Total organic carbon and its rate of loss from the soil profile. Aust. J. Soil Res. 24, 281±292. Doran, J., 1980. Soil microbial and biochemical changes associated with reduced tillage. Soil Sci. Soc. Am. J. 44, 765±771. Frioni, L., 1990. EcologõÂa microbiana del suelo. 1st ed. Universidad de la RepuÂblica, Montevideo, Uruguay, 519 pp. Gupta, V.V.C.R., Roper, M.M., Kinkegaard, J.A., Angus, J.F., 1994. Changes in microbial biomass and organic matter levels during the first year of modified tillage and stubble management practices on a red earth. Aust. J. Soil Res. 32, 1339±1354. Linn, D.M., Doran, J.D., 1984. Aerobic and anaerobic microbial populations in no-till and plowed soils. Soil Sci. Soc. Am. J. 48, 794±799. MartõÂnez, A.E., Godeas, A.M., Mc Allister, C., 1991. DescomposicioÂn de celulosa en un suelo de la Provincia de Buenos Aires. Revista Ciencia del Suelo, 9 N812: 33-40. Roper, M.M., Gupta, V.V.S.R., 1995. Management practices and soil biota. Aust. J. Soil Res. 33, 321±339. Voroney, R.P., Paul, E.A., Anderson, D.W., 1989. Decomposition of wheat straw and stabilization of microbial products. Can. J. Soil Sci. 69, 63±77.