Agua–suelo–clima

 

Influence of inorganic and organic fertilization on microbial biomass carbon and maize yield in two soils of contrasting pH

 

Influencia de la fertilización química y orgánica sobre el carbono de la biomasa microbiana y rendimiento del maíz en suelos de pH contrastante

 

Elena Arrieche–Luna¹*, Magaly Ruiz–Dager²

 

¹ Instituto Nacional de Investigaciones Agropecuarias (INIA). Estación Local Yaritagua. Km 3, El Rodeo, Yaritagua, Estado Yaracuy. Venezuela, * Autor responsable: (isarrieche99@hotmail.com).

² Centro de Investigación y Extensión en Suelos y Aguas (CIESA), Universidad Rómulo Gallegos, Carretera El Castrero. San Juan de los Morros, Estado Guárico. Venezuela. Email: (magaruiz@movistar.net.ve).

 

Received: February, 2009.
Approved: March, 2010.

 

ABSTRACT

Inappropriate soil management practices that have been applied in the Yafacuy River basin, Venezuela, have led to the loss of soil organic matter and crop productivity. The incorporation of composted organic residues has been suggested as an alternative management practice in order to increase organic matter (OM) content and crop yield in these soils. The objective of this study was to determine the effects of different application fates of an organic fertilizer obtained from sugarcane industry wastes, filter cake and bagasse with or without inorganic fertilizers, on microbial biomass carbon (MB–C), soil organic carbon (SOC), maize (Zea mays L.) yield and N, P, K content in the maize leaves, in Yafacuy River basin. The experiments were established in an acid soil and an alkaline soil, both of them with low OM contents. The experimental design was a randomized complete block with seven treatments and three repetitions. It was found that the highest MB–C, SOC and maize yield were obtained with the combination of organic and inorganic fertilizers: 160 kg N ha ^–1 +120 kg P₂O₅ ha^–1 + 80kg K₂O ha^–1 + 2000 kg ha^–1 organic fertilizer. In the alkaline soil this treatment increased the MB–C and SOC with respect to the control by approximately 97 %, and 43 %, and in the acid soil by 48 % and 43 %. A linear statistically significant correlation was found between MB–C and the SOC in the alkaline soil. Maize yield and MB–C, N, P, K content in the maize leaves were found to be significantly correlated in both soils.

Key words: maize yield, organic fertilization, soil microbial biomass carbon, soil organic carbon, sugar cane filter cake.

 

RESUMEN

Las prácticas inadecuadas de manejo del suelo usadas en la cuenca del Río Yaracuy en Venezuela han provocado el detrimento de la materia orgánica y de la productividad de los cultivos. Se ha sugerido la incorporación de residuos orgánicos compostados como práctica alternativa de manejo para aumentar el contenido de materia orgánica (MO) y el rendimiento de los cultivos en estos suelos. El objetivo de este estudio fue determinar los efectos de diversas tasas de aplicación de un fertilizante orgánico obtenido de desechos de la industria azucarera, de cachaza y bagazo, con o sin fertilizantes inorgánicos, sobre el carbono de la biomasa microbiana (CBM), carbono orgánico del suelo (COS), rendimiento del maíz (Zea mays L.) y contenido de N, P, K en las hojas del maíz, en la cuenca del Río Yaracuy. Los experimentos se establecieron en un suelo ácido y en uno alcalino, ambos con contenidos bajos de MO. El diseño fue de bloques completos aleatorizados con siete tratamientos y tres repeticiones. Los mayores rendimientos de maíz, CBM y COS se obtuvieron al combinar fertilizantes orgánicos e inorgánicos: 160 kg N ha^–1 + 120 kg P₂O₅ ha^–1 + 80 kg K₂O ha^–1 + 2000 kg ha^–1 de fertilizante orgánico. En el suelo alcalino este tratamiento aumentó el CBM y el COS un 97 % y 43 %, aproximadamente, con respecto al testigo, y 48 % y 43 % en el suelo ácido. En el suelo alcalino se encontró una correlación lineal estadísticamente significativa entre el CBM y el COS. El rendimiento de maíz y el contenido de CBM, N, P y K en las hojas de maíz estuvieron significativamente correlacionados en ambos suelos.

Palabras clave: rendimiento de maíz, fertilización orgánica, carbono de la biomasa microbiana del suelo, carbono orgánico del suelo, cachaza de caña de azúcar.

 

INTRODUCTION

In the Yaracuy River basin, located in the Central Western Region of Venezuela, inappropriate soil management practices have led to the degradation and deterioration of the soils, so that the yields of maize (Zea mays L.) have decreased mainly due to the low levels of soil organic matter. The application of composted organic residues is an alternative management practice used to increase the organic matter content. The effect of the application of sugarcane filter cake alone or in combination with chemical fertilizers has been studied (Matheus 2004; Kaur et al., 2005; Meunchang et al., 2006), and filter cake was found to be a valuable organic amendment that could be used to increase soil fertility, the organic matter status and the soil microbial biomass C and N. In Venezuela, slightly more than 300 000 Mg of filter cake are produced each year, most of them in the Central Western Region of Venezuela.

Intensive management practices and pollution can influence the soil microbial biomass. Soil fertility is noticeably affected by microbial activity (Leita et al, 1999). Changes in the size and activity of the biomass can affect C mineralization, turnover of organic matter and the cycling of N and P as well as their availability for plants because the biomass is a dynamic pool containing considerable reserves of these elements (Saffigna et al., 1989). The microbial biomass itself can be an important indicator of soil quality, and the ratio of microbial C to soil organic carbon can provide an early warning of the improvement or deterioration of soil quality (Powlson, 1994).

The aim of this study was to evaluate the effect of applying different rates of an organic fertilizer prepared from sugarcane industry wastes filter cake and bagasse, and mixed with inorganic fertilizers, on the microbial biomass carbon (MB–C), the soil organic carbon (SOC), maize yield and N, P, K content in maize leaves in two Alfisols with contrasting pH, cultivated with maize located in the Central Western Region of Venezuela.

 

MATERIALS AND METHODS

The experimental sites, located in Yaracuy State in the Central Western Region of Venezuela (10° 12' 15'N; 69° 01' 18'W), were seeded in June 2004. Two soils classified as Oxic Haplustalfs, one of them as acid soil and the other an alkaline one, both of them with low contents of phosphorous, and potassium and low organic matter, were used. These soils have been continuously cultivated with maize with yields not exceeding 3000 kg ha^–1. Soil samples were taken at the beginning of the experiment to a depth of 0–20 cm, air dried, and passed through a 2 mm sieve. Soil texture was determined according to the Bouyoucos method (López and López, 1978). Soil pH was measured potentiometrically using a 1:2.5 ratio of soiltwater. Measurement of SOC was done by wet digestion with sulfuric acid and potassium dichromate (Nelson and Sommers, 1996) (Table 1). The experimental organic fertilizer (OF) was obtained from the sugar cane industry waste, filter cake and bagasse, which were transformed into compost by an aerobic biodegradation process and a polienzimatic mixture composed of calcareous algae, macro and microelements from vegetal, humic acid, and enzymes (amylase, cellulose, lactase, lipase, pancrease, protease, invertase); organic nitrogen and nucleic acids.

The physical and chemical characteristics of the resulting composts were analyzed in triplicate (Table 2). Moisture content was determined by drying to constant weight at 105 °C. The pH and electrical conductivity (EC) were determined on a water extract from compost using a compost :water ratio of 1:5 by weight. Organic carbon (OC) from OF was measured by wet oxidation with K₂CrO₇ (Nelson and Sommers, 1996) and total nitrogen (N) was determined by the Kjeldahl technique (Cori et al., 1999). Macro nutrients and micronutrients were extracted with solution nitric acid–perchloric acid (HNO₃–HCIO₄), 1:2 ratio (AOAC 1997). Phosphorus was measured by spectrophotometry at 420 nm. Ca, Mg and extractable micronutrients were determined by atomic absorption spectrophotometry, and K and Na by flame emission spectroscopy. Cation exchange capacity was determined according to Rhoades (1982) using a solution of ammonium acetate pH 7.0.

The experimental design was a randomized complete block with seven treatments (T): T1= control without fertilization; T2=1000 kg ha^–1 OF; T3=2000 kg ha^–1 OF; T4=3000 kg ha^–1OF; T5=4000 kg ha^–1 OF; T6=160 kg N ha^–1 + 120 kg P₂O₅ ha^–1+ 80kg K₂O ha^–1; T7=160 kg N ha^–1+ 120 kg P₂O₅ ha^–1+ 80 kg K₂O ha^–1 + 2000 kg ha^–1 OF.

The sources for N, P, K were urea (46 % N), triple supper phosphate (46 % P₂O₅) and potassium chloride (60 % K₂O). All fertilizers were spread uniformly by hand on 3 x 14 m plots at sowing, except N (1/3 at sowing and 2/3 30 d later). In each type of soil, maize (Hybrid PB–8) was sown as an indicator plant. Each treatment was repeated three times.

To determine MB–C and SOC, composite samples from 15 simple samples were taken from a depth of 0–20 cm from each plot at the time of harvest (120 d after sowing; das). The samples were placed in plastic bags and transferred under refrigeration to the laboratory, where they were homogenized and divided into two portions: one was air–dried and passed by a 2 mm sieve to determine the SOC according to Nelson and Sommers (1996); the other was maintained in a field–moist condition at 4 °C in aerated polyethylene bags for a maximum of two weeks. The latter samples were sieved (< 2 mm), moisture was adjusted to 40 % of water holding capacity (WHC), and they were preincubated at 25 °C for 7 d before analysis (Schinner et al., 1995). Soil microbial biomass carbon was determined by the fumigation–incubation method (Jenkinson and Powlson, 1976).

Plant samples were collected 60 das and they were dried at 65 °C ground and sieved through a 40 mesh. Phosphorous and potassium in the leaves were determined according to AOAC (1997). Total N was determined by the micro–Kjeldahl method following digestion in sulfuric acid (Malavolta et al, 1997).

Samples of maize ears were collected from each plot and yield (kg ha^–1was calculated according to Gonzalez (2001)³.

The results are arithmetic means of triplicate analyses expressed on an oven–dry basis (105 °C, 24 h). Data were analyzed by ANOVA and Turkey's test (p<0.05) was applied. Simple linear regression and correlation analyses were used to test for relationships between variables. The statistical program used was STATISTLX for Windows version 8, 2003.

 

RESULTS AND DISCUSSION

The results of MB–C, SOC, MB–C/SOC, and yield in the alkaline and acid Oxic Haplustalfs are shown in Tables 3 and 4. The MB–C of the alkaline soil was significantly increased (p<0.05) as compared to control soil when the rate of organic fertilizer was higher than 3000 kg ha^–1. The lowest values of the MB–C were found with 1000 and 2000 kg ha^–1 OF (T2 andT3), and both of them were similar (p>0.05) to control. The highest value of the MB–C was observed in the treatment that included only 4000 kg ha^–1OF (T5).

The soils amended with chemical fertilizer combined with organic fertilizer (T7) showed the highest value of the MB–C, which was significantly different from the control (p<0.05). Application of organic fertilizer in combination with chemical fertilizers increased the MB–C by about 97 %.

The inorganic fertilization (T6) applied to the soil produced a lower value of MB–C than the T5 and T7 treatments, corresponding to the highest rate of organic fertilizer and the mixture (inorganic + organic fertilizer). This result agrees with that reported by Kaur et al. (2005), who indicate that there was an increase in soil MB–C in tropical soils which received sugarcane filter cake alone or in combination with chemical fertilizers, as compared to soils which received chemical fertilizers only. Mabuhay et al. (2006), Baaru et al. (2007) and Prakash et al. (2007) observed a significant increase in soil MB–C in response to the application of increasing amounts of other organic amendments (urban wastes, farmyard manure, municipal refuse compost, poultry manure, organic resources). The general increase in MB–C could be attributed to the application of easily biodegradable organic materials, which stimulate the autochthonous microbial activity of the soil, or to the incorporation of exogenous microorganisms (Perucci, 1992).

The trend of the MB–C for acid soil was similar to the one in the alkaline soil (Table 4). The MB–C in the acid soil was significantly (p<0.05) increased by the application of the organic fertilizer. This increase was significant with the rate of 2000 kg ha^–1 of OF and it was greater when higher rates of OF were added.

The greatest increase in the MB–C occurred in the soil amended with the combination of organic and inorganic fertilizers (T7). Similar results were found by Goyal et al. (2006) in a field experiment in Japan. They observed that the microbial biomass C and N increased significantly with the addition of pig slurry along with inorganic fertilizers, as compared to unfertilized soil. These authors indicated that there is evidence that fertilizer application, particularly N and increasing inputs of organic C residues, increase soil organic matter and microbial biomass (Mahmood et al, 1997; Graham et al, 2002). In our experiment, the incorporation of OF along with chemical fertilizers increased the MB–C by about 48 % related to control (T1).

The SOC level of the alkaline soil treated with inorganic fertilizers alone (T6) was not different (p>0.05) from the control (Table 3), which agrees with the result reported by Leita et al. (1999) in studies carried out on Italian soils treated with organic and inorganic fertilizer. The SOC was significantly increased in the soils treated with rates above 3000 kg ha^–1 related to control. However, there was a greater increase of the SOC when organic fertilizer was applied along with inorganic fertilizers (T7). Similar results were found by Kaur et al. (2005) in a study on soil chemical and biological properties after seven years with pearl millet–wheat cropping sequence in soils which received organic manures (farmyard manure, poultry manure, and sugarcane filter cake) with and without chemical fertilizers. Compost of sugar cane wastes in combination with chemical fertilizers increased the SOC by about 43 %; thus, the proportional increase in the MB–C resulting from incorporation of organic and inorganic fertilizers (96 %) was greater than those in the SOC. This result is in accordance with findings reported by Powlson et al. (1987), who found that straw incorporation increased the SOC by only 5 %, and the increases in MB–C were 45 and 37 %, in two field experiments in Denmark.

No significant differences were found (p>0.05) in the acid soil between the SOC content of control and inorganic–fertilizer treated soil (T6). But when the organic fertilizer was applied, either with or without inorganic fertilizer, significantly higher values of the SOC were obtained than in the control soil (Table 4). This result agrees with the findings of Dee et al. (2002), who reported that additions of sugar cane filter cake increased the organic C content of an acid soil in a pot experiment, the effect being greater at the higher rate. Arreóla–Enriquez et al. (2004) found that 10 and 15 t ha^–1 of filter cake enriched with inorganic fertilizer increased the SOC between 15 and 24 %. Significant increase in the SOC in response to the application of organic amendments with or without inorganic fertilizer also has been reported (Leita et al., 1999; Madejón et al, 2003; Prakash et al, 2007). The incorporation of OF in combination with chemical fertilizers increased the SOC by about 43 % compared to control (T1).

The MB–C/SOC ratio in the alkaline Alfisol ranged from 0.95 to 1.48 % (Table 3). There were no significant differences between MB–C/SOC ratio of control and soils receiving below 4000 kg ha^–1 of organic fertilizer. However, this variable was increased significantly in the soils treated with 4000 kg ha^–1 of OF (T5), OF combined with inorganic fertilizers (T7) or with inorganic–fertilizer treated soil (T6). The MB–C/SOC ratio forT5, T6 and T7 treatments was 47, 46 and 39 % higher than in the control (T1). These results agree with those reported by Powlson et al. (1987), and support the statement that application of organic materials to the soils increases the MB–C/SOC ratio. Similar results were found by Pascual et al (1997), who reported that the addition of urban wastes to the soil increased the values of MB–C to SOC ratio as compared with soils receiving no organic amendment. According to Anderson and Domsch (1989), the ratio of MB–C to SOC is an indicator of the relative availability of substrate for soil microorganisms. Pascual et al (1997) suggest that MB–C/SOC ratio (at least in the laboratory under forced humidity and temperature conditions) is a reflection of the potential of organic matter mineralization and not of the stability of the organic matter; the lower the ratio, the lower the tendency of the organic matter to mineralize.

In acid Alfisol, the MB–C/SOC ratio ranged from 1.86 to 2.45 % (Table 4). Significant differences were not detected (p>0.05) among the values of MB–C/SOC ratio of the Tl (control), T4, T5 and T7 treatments. The highest value in this case corresponded to the treatment in which only inorganic fertilizer was applied (T6). These results are opposed to those found by Anderson and Domsch (1989), who reported that green or farmyard manure increased MB–C/SOC ratio when compared to mineral fertilization under permanent monoculture plots and crop rotation plots, due to soil management practices. The values of MB–C/ SOC ratio were significantly greater (p<0.05) for the acid than for the alkaline Alfisol.

Jenkinson and Powlson (1976), Powlson et al (1987) and Smith and Paul (1990) reported a significant positive correlation between MB–C and SOC. In our study, a linear statistically significant correlation (r=0.76) was found between MB–C and the SOC in the alkaline soil (p<0.001) (Figure 1). Ruiz and Paolini (2004) and Leita et al. (1999) show similar correlations for agricultural alkaline soils in Venezuela and Italy, and Dominy et al. (2002) for soils in South Africa. However, no significant correlation was observed between MB–C and the SOC in the acid soil, a result that concurs with those of Insam et al. (1991), Smith et al. (2002) and Hargreaves et al. (2003) who reported no significant correlation between these two properties for agricultural and non–agricultural soils.

Yield of maize

The highest maize yield in both soils was found with the combination of inorganic fertilizer and OF (T7), followed by the application of only inorganic fertilizer (T6). The crop yields on the plots that only received OF were similar among themselves and higher than the control (T1) (Tables 3 and 4). However, in the acid soil, no significant differences were found between 1000 kg ha^–1 (T2) and the control (T1). The crop yields were higher in the alkaline Alfisol with all the treatments, than in the acid Alfisol.

Maize yield and MB–C were significantly correlated in both soils. The regression equations and simple correlation coefficients for these relationships are shown in Figure 2 for alkaline soil and in Figure 3 for acid soil. Similar results were reported by Insam et al. (1991) in acid soils from the USA, with different crops (R=0.77;p<0.01).

Content of N, P and K in the maize leaves are reported in Tables 5 and 6. In both soils, T6 and T7 showed the highest content of nutrients (N, P, K) with respect to the control.

With the exception of the correlation between maize yield and SOC in the acid soil, the correlations between yield and MB–C, SOC, N, P and K in maize plants were significant at p<0.001 in both soils (Table 7).

 

CONCLUSIONS

The highest values of microbial biomass carbon, soil organic carbon and maize yield were obtained with the combination of organic and inorganic fertilizers, in the following rate: 160 kg N ha^–1 + 120 kg P₂O₅ ha^–1 + 80 kg K₂O ha^–1 + 2000 kg ha^–1 organic fertilizer, as compared with an unfertilized control in the two soils.

The effect of fertilization on microbial biomass carbon, soil organic carbon and maize yield was more noticeable in the alkaline soil than in the acid soil, due to the highest availability of nutrients and better conditions for the microbial activity in the alkaline soil.

A linear significant correlation was found between microbial biomass carbon and soil organic carbon in the alkaline soil. Maize yield and microbial biomass carbon were significantly correlated in both soils. Furthermore, maize yield correlated with the nitrogen, phosphorus, and potassium content in the maize leaves.

 

LITERATURE CITED

Anderson, T. H., and K. H. Domsch. 1989. Ratios of microbial biomass carbon to total organic carbon in arable soils. Soil Biol. Biochem. 21: 471–479.        [ Links ]

AOAC (Association of Official Analytical Chemists). 1997. Official Methods of Analysis of AOAC International. Chapter 2. 16^th Ed., 3^rd rev. Gaithersburg, M.D. USA. pp: 5–6.        [ Links ]

Arreola–Enríquez, J., D. Palma–López, S. Salgado–García, W. Camacho–Chiu, J. Obrador–Olán, J. Juárez–López, y L. Pastrana–Aponte. 2004. Evaluación de abono órgano–mineral de cachaza en la producción y calidad de la caña de azúcar. Terra Latinoamericana 22: 351–357        [ Links ]

Baaru, M. W., D. N. Mungendi, A. Bationo, L. Verchot, and W. Waceke. 2007. Soil microbial biomass carbon and nitrogen as influenced by organic and inorganic inputs at Kabete, Kenya. In: Bationo A., B. Waswa, J. Kihara, and J. Kimetu (eds). Advances in Integrated Soil Fertility Management in sub–Saharan Africa: Challenges and Opportunities. Springer. Netherlands, pp: 827–832.        [ Links ]

Cori C, C. Arvelo, M. Ruiz, M. Zaragoza, L. Castillo, J. Escalona, E. Arteaga, M. Torres, C. Cañizales, I. Arrieche, y L. Saume. 1999. Definición de los métodos para analizar nitrógeno total en fertilizantes. Venesuelos 6: 33–38        [ Links ]

Dee, B. M., R. J. Haynes, and J. H. Meyer. 2002. Sugar mill wastes can be important soil amendments. In: South African Sugar Technologists Association, Abstracts of the 76th SASTA congress. Mount Edgecombe, South Africa. pp:15.        [ Links ]

Dominy, C. S., R. J. Haynes, and R. Van Antwerpen. 2002. Loss of soil organic matter and related soil properties under long–term sugarcane production on two contrasting soils. Biol. Fertility Soils 36:350–356.        [ Links ]

Goyal S., K. Sakamoto, K. Inubushi, and K. Kamewada. 2006. Long–term effects of inorganic fertilization and organic amendments on soil organic matter and soil microbial properties in Andisols. Arch. Agron. Soil Sci. 52(6): 617–625.        [ Links ]

Graham, M. H., R. J. Haynes, and J. H. Meyers. 2002. Soil organic matter content and quality: Effects of fertilizer applications, burning and trash retention on a long–term sugarcane experiment in South Africa. Soil Biol. Biochem. 34: 93–102.        [ Links ]

Hargreaves, P.R., PC. Brookes, G.J.S. Roos, and PR. Poulton. 2003. Evaluating soil microbial biomass carbon as an indicator of long–term environmental change. Soil Biol. Biochem. 35: 401–407.        [ Links ]

Insam, H., C. Mitchell, and J. Dormaar. 1991. Relationships of soil microbial biomass and activity with fertilization practice and crop yield of three Ultisols. Soil Biol. Biochem. 23: 459–464.        [ Links ]

Jenkinson, D. S., and D. Powlson. 1976. The effects of biocidal treatments on metabolism in soil. V. A method for measuring soil biomass. Soil Biol. Biochem. 8: 209–213.        [ Links ]

Kaur, K., K. K Kapoor, and A. P. Gupta. 2005. Impact of organic manures with and without mineral fertilizers on soil chemical and biological properties under tropical conditions. J. Plant Nutr. Soil Sci. 168:117–122        [ Links ]

Leita, L., M. De Nobili, C. Mondini, G. Muhlbachova, L. Marchiol, G. Bragato, and M. Contin. 1999. Influence of inorganic and organic fertilization on soil microbial biomass, metabolic quotient and heavy metal bioavailability. Biol. Fertility Soils 28:371–376.        [ Links ]

López R,. J., y J. López M. 1978. El Diagnóstico de Suelos y Plantas. Editorial Mundi–Prensa, Madrid, España 108 p.        [ Links ]

Mabuhay, J. A., N. Nakagoshi, and Y. Isagi. 2006. Microbial responses to organic and inorganic amendments in eroded soil. Land Degradation and Develop. 17: 321–332.        [ Links ]

Madejón, E., P. Burgos, R. López, and F. Cabrera. 2003. Agricultural use of three organic residues: effect on orange production and on properties of a soil of the Comarca Costa de Huelva (SE Spain). Nutrient Cycling in Agroecosystems 64: 281–288.        [ Links ]

Mahmood, T, F. Azam., F. Hussain, and K. Malik. 1997. Carbon availability and microbial biomass in soil under an irrigated wheat–maize cropping system receiving different fertilizer treatments. Biol. Fertility Soils 25: 63–68.        [ Links ]

Malavolta, E., G. Vitti, e S. De Oliveira, 1997. Avaliacao do Estado Nutricional das Plants. Principios e Aplicacoes. 2ªed. Associação Brasileira para pesquisa potassa e do fosfato. Piracicaba, Brasil. 201 p.        [ Links ]

Matheus, J. 2004. Evaluación agronómica del uso de compost de residuos de la industria azucarera (biofertilizante) en el cultivo de maíz (Zea mays L.). Bioagro 16: 219–224.        [ Links ]

Meunchang, S., S. Panichsakpatana, and R. W. Weaver. 2006. Tomato growth in soil amended with sugar mill by–products compost. Plant and Soil 280: 171–176.        [ Links ]

Nelson, D. W., and L.E. Sommers. 1996. Total carbon and organic matter. In: Woodwell, G. M. (ed). Methods of Soil Analysis Part 3 — Chemical Methods. Soil Science Society of America Book series No. 5. pp: 961–1010.        [ Links ]

Pascual, J., C. García, T. Hernández, and M. Ayuso. 1997. Changes in the microbial activity of an arid soil amended with urban organic wastes. Biol. Fertility Soils 24: 429–434.        [ Links ]

Perucci, P. 1992. Enzyme activity and microbial biomass in a field soil amended with municipal refuse. Biol. Fertility Soils 14: 54–60.        [ Links ]

Powlson, D. S., PC. Brookes, and B.T. Christensen. 1987. Measurement of soil microbial biomass provides an early indication of changes in the total soil matter due to straw incorporation. Soil Biol. Biochem. 19:159–164.        [ Links ]

Powlson, D. S. 1994. The soil microbial biomass: before, beyond and back. In: Ritz K., J. Dighton, and K. E. Giller (eds). Beyond the Biomass. Wiley. Chichester. pp: 3–20.        [ Links ]

Prakash, V., R. Bhattacharyya, G. Selvakumar, S. Kundu, and H. S Gupta. 2007. Long–term effects of fertilization on some soil properties under rainfed soybean–wheat cropping in the Indian Himalayas. J. Plant Nutr. Soil Sci. 170: 224– 233.        [ Links ]

Rhoades, J. 1982. Cation exchange capacity. In: Page, A.L. (ed). Methods of Soil Analysis. Part 2. Agronomy Monograph N° 9. Madison, WI. pp: 178–190.        [ Links ]

Ruiz, M., y J. Paolini. 2004. El cultivo y el agua de riego sobre el carbono de la biomasa microbiana. Agron. Trop. 54: 161–178.        [ Links ]

Saffigna, P. G., D. S. Powlson, P. C. Brookes, and G. A. Thomas. 1989. Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an Australian Vertisol. Soil Biol. Biochem. 21: 759–765.        [ Links ]

Schinner, R, E. Kandeler, R. Óhlinger, and R. Margesin. 1995. Soil sampling and sample preparation. In: Schinner F., R. Öhlinger, E. Kandeler, and R. Margesin (eds). Methods in Soil Biology. Springer. New York, pp: 7–11.        [ Links ]

Smith, J. L., and E. A. Paul. 1990. The significance of soil microbial biomass estimations. In: Soil Biochemestry, vol 6. Marcel Dekker, New York, pp: 357–396.        [ Links ]

Smith, J., J. Halvorson, and H. Bolton. 2002. Soil properties and microbial activity across a 500 m elevation gradient in a semi–arid environment. Soil Biol. Biochem. 34: 1749 –1757.        [ Links ]

 

Nota

³ Gonzalez, C. 2001. Estimación de cosecha de maíz. In: Memoria del VII Curso sobre producción de maíz. Capítulo 9: Manejo Postcosecha Araure, Portuguesa, Venezuela, pp: 354–359.