- Email: [email protected]
Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauí state, Brazil
european journal of soil biology 44 (2008) 225–230
available at www.sciencedirect.com
journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauı´ state, Brazil A.S.F. Arau´joa,*, V.B. Santosb, R.T.R. Monteiroc a
Universidade Federal do Piauı´, Campus Profa. Cinobelina Elvas, Agronomy, BR 135, km 3, Bom Jesus, PI 64900-000, Brazil Universidade Estadual do Piauı´, Campus de Parnaı´ba, Parnaı´ba, PI 64202-220, Brazil c Centro de Energia Nuclear na Agricultura, P.O. Box 96, Piracicaba, SP 13400-970, Brazil b
article info
abstract
Article history:
The aim of this work was to investigate the response of soil microbial biomass and activity
Received 24 November 2006
to practices in organic and conventional farming systems. The study was carried out at the
Accepted 6 June 2007
Irrigation District of Piauı´, Brazil. Five different plots planted with ‘‘acerola’’ orchard (Mal-
Published online 29 June 2007
pighia glaba) and established at the following management were evaluated: (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic man-
Keywords:
agement (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18
Microorganisms
months of soil organic management (ORG18); and (5) under 24 months of soil organic man-
Metabolic quotient
agement (ORG24). Soil microbial biomass C (Cmic), basal respiration, organic carbon (Corg),
Cmic-to-Corg ratio
Cmic-to-Corg ratio and metabolic quotient (qCO2) were evaluated in soil samples collected
Soil quality
at 0–10 cm depth. The highest Corg and Cmic levels occurred in organic system plots
Sustainability
ORG18 and ORG24 compared to the conventional system. Soil respiration and Cmic-toCorg ratio were significantly enhanced by the organic system plots. The qCO2 was greater in conventional than in organic system. These results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C. ª 2007 Elsevier Masson SAS. All rights reserved.
1.
Introduction
Conventional farming has played an important role in improving food and fibre productivity to meet human demands but has been largely dependent on intensive inputs of synthetic fertilizers and pesticides [23]. Problems arising from conventional practices have led to the development and promotion of organic farming system that account of the environment and public health as main concerns [17]. Organic farming is gaining worldwide acceptance and has been expanding at annual rate of 20% in the last decade,
accounting for over 24 million hectares worldwide [28]. Brazil counts as one of the leading countries worldwide in organic farming (about 100,000 ha) and occupies the 34th position in the world. In region north of Piauı´ state, organic fruit production has intensified to meet market demands over the last years. Organic practices for fruit production, in Piauı´ state, avoid applications of synthetic fertilizers and pesticides, rely on organic inputs and recycling for nutrient supply, and emphasize cropping system design and biological processes for pest management, as defined by organic farming regulation in the world. They may thus reduce some negative effects
* Corresponding author. Tel.: þ55 89 3562 2067; fax: þ55 89 3562 1103. E-mail address: [email protected] (A.S.F. Arau´jo). 1164-5563/$ – see front matter ª 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2007.06.001
226
european journal of soil biology 44 (2008) 225–230
attributed to conventional farming and have potential benefits in enhancing soil quality [18,15]. Soil quality is the capacity of soil to maintain some key ecological functions, such as decomposition and formation of soil organic matter [9]. Microbial processes are important for the management of farming system and to improve soil quality. The maintenance of the ecosystem productivity depends mainly on the organic matter transformation and, consequently, soil microbial biomass [25]. Soil microbial biomass, the living part of soil organic matter, functions as a transient nutrient sink and is responsible for releasing nutrient from organic matter for use by plants [20]. Changes in the structure of microbial communities may lead to changes of important functions such as organic matter decomposition and pollutant degradation [14]. Measurements of microbial activity in soils are based on the presence of intact and active microbial cells [1]. Soil respiration is one of the most frequently used parameters for quantifying microbial activities in soil [2] and reacts differently to treatment and cultivation methods and has been used most frequently for the assessment of the side effects of chemicals such as pesticides and heavy metals [5,4]. Additionally, the study of the relationship between microbial biomass C (Cmic) and total organic carbon (Corg), and the metabolic quotient (qCO2), which is the rate of CO2 per unit of biomass and time, can provide an understanding of the biological and chemical changes that occur under different agricultural practices [2]. The metabolic quotient (qCO2) indicates how efficiently the microbial biomass is utilizing available C for biosynthesis and is considered as a sensitive indicator for estimating biological activity and substrate quality [27]. An understanding of the microbial dynamics in organic and conventional systems may provide guidance for designing the best farming system strategies and minimizing yield losses. The aim of this paper was to investigate the change of microbial biomass and activity during different time periods of soil organic management using soil conventional management as control.
2.
Materials and methods
2.1.
Study area
The field study was carried out at the Irrigation District of Piauı´, Brazil (03 050 S; 41 470 W; 46 m). The climate is rather dry with a mean precipitation of 1000 mm year1 and an annual mean temperature of 30 C. The soil type is a Typic Quartzipsamment in the US soil taxonomy. Soil samples were obtained in March 2006 from five different plots planted with ‘‘acerola’’ orchard (Malpighia glaba) and established at the following management: (1) under 12 months of soil conventional management system (CNV); (2) under six months of soil organic management system (ORG6); (3) under 12 months of soil organic management system (ORG12); (4) under 18 months of soil organic management system (ORG18); and (5) under 24 months of soil organic management system (ORG24). The size of the organic and conventional areas varied between 0.05 and 0.1 ha. In the soil of all areas, the percentage contribution of sand and clay was
Table 1 – Annual inputs of fertilizers (N–P–K) on the conventionally and organically managed plots Fertilizer
Conventional system (kg ha1)
Organic system
N P
200 80
K
80
Composted cow manure (50 t ha1) Composted cow manure (50 t ha1) and rock phosphate (0.5 t ha1) Composted cow manure (50 t ha1)
relatively higher and lower, respectively. The plants of M. glaba are distanced 2.0 m long and 3.0 m wide. Conventional system is managed since 1999 (crop rotation cowpea/watermelon) and includes, annually, synthetic soil fertilizer applications (Table 1) and chemical control of pesticides. Annually, granulated dolomite lime is applied at a rate of 1 t ha1. The organic management was the immediate withdrawal of all conventional inputs and was managed since 2004. The organic plots (ORG6, ORG12, ORG18 and ORG24) were implanted in four different conversion periods to organic management (under 6, 12, 18 and 24 months of organic management). Initially, the areas in organic system received inputs of green manure (Crotalaria juncea, Vigna unguiculata, and Cajanus cajan), 0.5 t ha1 of rock phosphate and 1.2 t ha1 of ‘‘MB4’’ (calcareous plus micronutrients). Annually, organic practices included straw mulch for weed control; nutrients are supplied in the form of composted cow manure (50 t ha1), rock phosphate (0.5 t ha1), ‘‘MB4’’ (1.2 t ha1) and ‘‘carnauba’’ straw (100 t ha1). Table 2 shows the chemical characteristics of composted cow manure used in organic input. The organic amendments are applied in superficial form in the plant canopies. In the disease and insect control in organic management system are used alternative methods (plant extracts and bicontrol).
2.2.
Soil sampling and analyses
Soil subsamples were collected from each plot at 0–10 cm soil depth, and were selected using a grid pattern at 10 points in a 100 m2 quadrant (under the plant canopy) in each plot. The soil subsamples from each point were bulked and thoroughly mixed within a plastic bag, and a composite sample was taken. The composite samples, in four replicates for each plot, were transported to laboratory on ice in a cooler. Field moist soils were sieved through a 2-mm screen, and immediately stored in sealed plastic bags at 4 C. The soil chemical analysis was performed by an official laboratory (Soil Fertility Lab, Embrapa, Piauı´ state). Soil pH and electrical conductivity (EC) were determined in a 1:2.5
Table 2 – Some chemical characteristics of the composted cow manure added to organic plots Moisture (g kg1) pH (1:2.5) EC (1:2.5) Organic matter (g kg1) Total nitrogen (g kg1) C/N P available (g kg1) K available (g kg1)
198 6.9 1.8 281.6 8.32 19.1 4.1 3.5
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european journal of soil biology 44 (2008) 225–230
Table 3 – Soil chemical properties (0–10 cm) on different soil management systems Management systemb CNV ORG6 ORG12 ORG18 ORG24
pH 6.9 6.6 6.4 6.7 6.8
EC (dS m1)
aa a a a a
0.7 0.6 0.9 1.1 1.6
b b b b a
P (g kg1)
K (cmolc kg1)
Ca (cmolc kg1)
Mg (cmolc kg1)
CEC (cmolc kg1)
83.6 b 100.4 b 92.0 b 120.1 b 219.0 a
0.25 b 0.10 b 0.5 a 0.5 a 0.6 a
2.1 b 2.3 b 1.9 b 2.2 b 3.0 a
1.0 a 0.8 a 1.0 a 1.1 a 1.3 a
3.68 c 3.37 c 4.52 b 4.7 b 5.85 a
EC ¼ electrical conductivity and CEC ¼ cation exchange capacity. a Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
soil/water extract. Available P and K were determined using the method described by Tedesco et al. [22]. Soil moisture was determined from soils’ cores taken from five sites from the 0 to 10 cm depth. Soil subsamples (10 g) were weighed into aluminum dishes. These dishes were placed in a 105 þ 2 C oven for 24 h, and the dry weight was then recorded. Gravimetric soil moisture was the difference in soil weights before and after oven drying. Bulk density was determined according to USDA [24].
2.3.
Soil microbial biomass and organic C
Microbial biomass C (Cmic) was estimated by irradiation [12] and incubation methods [13]. Microbial biomass C was measured on 40 g (dry weight) subsamples that were irradiated in microwave for 5 min. Both irradiated and non-irradiated soils were incubated for 7 d at 25 C and 60% water holding capacity. CO2 trapped in NaOH was then determined titrimetrically. An efficiency coefficient of 0.45 was used to convert the difference in CO2 between the irradiated and the non-irradiated soils in microbial biomass C. Organic carbon (Corg) content was determined by the Walkley and Black [26] method.
2.4.
Results
3.1.
Soil chemical properties and organic carbon
Table 3 shows chemical properties of soil in conventional and organic management systems. Soil pH was not significantly different between conventional and organic management systems. Soil salinity, evaluated by electrical conductivity (EC), was greater in organic management system (ORG24) than the other evaluated plots. Organic management system (ORG24) had a higher soil P and Ca content than conventional management system (CNV). The highest organic C values were observed in ORG24 plots and significant differences among ORG24 plots and other ones are given (Table 4).
3.2.
Soil microbial biomass and activity
From the 12 months of organic management, the highest Cmic values were found and significant differences between conventional and organic management were observed (Table 4). The soil respiration values in the organic management system
Soil respiration and qCO2
Respiration was measured as CO2 evolution according to Alef [1]. Soil samples (100 g) were placed in 300-ml glass containers closed with rubber stoppers, moistened at 60% of the maximum water holding capacity and incubated for 7 d at 25 C. Glass vials holding 10 ml of 0.5 N NaOH to trap the evolved CO2 were placed in the above containers. On day 7 after the incubation, the glass vial was removed and the CO2 trapped in NaOH was then determined titrimetrically. qCO2 was calculated as the ratio of basal respiration to microbial biomass C. The qCO2 results were expressed as g CO2–C d1 g1 Cmic.
2.5.
3.
Statistical analyses
The study was carried out in a completely randomized design with four replicates and the data were analyzed statistically. When a significant F value was detected, the means were compared by the Duncan test ( p < 0.05).
Table 4 – Soil organic carbon (Corg), microbial biomass C (Cmic) and Cmic-to-Corg ratio of conventional and organic systems, at 0–10 cm depth Management systemb CNV ORG6 ORG12 ORG18 ORG24
Corg (g kg1)
Cmic (mg g1 soil)
Cmic-to-Corg (%)
11.6 2.1 ba 12.5 2.9 b 11.2 1.9 b 13.1 2.0 b 17.8 2.6 a
64.0 21 c 97.1 29 b 121.2 27 a 129.4 2.6 a 142.5 22 a
0.5 0.09 b 0.8 0.10 a 1.1 0.13 a 0.9 þ 0.10 a 0.8 0.11 a
a Mean standard error. Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
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european journal of soil biology 44 (2008) 225–230
were higher than in the conventional management system, except in ORG6 plots (Table 5).
3.3.
Microbial indices
The Cmic-to-Corg ratio and respiratory quotient (qCO2) were affected by the farming systems. Cmic-to-Corg ratios were the statistical differences between organically and conventional fertilized soils (Table 4), showing the highest Cmic-to-Corg ratios in organically fertilized soils. The qCO2 was higher in conventional management system than in organic management system (Table 5) and indicates low efficiency of the soil microbial biomass under conventional management system than organic management system in order to use available C for biosynthesis.
4.
Discussion
4.1.
Soil chemical properties and organic carbon
The results show that the soil salinity was higher in organic management system (ORG24) and these results were similar to the findings of Melero et al. [17], comparing soil under organic and conventional systems, who reported that neither inorganic nor organic fertilization appeared to cause soil salinization. The differences in available P and Ca between organic management system (ORG24) and conventional management system (CNV) suggest that permanent addition of rock phosphate and calcareous (‘‘MB4’’) to soil under organic management system increased available P and Ca. The permanent addition of compost contributed to the nutrient content of soil resulting in higher cation exchange capacity (CEC) in organic system with 24 months (ORG24) and indicates that soil fertility is improving slowly. Additionally, green manure inputs in soil, under organic system, add organic matter and nutrients to the soil, increasing soil fertility, conform to the reports of Ashraf et al. [6].
Table 5 – Soil basal respiration and respiratory quotient (qCO2) of conventional and organic systems, at 0–10 cm depth Management systemb CNV ORG6 ORG12 ORG18 ORG24
Basal respiration (mg CO2–C g1 d1) 144.4 22.1 157.3 18.3 230.9 26.3 227.4 20.9 226.3 21.1
ba b a a a
qCO2 (g CO2–C d1 g1 Cmic) 2.2 0.30 a 1.6 0.26 b 1.8 0.21 b 1.6 0.19 b 1.4 0.20 b
a Mean standard error. Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
Higher Corg content observed in organic management system (ORG24) indicates that the increase in Corg, following organic practices, was statistically significant after 2 years. This increase in Corg is important for semi-arid region, as Piauı´ state, due to the low levels of organic matter observed for our soils. The high organic C is important for sustainability because of organic matter influence on soil’s physical, chemical and biological properties [21]. These results are in accordance with Glover et al. [11] and Melero et al. [17] who observed that organic farming system has been shown to maintain soil organic matter at high levels than conventional system.
4.2.
Soil microbial biomass and activity
The long-term C inputs significantly impacted microbial biomass and activity in organic farming systems. These data indicate that microbial biomass C was significantly and rapidly enhanced in the organic system due to input of organic amendment, that supply of available C. Additionally, the increase of Cmic in organic plots is, probably, also due to the microbial biomass contained in the organic amendments. Microbial biomass is one of the most commonly used parameters in soil biology [4] and serves as an important reservoir of plant nutrients [16]. The results are in agreement with the findings of Melero et al. [17] and Tu et al. [23] in soils under organic farming systems. According to Fließbach and Mader [10], over the long term, microbial biomass C is significantly affected by the long-term management as well as by its intensity. The same author observed that microbial biomass C in the organic plots was 45–64% higher than in the respective conventional plots with manure amendment. Soil respiration is one of the oldest and still the most frequently used parameter for quantifying microbial activities in soils [1]. The higher soil respiration in the organic farming system with 12 (ORG12), 18 (ORG18) and 24 months (ORG24) indicates a higher soil microbial activity due to the permanent and continuous addition of an exogenous source of labile organic matter to the soil and the consequent stimulation of heterotrophic microorganisms [19]. According to Smith and Paul [20], the organic matter constitutes one of the most important sources of the energy and nutrients for the microbial development. Similar results were observed by Bettiol et al. [7], in a Brazilian soil under conventional and organic farming systems. Stimulation of microbial biomass and activities by organic C inputs has been reported in various organic substrates such as compost [8], cotton straw and animal manure [23] and solid waste compost [4]. Our results also showed that the cow compost and ‘‘carnauba’’ straw enhanced microbial biomass and activity.
4.3.
Microbial indices
The Cmic-to-Corg ratio is an indicator of the availability of carbon to microorganisms, input of organic matter to soil, conversion efficiency to microbial biomass and stabilization of carbon in soil. The differences in the Cmic-to-Corg ratio between organic and conventional management systems suggest that the soil amendment with compost and ‘‘carnauba’’ straw has been a significant factor for an increase
european journal of soil biology 44 (2008) 225–230
in the Cmic-to-Corg ratio, by the available organic carbon input which favored microbial biomass. Anderson and Domsch [2] speculated that the higher Cmic-to-Corg ratio in their crop rotation plots as compared to monoculture systems was a result of a higher efficiency of organic matter utilization for microbial growth, which they attributed to a more organic matter input. The qCO2 is a measure of the specific metabolic activity that varies according to the composition and physiological state of the microbial biomass, the availability of nutrients and various abiotic factors. According to Fließbach and Mader [10], the organic management system benefits soil microbial biomass because microorganisms are utilizing the available C more efficiently as indicated by a lower qCO2 and suggests better conditions within the soil organic matter which may contribute to nutrient mineralization and temporary storage of potentially leachable elements.
5.
Conclusions
The management according to organic and conventional systems resulted in changes in soil microbial and organic C. The results showed that the organic farming practices resulted, in short term, higher microbial biomass and activity, while organic C was more responsive in long term. This was caused by the higher inputs of organic matter, an energetic substrate for the present microbial communities that were activated to assure the turnover of applied nutrients. Additionally, the results of qCO2 show that a smaller microbial biomass respire at a greater rate in conventional plots, which could be related to lower nutrient availability for microorganisms in conventional plots and Cmic-to-Corg ratio suggests a better efficiency of microorganisms in the conversion of carbon sources to Cmic in organic plots. Finally, our results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C.
Acknowledgements We are grateful to D.B. Sampaio for her technical assistance in the analyses. We thank Dr. F.N. Silva for his input to this paper and for English review. This research was funded by Piauı´ State Research Foundation – FAPEPI (continue fluxes 2005). R.T.R. Monteiro is supported by personal grant from CNPq – Brazil.
references
[1] K. Alef, Estimation of soil respiration, in: K. Alef, P. Nannipieri (Eds.), Methods in Soil Microbiology and Biochemistry, Academic Press, New York, 1995, pp. 464–470. [2] J.M. Anderson, K.H. Domsch, Application of ecophysiological quotients (qCO and qD) on microbial biomass from soils of different cropping histories, Soil Biol. Biochem. 22 (1990) 251–255.
229
[4] A.S.F. Arau´jo, R.T.R. Monteiro, Microbial biomass and activity in a Brazilian soil amended with untreated and composted textile sludge, Chemosphere 64 (2006) 1028–1032. [5] A.S.F. Arau´jo, R.T.R. Monteiro, R.B. Abarkeli, Effect of glyphosate on the microbial activity of two Brazilian soils, Chemosphere 52 (2003) 799–804. [6] M. Ashraf, T. Mahmood, F. Azam, R.M. Qureshi, Comparative effects of applying leguminous and non-leguminous green manures and inorganic N on biomass yield and nitrogen uptake in flooded rice (Oryza sativa L.), Biol. Fertil. Soils 40 (2004) 147–152. [7] W. Bettiol, R. Ghini, J.A.H. Galva˜o, M.A.V. Ligo, J.L.C. Mineiro, Soil organisms in organic and conventional cropping systems, Sci. Agric. 59 (2002) 565–572. [8] M.A.H. Chowdhury, K. Kouno, T. Ando, T. Nagaoka, Microbial biomass, S mineralization and S uptake by African millet from soil amended with various composts, Soil Biol. Biochem. 32 (2000) 845–852. [9] J.W. Doran, M. Sarrantonio, M.A. Liebig, Soil health and sustainability, in: D.L. Sparks (Ed.), Advances in Agronomy, vol. 56, Academic Press, San Diego, 1996, pp. 25–37. [10] A. Fließbach, P. Mader, Microbial biomass and size–density fractions differ between soils of organic and conventional agricultural systems, Soil Biol. Biochem. 32 (2000) 757–768. [11] J.D. Glover, J.P. Reganold, P.K. Andrews, Systematic method for rating soil quality of conventional, organic, and integrated apple orchards in Washington State, Agric. Ecosyst. Environ. 80 (2000) 29–45. [12] K.R. Islam, R.R. Weil, A rapid microwave digestion method for colorimetric measurement of soil organic carbon, Commun. Soil Sci. Plant Anal. 29 (1988) 2269–2284. [13] D.S. Jenkinson, D.S. Powlson, The effects of biocidal treatments on metabolism in soil. I: Fumigation with chloroform, Soil Biol. Biochem. 8 (1976) 167–177. [14] J.L. MacAlady, M.E. Muller, K.M. Scow, Effects of metam sodium fumigation on soil microbial activity and community structure, J. Environ. Qual. 27 (1998) 54–63. [15] P. Mader, A. Fließbach, D. Dubois, L. Gunst, P. Fried, U. Niggli, Soil fertility and biodiversity in organic farming, Science 296 (2002) 1694–1697. [16] T. Marumoto, J.P.E. Anderson, K.H. Domsch, Mineralization of nutrients from soil microbial biomass, Soil Biol. Biochem. 14 (1982) 469–475. [17] S. Melero, J.C.R. Porras, J.F. Herencia, E. Madejon, Chemical and biochemical properties in a silty loam soil under conventional and organic management, Soil Tillage Res. 90 (2005) 162–170. [18] D. Rigby, D. Ca´ceres, Organic farming and the sustainability of agricultural systems, Agric. Syst. 68 (2001) 21–40. [19] P.G. Saffigna, D.S. Powlson, P.C. Brookes, G.A. Thomas, Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an Australian vertissol, Soil Biol. Biochem. 21 (1989) 759–765. [20] L. Smith, E.A. Paul, The significance of soil microbial biomass estimations, in: J.M. Bollag, G. Stotzky (Eds.), Soil Biochemistry, Dekker, New York, 1990, pp. 357–396. [21] G.P. Sparling, T.G. Shephed, H.A. Kettles, Changes in soil organic C, microbial C and aggregate stability under continuous maize and cereal cropping, and after restoration to pasture in soil from the Manawatu region, New Zeal. Soil Tillage 24 (1992) 225–241. [22] M.J. Tedesco, C. Gianello, C.A. Bissani, Ana´lises de solos, plantas e outros materiais, UFRGS, Porto Alegre, 1995. [23] C. Tu, J.B. Ristaino, S. Hu, Soil microbial biomass and activity in organic tomato farming systems: effects of organic inputs and straw mulching, Soil Biol. Biochem. 38 (2006) 247–255. [24] United State Department of Agriculture, Soil Quality Test Kit, Agricultural Research Service, Washington, 1998.
230
european journal of soil biology 44 (2008) 225–230
[25] M.A.R. Valpassos, E.G.S. Cavalcante, A.M. Cassiolato, M.C. Alves, Effects of soil management systems on soil microbial activity, bulk density and chemical properties, Pesqui. Agropecu. Bras. 36 (2001) 1539–1545. [26] A. Walkley, J.A. Black, An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method, Soil Sci. 37 (1934) 29–73.
[27] D.A. Wardle, A. Ghani, A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development, Soil Biol. Biochem. 27 (1995) 1601–1610. [28] H. Willer, M. Yussefi, The World of Organic Agriculture: Statistics and Emerging Trends, International Federation of Organic Agriculture Movements, Bonn, Germany, 2004, p. 167.
available at www.sciencedirect.com
journal homepage: http://www.elsevier.com/locate/ejsobi
Original article
Responses of soil microbial biomass and activity for practices of organic and conventional farming systems in Piauı´ state, Brazil A.S.F. Arau´joa,*, V.B. Santosb, R.T.R. Monteiroc a
Universidade Federal do Piauı´, Campus Profa. Cinobelina Elvas, Agronomy, BR 135, km 3, Bom Jesus, PI 64900-000, Brazil Universidade Estadual do Piauı´, Campus de Parnaı´ba, Parnaı´ba, PI 64202-220, Brazil c Centro de Energia Nuclear na Agricultura, P.O. Box 96, Piracicaba, SP 13400-970, Brazil b
article info
abstract
Article history:
The aim of this work was to investigate the response of soil microbial biomass and activity
Received 24 November 2006
to practices in organic and conventional farming systems. The study was carried out at the
Accepted 6 June 2007
Irrigation District of Piauı´, Brazil. Five different plots planted with ‘‘acerola’’ orchard (Mal-
Published online 29 June 2007
pighia glaba) and established at the following management were evaluated: (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic man-
Keywords:
agement (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18
Microorganisms
months of soil organic management (ORG18); and (5) under 24 months of soil organic man-
Metabolic quotient
agement (ORG24). Soil microbial biomass C (Cmic), basal respiration, organic carbon (Corg),
Cmic-to-Corg ratio
Cmic-to-Corg ratio and metabolic quotient (qCO2) were evaluated in soil samples collected
Soil quality
at 0–10 cm depth. The highest Corg and Cmic levels occurred in organic system plots
Sustainability
ORG18 and ORG24 compared to the conventional system. Soil respiration and Cmic-toCorg ratio were significantly enhanced by the organic system plots. The qCO2 was greater in conventional than in organic system. These results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C. ª 2007 Elsevier Masson SAS. All rights reserved.
1.
Introduction
Conventional farming has played an important role in improving food and fibre productivity to meet human demands but has been largely dependent on intensive inputs of synthetic fertilizers and pesticides [23]. Problems arising from conventional practices have led to the development and promotion of organic farming system that account of the environment and public health as main concerns [17]. Organic farming is gaining worldwide acceptance and has been expanding at annual rate of 20% in the last decade,
accounting for over 24 million hectares worldwide [28]. Brazil counts as one of the leading countries worldwide in organic farming (about 100,000 ha) and occupies the 34th position in the world. In region north of Piauı´ state, organic fruit production has intensified to meet market demands over the last years. Organic practices for fruit production, in Piauı´ state, avoid applications of synthetic fertilizers and pesticides, rely on organic inputs and recycling for nutrient supply, and emphasize cropping system design and biological processes for pest management, as defined by organic farming regulation in the world. They may thus reduce some negative effects
* Corresponding author. Tel.: þ55 89 3562 2067; fax: þ55 89 3562 1103. E-mail address: [email protected] (A.S.F. Arau´jo). 1164-5563/$ – see front matter ª 2007 Elsevier Masson SAS. All rights reserved. doi:10.1016/j.ejsobi.2007.06.001
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european journal of soil biology 44 (2008) 225–230
attributed to conventional farming and have potential benefits in enhancing soil quality [18,15]. Soil quality is the capacity of soil to maintain some key ecological functions, such as decomposition and formation of soil organic matter [9]. Microbial processes are important for the management of farming system and to improve soil quality. The maintenance of the ecosystem productivity depends mainly on the organic matter transformation and, consequently, soil microbial biomass [25]. Soil microbial biomass, the living part of soil organic matter, functions as a transient nutrient sink and is responsible for releasing nutrient from organic matter for use by plants [20]. Changes in the structure of microbial communities may lead to changes of important functions such as organic matter decomposition and pollutant degradation [14]. Measurements of microbial activity in soils are based on the presence of intact and active microbial cells [1]. Soil respiration is one of the most frequently used parameters for quantifying microbial activities in soil [2] and reacts differently to treatment and cultivation methods and has been used most frequently for the assessment of the side effects of chemicals such as pesticides and heavy metals [5,4]. Additionally, the study of the relationship between microbial biomass C (Cmic) and total organic carbon (Corg), and the metabolic quotient (qCO2), which is the rate of CO2 per unit of biomass and time, can provide an understanding of the biological and chemical changes that occur under different agricultural practices [2]. The metabolic quotient (qCO2) indicates how efficiently the microbial biomass is utilizing available C for biosynthesis and is considered as a sensitive indicator for estimating biological activity and substrate quality [27]. An understanding of the microbial dynamics in organic and conventional systems may provide guidance for designing the best farming system strategies and minimizing yield losses. The aim of this paper was to investigate the change of microbial biomass and activity during different time periods of soil organic management using soil conventional management as control.
2.
Materials and methods
2.1.
Study area
The field study was carried out at the Irrigation District of Piauı´, Brazil (03 050 S; 41 470 W; 46 m). The climate is rather dry with a mean precipitation of 1000 mm year1 and an annual mean temperature of 30 C. The soil type is a Typic Quartzipsamment in the US soil taxonomy. Soil samples were obtained in March 2006 from five different plots planted with ‘‘acerola’’ orchard (Malpighia glaba) and established at the following management: (1) under 12 months of soil conventional management system (CNV); (2) under six months of soil organic management system (ORG6); (3) under 12 months of soil organic management system (ORG12); (4) under 18 months of soil organic management system (ORG18); and (5) under 24 months of soil organic management system (ORG24). The size of the organic and conventional areas varied between 0.05 and 0.1 ha. In the soil of all areas, the percentage contribution of sand and clay was
Table 1 – Annual inputs of fertilizers (N–P–K) on the conventionally and organically managed plots Fertilizer
Conventional system (kg ha1)
Organic system
N P
200 80
K
80
Composted cow manure (50 t ha1) Composted cow manure (50 t ha1) and rock phosphate (0.5 t ha1) Composted cow manure (50 t ha1)
relatively higher and lower, respectively. The plants of M. glaba are distanced 2.0 m long and 3.0 m wide. Conventional system is managed since 1999 (crop rotation cowpea/watermelon) and includes, annually, synthetic soil fertilizer applications (Table 1) and chemical control of pesticides. Annually, granulated dolomite lime is applied at a rate of 1 t ha1. The organic management was the immediate withdrawal of all conventional inputs and was managed since 2004. The organic plots (ORG6, ORG12, ORG18 and ORG24) were implanted in four different conversion periods to organic management (under 6, 12, 18 and 24 months of organic management). Initially, the areas in organic system received inputs of green manure (Crotalaria juncea, Vigna unguiculata, and Cajanus cajan), 0.5 t ha1 of rock phosphate and 1.2 t ha1 of ‘‘MB4’’ (calcareous plus micronutrients). Annually, organic practices included straw mulch for weed control; nutrients are supplied in the form of composted cow manure (50 t ha1), rock phosphate (0.5 t ha1), ‘‘MB4’’ (1.2 t ha1) and ‘‘carnauba’’ straw (100 t ha1). Table 2 shows the chemical characteristics of composted cow manure used in organic input. The organic amendments are applied in superficial form in the plant canopies. In the disease and insect control in organic management system are used alternative methods (plant extracts and bicontrol).
2.2.
Soil sampling and analyses
Soil subsamples were collected from each plot at 0–10 cm soil depth, and were selected using a grid pattern at 10 points in a 100 m2 quadrant (under the plant canopy) in each plot. The soil subsamples from each point were bulked and thoroughly mixed within a plastic bag, and a composite sample was taken. The composite samples, in four replicates for each plot, were transported to laboratory on ice in a cooler. Field moist soils were sieved through a 2-mm screen, and immediately stored in sealed plastic bags at 4 C. The soil chemical analysis was performed by an official laboratory (Soil Fertility Lab, Embrapa, Piauı´ state). Soil pH and electrical conductivity (EC) were determined in a 1:2.5
Table 2 – Some chemical characteristics of the composted cow manure added to organic plots Moisture (g kg1) pH (1:2.5) EC (1:2.5) Organic matter (g kg1) Total nitrogen (g kg1) C/N P available (g kg1) K available (g kg1)
198 6.9 1.8 281.6 8.32 19.1 4.1 3.5
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Table 3 – Soil chemical properties (0–10 cm) on different soil management systems Management systemb CNV ORG6 ORG12 ORG18 ORG24
pH 6.9 6.6 6.4 6.7 6.8
EC (dS m1)
aa a a a a
0.7 0.6 0.9 1.1 1.6
b b b b a
P (g kg1)
K (cmolc kg1)
Ca (cmolc kg1)
Mg (cmolc kg1)
CEC (cmolc kg1)
83.6 b 100.4 b 92.0 b 120.1 b 219.0 a
0.25 b 0.10 b 0.5 a 0.5 a 0.6 a
2.1 b 2.3 b 1.9 b 2.2 b 3.0 a
1.0 a 0.8 a 1.0 a 1.1 a 1.3 a
3.68 c 3.37 c 4.52 b 4.7 b 5.85 a
EC ¼ electrical conductivity and CEC ¼ cation exchange capacity. a Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
soil/water extract. Available P and K were determined using the method described by Tedesco et al. [22]. Soil moisture was determined from soils’ cores taken from five sites from the 0 to 10 cm depth. Soil subsamples (10 g) were weighed into aluminum dishes. These dishes were placed in a 105 þ 2 C oven for 24 h, and the dry weight was then recorded. Gravimetric soil moisture was the difference in soil weights before and after oven drying. Bulk density was determined according to USDA [24].
2.3.
Soil microbial biomass and organic C
Microbial biomass C (Cmic) was estimated by irradiation [12] and incubation methods [13]. Microbial biomass C was measured on 40 g (dry weight) subsamples that were irradiated in microwave for 5 min. Both irradiated and non-irradiated soils were incubated for 7 d at 25 C and 60% water holding capacity. CO2 trapped in NaOH was then determined titrimetrically. An efficiency coefficient of 0.45 was used to convert the difference in CO2 between the irradiated and the non-irradiated soils in microbial biomass C. Organic carbon (Corg) content was determined by the Walkley and Black [26] method.
2.4.
Results
3.1.
Soil chemical properties and organic carbon
Table 3 shows chemical properties of soil in conventional and organic management systems. Soil pH was not significantly different between conventional and organic management systems. Soil salinity, evaluated by electrical conductivity (EC), was greater in organic management system (ORG24) than the other evaluated plots. Organic management system (ORG24) had a higher soil P and Ca content than conventional management system (CNV). The highest organic C values were observed in ORG24 plots and significant differences among ORG24 plots and other ones are given (Table 4).
3.2.
Soil microbial biomass and activity
From the 12 months of organic management, the highest Cmic values were found and significant differences between conventional and organic management were observed (Table 4). The soil respiration values in the organic management system
Soil respiration and qCO2
Respiration was measured as CO2 evolution according to Alef [1]. Soil samples (100 g) were placed in 300-ml glass containers closed with rubber stoppers, moistened at 60% of the maximum water holding capacity and incubated for 7 d at 25 C. Glass vials holding 10 ml of 0.5 N NaOH to trap the evolved CO2 were placed in the above containers. On day 7 after the incubation, the glass vial was removed and the CO2 trapped in NaOH was then determined titrimetrically. qCO2 was calculated as the ratio of basal respiration to microbial biomass C. The qCO2 results were expressed as g CO2–C d1 g1 Cmic.
2.5.
3.
Statistical analyses
The study was carried out in a completely randomized design with four replicates and the data were analyzed statistically. When a significant F value was detected, the means were compared by the Duncan test ( p < 0.05).
Table 4 – Soil organic carbon (Corg), microbial biomass C (Cmic) and Cmic-to-Corg ratio of conventional and organic systems, at 0–10 cm depth Management systemb CNV ORG6 ORG12 ORG18 ORG24
Corg (g kg1)
Cmic (mg g1 soil)
Cmic-to-Corg (%)
11.6 2.1 ba 12.5 2.9 b 11.2 1.9 b 13.1 2.0 b 17.8 2.6 a
64.0 21 c 97.1 29 b 121.2 27 a 129.4 2.6 a 142.5 22 a
0.5 0.09 b 0.8 0.10 a 1.1 0.13 a 0.9 þ 0.10 a 0.8 0.11 a
a Mean standard error. Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
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were higher than in the conventional management system, except in ORG6 plots (Table 5).
3.3.
Microbial indices
The Cmic-to-Corg ratio and respiratory quotient (qCO2) were affected by the farming systems. Cmic-to-Corg ratios were the statistical differences between organically and conventional fertilized soils (Table 4), showing the highest Cmic-to-Corg ratios in organically fertilized soils. The qCO2 was higher in conventional management system than in organic management system (Table 5) and indicates low efficiency of the soil microbial biomass under conventional management system than organic management system in order to use available C for biosynthesis.
4.
Discussion
4.1.
Soil chemical properties and organic carbon
The results show that the soil salinity was higher in organic management system (ORG24) and these results were similar to the findings of Melero et al. [17], comparing soil under organic and conventional systems, who reported that neither inorganic nor organic fertilization appeared to cause soil salinization. The differences in available P and Ca between organic management system (ORG24) and conventional management system (CNV) suggest that permanent addition of rock phosphate and calcareous (‘‘MB4’’) to soil under organic management system increased available P and Ca. The permanent addition of compost contributed to the nutrient content of soil resulting in higher cation exchange capacity (CEC) in organic system with 24 months (ORG24) and indicates that soil fertility is improving slowly. Additionally, green manure inputs in soil, under organic system, add organic matter and nutrients to the soil, increasing soil fertility, conform to the reports of Ashraf et al. [6].
Table 5 – Soil basal respiration and respiratory quotient (qCO2) of conventional and organic systems, at 0–10 cm depth Management systemb CNV ORG6 ORG12 ORG18 ORG24
Basal respiration (mg CO2–C g1 d1) 144.4 22.1 157.3 18.3 230.9 26.3 227.4 20.9 226.3 21.1
ba b a a a
qCO2 (g CO2–C d1 g1 Cmic) 2.2 0.30 a 1.6 0.26 b 1.8 0.21 b 1.6 0.19 b 1.4 0.20 b
a Mean standard error. Means followed by the same letter within each column are not significantly different at 5% level by Duncan’s test. b The management systems evaluated were (1) under 12 months of soil conventional management (CNV); (2) under six months of soil organic management (ORG6); (3) under 12 months of soil organic management (ORG12); (4) under 18 months of soil organic management (ORG18); and (5) under 24 months of soil organic management (ORG24).
Higher Corg content observed in organic management system (ORG24) indicates that the increase in Corg, following organic practices, was statistically significant after 2 years. This increase in Corg is important for semi-arid region, as Piauı´ state, due to the low levels of organic matter observed for our soils. The high organic C is important for sustainability because of organic matter influence on soil’s physical, chemical and biological properties [21]. These results are in accordance with Glover et al. [11] and Melero et al. [17] who observed that organic farming system has been shown to maintain soil organic matter at high levels than conventional system.
4.2.
Soil microbial biomass and activity
The long-term C inputs significantly impacted microbial biomass and activity in organic farming systems. These data indicate that microbial biomass C was significantly and rapidly enhanced in the organic system due to input of organic amendment, that supply of available C. Additionally, the increase of Cmic in organic plots is, probably, also due to the microbial biomass contained in the organic amendments. Microbial biomass is one of the most commonly used parameters in soil biology [4] and serves as an important reservoir of plant nutrients [16]. The results are in agreement with the findings of Melero et al. [17] and Tu et al. [23] in soils under organic farming systems. According to Fließbach and Mader [10], over the long term, microbial biomass C is significantly affected by the long-term management as well as by its intensity. The same author observed that microbial biomass C in the organic plots was 45–64% higher than in the respective conventional plots with manure amendment. Soil respiration is one of the oldest and still the most frequently used parameter for quantifying microbial activities in soils [1]. The higher soil respiration in the organic farming system with 12 (ORG12), 18 (ORG18) and 24 months (ORG24) indicates a higher soil microbial activity due to the permanent and continuous addition of an exogenous source of labile organic matter to the soil and the consequent stimulation of heterotrophic microorganisms [19]. According to Smith and Paul [20], the organic matter constitutes one of the most important sources of the energy and nutrients for the microbial development. Similar results were observed by Bettiol et al. [7], in a Brazilian soil under conventional and organic farming systems. Stimulation of microbial biomass and activities by organic C inputs has been reported in various organic substrates such as compost [8], cotton straw and animal manure [23] and solid waste compost [4]. Our results also showed that the cow compost and ‘‘carnauba’’ straw enhanced microbial biomass and activity.
4.3.
Microbial indices
The Cmic-to-Corg ratio is an indicator of the availability of carbon to microorganisms, input of organic matter to soil, conversion efficiency to microbial biomass and stabilization of carbon in soil. The differences in the Cmic-to-Corg ratio between organic and conventional management systems suggest that the soil amendment with compost and ‘‘carnauba’’ straw has been a significant factor for an increase
european journal of soil biology 44 (2008) 225–230
in the Cmic-to-Corg ratio, by the available organic carbon input which favored microbial biomass. Anderson and Domsch [2] speculated that the higher Cmic-to-Corg ratio in their crop rotation plots as compared to monoculture systems was a result of a higher efficiency of organic matter utilization for microbial growth, which they attributed to a more organic matter input. The qCO2 is a measure of the specific metabolic activity that varies according to the composition and physiological state of the microbial biomass, the availability of nutrients and various abiotic factors. According to Fließbach and Mader [10], the organic management system benefits soil microbial biomass because microorganisms are utilizing the available C more efficiently as indicated by a lower qCO2 and suggests better conditions within the soil organic matter which may contribute to nutrient mineralization and temporary storage of potentially leachable elements.
5.
Conclusions
The management according to organic and conventional systems resulted in changes in soil microbial and organic C. The results showed that the organic farming practices resulted, in short term, higher microbial biomass and activity, while organic C was more responsive in long term. This was caused by the higher inputs of organic matter, an energetic substrate for the present microbial communities that were activated to assure the turnover of applied nutrients. Additionally, the results of qCO2 show that a smaller microbial biomass respire at a greater rate in conventional plots, which could be related to lower nutrient availability for microorganisms in conventional plots and Cmic-to-Corg ratio suggests a better efficiency of microorganisms in the conversion of carbon sources to Cmic in organic plots. Finally, our results indicate that the organic practices rapidly improved soil microbial characteristics and slowly increase soil organic C.
Acknowledgements We are grateful to D.B. Sampaio for her technical assistance in the analyses. We thank Dr. F.N. Silva for his input to this paper and for English review. This research was funded by Piauı´ State Research Foundation – FAPEPI (continue fluxes 2005). R.T.R. Monteiro is supported by personal grant from CNPq – Brazil.
references
[1] K. Alef, Estimation of soil respiration, in: K. Alef, P. Nannipieri (Eds.), Methods in Soil Microbiology and Biochemistry, Academic Press, New York, 1995, pp. 464–470. [2] J.M. Anderson, K.H. Domsch, Application of ecophysiological quotients (qCO and qD) on microbial biomass from soils of different cropping histories, Soil Biol. Biochem. 22 (1990) 251–255.
229
[4] A.S.F. Arau´jo, R.T.R. Monteiro, Microbial biomass and activity in a Brazilian soil amended with untreated and composted textile sludge, Chemosphere 64 (2006) 1028–1032. [5] A.S.F. Arau´jo, R.T.R. Monteiro, R.B. Abarkeli, Effect of glyphosate on the microbial activity of two Brazilian soils, Chemosphere 52 (2003) 799–804. [6] M. Ashraf, T. Mahmood, F. Azam, R.M. Qureshi, Comparative effects of applying leguminous and non-leguminous green manures and inorganic N on biomass yield and nitrogen uptake in flooded rice (Oryza sativa L.), Biol. Fertil. Soils 40 (2004) 147–152. [7] W. Bettiol, R. Ghini, J.A.H. Galva˜o, M.A.V. Ligo, J.L.C. Mineiro, Soil organisms in organic and conventional cropping systems, Sci. Agric. 59 (2002) 565–572. [8] M.A.H. Chowdhury, K. Kouno, T. Ando, T. Nagaoka, Microbial biomass, S mineralization and S uptake by African millet from soil amended with various composts, Soil Biol. Biochem. 32 (2000) 845–852. [9] J.W. Doran, M. Sarrantonio, M.A. Liebig, Soil health and sustainability, in: D.L. Sparks (Ed.), Advances in Agronomy, vol. 56, Academic Press, San Diego, 1996, pp. 25–37. [10] A. Fließbach, P. Mader, Microbial biomass and size–density fractions differ between soils of organic and conventional agricultural systems, Soil Biol. Biochem. 32 (2000) 757–768. [11] J.D. Glover, J.P. Reganold, P.K. Andrews, Systematic method for rating soil quality of conventional, organic, and integrated apple orchards in Washington State, Agric. Ecosyst. Environ. 80 (2000) 29–45. [12] K.R. Islam, R.R. Weil, A rapid microwave digestion method for colorimetric measurement of soil organic carbon, Commun. Soil Sci. Plant Anal. 29 (1988) 2269–2284. [13] D.S. Jenkinson, D.S. Powlson, The effects of biocidal treatments on metabolism in soil. I: Fumigation with chloroform, Soil Biol. Biochem. 8 (1976) 167–177. [14] J.L. MacAlady, M.E. Muller, K.M. Scow, Effects of metam sodium fumigation on soil microbial activity and community structure, J. Environ. Qual. 27 (1998) 54–63. [15] P. Mader, A. Fließbach, D. Dubois, L. Gunst, P. Fried, U. Niggli, Soil fertility and biodiversity in organic farming, Science 296 (2002) 1694–1697. [16] T. Marumoto, J.P.E. Anderson, K.H. Domsch, Mineralization of nutrients from soil microbial biomass, Soil Biol. Biochem. 14 (1982) 469–475. [17] S. Melero, J.C.R. Porras, J.F. Herencia, E. Madejon, Chemical and biochemical properties in a silty loam soil under conventional and organic management, Soil Tillage Res. 90 (2005) 162–170. [18] D. Rigby, D. Ca´ceres, Organic farming and the sustainability of agricultural systems, Agric. Syst. 68 (2001) 21–40. [19] P.G. Saffigna, D.S. Powlson, P.C. Brookes, G.A. Thomas, Influence of sorghum residues and tillage on soil organic matter and soil microbial biomass in an Australian vertissol, Soil Biol. Biochem. 21 (1989) 759–765. [20] L. Smith, E.A. Paul, The significance of soil microbial biomass estimations, in: J.M. Bollag, G. Stotzky (Eds.), Soil Biochemistry, Dekker, New York, 1990, pp. 357–396. [21] G.P. Sparling, T.G. Shephed, H.A. Kettles, Changes in soil organic C, microbial C and aggregate stability under continuous maize and cereal cropping, and after restoration to pasture in soil from the Manawatu region, New Zeal. Soil Tillage 24 (1992) 225–241. [22] M.J. Tedesco, C. Gianello, C.A. Bissani, Ana´lises de solos, plantas e outros materiais, UFRGS, Porto Alegre, 1995. [23] C. Tu, J.B. Ristaino, S. Hu, Soil microbial biomass and activity in organic tomato farming systems: effects of organic inputs and straw mulching, Soil Biol. Biochem. 38 (2006) 247–255. [24] United State Department of Agriculture, Soil Quality Test Kit, Agricultural Research Service, Washington, 1998.
230
european journal of soil biology 44 (2008) 225–230
[25] M.A.R. Valpassos, E.G.S. Cavalcante, A.M. Cassiolato, M.C. Alves, Effects of soil management systems on soil microbial activity, bulk density and chemical properties, Pesqui. Agropecu. Bras. 36 (2001) 1539–1545. [26] A. Walkley, J.A. Black, An examination of the Degtjareff method for determining soil organic matter and a proposed modification of the chromic acid titration method, Soil Sci. 37 (1934) 29–73.
[27] D.A. Wardle, A. Ghani, A critique of the microbial metabolic quotient (qCO2) as a bioindicator of disturbance and ecosystem development, Soil Biol. Biochem. 27 (1995) 1601–1610. [28] H. Willer, M. Yussefi, The World of Organic Agriculture: Statistics and Emerging Trends, International Federation of Organic Agriculture Movements, Bonn, Germany, 2004, p. 167.