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Organic matter, nutrient contents and cation exchange capacity in fine fractions from semiarid calcareous soils
Geoderma 93 Ž1999. 161–176
Organic matter, nutrient contents and cation exchange capacity in fine fractions from semiarid calcareous soils F. Caravaca ) , A. Lax, J. Albaladejo Department of Soil and Water ConserÕation and Organic Waste Management, CEBAS-CSIC, Apdo. 4195, 30080 Murcia, Spain Received 23 July 1998; received in revised form 25 February 1999; accepted 12 May 1999
Abstract Soil erosion, which is a widespread problem in semiarid areas, may lead to a decline in soil productivity since the finest and most fertile soil particles are those which are generally removed. Our objective was to determine the distribution of soil organic matter, phosphorus, potassium and cation exchange capacity within the fine fractions Ž- 2 mm and 2–20 mm. of the soil. Samples were taken from the top 20 cm of 14 cultivated soils and six forest soils. The organo-mineral size fractions from soil samples were isolated without chemical pretreatment by ultrasonic dispersion in water followed by sedimentation–syphonation. The distribution of organic matter within size fractions varied with land use. The cultivated soils had a greater percentage Žon average, about 30%. of total soil C in the - 2 mm fraction than the soils under natural vegetation Žon average, about 18%., in which the total soil C was associated with the 2–20 mm fraction to a greater extent than in cultivated soils. The distribution of the soil N between the clay and fine silt size fractions followed a similar pattern to that shown by soil C. The CrN ratio became smaller as particle size decreased. The higher CrN ratio obtained for the 2–20 mm fraction for both forest and cultivated soils suggests the presence of less decomposed organic matter, while the organic matter associated with the - 2 mm fraction can be considered to be more humified. The cation exchange capacity of whole soil and organo-mineral fractions were closely correlated with their respective C contents. The clay-size fraction had the highest CEC, which was related to its mineralogical composition. The data confirm that the proportion of soil organic matter depends on the stabilizing capacity of the different size fractions, both the clay and fine silt size fractions playing an important role in semiarid soils. To the detriment of the soil’s organic matter content these fractions are easily
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Corresponding author. Fax: q34-968-266613. E-mail address: [email protected] ŽF. Caravaca.
0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 - 7 0 6 1 Ž 9 9 . 0 0 0 4 5 - 2
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eroded in soils under arid and semiarid conditions, which may render them unsuitable for agricultural purposes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: ultrasonic dispersion; soil particle size; carbon and nitrogen distribution; calcareous soils
1. Introduction It has been suggested that small particles with a relatively large specific surface are particularly effective in stabilizing soil organic matter ŽChristensen, 1988; Cheshire et al., 1990; Leinweber et al., 1993. and hence in the formation of organo-mineral associations. Certainly, such fractions can preserve large amounts of soil organic matter in biologically resistant forms and thus provide a pool of moderately available nutrients Ž Anderson and Paul, 1984. . In arid and semiarid zones, water erosion processes lead to a substantial loss of these fine fractions Ž- 20 mm., which are the most fertile and, as a consequence, soil fertility declines ŽStocking, 1984.. Likewise, the amounts of C and N that may be associated to clay and silt size fractions are affected by many factors including soil texture Ž Christensen, 1992; Hassink, 1995., the dominant type of clay mineral ŽHassink, 1997. and the use to which the land is put ŽHassink, 1994; Parfitt et al., 1997. . Thus, in arable soils most of the soil organic matter can be found in the clay and silt fractions, whereas in forest and grassland soils the contribution of sand size organic matter to total soil organic matter is greater ŽChristensen, 1992; Hassink, 1997. . On the other hand, soils dominated by clays with a high specific surface area and numerous reaction sites probably adsorb more humic substances than soils dominated by clays with a low specific surface area ŽTate and Theng, 1980. . However, the amounts of C and N in coarser size fractions are related to the amount of debris that is incorporated into the soil and not to soil characteristics ŽBonde et al., 1992; Hassink, 1995; Quiroga et al., 1996. . Ultrasonic dispersion in water followed by sedimentation has been used successfully ŽEdwards and Bremner, 1967; Anderson and Paul, 1984; Gregorich et al., 1988; Bremner and Genrich, 1990; Leinweber et al., 1993. to isolate the organo-mineral complexes associated with differently sized fractions. This method is less destructive and more selective for isolating soil organic matter bound to mineral components than techniques involving alkaline extraction or shaking in water ŽAnderson et al., 1981; Christensen, 1985; Bremner and Genrich, 1990. . Hence, the results obtained from physically separated soil fractions may relate more directly to the structure and function of soil organic matter Ž Christensen, 1992. . However, this method has not been widely assessed in semiarid calcareous soils, which require a high dispersion energy due to their high degree of Ca-saturation.
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The aim of this study was to determine the distribution of soil organic carbon, total nitrogen, extractable phosphorus and potassium and cation exchange capacity in the fine particle size fractions isolated from different soil types and subjected to different uses. Both the clay Ž- 2 mm. and fine silt Ž 2–20 mm. fractions were considered in relation to the capacity of a soil to physically preserve organic matter, taking into account the close mineralogical and chemical similarities between the mineral particles of the fractions.
2. Materials and methods 2.1. Soils Twenty soils were sampled in the province of Murcia ŽS.E. Spain.. Soil samples were taken from the top 20 cm at several randomly selected points for each soil, air-dried and sieved to 2 mm. All the localities were characterized by high annual mean temperature Ž16.5–17.58C. and low annual rainfall Ž250–300 mm.. Differences between the soils were due to the nature of their parent material and the uses to which the land was put Žforestry or agricultural. Ž Table 1.. The soils were selected to represent a wide range in the study area of the principal clay types and organic-carbon contents. They included soils developed from marls, Holocene sediments, Quaternary sediments and limestones. The following analytical methods were used: Total nitrogen Ž NT. and total organic carbon Ž TOC. were assessed in an Automatic Nitrogen and Carbon Analyzer after pretreatment with HCl to eliminate carbonates ŽNavarro et al., 1991. and combustion at 10208C. Phosphorus ŽPex . was extracted with sodium bicarbonate Ž Olsen et al., 1954. and determined by colorimetry according to Murphy and Riley Ž 1962. . Potassium Ž K ex . was determined by ammonium acetate extraction 1 N at pH 7 and flame photometer Ž Schollemberger and Simon, 1954. . Electrical conductivity was potentiometrically evaluated from the 1:5 water extract, while the pH values of the soils were determined in H 2 O saturated paste. Cation exchange capacity was determined by Ba2q retention after percolation with a solution of 0.2 N BaCl 2 –triethanolamine at pH 8.1 ŽCarpena et al., 1972. . Particle size distribution was determined using the pipette method after oxidation of the organic matter with H 2 O 2 and stirring in a sodium hexametaphosphate solution. 2.2. Soil fractions For particle-size fractionation, 60 g of soil were dispersed in 300 mL of distilled water with a Branson-450 probe-type sonifier Ž Branson Ultrasonics, Danbury, CT, USA. set at 400 W power, operating for 30 min in continuous mode. Under these conditions, the power input was ; 840 J mly1 suspension.
Soil no.
Location
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Table 1 Some characteristics of the top 20 cm of the soils that were sampled TOC: total organic carbon. Granular composition % particles Soil type ŽFAO, 1988.
pH
TOC
CaCO 3
- 2 mm
2–20 mm
ŽH 2 O.
Žg kgy1 .
Ž%.
Land use
Beniajan ´ Torre Pacheco Alguazas Cieza Mula Mula La Paca C. de San Juan Cehegın ´ Calasparra Fortuna Sangonera El Algar Abanilla
31.1 30.9 26.9 14.5 18.9 26.3 15.9 13.7 14.9 20.6 33.4 20.9 23.8 29.7 23.0
48.2 40.3 40.9 30.8 28.9 39.9 19.6 15.8 32.3 46.6 44.1 46.8 24.4 30.1 34.9
Calcaric Fluvisol Haplic Calcisol Calcaric Fluvisol Calcaric Fluvisol Haplic Calcisol Calcaric Fluvisol Petric Calcisol Calcaric Cambisol Calcaric Fluvisol Calcaric Regosol Calcaric Regosol Calcaric Fluvisol Calcic Luvisol Calcaric Regosol
7.80 7.85 7.89 7.98 7.62 8.14 7.91 7.89 7.98 7.51 7.64 8.04 7.96 7.92 7.87
18.1 8.7 17.7 15.9 6.6 12.4 7.3 10.4 9.8 4.4 6.2 10.4 7.4 5.6 10.1
43.0 44.5 50.0 59.0 69.5 42.0 36.5 14.0 64.5 68.5 57.0 36.0 6.0 60.0 46.5
Citrus Arable Prunus Prunus Arable Citrus Olea Arable Prunus Prunus Olea Arable Citrus Arable
Forest 15 16 17 18 19 20 X
S. Espuna ˜ S. Espuna ˜ S. de la Pila S. de la Pila S. de la Pila S. de la Pila
10.2 9.5 12.4 11.9 15.6 21.6 13.5
30.0 31.3 31.3 38.1 21.6 23.4 29.3
Calcic Kastannozem Calcic Kastannozem Calcaric Phaeozem Calcaric Phaeozem Calcaric Phaeozem Calcaric Phaeozem
7.95 7.99 8.15 8.04 8.05 8.01 8.03
41.1 34.7 61.1 60.2 45.2 50.6 48.8
22.0 25.0 50.0 64.0 40.5 49.0 41.8
Pinus Pinus Pinus Pinus Pinus Pinus
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CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
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The sand fraction Ž50–2000 mm. was separated by wet sieving. The remaining suspension was separated into clay Ž - 2 mm. and fine silt Ž 2–20 mm. fractions by repeated sedimentation–syphonation cycles. After collection the size-fraction samples were dried at 558C, ground and stored for further analysis. They were analyzed for total organic carbon, total nitrogen, available phosphorus, available potassium and cation exchange capacity in the same way as the whole soil samples. All the procedures were replicated three times for each soil. Mineralogical analyses of the clay and fine silt samples were performed by X-ray diffractometry using a Philips PW1710, with Cu anticathode, Ni filter, 40 kV, 24A. The clay samples were submitted to the following treatments before the diffractograms were made: saturation with Mgq2 Ž Jackson, 1958. , solvation with ethylene glycol Ž Bridley, 1996. , heating to 5508C ŽWhitting, 1965. and acid attack Ž Martin Vivaldi and Rodriguez Gallego, 1961.. The samples were then studied as oriented specimens on glass slides. Diffractometric analysis of the fine silt samples was carried out on powder samples.
3. Results and discussion 3.1. Soils All the studied soils under agricultural use had an ochric epipedon whereas the soils under natural vegetation had a mollic epipedon. The soils selected belong to the most widespread soils units Ž FAO, 1988. found in the semiarid Mediterranean area Ž Table 1.. They were similar in texture, and classified as silt loam and silty clay loam. The fine silt content was generally higher than the clay content and only in some soil samples were similar values for both fractions observed Žsoils 7, 8, 18, 19 and 20.. The agricultural soils tended to have a higher percentage of fine fractions than the forestry soils. Thus, the percentage of the - 20 mm size fractions amounted to about 58% on average in cultivated soils, while in forest soils the percentage of such fractions was lower, about 43%. pH values were around 8 in most of the soils due to the high calcium carbonate content. Total organic carbon contents were generally low because of the semiarid climatological characteristics Ž low rainfall and high temperatures. of the province of Murcia, which reduce the input of organic matter. Furthermore, the organic matter content varied greatly according to the differences in land use, the total amount of C in the top 20 cm ranging from 4.4 to 18.1 for cultivated and 34.7 to 61.1 g kgy1 for forest soils. The level of organic matter depends on soil management practices as Quiroga et al. Ž 1996. showed for soils of the semiarid Argentinean Pampas, which ranged in texture from sand to loam. Thus, the organic C content varied considerably between soils even when they
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had a similar clay and fine silt content and the values were generally greater in forest soils than in the cultivated soils. The mineralogical composition of the clay fraction in most of the soils analysed was characterised by, in order of abundance, illite, interstratifications of illite and a swelling mineral of the montmorillonite group, and kaolinite. The interstratifications were identified as Ž10–14 M.14 M y Ž10–14 M. according to Thorez Ž1976. . Among non-layer minerals, calcite and quartz were identified. The X-ray diffraction patterns of the fine silt powder samples showed reflections ˚ ., quartz Ž4.20 A˚ ., calcite Ž3.03 A˚ ., dolomite Ž2.89 corresponding to illite Ž 10 A ˚A. and feldspars Ž3.19 A˚ .. The diffractograms indicate that calcite and quartz were the most abundant non-layer minerals. Furthermore, most of the samples had a similar mineralogical composition. 3.2. Soil fractions 3.2.1. Organic matter of the fractions The average TOC content was higher in the 2–20 mm fraction than in the - 2 mm fraction ŽTable 2., as was also shown for a calcareous clay soil by Ahmed and Oades Ž 1984. . The data showed that, generally, in cultivated soils the TOC content increased as particle size decreased. The fine silt size fraction contained less organic C than clay fraction. However, the contents of organic C were concentrated in the 2–20 mm fraction in most forest soils. These findings agree with those of several authors in their studies of a great variety of soils ŽAndreux et al., 1980; Bartoli et al., 1988; Elliott et al., 1991. . The results also confirm that the amount of carbon associated to each fraction varies with its mineral composition and the use to which the soil is put. The average amount of N in the - 2 mm and 2–20 mm fractions of forest soils was greater than in the corresponding fractions of cultivated soils. From Table 2, it can also be seen that total N values were similar in the two soil size fractions considered. In the present study we observed no relation between the dominant clay type in the soil and the amount of organic C and N than was associated with the clay and fine silt fractions. The CrN ratios of the soil fractions analyzed decreased markedly with smaller particle size, as was also demonstrated by Anderson et al. Ž 1981. and Christensen Ž1985. . Thus, the organic matter associated with the clay size fraction can be considered more humified than that associated with the fine silt size fraction. On the other hand, the CrN ratios were generally lower in agricultural soils ŽTable 2.. Although this may have been partly the result of the inputs of nitrogen during N fertilization, it is accepted ŽClement and Williams, 1967; Hassink and Neeteson, 1991; Hassink, 1994. that the accumulation of N is independent of the input of inorganic nitrogen. In cultivated soils, the CrN ratio was, on average, 8.5 for the - 2 mm fraction and 13.6 for the 2–20 mm
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Table 2 C and N contents Žg kgy1 . and CrN ratio in the - 2 mm and 2–20 mm fractions TOC: total organic carbon; TN: total nitrogen. Soil no.
TOC in fraction
TN in fraction
CrN in fraction
- 2 mm
- 2 mm
2–20 mm
- 2 mm
2–20 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
17.2 9.4 21.0 24.9 9.6 16.0 13.4 19.0 16.8 6.8 8.4 11.2 9.4 7.0 13.6
15.8 9.5 17.7 20.5 8.1 14.7 9.4 20.0 14.8 4.8 7.3 12.6 17.2 6.0 12.7
2.1 1.0 2.3 3.2 1.2 1.5 1.2 2.4 2.2 1.0 1.1 1.3 1.0 1.0 1.6
1.5 0.7 1.7 1.6 0.6 1.0 0.7 1.5 1.3 0.2 0.4 1.1 1.4 0.6 1.0
8.2 9.4 9.1 7.8 8.0 10.7 11.2 7.9 7.6 6.8 7.6 8.6 9.4 7.0 8.5
10.5 13.6 10.4 12.8 13.5 14.7 13.4 13.3 11.4 24.0 18.3 11.5 12.3 10.0 13.6
Forest 15 16 17 18 19 20 X
54.4 63.3 76.1 65.5 39.8 74.2 62.2
57.8 49.1 109.6 94.7 68.0 95.1 79.1
5.1 2.9 6.8 6.3 3.4 3.5 4.7
4.0 2.0 6.5 6.7 2.7 4.0 4.3
10.7 21.8 11.2 10.4 11.7 21.2 14.5
14.5 24.6 16.9 14.1 25.2 23.8 19.9
fraction, the higher CrN ratio obtained for the 2–20 mm fraction suggesting the presence of some less decomposed organic matter. Shiel Ž 1986. found higher C and N contents in all the size fractions studied of acid grassland soils, but the CrN ratio was similar to that established in the soils studied here, being in both cases higher in the coarser fraction. These results are consistent with data referring to soils dominated by layer silicate clay minerals of low specific surface area ŽChristensen, 1992. . The percentages of whole soil C and N associated with the clay and fine silt size fractions are shown in Figs. 1 and 2. On average more than 40% of TOC was concentrated in the fine silt size fraction and about 27% in the clay size fraction. We can conclude, therefore, that soil organic matter is preferably associated with the smallest Ž- 20 mm. or fine soil fractions Ž71% of the total soil organic matter being associated to them. because their chemical and mineralogical characteristics favour such a union.
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Fig. 1. Percentage distribution of TOC among the clay and fine silt size fractions.
The cultivated soils had a greater percentage Žon average, about 30%. of total soil C in the - 2 mm fraction than forest soils Ž on average, about 18%. . This may be due to the fact that the percentage of clay is lower in forest soils than cultivated soils. In forest soils, on the other hand, the total soil C was associated with the 2–20 mm fraction to a greater extent than in cultivated soils. The percentages of soil N found in the clay size fraction for cultivated and forest soils were, on average, about 36 and 19%, respectively, while around 33 and 39% of soil N was found in the corresponding fine silt size fractions. The distribution of the soil N between the clay and fine silt size fractions was similar to that shown by soil C. The percentage of whole soil C and N in the - 20 mm fraction was higher than in the ) 20 mm fraction, confirming that the soil organic matter associated with the - 20 mm fraction is physically better protected against microbial
Fig. 2. Percentage distribution of TN among the clay and fine silt size fractions.
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decomposition Ž van Veen and Paul, 1981; Cambardella and Elliott, 1994; Hassink, 1997. . 3.2.2. Phosphorus and potassium contents of the fractions The extractable phosphorus and potassium of the clay and fine silt fractions and whole soil are presented in Table 3. The extractable K content of the clay fraction was highest, followed, by that of whole soil and the fine silt fraction. This trend was observed in most soils. The concentrations of Pex in both size-fractions were almost always higher for the cultivated soils than for the forest soils. The amount of sodium bicarbonate extractable P in both size fractions of forest soils was higher than in whole soil. In these soils, the Pex content may be due to the continued mineralization of soil organic matter,
Table 3 Extractable phosphorus Žppm P2 O5 . and potassium Žcmol c kgy1 . in the - 2 mm and 2–20 mm fractions and whole soil Pex : extractable phosphorus; K ex : extractable potassium. Soil no.
Pex in fraction
Pex in soil
- 2 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
63 79 160 434 28 396 61 71 456 39 54 74 50 14 141
80 81 122 281 30 143 40 60 326 11 60 84 105 15 103
Forest 15 16 17 18 19 20 X
94 17 94 110 14 27 59
60 44 75 60 49 76 61
K ex in fraction
K ex in soil
- 2 mm
2–20 mm
61 57 118 288 15 247 25 45 266 18 45 60 38 9 92
2.07 2.07 2.61 3.15 1.36 3.30 1.61 2.58 2.28 1.84 1.97 3.56 3.27 1.87 2.40
0.37 0.29 0.22 0.19 0.10 0.18 0.12 0.20 0.12 0.20 0.12 0.43 0.82 0.12 0.25
1.25 0.92 1.51 1.38 0.49 1.66 0.61 0.69 0.72 0.92 1.30 1.89 1.92 0.72 1.14
28 16 35 37 13 38 28
2.92 2.63 2.10 1.92 2.07 2.43 2.35
0.87 0.33 0.32 0.29 0.24 0.23 0.38
1.18 1.41 1.00 0.97 1.02 1.41 1.17
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Fig. 3. Percentage distribution of Pex among the clay and fine silt size fractions.
whereas in cultivated soils it would also result from P fertilization Ž Elliott, 1986.. The percentage distribution of available phosphorus and potassium in the clay and fine silt fractions isolated from the soils studied is represented in Figs. 3 and 4. Fig. 3 shows that, on average, around half of Pex was recovered from the fine silt size fraction and less from the clay size fraction. The percentage distribution of soil K ex among the - 20 mm fractions varied with the mineralogical composition of the fractions but did not depend on land use. It was highly associated with the - 2 mm fraction because the clay size fraction of most of the soils studied contained illite, which is rich in potassium. 3.2.3. Cation exchange capacity of the fractions The CEC of the organo-mineral particle-size fractions from cultivated soils ŽTable 4. decreased with increasing equivalent diameters of the particle size.
Fig. 4. Percentage distribution of K ex among the clay and fine silt size fractions.
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Table 4 Cation exchange capacity ŽCEC. in the - 2 mm and 2–20 mm fractions and whole soil Žcmol c kgy1 . and clay minerals types in the - 2 mm fraction I: illite; K: kaolinite; C: chlorite; S: smectites; In: interstratified. Soil no.
CEC in fraction
CEC in soil
Main clay type
- 2 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
- 2 mm
33.5 31.8 33.7 35.9 24.2 27.9 32.5 44.4 29.7 27.9 30.0 34.0 29.4 27.8 31.6
12.4 9.3 9.6 9.7 5.4 5.1 7.3 6.0 5.9 5.4 0.8 9.0 17.5 1.7 7.5
19.2 15.2 16.5 13.8 8.7 8.6 10.8 12.1 11.2 17.7 18.3 16.4 15.8 13.8 14.2
I–K–In I–C–K–In I–K–In I–K–In I–K–In I–K–C I–K–C I–K–In I–K I–K–S I–In I–K–In I–K–C I–S–C
Forest 15 16 17 18 19 20 X
40.8 40.9 64.6 50.6 45.8 41.2 47.3
47.0 39.6 72.9 50.4 44.9 57.2 52.0
21.7 25.8 41.6 31.7 41.7 39.6 33.7
I–K I–K In I–K–In I–K–In I–In
The cation exchange capacity of the clay size fraction yielded values of between 24.2 and 44.4 cmol c kgy1 of clay, whereas the CEC of the fine silt size fraction ranged from 0.8 to 17.5 cmol c kgy1 of fine silt. The CEC of the clay size-fraction, then was on average four times greater than that of the fine silt size fraction Žsee Table 4.. The CEC of the soils and their fractions were closely related to their organic C contents. The coefficients for the correlations between organic C and CEC values are shown in Table 5. According to the regression coefficients of the soil regression equation and assuming an average of 22 g kgy1 C for all soils, around 11 cmol c kgy1 were derived from the organic matter and around 9 cmol c kgy1 originated from soil mineral particles. The correlation coefficients between the CEC and the TOC content of the clay and fine silt size fractions were highly significant Ž r s 0.82UUU and
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Table 5 Linear regression equations describing the dependence of CEC on the TOC content in organomineral particle-size fractions and whole soil UUU p- 0.001. Significance level: R Soil - 2 mm 2–20 mm
CEC s 0.49PTOCq9.45 CEC s 0.33PTOCq27.13 CEC s 0.64PTOCq0.12
UUU
0.89 0.82UUU 0.97UUU
R 2 Ž%.
"SE Ž%.
79 68 95
4.92 5.63 5.12
r s 0.97UUU , respectively. . Using the regression equations it was possible to evaluate the contribution of the different size-fractions to the total CEC of the organic and mineral components. The regression constants, taken as an indication of the effective CEC of the minerals, showed that in the 2–20 mm fraction, the fine silt accounted for only a small percentage of the cation exchange capacity Ž0.12 cmol c kgy1 . , which depended almost exclusively on the organic matter content. However, in the - 2 mm fraction both the associated organic matter content and the mineralogical composition of the clay clearly influenced the CEC Ž27.13 cmol c kgy1 .. The estimated CEC of the clay and fine silt particles were smaller than the CEC reported for such mineral particles by Leinweber et al. Ž 1993. , despite similarities in the mineralogical composition recorded. The contribution of the organo-mineral fractions to the total exchange capacity of the soil was calculated by relating the CEC of the fine silt or clay with the percentage of these fractions calculated according to the granulometric analysis, and the total CEC of the soil ŽMorras, ´ 1995.. The results obtained ŽFig. 5.
Fig. 5. Percentage distribution of CEC among the clay and fine silt size fractions.
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173
showed that the fine silt size fraction contributed between 2 and 65% to soil CEC, whilst the contribution of the clay size fraction represented between 15 and 85%. The fine silt-size fraction, then represented a substantial contribution to the soil CEC. The range found was greater than that observed by Leinweber et al. Ž1993., who found 13–23% of the total soil CEC to be associated with fine silt. These comparative data suggest that the contribution of the fine silt size fraction to soil CEC is greater in semiarid calcareous soils than in decarbonated acidic soils, in which the CEC of the clay fraction is much higher. In fact, the greatest part of the soil’s CEC lies in these two fractions and their corresponding OM content. The cation exchange capacity is a very important soil property for nutrient retention and supply. In addition, the exchange cations of organo-mineral particle-size fractions act as bridges between soil and plant. The mineralogical composition of the clay fraction with high amount of interstratified minerals of high specific surface area may explain its substantial contribution to soil CEC. As regards the fine silt size fraction, according with the mineralogical composition, the CEC may also be due to the presence of illite, illite–montmorillonite interstratified minerals and feldspars, as suggested by Morras ´ Ž1995.. The fact that the contribution of the fine silt size fraction to the total soil CEC was greater in forest soil samples can be explained because the OM of such soils is preferentially distributed in this fraction. Thus, differences in soil uses also affect the distribution of the soil CEC among the size-fractions, as demonstrated by Leinweber et al. Ž1993..
4. Conclusions In general, most of the soil organic matter in the soils studied was associated with the - 2 mm and 2–20 mm fractions, where it probably occurred as a complex with the mineral constituents, thus making them the most fertile soil fractions. The redistribution of soil organic matter between particle-size fractions is most probably affected by land use, as is demonstrated by the fact that the TOC content of cultivated soils was higher in the - 2 mm fraction, while in forest soils it was higher in the 2–20 mm fraction. The observation that the percentage of whole soil organic matter associated with the clay and fine silt size fractions of cultivated soils was generally larger than the percentage associated with the ) 20 mm fractions strongly suggests that clay and fine silt particles protect organic matter against microbial degradation. Thus, the proportion of organic matter in the - 20 mm fractions was less affected by cultivation in most soils. On the other hand, although forest soils generally had higher organic matter content than cultivated soils, the contribution of the - 20 mm fractions to soil organic matter was lower. This also
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suggests that the input of C provides by plant debris in forest soils was accumulated in the ) 20 mm fractions. In further studies it will be necessary to examine the role played by organo-mineral size particles in nutrient availability and soil organic matter turnover in the soils of Southeast Spain, which are characterized by large losses of such particles due to water erosion and unsuitable agricultural management.
Acknowledgements F. Caravaca acknowledges a grant from Caja de Ahorros del Mediterraneo ´ ŽCAM..
References Ahmed, M., Oades, J.M., 1984. Distribution of organic matter and adenosine triphosphate after fractionation of soils by physical procedures. Soil Biol. Biochem. 16, 465–470. Anderson, D.W., Paul, E.A., 1984. Organo-mineral complexes and their study by radiocarbon dating. Soil Sci. Soc. Am. J. 48, 298–301. Anderson, D.W., Saggar, S., Bettany, J.R., Stewart, J.W.B., 1981. Particle size fractions and their use in studies of soil organic matter: I. The nature and distribution of forms of carbon, nitrogen, and sulphur. Soil Sci. Soc. Am. J. 45, 767–772. Andreux, F., Bruckert, S., Correa, A., Souchier, B., 1980. Sur une methode de fractionnement ´ physique et chimique des agregats des sols: origine possible de la matiere ´ ´ organique des fractions obtenues. C.R. Acad. Sci. Paris 291, 381–385, serie ´ D. Bartoli, F., Philippy, R., Bartin, G., 1988. Aggregation in soils with small amount of swelling clays: I. Aggregate stability. J. Soil Sci. 39, 593–616. Bonde, T.A., Christensen, B.T., Cem, C.C., 1992. Dynamics of soil organic matter as reflected by natural 13C abundance in particle size fractions of forested and cultivated oxisols. Soil Biol. Biochem. 24, 275–277. Bremner, J.M., Genrich, D.A., 1990. Characterisation of the sand, silt, and clay fractions of some Mollisols. Soil Colloids and their Associations in Aggregates. Bridley, G.H., 1996. Ethylene glycol and glycerol complexes of smectites and vermiculites. Clay Miner. 6, 237–260. Cambardella, C.A., Elliott, E.T., 1994. Carbon and nitrogen dynamics of soil organic matter fractions from cultivated grassland soils. Soil Sci. Soc. Am. J. 58, 123–130. Carpena, O., Lax, A., Vahtras, K., 1972. Determination of exchangeable cations in calcareous soils. Soil Sci. 113, 194–199. Cheshire, M.V., Christensen, B.T., Sorensen, L.H., 1990. Labelled and native sugars in particle-size fractions from soils incubated with 14C straw for 6 to 18 years. J. Soil Sci. 41, 29–39. Christensen, B.T., 1985. Carbon and nitrogen in particle size fractions isolated from Danish arable soils by ultrasonic dispersion and gravity-sedimentation. Acta Agric. Scand. 35, 175–187. Christensen, B.T., 1988. Effects of animal manure and mineral fertilizer on the total carbon and nitrogen contents of soil size fractions. Biology and Fertility of Soils 5, 304–307. Christensen, B.T., 1992. Physical fractionation of soil and organic matter in primary particle size and density separates. Adv. Soil Sci. 20, 1–89.
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Clement, C.R., Williams, T.E., 1967. Leys and soil organic matter: II. The accumulation of nitrogen in soils under different leys. J. Agricult. Sci. ŽCambridge. 69, 133–138. Edwards, A.P., Bremner, J.M., 1967. Dispersion of soil particles by sonic vibration. J. Soil Sci. 18, 47–63. Elliott, E.T., 1986. Aggregate structure and carbon, nitrogen, and phosphorus in native and cultivated soils. Soil Sci. Soc. Am. J. 50, 627–633. Elliott, E.T., Palm, C.A., Reuss, D.E., Monz, C.A., 1991. Organic matter contained in soil aggregates from a tropical chronosequence: correction for sand and light fraction. Agric. Ecosyst. Environ. 34, 443–451. FAO, 1988. Soil map of the world: revised legend, Final Draft. Food Agriculture Organization of the United Nation, 119 pp. Gregorich, E.G., Kachanoski, R.G., Voroney, R.P., 1988. Ultrasonic dispersion of aggregates: distribution of organic matter in size fractions. Can. J. Soil Sci. 68, 395–403. Hassink, J., 1994. Effects of soil texture and grassland management on soil organic C and N and rates of C and N mineralization. Soil Biol. Biochem. 26, 1221–1231. Hassink, J., 1995. Density fractions of soil macroorganic matter and microbial biomass as predictors of C and N mineralization. Soil Biol. Biochem. 27, 1099–1108. Hassink, J., 1997. The capacity of soils to preserve organic C and N by their association with clay and silt particles. Plant and Soil 191, 77–87. Hassink, J., Neeteson, J.J., 1991. Effect of grassland management on the amounts of soil organic N and C. Netherlands J. Agricult. Sci. 39, 225–236. Jackson, M.L., 1958. Soil Chemical analysis. Advanced Course. Dept. of Soils, Univ. of Wisconsin, Madison. Leinweber, P., Reuter, G., Brozio, K., 1993. Cation exchange capacities of organo-mineral particle-size fractions in soils from long-term experiments. J. Soil Sci. 44, 111–119. Martin Vivaldi, J.L., Rodriguez Gallego, M., 1961. Some problems in the identification of clay minerals in mixtures by X-ray difraction: I. Chlorite–kaolinite mixtures. Clay Miner. Bull. 26, 288–292. Morras, ´ H.J.M., 1995. Mineralogy and cation exchange capacity of the fine silt fraction in two soils from the southern Chaco Region ŽArgentina.. Geoderma 64, 281–295. Murphy, J., Riley, J.P., 1962. A modified single solution method for determination of phosphate in natural waters. Anal. Chim. Acta 27, 31–36. Navarro, A.F., Cegarra, J., Roig, A., Bernal, M.P., 1991. An automatic microanalysis method for the determination of organic carbon in wastes. Commun. Soil Sci. Plant Anal. 22, 2137–2144. Olsen, S.R., Cole, C.V., Watanabe, F.S., Dean, L.A., 1954. Estimation of available phosphorus in soils by extraction with sodium bicarbonate. United States Department of Agriculture, Washington, DC, Circular 939. Parfitt, R.L., Theng, B.K.G., Whitton, J.S., Shepherd, T.G., 1997. Effects of clay minerals and land use on organic matter pools. Geoderma 75, 1–12. Quiroga, A.R., Buschiazzo, D.E., Peinemann, N., 1996. Soil organic matter particle size fractions in soils of the semiarid Argentinean Pampas. Soil Sci. 161, 104–108. Schollemberger, C.J., Simon, R.H., 1954. Determination of exchange capacity and exchangeable bases in soils. Soil Sci. 59, 13–24. Shiel, R.S., 1986. Variations in amounts of carbon and nitrogen associated with particle size fractions of soils from the Palace Leas meadow hay plots. J. Soil Sci. 37, 249–257. Stocking, M.A., 1984. Erosion and soil productivity: A review. Consultants´ Working Paper 1. Land and Water Development Division, Food and Agriculture Organization, Rome. Tate, K.R., Theng, B.K.G., 1980. Organic matter and its interactions with inorganic soil constituents. In: Theng, G.K.G. ŽEd.., Soil with a Variable Charge. New Zealand Soc. Soil Sci., Lower Hutt, pp. 225–249.
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Thorez, J., 1976. Practical identification of clay minerals. In: Lelotte, G. ŽEd.., Dison, Belqique, p. 90. van Veen, J.A., Paul, E.A., 1981. Organic carbon dynamics in grassland soils: 1. Background information and computer simulation. Can. J. Soil Sci. 61, 185–201. Whitting, L.D., 1965. X-ray diffraction techniques for mineral identification and mineralogical composition. In: Black, C.A. ŽEd.., Methods of soil analysis. Part I. Soil Sci. Soc. Am. Madison, pp. 671–698.
Organic matter, nutrient contents and cation exchange capacity in fine fractions from semiarid calcareous soils F. Caravaca ) , A. Lax, J. Albaladejo Department of Soil and Water ConserÕation and Organic Waste Management, CEBAS-CSIC, Apdo. 4195, 30080 Murcia, Spain Received 23 July 1998; received in revised form 25 February 1999; accepted 12 May 1999
Abstract Soil erosion, which is a widespread problem in semiarid areas, may lead to a decline in soil productivity since the finest and most fertile soil particles are those which are generally removed. Our objective was to determine the distribution of soil organic matter, phosphorus, potassium and cation exchange capacity within the fine fractions Ž- 2 mm and 2–20 mm. of the soil. Samples were taken from the top 20 cm of 14 cultivated soils and six forest soils. The organo-mineral size fractions from soil samples were isolated without chemical pretreatment by ultrasonic dispersion in water followed by sedimentation–syphonation. The distribution of organic matter within size fractions varied with land use. The cultivated soils had a greater percentage Žon average, about 30%. of total soil C in the - 2 mm fraction than the soils under natural vegetation Žon average, about 18%., in which the total soil C was associated with the 2–20 mm fraction to a greater extent than in cultivated soils. The distribution of the soil N between the clay and fine silt size fractions followed a similar pattern to that shown by soil C. The CrN ratio became smaller as particle size decreased. The higher CrN ratio obtained for the 2–20 mm fraction for both forest and cultivated soils suggests the presence of less decomposed organic matter, while the organic matter associated with the - 2 mm fraction can be considered to be more humified. The cation exchange capacity of whole soil and organo-mineral fractions were closely correlated with their respective C contents. The clay-size fraction had the highest CEC, which was related to its mineralogical composition. The data confirm that the proportion of soil organic matter depends on the stabilizing capacity of the different size fractions, both the clay and fine silt size fractions playing an important role in semiarid soils. To the detriment of the soil’s organic matter content these fractions are easily
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Corresponding author. Fax: q34-968-266613. E-mail address: [email protected] ŽF. Caravaca.
0016-7061r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 0 1 6 - 7 0 6 1 Ž 9 9 . 0 0 0 4 5 - 2
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eroded in soils under arid and semiarid conditions, which may render them unsuitable for agricultural purposes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: ultrasonic dispersion; soil particle size; carbon and nitrogen distribution; calcareous soils
1. Introduction It has been suggested that small particles with a relatively large specific surface are particularly effective in stabilizing soil organic matter ŽChristensen, 1988; Cheshire et al., 1990; Leinweber et al., 1993. and hence in the formation of organo-mineral associations. Certainly, such fractions can preserve large amounts of soil organic matter in biologically resistant forms and thus provide a pool of moderately available nutrients Ž Anderson and Paul, 1984. . In arid and semiarid zones, water erosion processes lead to a substantial loss of these fine fractions Ž- 20 mm., which are the most fertile and, as a consequence, soil fertility declines ŽStocking, 1984.. Likewise, the amounts of C and N that may be associated to clay and silt size fractions are affected by many factors including soil texture Ž Christensen, 1992; Hassink, 1995., the dominant type of clay mineral ŽHassink, 1997. and the use to which the land is put ŽHassink, 1994; Parfitt et al., 1997. . Thus, in arable soils most of the soil organic matter can be found in the clay and silt fractions, whereas in forest and grassland soils the contribution of sand size organic matter to total soil organic matter is greater ŽChristensen, 1992; Hassink, 1997. . On the other hand, soils dominated by clays with a high specific surface area and numerous reaction sites probably adsorb more humic substances than soils dominated by clays with a low specific surface area ŽTate and Theng, 1980. . However, the amounts of C and N in coarser size fractions are related to the amount of debris that is incorporated into the soil and not to soil characteristics ŽBonde et al., 1992; Hassink, 1995; Quiroga et al., 1996. . Ultrasonic dispersion in water followed by sedimentation has been used successfully ŽEdwards and Bremner, 1967; Anderson and Paul, 1984; Gregorich et al., 1988; Bremner and Genrich, 1990; Leinweber et al., 1993. to isolate the organo-mineral complexes associated with differently sized fractions. This method is less destructive and more selective for isolating soil organic matter bound to mineral components than techniques involving alkaline extraction or shaking in water ŽAnderson et al., 1981; Christensen, 1985; Bremner and Genrich, 1990. . Hence, the results obtained from physically separated soil fractions may relate more directly to the structure and function of soil organic matter Ž Christensen, 1992. . However, this method has not been widely assessed in semiarid calcareous soils, which require a high dispersion energy due to their high degree of Ca-saturation.
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The aim of this study was to determine the distribution of soil organic carbon, total nitrogen, extractable phosphorus and potassium and cation exchange capacity in the fine particle size fractions isolated from different soil types and subjected to different uses. Both the clay Ž- 2 mm. and fine silt Ž 2–20 mm. fractions were considered in relation to the capacity of a soil to physically preserve organic matter, taking into account the close mineralogical and chemical similarities between the mineral particles of the fractions.
2. Materials and methods 2.1. Soils Twenty soils were sampled in the province of Murcia ŽS.E. Spain.. Soil samples were taken from the top 20 cm at several randomly selected points for each soil, air-dried and sieved to 2 mm. All the localities were characterized by high annual mean temperature Ž16.5–17.58C. and low annual rainfall Ž250–300 mm.. Differences between the soils were due to the nature of their parent material and the uses to which the land was put Žforestry or agricultural. Ž Table 1.. The soils were selected to represent a wide range in the study area of the principal clay types and organic-carbon contents. They included soils developed from marls, Holocene sediments, Quaternary sediments and limestones. The following analytical methods were used: Total nitrogen Ž NT. and total organic carbon Ž TOC. were assessed in an Automatic Nitrogen and Carbon Analyzer after pretreatment with HCl to eliminate carbonates ŽNavarro et al., 1991. and combustion at 10208C. Phosphorus ŽPex . was extracted with sodium bicarbonate Ž Olsen et al., 1954. and determined by colorimetry according to Murphy and Riley Ž 1962. . Potassium Ž K ex . was determined by ammonium acetate extraction 1 N at pH 7 and flame photometer Ž Schollemberger and Simon, 1954. . Electrical conductivity was potentiometrically evaluated from the 1:5 water extract, while the pH values of the soils were determined in H 2 O saturated paste. Cation exchange capacity was determined by Ba2q retention after percolation with a solution of 0.2 N BaCl 2 –triethanolamine at pH 8.1 ŽCarpena et al., 1972. . Particle size distribution was determined using the pipette method after oxidation of the organic matter with H 2 O 2 and stirring in a sodium hexametaphosphate solution. 2.2. Soil fractions For particle-size fractionation, 60 g of soil were dispersed in 300 mL of distilled water with a Branson-450 probe-type sonifier Ž Branson Ultrasonics, Danbury, CT, USA. set at 400 W power, operating for 30 min in continuous mode. Under these conditions, the power input was ; 840 J mly1 suspension.
Soil no.
Location
164
Table 1 Some characteristics of the top 20 cm of the soils that were sampled TOC: total organic carbon. Granular composition % particles Soil type ŽFAO, 1988.
pH
TOC
CaCO 3
- 2 mm
2–20 mm
ŽH 2 O.
Žg kgy1 .
Ž%.
Land use
Beniajan ´ Torre Pacheco Alguazas Cieza Mula Mula La Paca C. de San Juan Cehegın ´ Calasparra Fortuna Sangonera El Algar Abanilla
31.1 30.9 26.9 14.5 18.9 26.3 15.9 13.7 14.9 20.6 33.4 20.9 23.8 29.7 23.0
48.2 40.3 40.9 30.8 28.9 39.9 19.6 15.8 32.3 46.6 44.1 46.8 24.4 30.1 34.9
Calcaric Fluvisol Haplic Calcisol Calcaric Fluvisol Calcaric Fluvisol Haplic Calcisol Calcaric Fluvisol Petric Calcisol Calcaric Cambisol Calcaric Fluvisol Calcaric Regosol Calcaric Regosol Calcaric Fluvisol Calcic Luvisol Calcaric Regosol
7.80 7.85 7.89 7.98 7.62 8.14 7.91 7.89 7.98 7.51 7.64 8.04 7.96 7.92 7.87
18.1 8.7 17.7 15.9 6.6 12.4 7.3 10.4 9.8 4.4 6.2 10.4 7.4 5.6 10.1
43.0 44.5 50.0 59.0 69.5 42.0 36.5 14.0 64.5 68.5 57.0 36.0 6.0 60.0 46.5
Citrus Arable Prunus Prunus Arable Citrus Olea Arable Prunus Prunus Olea Arable Citrus Arable
Forest 15 16 17 18 19 20 X
S. Espuna ˜ S. Espuna ˜ S. de la Pila S. de la Pila S. de la Pila S. de la Pila
10.2 9.5 12.4 11.9 15.6 21.6 13.5
30.0 31.3 31.3 38.1 21.6 23.4 29.3
Calcic Kastannozem Calcic Kastannozem Calcaric Phaeozem Calcaric Phaeozem Calcaric Phaeozem Calcaric Phaeozem
7.95 7.99 8.15 8.04 8.05 8.01 8.03
41.1 34.7 61.1 60.2 45.2 50.6 48.8
22.0 25.0 50.0 64.0 40.5 49.0 41.8
Pinus Pinus Pinus Pinus Pinus Pinus
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CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
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The sand fraction Ž50–2000 mm. was separated by wet sieving. The remaining suspension was separated into clay Ž - 2 mm. and fine silt Ž 2–20 mm. fractions by repeated sedimentation–syphonation cycles. After collection the size-fraction samples were dried at 558C, ground and stored for further analysis. They were analyzed for total organic carbon, total nitrogen, available phosphorus, available potassium and cation exchange capacity in the same way as the whole soil samples. All the procedures were replicated three times for each soil. Mineralogical analyses of the clay and fine silt samples were performed by X-ray diffractometry using a Philips PW1710, with Cu anticathode, Ni filter, 40 kV, 24A. The clay samples were submitted to the following treatments before the diffractograms were made: saturation with Mgq2 Ž Jackson, 1958. , solvation with ethylene glycol Ž Bridley, 1996. , heating to 5508C ŽWhitting, 1965. and acid attack Ž Martin Vivaldi and Rodriguez Gallego, 1961.. The samples were then studied as oriented specimens on glass slides. Diffractometric analysis of the fine silt samples was carried out on powder samples.
3. Results and discussion 3.1. Soils All the studied soils under agricultural use had an ochric epipedon whereas the soils under natural vegetation had a mollic epipedon. The soils selected belong to the most widespread soils units Ž FAO, 1988. found in the semiarid Mediterranean area Ž Table 1.. They were similar in texture, and classified as silt loam and silty clay loam. The fine silt content was generally higher than the clay content and only in some soil samples were similar values for both fractions observed Žsoils 7, 8, 18, 19 and 20.. The agricultural soils tended to have a higher percentage of fine fractions than the forestry soils. Thus, the percentage of the - 20 mm size fractions amounted to about 58% on average in cultivated soils, while in forest soils the percentage of such fractions was lower, about 43%. pH values were around 8 in most of the soils due to the high calcium carbonate content. Total organic carbon contents were generally low because of the semiarid climatological characteristics Ž low rainfall and high temperatures. of the province of Murcia, which reduce the input of organic matter. Furthermore, the organic matter content varied greatly according to the differences in land use, the total amount of C in the top 20 cm ranging from 4.4 to 18.1 for cultivated and 34.7 to 61.1 g kgy1 for forest soils. The level of organic matter depends on soil management practices as Quiroga et al. Ž 1996. showed for soils of the semiarid Argentinean Pampas, which ranged in texture from sand to loam. Thus, the organic C content varied considerably between soils even when they
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had a similar clay and fine silt content and the values were generally greater in forest soils than in the cultivated soils. The mineralogical composition of the clay fraction in most of the soils analysed was characterised by, in order of abundance, illite, interstratifications of illite and a swelling mineral of the montmorillonite group, and kaolinite. The interstratifications were identified as Ž10–14 M.14 M y Ž10–14 M. according to Thorez Ž1976. . Among non-layer minerals, calcite and quartz were identified. The X-ray diffraction patterns of the fine silt powder samples showed reflections ˚ ., quartz Ž4.20 A˚ ., calcite Ž3.03 A˚ ., dolomite Ž2.89 corresponding to illite Ž 10 A ˚A. and feldspars Ž3.19 A˚ .. The diffractograms indicate that calcite and quartz were the most abundant non-layer minerals. Furthermore, most of the samples had a similar mineralogical composition. 3.2. Soil fractions 3.2.1. Organic matter of the fractions The average TOC content was higher in the 2–20 mm fraction than in the - 2 mm fraction ŽTable 2., as was also shown for a calcareous clay soil by Ahmed and Oades Ž 1984. . The data showed that, generally, in cultivated soils the TOC content increased as particle size decreased. The fine silt size fraction contained less organic C than clay fraction. However, the contents of organic C were concentrated in the 2–20 mm fraction in most forest soils. These findings agree with those of several authors in their studies of a great variety of soils ŽAndreux et al., 1980; Bartoli et al., 1988; Elliott et al., 1991. . The results also confirm that the amount of carbon associated to each fraction varies with its mineral composition and the use to which the soil is put. The average amount of N in the - 2 mm and 2–20 mm fractions of forest soils was greater than in the corresponding fractions of cultivated soils. From Table 2, it can also be seen that total N values were similar in the two soil size fractions considered. In the present study we observed no relation between the dominant clay type in the soil and the amount of organic C and N than was associated with the clay and fine silt fractions. The CrN ratios of the soil fractions analyzed decreased markedly with smaller particle size, as was also demonstrated by Anderson et al. Ž 1981. and Christensen Ž1985. . Thus, the organic matter associated with the clay size fraction can be considered more humified than that associated with the fine silt size fraction. On the other hand, the CrN ratios were generally lower in agricultural soils ŽTable 2.. Although this may have been partly the result of the inputs of nitrogen during N fertilization, it is accepted ŽClement and Williams, 1967; Hassink and Neeteson, 1991; Hassink, 1994. that the accumulation of N is independent of the input of inorganic nitrogen. In cultivated soils, the CrN ratio was, on average, 8.5 for the - 2 mm fraction and 13.6 for the 2–20 mm
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Table 2 C and N contents Žg kgy1 . and CrN ratio in the - 2 mm and 2–20 mm fractions TOC: total organic carbon; TN: total nitrogen. Soil no.
TOC in fraction
TN in fraction
CrN in fraction
- 2 mm
- 2 mm
2–20 mm
- 2 mm
2–20 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
17.2 9.4 21.0 24.9 9.6 16.0 13.4 19.0 16.8 6.8 8.4 11.2 9.4 7.0 13.6
15.8 9.5 17.7 20.5 8.1 14.7 9.4 20.0 14.8 4.8 7.3 12.6 17.2 6.0 12.7
2.1 1.0 2.3 3.2 1.2 1.5 1.2 2.4 2.2 1.0 1.1 1.3 1.0 1.0 1.6
1.5 0.7 1.7 1.6 0.6 1.0 0.7 1.5 1.3 0.2 0.4 1.1 1.4 0.6 1.0
8.2 9.4 9.1 7.8 8.0 10.7 11.2 7.9 7.6 6.8 7.6 8.6 9.4 7.0 8.5
10.5 13.6 10.4 12.8 13.5 14.7 13.4 13.3 11.4 24.0 18.3 11.5 12.3 10.0 13.6
Forest 15 16 17 18 19 20 X
54.4 63.3 76.1 65.5 39.8 74.2 62.2
57.8 49.1 109.6 94.7 68.0 95.1 79.1
5.1 2.9 6.8 6.3 3.4 3.5 4.7
4.0 2.0 6.5 6.7 2.7 4.0 4.3
10.7 21.8 11.2 10.4 11.7 21.2 14.5
14.5 24.6 16.9 14.1 25.2 23.8 19.9
fraction, the higher CrN ratio obtained for the 2–20 mm fraction suggesting the presence of some less decomposed organic matter. Shiel Ž 1986. found higher C and N contents in all the size fractions studied of acid grassland soils, but the CrN ratio was similar to that established in the soils studied here, being in both cases higher in the coarser fraction. These results are consistent with data referring to soils dominated by layer silicate clay minerals of low specific surface area ŽChristensen, 1992. . The percentages of whole soil C and N associated with the clay and fine silt size fractions are shown in Figs. 1 and 2. On average more than 40% of TOC was concentrated in the fine silt size fraction and about 27% in the clay size fraction. We can conclude, therefore, that soil organic matter is preferably associated with the smallest Ž- 20 mm. or fine soil fractions Ž71% of the total soil organic matter being associated to them. because their chemical and mineralogical characteristics favour such a union.
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Fig. 1. Percentage distribution of TOC among the clay and fine silt size fractions.
The cultivated soils had a greater percentage Žon average, about 30%. of total soil C in the - 2 mm fraction than forest soils Ž on average, about 18%. . This may be due to the fact that the percentage of clay is lower in forest soils than cultivated soils. In forest soils, on the other hand, the total soil C was associated with the 2–20 mm fraction to a greater extent than in cultivated soils. The percentages of soil N found in the clay size fraction for cultivated and forest soils were, on average, about 36 and 19%, respectively, while around 33 and 39% of soil N was found in the corresponding fine silt size fractions. The distribution of the soil N between the clay and fine silt size fractions was similar to that shown by soil C. The percentage of whole soil C and N in the - 20 mm fraction was higher than in the ) 20 mm fraction, confirming that the soil organic matter associated with the - 20 mm fraction is physically better protected against microbial
Fig. 2. Percentage distribution of TN among the clay and fine silt size fractions.
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decomposition Ž van Veen and Paul, 1981; Cambardella and Elliott, 1994; Hassink, 1997. . 3.2.2. Phosphorus and potassium contents of the fractions The extractable phosphorus and potassium of the clay and fine silt fractions and whole soil are presented in Table 3. The extractable K content of the clay fraction was highest, followed, by that of whole soil and the fine silt fraction. This trend was observed in most soils. The concentrations of Pex in both size-fractions were almost always higher for the cultivated soils than for the forest soils. The amount of sodium bicarbonate extractable P in both size fractions of forest soils was higher than in whole soil. In these soils, the Pex content may be due to the continued mineralization of soil organic matter,
Table 3 Extractable phosphorus Žppm P2 O5 . and potassium Žcmol c kgy1 . in the - 2 mm and 2–20 mm fractions and whole soil Pex : extractable phosphorus; K ex : extractable potassium. Soil no.
Pex in fraction
Pex in soil
- 2 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
63 79 160 434 28 396 61 71 456 39 54 74 50 14 141
80 81 122 281 30 143 40 60 326 11 60 84 105 15 103
Forest 15 16 17 18 19 20 X
94 17 94 110 14 27 59
60 44 75 60 49 76 61
K ex in fraction
K ex in soil
- 2 mm
2–20 mm
61 57 118 288 15 247 25 45 266 18 45 60 38 9 92
2.07 2.07 2.61 3.15 1.36 3.30 1.61 2.58 2.28 1.84 1.97 3.56 3.27 1.87 2.40
0.37 0.29 0.22 0.19 0.10 0.18 0.12 0.20 0.12 0.20 0.12 0.43 0.82 0.12 0.25
1.25 0.92 1.51 1.38 0.49 1.66 0.61 0.69 0.72 0.92 1.30 1.89 1.92 0.72 1.14
28 16 35 37 13 38 28
2.92 2.63 2.10 1.92 2.07 2.43 2.35
0.87 0.33 0.32 0.29 0.24 0.23 0.38
1.18 1.41 1.00 0.97 1.02 1.41 1.17
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Fig. 3. Percentage distribution of Pex among the clay and fine silt size fractions.
whereas in cultivated soils it would also result from P fertilization Ž Elliott, 1986.. The percentage distribution of available phosphorus and potassium in the clay and fine silt fractions isolated from the soils studied is represented in Figs. 3 and 4. Fig. 3 shows that, on average, around half of Pex was recovered from the fine silt size fraction and less from the clay size fraction. The percentage distribution of soil K ex among the - 20 mm fractions varied with the mineralogical composition of the fractions but did not depend on land use. It was highly associated with the - 2 mm fraction because the clay size fraction of most of the soils studied contained illite, which is rich in potassium. 3.2.3. Cation exchange capacity of the fractions The CEC of the organo-mineral particle-size fractions from cultivated soils ŽTable 4. decreased with increasing equivalent diameters of the particle size.
Fig. 4. Percentage distribution of K ex among the clay and fine silt size fractions.
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Table 4 Cation exchange capacity ŽCEC. in the - 2 mm and 2–20 mm fractions and whole soil Žcmol c kgy1 . and clay minerals types in the - 2 mm fraction I: illite; K: kaolinite; C: chlorite; S: smectites; In: interstratified. Soil no.
CEC in fraction
CEC in soil
Main clay type
- 2 mm
2–20 mm
CultiÕated 1 2 3 4 5 6 7 8 9 10 11 12 13 14 X
- 2 mm
33.5 31.8 33.7 35.9 24.2 27.9 32.5 44.4 29.7 27.9 30.0 34.0 29.4 27.8 31.6
12.4 9.3 9.6 9.7 5.4 5.1 7.3 6.0 5.9 5.4 0.8 9.0 17.5 1.7 7.5
19.2 15.2 16.5 13.8 8.7 8.6 10.8 12.1 11.2 17.7 18.3 16.4 15.8 13.8 14.2
I–K–In I–C–K–In I–K–In I–K–In I–K–In I–K–C I–K–C I–K–In I–K I–K–S I–In I–K–In I–K–C I–S–C
Forest 15 16 17 18 19 20 X
40.8 40.9 64.6 50.6 45.8 41.2 47.3
47.0 39.6 72.9 50.4 44.9 57.2 52.0
21.7 25.8 41.6 31.7 41.7 39.6 33.7
I–K I–K In I–K–In I–K–In I–In
The cation exchange capacity of the clay size fraction yielded values of between 24.2 and 44.4 cmol c kgy1 of clay, whereas the CEC of the fine silt size fraction ranged from 0.8 to 17.5 cmol c kgy1 of fine silt. The CEC of the clay size-fraction, then was on average four times greater than that of the fine silt size fraction Žsee Table 4.. The CEC of the soils and their fractions were closely related to their organic C contents. The coefficients for the correlations between organic C and CEC values are shown in Table 5. According to the regression coefficients of the soil regression equation and assuming an average of 22 g kgy1 C for all soils, around 11 cmol c kgy1 were derived from the organic matter and around 9 cmol c kgy1 originated from soil mineral particles. The correlation coefficients between the CEC and the TOC content of the clay and fine silt size fractions were highly significant Ž r s 0.82UUU and
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Table 5 Linear regression equations describing the dependence of CEC on the TOC content in organomineral particle-size fractions and whole soil UUU p- 0.001. Significance level: R Soil - 2 mm 2–20 mm
CEC s 0.49PTOCq9.45 CEC s 0.33PTOCq27.13 CEC s 0.64PTOCq0.12
UUU
0.89 0.82UUU 0.97UUU
R 2 Ž%.
"SE Ž%.
79 68 95
4.92 5.63 5.12
r s 0.97UUU , respectively. . Using the regression equations it was possible to evaluate the contribution of the different size-fractions to the total CEC of the organic and mineral components. The regression constants, taken as an indication of the effective CEC of the minerals, showed that in the 2–20 mm fraction, the fine silt accounted for only a small percentage of the cation exchange capacity Ž0.12 cmol c kgy1 . , which depended almost exclusively on the organic matter content. However, in the - 2 mm fraction both the associated organic matter content and the mineralogical composition of the clay clearly influenced the CEC Ž27.13 cmol c kgy1 .. The estimated CEC of the clay and fine silt particles were smaller than the CEC reported for such mineral particles by Leinweber et al. Ž 1993. , despite similarities in the mineralogical composition recorded. The contribution of the organo-mineral fractions to the total exchange capacity of the soil was calculated by relating the CEC of the fine silt or clay with the percentage of these fractions calculated according to the granulometric analysis, and the total CEC of the soil ŽMorras, ´ 1995.. The results obtained ŽFig. 5.
Fig. 5. Percentage distribution of CEC among the clay and fine silt size fractions.
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showed that the fine silt size fraction contributed between 2 and 65% to soil CEC, whilst the contribution of the clay size fraction represented between 15 and 85%. The fine silt-size fraction, then represented a substantial contribution to the soil CEC. The range found was greater than that observed by Leinweber et al. Ž1993., who found 13–23% of the total soil CEC to be associated with fine silt. These comparative data suggest that the contribution of the fine silt size fraction to soil CEC is greater in semiarid calcareous soils than in decarbonated acidic soils, in which the CEC of the clay fraction is much higher. In fact, the greatest part of the soil’s CEC lies in these two fractions and their corresponding OM content. The cation exchange capacity is a very important soil property for nutrient retention and supply. In addition, the exchange cations of organo-mineral particle-size fractions act as bridges between soil and plant. The mineralogical composition of the clay fraction with high amount of interstratified minerals of high specific surface area may explain its substantial contribution to soil CEC. As regards the fine silt size fraction, according with the mineralogical composition, the CEC may also be due to the presence of illite, illite–montmorillonite interstratified minerals and feldspars, as suggested by Morras ´ Ž1995.. The fact that the contribution of the fine silt size fraction to the total soil CEC was greater in forest soil samples can be explained because the OM of such soils is preferentially distributed in this fraction. Thus, differences in soil uses also affect the distribution of the soil CEC among the size-fractions, as demonstrated by Leinweber et al. Ž1993..
4. Conclusions In general, most of the soil organic matter in the soils studied was associated with the - 2 mm and 2–20 mm fractions, where it probably occurred as a complex with the mineral constituents, thus making them the most fertile soil fractions. The redistribution of soil organic matter between particle-size fractions is most probably affected by land use, as is demonstrated by the fact that the TOC content of cultivated soils was higher in the - 2 mm fraction, while in forest soils it was higher in the 2–20 mm fraction. The observation that the percentage of whole soil organic matter associated with the clay and fine silt size fractions of cultivated soils was generally larger than the percentage associated with the ) 20 mm fractions strongly suggests that clay and fine silt particles protect organic matter against microbial degradation. Thus, the proportion of organic matter in the - 20 mm fractions was less affected by cultivation in most soils. On the other hand, although forest soils generally had higher organic matter content than cultivated soils, the contribution of the - 20 mm fractions to soil organic matter was lower. This also
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suggests that the input of C provides by plant debris in forest soils was accumulated in the ) 20 mm fractions. In further studies it will be necessary to examine the role played by organo-mineral size particles in nutrient availability and soil organic matter turnover in the soils of Southeast Spain, which are characterized by large losses of such particles due to water erosion and unsuitable agricultural management.
Acknowledgements F. Caravaca acknowledges a grant from Caja de Ahorros del Mediterraneo ´ ŽCAM..
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