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Soil humic acids formation and characteristics in a xeric mollisol reforested with two tree species
Developments in Soil Science, Volume 28B Editors: A. Violante, P.M. Huang, J.-M. Bollag and L. Gianfreda © 2002 Elsevier Science B.V. All rights reserved.
393
SOIL HUMIC ACIDS FORMATION AND CHARACTERISTICS IN A XERIC MOLLISOL REFORESTED WITH TWO TREE SPECIES M. T. Deir Abate^ F. Pinzari^ A. Benedetti^ B. M. Petronio^ and C. Dazzi' ^Istituto Sperimentale per la Nutrizione delle Piante di Roma, Via della Navicella 2/4, 00184 Roma, Italy ^Dipartimento di Chimica, Universita di Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy ^Dipartimento di Agronomia, Coltivazioni Erbacee e Pedologia, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy
The present study deals with the influence on humic acids characteristics of different plant species [Pinus halepensis Miller and Cedrus atlantica (Endl) Carriere] used in the restoration of a Mollisol under xeric conditions. The area studied is in Sicily (Italy), and two stands were compared 40 years after planting. The objective was to contribute to the comprehension of the different levels at which the processes of pedogenesis acted in the study area. The differences between soil profile development in the two stands were investigated. Moreover, some chemical and spectroscopic characteristics of humic acids (HA) were analyzed. The results showed that the evolution of soils differed under the two tree stands, both in the formation of different humus forms and in different C storage in the forest floors. The HA chemical composition showed differences in the interactions between humic acids and inorganic matter in relation to the tree species and depth. Since the pedogenetic factors (mainly climate and parent material) were the same for the two pedons, and the HA extracting procedure adopted was the same for all samples, the data seem to suggest a different role for the microflora and plant coverage in the soil profile evolution of the two stands, which could address the pedogenesis process.
1. INTRODUCTION The influence exerted by vegetation type on some soil properties due to both different organic matter dynamics and nature has been discussed and exploited by many authors [1,2]. The study of the relationship between soil and vegetation and the natural succession of biocenosis in the reforested sites allows us to define the fitness of the stands for the specific environmental condition. In fact, the suitability of the tree species commonly used in reforestation for improving soil formation and avoiding soil degradation differs significantly, according to the characteristics of the areas, i.e., climate, soils physics, and chemistry. In particular, the formation of different humus form profiles, which are recognizable from a taxonomic point of view [3], is the consequence of both the rate and the type of organic
394 carbon turnover, which depend on both the amount and the chemical characteristics of the Utter. Since humic substances represent the final product of the organic matter degradation/building up processes that occurred in the soil following anthropogenic activity, such as reforestation practices, their amount and qualitative characteristics can yield some information on pedogenesis. hi the present study, the effects fi-om humic acids characteristics of mineral-organic matter-biota interactions due to different plant species in the restoration of a Mollisol under xeric conditions were investigated to evaluate the effects of the tree species on site remediation and pedogenesis. The differences between soil profile development were investigated by chemical and biochemical methods in order to understand the different levels at which the processes of pedogenesis acted in the study area, hi particular, some chemical and spectroscopic characteristics of humic acids (HA), separatedfi*omtwo selected pedons, were analyzed.
2. MATERIALS AND METHODS 2.1 Study area and sampling. The study area (5000 m^) is located in the Sicani range (western Sicily, Italy), within a wider area subjected to a reforestation program for soil conservation purposes fi-om 1944 to 1960 (Figure 1). The climate, from data relative to years 1951-1990 recorded in Piano del Leone (slightly to the west of the test area), provides average monthly temperatures whose maximum values are found in July (21.7°C) and August (22°C) and minimum values in January (6°C) and February (6.5°C). Average rainfall is 800 mm, with maximum levels recorded in January and December (136 and 123 mm, respectively) and minimum in June and July (12 and 8 mm, respectively). The climate of the area was defined according to BagnoulsGaussen as meso-mediterranean, the udometric regime as xeric, and the thermometric regime as thermic.
<-' P. .'-Pi
//
i P'
'^^ 'v*^^
Sicani's range
v.
Figure 1. Geographical location of the study area.
395 The testing area consists of two subunits, separated by a footpath, that were reforested in rows with Pinus halepensis Miller in one subunit and Cedrus atlantica (Endl) Carriere in the other during the winter of 1956. The study area is nearly uniform in terms of topography, lithology, slope, exposition, soil, climate, etc. Soils were classified as Lithic Haploxerolls [4]. Twenty pedons were selected, taking care to respect some similarities such as light exposition, pitch, etc. and the distance from the tree's trunk (1 meter) to avoid possible differences in soil characteristics due to stem flow [5]. Soils were described in the field in hand-dug pits; samples were collected from each organic horizon [3] and from each mineral horizon [6]. The pedological characteristics of the soil profiles are reported in Table 1. Each sample was airdried and manually sieved (< 2 mm).
Table 1 Soil classification, description of soil-forming factors and morphological properties of twenty pedons in the testing area on the Sicani range, Sicily, Italy Pedon N°
Site Parent Topography elevation Material (m)
1-10
1030
11-20
1030
Dolomitic limestone Dolomitic limestone
Pedon Horizon Depth* N° (g^) Lv 6-3 1-10
11-20
Fm Ah Bw
3-0 0-8 8-30
R
30+
Fa Ah Bw
4-0 0-7 7-30
Vegetation Mean annual temp. (T°C)
south-facing Pinus 13.5 slope of 8% halepensis south-facing Cedrus 13.5 slope of 8% atlantica Bound- Color* dry ary^
Mean annual precip. (mm)
Classification (SSS, 1997)
800
Lithic Haploxeroll, loamy, mixed, mesic Lithic Haploxeroll, loamy, mixed, mesic
800
Color* Texture Structure* Consistence* Roots* moist
gs ai
5YR3/3 5YR2/2 scl 7,5YR3/2 7,5YR2/2 cl
3 fmcr 3 mabk
sh fr ss ps shfrssps
2fm Ico 2fm
gs ai
5YR3/3 5YR2/2 scl 7,5YR3/2 7,5YR2/2 cl
1 fmcr 2 mabk
shfrss ps 2fm shfrssps Ico 2fm
30+ R Abbreviations: Boundary: a=abrupt; c=clear; g=gradual; d=diffuse; Ss=smooth; w=wavy; i=irregular. Texture: s=sand; l=loam; c=clay. Structure: l=weak; 2=moderate; 3=strong; vf=very fine; f=fine; m=medium; c=coarse; cr=crumby; pr=prismatic; abk=angular blocky; sbk=subangular blocky. Consistence: (dry) so=soft; sh=slightly hard; vh=very hard; (moist) vfr=very friable; fr=friable; fi=firm; (wet) so=nonsticky; ss=slightly sticky; po=nonplastic; ps=sHghtly plastic. Root: l=few; 2=common; f=fine; m=medium; co=coarse. * average of prevalent.
2.2 Humic acids Two pedons, under Cedrus and Pinus, respectively, were selected to carry out a comparative investigation on some chemical characteristics of humic acids. HA were
396 extracted and purified according to Stevenson [7]. Thus, soil was shaken for 1 h with 0.1 M HCl (soil:solution ratio 1:10) to eliminate carbonates and alkaline-earth metals. After washing for the ehmination of chloride, a solution of 0.1 M NaOH was added under N2, to give a soil:extractant ratio of 1:10. The suspension was shaken at room temperature for 4 h and then allowed to stand for 12 h. After centriftigation, HA were precipitated from the supernatant by acidification with 6 M HCl to pH 1, then separated by centrifugation. Several purification steps with a solution of 0.1 M HCl and 0.3 M HF were included to reduce the amounts of inorganic impurities in the humic extracts. The HA obtained were then dialyzed by Spectrapor membrane (size exclusion limit, 1000 Dalton) to remove salts, and finally lyophiHzed. 2.3 Chemical analyses Total organic carbon (Corg) was determined by mineralization with 2 N K2Cr207 and H2SO4 solutions at 160°C according to Springer and Klee [8]. The extraction of the soil organic matter (SOM) was carried out according to Ciavatta et al. [9], by using a reagent containing 0.1 N NaOH and 0.1 N Na4P207. Total extractable carbon (Cext) and the combined humic and fulvic fractions carbon (C(HA+FA)) were determined by dichromate oxidation method [9]. The humification rate (HR) and degree of humification (DH) were calculated according to Ciavatta et al. (1990), as follows: H R ( % ) = C(HA+FA)X 100/Corg
(1)
D H ( % ) = C(HA+FA)X 100/Cext
(2)
HA carbon, hydrogen, nitrogen and sulftir were determined by a Perkin-Elmer 240 B elemental analyzer. 2.4 FTIR analysis FTIR spectra of HA were recorded on KBr pellets in the 4000 to 400 cm'^ wavenumber range using a Philips PU9800 FTIR spectrometer working in diffuse reflectance conditions. The results are given in Kubelka Munk units deriving from a mathematical formula applied to the diffuse reflectance spectra (Kortum, cited in [10]). The KBr pellets were prepared by pressing a mixture of 1 mg HA and 100 mg anhydrous KBr, spectrometry grade. The operative conditions for spectra acquisition were 100 scans with a 4 cm'^ resolution. Each analysis was replicated twice. 2.5 Thermal analysis Differential scanning calorimetry (DSC) and thermogravimetry (TG) measurements of HA were carried out with a Netzsch Simultaneous Analyzer STA 409 equipped with a TG/DSC sample carrier supporting a type S thermocouple of PtRhlO-Pt. The following conditions were used: heating rate of 10°C min'^ from 20°C to 900°C, static air atmosphere, alumina crucible, calcined kaolinite as reference, sample weight 5 mg. The thermo-balance was calibrated for buoyancy effects to obtain quantitative estimation of weight changes. Heat production in the "heat-flux" DSC was calibrated under the same measurement conditions by using a sapphire standard and subtracting a baseline obtained by an additional run for the empty crucibles. Netzsch applied software SW/cp/311.01 was used for data processing. Duplicate thermal runs were carried out to verify the measurement reproducibility.
397 3. RESULTS AND DISCUSSION The two sites were previously compared on the basis of their contents of both total and exchangeable Ca, Mg, K, Na, N, and P; pH; humus forms; microbial biomass activity [11, 12]. Higher concentrations of the above mentioned mineral elements were found in the forest floor under Cedrus than under Pinus, with significant difference (P < 0.01) for total K contents (10.61 cmolkg'^ and 7.83 cmolkg"^ respectively). Exchangeable K was also higher in the A horizon under Cedrus (1.75 cmolkg"^) than under Pinus (1.52 cmolkg'^). In the B horizon, a significantly lower pH value under Cedrus (6.34) was found with respect to that under Pinus (7.24). Under the Cedrus stand, only two soil profiles showed an Lv horizon. The taxonomic classification of the humus forms [3] also revealed differences in the forest floors under the two tree stands: Hemimor under Pinus and Mormoder under Cedrus [4]. In Table 2, the contents of soil organic matter fractions in the two tree stands are reported. In the Pinus stand, a significantly higher amount of total organic carbon (Corg) was stored in both the Fm and A horizons than in the Cedrus horizons. In addition, the Pinus stand exhibited the highest contents of organic carbon extracted with a NaOH plus Na4P207 0.1 N solution (Cext) and the highest humified fraction [C(HA+FA)]- The organic matter from the Pinus Fm horizon showed a slightly lower chemical extractability, indicated by the Cext/Corg ratio, which might be due to the mycological characteristics of this horizon that were recognized in the field observations. Table 2 Average values of total organic carbon (Corg), total extractable carbon (Cext), carbon of humic plus fulvic fraction (C(HA+FA)), Corg to N ratio, Cext to Corg ratio, humification rate (HR) and degree of humification (DH) along the profiles (average values of 10 profiles for each stand). DH HR Ecosystem Corg/N Corg Cext'^-'org C(HA+FA) Cext (%) (gkg'^) (%) (gkg-^) (gkg'^) Lv horizons 61 39 32.1 343.2 Cedrus"^ 0.637 133.9 218.7 43 71 0.602 28.4 303.5 Pinus 129.6 182.8 F horizons Cedrus Pinus Student t Significance
253.4 351.6 3.18 P<0.010
25.0 27.5 0.75 NS
162.7 202.3 3.15 P<0.010
107.7 143.3 3.73 P<0.005
0.642 0.575 1.29 NS
43 41 0.52 NS
66 71 1.69 NS
A horizons Cedrus Pinus Student t Significance
65 93 3.61 P<0.005
20.2 21.4 0.46 NS
42 57 3.10 P<0.010
29 43 2.78 P<0.025
0.657 0.617 1.70 NS
45 46 0.79 NS
69 75 0.70 NS
B horizons Cedrus Pinus Student t Significance
29 31 1.41 NS
13.8 14.2 0.47 NS
20 21 0.59 NS
13 14 1.06 NS
0.742 0.669 1.75 NS
45 46 0.43 NS
65 67 0.88 NS
398 According to the traditional approach, based on the different solubihty at acidic conditions of extracted organic macromolecules, the calculation of the humification parameters [9] might give some indication of the extent to which the humification processes occur. From results reported in Table 2, it can be deduced that the HR and DH did not show statistically significant differences, although the small differences of DH could reflect a different quality of the extracted organic matter. Thus, the highest amount of humified fractions extracted under the Pinus stand did not correspond to a higher humification of the extracted organic matter on a gross scale. Table 3 reports the results of correlation analysis for each A and B horizon between parameters related to the C cycle and the pH and K^ values (ten values for each parameter and for each tree stand). The parameters related to the C cycle showed a significant correlation with the pH in A horizon under the Cedrus plantation. On the other hand, no correlation was found under Pinus. Since there were no significant pH differences in A horizons between the two tree stands, but since Pinzari et al. [12] reported significant differences in the microbial biomass activity and in the carbon mineralization rates between the A horizons, it was hypothesized that microbial biomass activity could play the main role in pedogenesis under Cedrus. Li particular, a different litter quality, more favourable to microbial biomass activity and leading to better substrate utilization in the A horizons of the site with the Cedrus plantation, could determine different mineral-organic matter-biota interactions in the two stands [12].
Table 3 Correlation coefficients between C-parameters and the three chemical parameters that differed significantly between the two soils. The bold values correspond to a significant correlation (0.05>a>0.01). Parameters K; pH Cedrus Cedrus Pinus Pinus Cedrus Cedrus Pinus Pinus A B A B B A B A -0.10 0.46 0.17 0.75 0.59 0.19 0.20 0.03 Corg(l) 0.35 0.00 -0.31 0.69 -0.60 -0.16 0.47 0.06 Cext (2) -0.02 -0.23 0.72 0.51 0.51 0.04 -0.22 -0.09 C(HA+FA) (3) -0.39 0.21 0.77 0.12 0.53 0.09 -0.27 0.13 ^mic \y) C28*(5) 0.50 -0.07 -0.02 0.79 0.32 -0.05 0.11 0.05 Co (6) 0.31 0.81 0.67 0.07 0.53 0.04 0.14 0.12 K(7) -0.74 -0.17 -0.10 -0.11 -0.44 0.14 -0.04 0.22 CI (8) 0.64 0.70 0.28 0.35 0.34 0.19 0.29 0.32 (1-2-3) see Table 3; (4) soil microbial biomass carbon; (5) microbial biomass basal respiration; (6) potentially mineralizable C; (7) rate constant of carbon mineralizable; (8) carbon respiration flush at the first day measurement. Data referred to (4) to (8) parameters are reported in Pinzari et al. [12]. The investigation carried out on HA from the two pedons selected under the two plant coverages allowed us to highlight some differences in their chemical composition. A different elemental composition of HA (Table 4) was found. In particular, the very high ash content of HA from the mineral horizons, despite the purification procedure adopted, demonstrates a
399 strong linkage between humic acids and soil mineral components, more marked for HA formed under Pinus plants. This high HA ash content, different in both the profile and between the two studied pedons, makes the HA composition not directly comparable with respect to literature data, whereas the atomic ratio indicates the incorporation of N into the humic molecule (increase of the C/N ratio) and an increase in aliphatic (H/C) with depth.
Table 4 Compositions of humic acids (HA) and atomic ratio Elemental composition (%) Horizons H C N S Cedrus atlantica Lv 52.49 5.63 0 3.46 Fa 52.40 5.25 3.10 0 A 30.81 2.99 0 1.93 Bw 17.14 2.78 1.35 0 Pinus halepensis Lv 51.68 5.11 1.9 0 Fm 51.70 5.48 0 2.82 A 21.59 2.81 1.27 0 Bw 8.26 1.79 0 0.77 "'Ash was measured by thermogravimetric analysis.
Ash*
Atomic ratio N/C H/C
2.87 4.50 36.62 56.63
1.289 1.202 1.165 1.946
0,057 0.051 0.054 0.068
5.01 6.7 50.32 71.41
1.187 1.272 1.562 2.600
0.032 0.047 0.050 0.080
The FTIR spectra of the HAs studied are shown in Figures 2a and 2b. In all samples, the more intense bands were recorded in the following regions: at about 3400 cm'^ the -OH stretch, at 1730 cm'^ the carboxylic and/or carbonilic -C=0 stretching, at 1630 cm'^ the conjugated and aromatic C=C vibrations, and at 1050 cm"^ the polysaccharide C-0 stretch and/or the Si-0 vibrations of associated clays. Less intense absorption was recorded at about 2900 cm'^ (aliphatic C-H stretching), 1520 cm'^ (aromatic C=C bonds), 1450 cm'^ (methyUc and methylenic -C-H bending). The presence of a higher ash content in HA from the Pinus pedon was also confirmed. In fact, the increasing intensity of the absorption at 1050 cm'^ along the profile, accompanied by the decrease of the 3400 cm'^ signal, due to the aliphatic OH and/or NH groups, allowed us to assign the 1050 cm'^ band to the Si-0 vibrations of associated clays. This behavior was not recorded for Cedrus HA. Moreover, HA under Pinus showed a more aromatic structure and a minor presence of alcoholic, carboxylic and carbonylic groups, in contrast to Cedrus HA, which were more hydrous and rich in both free and bound alcoholic groups. The minor presence of OH groups is considered indicative of a higher humification [13]. Differential scanning calorimetry and thermogravimetry measurements confirmed the observed differences between HA of the two pedons (Figures 3a and 3b). The oxidative thermal decomposition of HA from organic horizons (Lv and F) showed multisteps patterns that were reduced to two main reactions for HA from horizons A and B. In particular, the DSC curve of HA from Pinus Lv showed three well-resolved exothermic peaks at 331°, 456° and 531.7°C, each of which was associated with distinct weight losses registered on the TG curve. The correspondence between the heat flux rate on the DSC curve and the weight loss rate was checked by the calculation of the first derivative of the TG curve (DTG). The lower
400
4000 3500 3000 2500 2000 1500 1000 500
0.00 4; 4000 3500 3000 2500 2000 1500 1000 500 0.25
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500
0.30-f 0.25
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500 0.40 0.30 , 0.20 ..
0.05 A
0.10 . 0.00^
4000 3500 3000 2500 2000 1500 1000 500 Figure 2a. FTIR spectra of HA separated from the horizons Lv, Fa, A and B of the Cedrus pedon. (Y axis: Kubelka Munk units; X axis: wavenumbers).
^
11 /
»1
j/V^vi y
4000 3500 3000 2500 2000 1500 1000 500 Figure 2b. FTIR spectra of HA separated from the horizons Lv, Fm, A and B of the Pinus pedon. (Y axis: Kubelka Munk units; X axis: wavenumbers
401 DSC/ uV 50
0
100 200 300 400 500 600 700 800 Temperature/ °C
TG/ %
0
100 200 300 400 500 600 700 800 °C
Fm
0 100 200 300 400 500 600 700 800 °C DSC/uV
0 100 200 300 400 500 600 700 800 °C
0
100 200 300 400 500 600 700 800 °C
0 100 200 300 400 500 600 700 800 °C
0
100 200 300 400 500 600 700 800 °C
Figure 3 a Cedrus pedon: DSC, TG and DTG traces of HA from the horizons Lv, Fa, A and B.
Figure 3b Pinus pedon: DSC, TG and DTG traces of HA from the horizons Lv, Fm, A and B.
402
temperature exotherm was attributed to aliphatics and carbohydrates and the effects at higher temperature to thermal disassociation and the breakdown of aromatics [14]. HA from Cedrus Lv showed the most intense exotherm at 490.7°C and minor thermal effects at 435° and 350°C. Thermal behavior of HA from F horizons of both pedons was more complex in terms of number of reactions, which occurred over a wider temperature range. This could be the result of the organic resynthesis carried out by the microbial biomass acting in these horizons. Finally, thermograms of HA from Pinus mineral horizons showed two main reactions, which differed from those from Cedrus horizons. The thermogravimetric data of weight losses that occurred during each thermally induced reaction allowed us to quantify the sizes of the different organic pools involved (Table 5). The results confirmed the difference in chemical composition between HA developed in the organic horizons under the two plantations and the strong linkage existing between humic acids and mineral components, as show by the high ash contents of HA; these data are reported in Table 4 and discussed above.
Table 5 Results of thermogravimetric measurements on HA: weight losses (%) related to the exothermic reactions; mean values of two replications. Horizons Cedrus atlantica Pinus halepensis TG weight loss
31.55 19.0 41.45
Temperature range (°C) 146-365.1 365.1-497.5 497.5- 640.4
144.9-344 344-437 437-532.9 532.9-678
28.2 18.24 26.1 17.8
152.4-352 352-371.8 371.8-441.7 441.7-518.0
27.29 3.17
A
145.2- 309.4 309.4-558.7
16.1 42.6
154.5-328.1 322.5-583.8
20.19 20.7
B
157-300 300-607.9
12.4 25.4
143.8-601.4
22.03
Lv
F
Temperature range (°C) 149-366 366-448.2 448.2-602.2
TG weight loss
30.2 34.94 20.57
n.4 15.07
4. CONCLUSIONS The results obtained in this study showed a different evolution of soil profile under the two tree stands, in the formation of different humus forms and in different C storage in the forest floor and in some chemical parameters in the mineral horizons, such as pH. In particular, the parameters related to soil organic matter characteristics showed a significant correlation with pH in A horizons under the Cedrus plantation. It was hypothesized that microbial biomass activity could play a major role in pedogenesis under Cedrus, possibly due to the different litter quality more favorable to microbial biomass activity and leading to a
403
better substrate utilization, as was found by Pinzari et al. [12]. The plantation difference influenced the amount of the humified organic fraction stored in the two stand soils: it was higher in both organic and A horizons under Pinus. However, the highest amount of humified fractions extracted under the Pinus stand did not correspond to a higher humification level of the extracted organic matter on a gross scale, as shown by the calculation of the humification parameters. Thus, it was not possible to define if either the quantity of organic matter stored in soil or its chemical characteristics were relevant for pedogenesis. On the other hand, data on plant biomass production were not available; thus this aspect was not investigated. The investigation carried out on HA from the two pedons selected under the two plant stands allowed us to highlight some differences in their chemical composition. A different elemental composition of HA was shown. The atomic ratio indicated the incorporation of N into the humic molecule (increase of the C/N ratio) and an increase in aliphatic (H/C) with depth. Spectroscopic investigation carried out by FTIR showed a more aromatic structure and a minor presence of alcoholic, carboxylic and carbonylic groups for HA under Pinus, in contrast to Cedrus HA, which were more hydrous and rich in both free and bound alcoholic groups. Finally, HA thermal behavior was consistently different between HA from Lv horizons: in particular, under Pinus, more aromatic structures were revealed. Despite the HA purification procedure adopted, a very high ash content in the mineral horizons HA was found by thermogravimetry; this finding demonstrates a strong linkage between humic acids and soil mineral components, which was more marked for HA formed under Pinus plants. Since the pedogenetic factors (mainly climate and parent material) were the same for the two pedons, and the HA extracting procedure adopted was the same for all samples, the formation of humic acids having different chemical composition could derive from different mineral-organic matter-biota interactions in the two tree plantations, hi other words, a different role for the microflora and plant coverage in the soil profile evolution of the two stands could address the pedogenesis process. The results of investigations on energy use in the soil profile by soil microbial biomass can be found in Pinzari et al. [12].
REFERENCES 1. Fyles, J.W., Cote, B., 1994. Forest floor and soil nutrient status under Norway spruce and red pine in a plantation in southern Quebec. Can. J. Soil Sci. 74, 387-392. 2. Howard, P.J.A., Howard, D.M., Lowe, L.E., 1998. Effects of tree species and soil physico-chemical conditions on the nature of soil organic matter. Soil Biol. Biochem. 30, 285-297. 3. Green, R.N., Trowbridge, R.L., Klinka, K., 1993. Towards a Taxonomic Classification of Humus Forms. Supplement to Forest Science 29, 1-49. 4. Dazzi, C , Di Pisa, A., La Mela Veca, S., Vacante, G., 1996. Caratterisfiche e distribuzione dei suoli forestali del bosco Ledera sui monti Sicani (Sicilia occidentale). Boll. Soc. Ital. Sc. Suolo, Palermo, 8, 295-307. 5. Pritchett, W.L., Fisher, R.F., 1985. Properties and Management of Forest Soils, 2"^ Ed. Wiley & Sons, New York. 6. Soil Survey Staff, 1993. Soil Survey Manual, USDA Handbook, n. 18, Washington, D.C.. 7. Stevenson, F. J., 1982. Humus Chemistry, John Wiley & Sons, New York.
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8. Springer, U., Klee, J., 1954. Priifung der Leistungsfahigkeit von einigen wichtigeren Verfahren zur Bestimmung des Kohlemstoffs mittels Chromschwefelsaure sowie Vorschlag einer neuen Schnellmethode, Z. Pflanzenemahr. Dang. Bodenk., 64, 1-8. 9. Ciavatta, C, Govi, M., Vittori Antisari, L., Sequi, P., 1990. Characterization of humified compounds by extraction and fractionation on solid polyvynilpyrrolidone. J. Chromat. 509, 141-146. 10. Painter, P., Starsinic, M., Coleman, M., 1985, Determination of functional groups in coal by Fourier transform interferometry. In: Ferraro, J.R., Basile, L.J. (Eds.), Fourier Transform Spectroscopy. Application to Chemical Systems. Vol. 4. Academic Press, Orlando, FL, pp. 169-241. ll.Dazzi, C, 1996. Tassonomia del "Forest floor" e caratteristiche di fertilita di suoH forestall sotto pino d'Aleppo {Pinus halepensis Miller) e Cedro dell'Atlante [Cedrus atlantica (Endl.) Carriere], Boll. Soc. Ital. Sc. Suolo, Palermo, 8, 51-61. 12. Pinzari, F., Dell'Abate, M.T., Benedetti, A., Dazzi, C, 2002. Energy use in the A and B horizons of a soil under a pine and a cedar stand. In: Violante, A., Huang, P.M., Bollag, J.-M., Gianfreda, L. (Eds.), Proceedings of ISMOM Congress. 13. Schnitzer, M., Desjardin, J.G., 1965. Carboxyl and fenolic hydroxyl groups in some organic soils and their relation to the degree of humification. Can. J. Soil Sci. 45, 257-264. 14. Calace, N., Petronio, B.M., 1997. Characterization of high molecular weight organic compounds in landfill leachate: humic substances. J. Environ. Sci. Health, A32(8), 22292244.
393
SOIL HUMIC ACIDS FORMATION AND CHARACTERISTICS IN A XERIC MOLLISOL REFORESTED WITH TWO TREE SPECIES M. T. Deir Abate^ F. Pinzari^ A. Benedetti^ B. M. Petronio^ and C. Dazzi' ^Istituto Sperimentale per la Nutrizione delle Piante di Roma, Via della Navicella 2/4, 00184 Roma, Italy ^Dipartimento di Chimica, Universita di Roma "La Sapienza", Piazzale Aldo Moro 5, 00185 Roma, Italy ^Dipartimento di Agronomia, Coltivazioni Erbacee e Pedologia, Universita di Palermo, Viale delle Scienze, 90128 Palermo, Italy
The present study deals with the influence on humic acids characteristics of different plant species [Pinus halepensis Miller and Cedrus atlantica (Endl) Carriere] used in the restoration of a Mollisol under xeric conditions. The area studied is in Sicily (Italy), and two stands were compared 40 years after planting. The objective was to contribute to the comprehension of the different levels at which the processes of pedogenesis acted in the study area. The differences between soil profile development in the two stands were investigated. Moreover, some chemical and spectroscopic characteristics of humic acids (HA) were analyzed. The results showed that the evolution of soils differed under the two tree stands, both in the formation of different humus forms and in different C storage in the forest floors. The HA chemical composition showed differences in the interactions between humic acids and inorganic matter in relation to the tree species and depth. Since the pedogenetic factors (mainly climate and parent material) were the same for the two pedons, and the HA extracting procedure adopted was the same for all samples, the data seem to suggest a different role for the microflora and plant coverage in the soil profile evolution of the two stands, which could address the pedogenesis process.
1. INTRODUCTION The influence exerted by vegetation type on some soil properties due to both different organic matter dynamics and nature has been discussed and exploited by many authors [1,2]. The study of the relationship between soil and vegetation and the natural succession of biocenosis in the reforested sites allows us to define the fitness of the stands for the specific environmental condition. In fact, the suitability of the tree species commonly used in reforestation for improving soil formation and avoiding soil degradation differs significantly, according to the characteristics of the areas, i.e., climate, soils physics, and chemistry. In particular, the formation of different humus form profiles, which are recognizable from a taxonomic point of view [3], is the consequence of both the rate and the type of organic
394 carbon turnover, which depend on both the amount and the chemical characteristics of the Utter. Since humic substances represent the final product of the organic matter degradation/building up processes that occurred in the soil following anthropogenic activity, such as reforestation practices, their amount and qualitative characteristics can yield some information on pedogenesis. hi the present study, the effects fi-om humic acids characteristics of mineral-organic matter-biota interactions due to different plant species in the restoration of a Mollisol under xeric conditions were investigated to evaluate the effects of the tree species on site remediation and pedogenesis. The differences between soil profile development were investigated by chemical and biochemical methods in order to understand the different levels at which the processes of pedogenesis acted in the study area, hi particular, some chemical and spectroscopic characteristics of humic acids (HA), separatedfi*omtwo selected pedons, were analyzed.
2. MATERIALS AND METHODS 2.1 Study area and sampling. The study area (5000 m^) is located in the Sicani range (western Sicily, Italy), within a wider area subjected to a reforestation program for soil conservation purposes fi-om 1944 to 1960 (Figure 1). The climate, from data relative to years 1951-1990 recorded in Piano del Leone (slightly to the west of the test area), provides average monthly temperatures whose maximum values are found in July (21.7°C) and August (22°C) and minimum values in January (6°C) and February (6.5°C). Average rainfall is 800 mm, with maximum levels recorded in January and December (136 and 123 mm, respectively) and minimum in June and July (12 and 8 mm, respectively). The climate of the area was defined according to BagnoulsGaussen as meso-mediterranean, the udometric regime as xeric, and the thermometric regime as thermic.
<-' P. .'-Pi
//
i P'
'^^ 'v*^^
Sicani's range
v.
Figure 1. Geographical location of the study area.
395 The testing area consists of two subunits, separated by a footpath, that were reforested in rows with Pinus halepensis Miller in one subunit and Cedrus atlantica (Endl) Carriere in the other during the winter of 1956. The study area is nearly uniform in terms of topography, lithology, slope, exposition, soil, climate, etc. Soils were classified as Lithic Haploxerolls [4]. Twenty pedons were selected, taking care to respect some similarities such as light exposition, pitch, etc. and the distance from the tree's trunk (1 meter) to avoid possible differences in soil characteristics due to stem flow [5]. Soils were described in the field in hand-dug pits; samples were collected from each organic horizon [3] and from each mineral horizon [6]. The pedological characteristics of the soil profiles are reported in Table 1. Each sample was airdried and manually sieved (< 2 mm).
Table 1 Soil classification, description of soil-forming factors and morphological properties of twenty pedons in the testing area on the Sicani range, Sicily, Italy Pedon N°
Site Parent Topography elevation Material (m)
1-10
1030
11-20
1030
Dolomitic limestone Dolomitic limestone
Pedon Horizon Depth* N° (g^) Lv 6-3 1-10
11-20
Fm Ah Bw
3-0 0-8 8-30
R
30+
Fa Ah Bw
4-0 0-7 7-30
Vegetation Mean annual temp. (T°C)
south-facing Pinus 13.5 slope of 8% halepensis south-facing Cedrus 13.5 slope of 8% atlantica Bound- Color* dry ary^
Mean annual precip. (mm)
Classification (SSS, 1997)
800
Lithic Haploxeroll, loamy, mixed, mesic Lithic Haploxeroll, loamy, mixed, mesic
800
Color* Texture Structure* Consistence* Roots* moist
gs ai
5YR3/3 5YR2/2 scl 7,5YR3/2 7,5YR2/2 cl
3 fmcr 3 mabk
sh fr ss ps shfrssps
2fm Ico 2fm
gs ai
5YR3/3 5YR2/2 scl 7,5YR3/2 7,5YR2/2 cl
1 fmcr 2 mabk
shfrss ps 2fm shfrssps Ico 2fm
30+ R Abbreviations: Boundary: a=abrupt; c=clear; g=gradual; d=diffuse; Ss=smooth; w=wavy; i=irregular. Texture: s=sand; l=loam; c=clay. Structure: l=weak; 2=moderate; 3=strong; vf=very fine; f=fine; m=medium; c=coarse; cr=crumby; pr=prismatic; abk=angular blocky; sbk=subangular blocky. Consistence: (dry) so=soft; sh=slightly hard; vh=very hard; (moist) vfr=very friable; fr=friable; fi=firm; (wet) so=nonsticky; ss=slightly sticky; po=nonplastic; ps=sHghtly plastic. Root: l=few; 2=common; f=fine; m=medium; co=coarse. * average of prevalent.
2.2 Humic acids Two pedons, under Cedrus and Pinus, respectively, were selected to carry out a comparative investigation on some chemical characteristics of humic acids. HA were
396 extracted and purified according to Stevenson [7]. Thus, soil was shaken for 1 h with 0.1 M HCl (soil:solution ratio 1:10) to eliminate carbonates and alkaline-earth metals. After washing for the ehmination of chloride, a solution of 0.1 M NaOH was added under N2, to give a soil:extractant ratio of 1:10. The suspension was shaken at room temperature for 4 h and then allowed to stand for 12 h. After centriftigation, HA were precipitated from the supernatant by acidification with 6 M HCl to pH 1, then separated by centrifugation. Several purification steps with a solution of 0.1 M HCl and 0.3 M HF were included to reduce the amounts of inorganic impurities in the humic extracts. The HA obtained were then dialyzed by Spectrapor membrane (size exclusion limit, 1000 Dalton) to remove salts, and finally lyophiHzed. 2.3 Chemical analyses Total organic carbon (Corg) was determined by mineralization with 2 N K2Cr207 and H2SO4 solutions at 160°C according to Springer and Klee [8]. The extraction of the soil organic matter (SOM) was carried out according to Ciavatta et al. [9], by using a reagent containing 0.1 N NaOH and 0.1 N Na4P207. Total extractable carbon (Cext) and the combined humic and fulvic fractions carbon (C(HA+FA)) were determined by dichromate oxidation method [9]. The humification rate (HR) and degree of humification (DH) were calculated according to Ciavatta et al. (1990), as follows: H R ( % ) = C(HA+FA)X 100/Corg
(1)
D H ( % ) = C(HA+FA)X 100/Cext
(2)
HA carbon, hydrogen, nitrogen and sulftir were determined by a Perkin-Elmer 240 B elemental analyzer. 2.4 FTIR analysis FTIR spectra of HA were recorded on KBr pellets in the 4000 to 400 cm'^ wavenumber range using a Philips PU9800 FTIR spectrometer working in diffuse reflectance conditions. The results are given in Kubelka Munk units deriving from a mathematical formula applied to the diffuse reflectance spectra (Kortum, cited in [10]). The KBr pellets were prepared by pressing a mixture of 1 mg HA and 100 mg anhydrous KBr, spectrometry grade. The operative conditions for spectra acquisition were 100 scans with a 4 cm'^ resolution. Each analysis was replicated twice. 2.5 Thermal analysis Differential scanning calorimetry (DSC) and thermogravimetry (TG) measurements of HA were carried out with a Netzsch Simultaneous Analyzer STA 409 equipped with a TG/DSC sample carrier supporting a type S thermocouple of PtRhlO-Pt. The following conditions were used: heating rate of 10°C min'^ from 20°C to 900°C, static air atmosphere, alumina crucible, calcined kaolinite as reference, sample weight 5 mg. The thermo-balance was calibrated for buoyancy effects to obtain quantitative estimation of weight changes. Heat production in the "heat-flux" DSC was calibrated under the same measurement conditions by using a sapphire standard and subtracting a baseline obtained by an additional run for the empty crucibles. Netzsch applied software SW/cp/311.01 was used for data processing. Duplicate thermal runs were carried out to verify the measurement reproducibility.
397 3. RESULTS AND DISCUSSION The two sites were previously compared on the basis of their contents of both total and exchangeable Ca, Mg, K, Na, N, and P; pH; humus forms; microbial biomass activity [11, 12]. Higher concentrations of the above mentioned mineral elements were found in the forest floor under Cedrus than under Pinus, with significant difference (P < 0.01) for total K contents (10.61 cmolkg'^ and 7.83 cmolkg"^ respectively). Exchangeable K was also higher in the A horizon under Cedrus (1.75 cmolkg"^) than under Pinus (1.52 cmolkg'^). In the B horizon, a significantly lower pH value under Cedrus (6.34) was found with respect to that under Pinus (7.24). Under the Cedrus stand, only two soil profiles showed an Lv horizon. The taxonomic classification of the humus forms [3] also revealed differences in the forest floors under the two tree stands: Hemimor under Pinus and Mormoder under Cedrus [4]. In Table 2, the contents of soil organic matter fractions in the two tree stands are reported. In the Pinus stand, a significantly higher amount of total organic carbon (Corg) was stored in both the Fm and A horizons than in the Cedrus horizons. In addition, the Pinus stand exhibited the highest contents of organic carbon extracted with a NaOH plus Na4P207 0.1 N solution (Cext) and the highest humified fraction [C(HA+FA)]- The organic matter from the Pinus Fm horizon showed a slightly lower chemical extractability, indicated by the Cext/Corg ratio, which might be due to the mycological characteristics of this horizon that were recognized in the field observations. Table 2 Average values of total organic carbon (Corg), total extractable carbon (Cext), carbon of humic plus fulvic fraction (C(HA+FA)), Corg to N ratio, Cext to Corg ratio, humification rate (HR) and degree of humification (DH) along the profiles (average values of 10 profiles for each stand). DH HR Ecosystem Corg/N Corg Cext'^-'org C(HA+FA) Cext (%) (gkg'^) (%) (gkg-^) (gkg'^) Lv horizons 61 39 32.1 343.2 Cedrus"^ 0.637 133.9 218.7 43 71 0.602 28.4 303.5 Pinus 129.6 182.8 F horizons Cedrus Pinus Student t Significance
253.4 351.6 3.18 P<0.010
25.0 27.5 0.75 NS
162.7 202.3 3.15 P<0.010
107.7 143.3 3.73 P<0.005
0.642 0.575 1.29 NS
43 41 0.52 NS
66 71 1.69 NS
A horizons Cedrus Pinus Student t Significance
65 93 3.61 P<0.005
20.2 21.4 0.46 NS
42 57 3.10 P<0.010
29 43 2.78 P<0.025
0.657 0.617 1.70 NS
45 46 0.79 NS
69 75 0.70 NS
B horizons Cedrus Pinus Student t Significance
29 31 1.41 NS
13.8 14.2 0.47 NS
20 21 0.59 NS
13 14 1.06 NS
0.742 0.669 1.75 NS
45 46 0.43 NS
65 67 0.88 NS
398 According to the traditional approach, based on the different solubihty at acidic conditions of extracted organic macromolecules, the calculation of the humification parameters [9] might give some indication of the extent to which the humification processes occur. From results reported in Table 2, it can be deduced that the HR and DH did not show statistically significant differences, although the small differences of DH could reflect a different quality of the extracted organic matter. Thus, the highest amount of humified fractions extracted under the Pinus stand did not correspond to a higher humification of the extracted organic matter on a gross scale. Table 3 reports the results of correlation analysis for each A and B horizon between parameters related to the C cycle and the pH and K^ values (ten values for each parameter and for each tree stand). The parameters related to the C cycle showed a significant correlation with the pH in A horizon under the Cedrus plantation. On the other hand, no correlation was found under Pinus. Since there were no significant pH differences in A horizons between the two tree stands, but since Pinzari et al. [12] reported significant differences in the microbial biomass activity and in the carbon mineralization rates between the A horizons, it was hypothesized that microbial biomass activity could play the main role in pedogenesis under Cedrus. Li particular, a different litter quality, more favourable to microbial biomass activity and leading to better substrate utilization in the A horizons of the site with the Cedrus plantation, could determine different mineral-organic matter-biota interactions in the two stands [12].
Table 3 Correlation coefficients between C-parameters and the three chemical parameters that differed significantly between the two soils. The bold values correspond to a significant correlation (0.05>a>0.01). Parameters K; pH Cedrus Cedrus Pinus Pinus Cedrus Cedrus Pinus Pinus A B A B B A B A -0.10 0.46 0.17 0.75 0.59 0.19 0.20 0.03 Corg(l) 0.35 0.00 -0.31 0.69 -0.60 -0.16 0.47 0.06 Cext (2) -0.02 -0.23 0.72 0.51 0.51 0.04 -0.22 -0.09 C(HA+FA) (3) -0.39 0.21 0.77 0.12 0.53 0.09 -0.27 0.13 ^mic \y) C28*(5) 0.50 -0.07 -0.02 0.79 0.32 -0.05 0.11 0.05 Co (6) 0.31 0.81 0.67 0.07 0.53 0.04 0.14 0.12 K(7) -0.74 -0.17 -0.10 -0.11 -0.44 0.14 -0.04 0.22 CI (8) 0.64 0.70 0.28 0.35 0.34 0.19 0.29 0.32 (1-2-3) see Table 3; (4) soil microbial biomass carbon; (5) microbial biomass basal respiration; (6) potentially mineralizable C; (7) rate constant of carbon mineralizable; (8) carbon respiration flush at the first day measurement. Data referred to (4) to (8) parameters are reported in Pinzari et al. [12]. The investigation carried out on HA from the two pedons selected under the two plant coverages allowed us to highlight some differences in their chemical composition. A different elemental composition of HA (Table 4) was found. In particular, the very high ash content of HA from the mineral horizons, despite the purification procedure adopted, demonstrates a
399 strong linkage between humic acids and soil mineral components, more marked for HA formed under Pinus plants. This high HA ash content, different in both the profile and between the two studied pedons, makes the HA composition not directly comparable with respect to literature data, whereas the atomic ratio indicates the incorporation of N into the humic molecule (increase of the C/N ratio) and an increase in aliphatic (H/C) with depth.
Table 4 Compositions of humic acids (HA) and atomic ratio Elemental composition (%) Horizons H C N S Cedrus atlantica Lv 52.49 5.63 0 3.46 Fa 52.40 5.25 3.10 0 A 30.81 2.99 0 1.93 Bw 17.14 2.78 1.35 0 Pinus halepensis Lv 51.68 5.11 1.9 0 Fm 51.70 5.48 0 2.82 A 21.59 2.81 1.27 0 Bw 8.26 1.79 0 0.77 "'Ash was measured by thermogravimetric analysis.
Ash*
Atomic ratio N/C H/C
2.87 4.50 36.62 56.63
1.289 1.202 1.165 1.946
0,057 0.051 0.054 0.068
5.01 6.7 50.32 71.41
1.187 1.272 1.562 2.600
0.032 0.047 0.050 0.080
The FTIR spectra of the HAs studied are shown in Figures 2a and 2b. In all samples, the more intense bands were recorded in the following regions: at about 3400 cm'^ the -OH stretch, at 1730 cm'^ the carboxylic and/or carbonilic -C=0 stretching, at 1630 cm'^ the conjugated and aromatic C=C vibrations, and at 1050 cm"^ the polysaccharide C-0 stretch and/or the Si-0 vibrations of associated clays. Less intense absorption was recorded at about 2900 cm'^ (aliphatic C-H stretching), 1520 cm'^ (aromatic C=C bonds), 1450 cm'^ (methyUc and methylenic -C-H bending). The presence of a higher ash content in HA from the Pinus pedon was also confirmed. In fact, the increasing intensity of the absorption at 1050 cm'^ along the profile, accompanied by the decrease of the 3400 cm'^ signal, due to the aliphatic OH and/or NH groups, allowed us to assign the 1050 cm'^ band to the Si-0 vibrations of associated clays. This behavior was not recorded for Cedrus HA. Moreover, HA under Pinus showed a more aromatic structure and a minor presence of alcoholic, carboxylic and carbonylic groups, in contrast to Cedrus HA, which were more hydrous and rich in both free and bound alcoholic groups. The minor presence of OH groups is considered indicative of a higher humification [13]. Differential scanning calorimetry and thermogravimetry measurements confirmed the observed differences between HA of the two pedons (Figures 3a and 3b). The oxidative thermal decomposition of HA from organic horizons (Lv and F) showed multisteps patterns that were reduced to two main reactions for HA from horizons A and B. In particular, the DSC curve of HA from Pinus Lv showed three well-resolved exothermic peaks at 331°, 456° and 531.7°C, each of which was associated with distinct weight losses registered on the TG curve. The correspondence between the heat flux rate on the DSC curve and the weight loss rate was checked by the calculation of the first derivative of the TG curve (DTG). The lower
400
4000 3500 3000 2500 2000 1500 1000 500
0.00 4; 4000 3500 3000 2500 2000 1500 1000 500 0.25
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500
0.30-f 0.25
4000 3500 3000 2500 2000 1500 1000 500
4000 3500 3000 2500 2000 1500 1000 500 0.40 0.30 , 0.20 ..
0.05 A
0.10 . 0.00^
4000 3500 3000 2500 2000 1500 1000 500 Figure 2a. FTIR spectra of HA separated from the horizons Lv, Fa, A and B of the Cedrus pedon. (Y axis: Kubelka Munk units; X axis: wavenumbers).
^
11 /
»1
j/V^vi y
4000 3500 3000 2500 2000 1500 1000 500 Figure 2b. FTIR spectra of HA separated from the horizons Lv, Fm, A and B of the Pinus pedon. (Y axis: Kubelka Munk units; X axis: wavenumbers
401 DSC/ uV 50
0
100 200 300 400 500 600 700 800 Temperature/ °C
TG/ %
0
100 200 300 400 500 600 700 800 °C
Fm
0 100 200 300 400 500 600 700 800 °C DSC/uV
0 100 200 300 400 500 600 700 800 °C
0
100 200 300 400 500 600 700 800 °C
0 100 200 300 400 500 600 700 800 °C
0
100 200 300 400 500 600 700 800 °C
Figure 3 a Cedrus pedon: DSC, TG and DTG traces of HA from the horizons Lv, Fa, A and B.
Figure 3b Pinus pedon: DSC, TG and DTG traces of HA from the horizons Lv, Fm, A and B.
402
temperature exotherm was attributed to aliphatics and carbohydrates and the effects at higher temperature to thermal disassociation and the breakdown of aromatics [14]. HA from Cedrus Lv showed the most intense exotherm at 490.7°C and minor thermal effects at 435° and 350°C. Thermal behavior of HA from F horizons of both pedons was more complex in terms of number of reactions, which occurred over a wider temperature range. This could be the result of the organic resynthesis carried out by the microbial biomass acting in these horizons. Finally, thermograms of HA from Pinus mineral horizons showed two main reactions, which differed from those from Cedrus horizons. The thermogravimetric data of weight losses that occurred during each thermally induced reaction allowed us to quantify the sizes of the different organic pools involved (Table 5). The results confirmed the difference in chemical composition between HA developed in the organic horizons under the two plantations and the strong linkage existing between humic acids and mineral components, as show by the high ash contents of HA; these data are reported in Table 4 and discussed above.
Table 5 Results of thermogravimetric measurements on HA: weight losses (%) related to the exothermic reactions; mean values of two replications. Horizons Cedrus atlantica Pinus halepensis TG weight loss
31.55 19.0 41.45
Temperature range (°C) 146-365.1 365.1-497.5 497.5- 640.4
144.9-344 344-437 437-532.9 532.9-678
28.2 18.24 26.1 17.8
152.4-352 352-371.8 371.8-441.7 441.7-518.0
27.29 3.17
A
145.2- 309.4 309.4-558.7
16.1 42.6
154.5-328.1 322.5-583.8
20.19 20.7
B
157-300 300-607.9
12.4 25.4
143.8-601.4
22.03
Lv
F
Temperature range (°C) 149-366 366-448.2 448.2-602.2
TG weight loss
30.2 34.94 20.57
n.4 15.07
4. CONCLUSIONS The results obtained in this study showed a different evolution of soil profile under the two tree stands, in the formation of different humus forms and in different C storage in the forest floor and in some chemical parameters in the mineral horizons, such as pH. In particular, the parameters related to soil organic matter characteristics showed a significant correlation with pH in A horizons under the Cedrus plantation. It was hypothesized that microbial biomass activity could play a major role in pedogenesis under Cedrus, possibly due to the different litter quality more favorable to microbial biomass activity and leading to a
403
better substrate utilization, as was found by Pinzari et al. [12]. The plantation difference influenced the amount of the humified organic fraction stored in the two stand soils: it was higher in both organic and A horizons under Pinus. However, the highest amount of humified fractions extracted under the Pinus stand did not correspond to a higher humification level of the extracted organic matter on a gross scale, as shown by the calculation of the humification parameters. Thus, it was not possible to define if either the quantity of organic matter stored in soil or its chemical characteristics were relevant for pedogenesis. On the other hand, data on plant biomass production were not available; thus this aspect was not investigated. The investigation carried out on HA from the two pedons selected under the two plant stands allowed us to highlight some differences in their chemical composition. A different elemental composition of HA was shown. The atomic ratio indicated the incorporation of N into the humic molecule (increase of the C/N ratio) and an increase in aliphatic (H/C) with depth. Spectroscopic investigation carried out by FTIR showed a more aromatic structure and a minor presence of alcoholic, carboxylic and carbonylic groups for HA under Pinus, in contrast to Cedrus HA, which were more hydrous and rich in both free and bound alcoholic groups. Finally, HA thermal behavior was consistently different between HA from Lv horizons: in particular, under Pinus, more aromatic structures were revealed. Despite the HA purification procedure adopted, a very high ash content in the mineral horizons HA was found by thermogravimetry; this finding demonstrates a strong linkage between humic acids and soil mineral components, which was more marked for HA formed under Pinus plants. Since the pedogenetic factors (mainly climate and parent material) were the same for the two pedons, and the HA extracting procedure adopted was the same for all samples, the formation of humic acids having different chemical composition could derive from different mineral-organic matter-biota interactions in the two tree plantations, hi other words, a different role for the microflora and plant coverage in the soil profile evolution of the two stands could address the pedogenesis process. The results of investigations on energy use in the soil profile by soil microbial biomass can be found in Pinzari et al. [12].
REFERENCES 1. Fyles, J.W., Cote, B., 1994. Forest floor and soil nutrient status under Norway spruce and red pine in a plantation in southern Quebec. Can. J. Soil Sci. 74, 387-392. 2. Howard, P.J.A., Howard, D.M., Lowe, L.E., 1998. Effects of tree species and soil physico-chemical conditions on the nature of soil organic matter. Soil Biol. Biochem. 30, 285-297. 3. Green, R.N., Trowbridge, R.L., Klinka, K., 1993. Towards a Taxonomic Classification of Humus Forms. Supplement to Forest Science 29, 1-49. 4. Dazzi, C , Di Pisa, A., La Mela Veca, S., Vacante, G., 1996. Caratterisfiche e distribuzione dei suoli forestali del bosco Ledera sui monti Sicani (Sicilia occidentale). Boll. Soc. Ital. Sc. Suolo, Palermo, 8, 295-307. 5. Pritchett, W.L., Fisher, R.F., 1985. Properties and Management of Forest Soils, 2"^ Ed. Wiley & Sons, New York. 6. Soil Survey Staff, 1993. Soil Survey Manual, USDA Handbook, n. 18, Washington, D.C.. 7. Stevenson, F. J., 1982. Humus Chemistry, John Wiley & Sons, New York.
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8. Springer, U., Klee, J., 1954. Priifung der Leistungsfahigkeit von einigen wichtigeren Verfahren zur Bestimmung des Kohlemstoffs mittels Chromschwefelsaure sowie Vorschlag einer neuen Schnellmethode, Z. Pflanzenemahr. Dang. Bodenk., 64, 1-8. 9. Ciavatta, C, Govi, M., Vittori Antisari, L., Sequi, P., 1990. Characterization of humified compounds by extraction and fractionation on solid polyvynilpyrrolidone. J. Chromat. 509, 141-146. 10. Painter, P., Starsinic, M., Coleman, M., 1985, Determination of functional groups in coal by Fourier transform interferometry. In: Ferraro, J.R., Basile, L.J. (Eds.), Fourier Transform Spectroscopy. Application to Chemical Systems. Vol. 4. Academic Press, Orlando, FL, pp. 169-241. ll.Dazzi, C, 1996. Tassonomia del "Forest floor" e caratteristiche di fertilita di suoH forestall sotto pino d'Aleppo {Pinus halepensis Miller) e Cedro dell'Atlante [Cedrus atlantica (Endl.) Carriere], Boll. Soc. Ital. Sc. Suolo, Palermo, 8, 51-61. 12. Pinzari, F., Dell'Abate, M.T., Benedetti, A., Dazzi, C, 2002. Energy use in the A and B horizons of a soil under a pine and a cedar stand. In: Violante, A., Huang, P.M., Bollag, J.-M., Gianfreda, L. (Eds.), Proceedings of ISMOM Congress. 13. Schnitzer, M., Desjardin, J.G., 1965. Carboxyl and fenolic hydroxyl groups in some organic soils and their relation to the degree of humification. Can. J. Soil Sci. 45, 257-264. 14. Calace, N., Petronio, B.M., 1997. Characterization of high molecular weight organic compounds in landfill leachate: humic substances. J. Environ. Sci. Health, A32(8), 22292244.