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Soil Science Society of America Journal 66:948-958 (2002)
© 2002 Soil Science Society of America

DIVISION S-6—SOIL & WATER MANAGEMENT & CONSERVATION

Soil Quality in Mediterranean Mountain Environments

Effects of Land Use Change

M. Sánchez-Marañón*,a, M. Sorianoa, G. Delgadob and R. Delgadob

a Departamento de Edafología y Química Agrícola, CITE II-B, Universidad de Almería, 04120 Almería, Spain
b Departamento de Edafología y Química Agrícola, Facultad de Farmacia, Universidad de Granada, 18071 Granada, Spain

* Corresponding author (masanche{at}ual.es)


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
Unsuitable land use worldwide has led to environmental degradation. Land use effects on the environmental component of soil quality were assessed in 47 benchmark soils of three natural environments (Xerolls, Xerepts, and Cryepts) in southern Spain. Within each environment, soil groups with traditional land uses were compared with native sites. Soil quality changes were inferred by measuring the relative changes in chemical and physical soil properties. Irrigated terraces, rainfed cropland, and grassland, all within Xerepts, and grazed thyme scrub land were degraded with respect to native sites (significant differences at P < 0.05). In all cases, total porosity, macroporosity, and cation exchange capacity declined by more than 18% (-0.11 cm3 cm-3), 30% (-0.11 cm3 cm-3), and 48% [-10.5 cmol(+) kg-1], respectively. Except in irrigated terraces, soil erodibility increased by as much as 59% (+0.16 USLE factor). Substantial losses of soil organic C (37%, -14.7 Mg ha-1), available water (52%, -36.2 mm), total N (65%, -1.7 Mg ha-1), and rooting depth (68%, -39 cm) were also observed in grassland and thyme scrub land. These changes suggest adverse effects on environmental protection functions of soil because of soil compaction and/or elimination of structural binding agents. Scanning electron microscopy images confirmed these morphological changes in microaggregates. Similar changes did not occur in cropland and grassland within Cryepts or in planted pine forest land. Reforestation with pine (Pinus nigra Arnold, Pinus sylvestris L., and Pinus uncinata Mill. ex Mirb.) provided organic cements and fungal hyphae that reinforce soil aggregation as well as ecologically valuable humus. Because cropland and grassland were able to recover the natural soil properties without human activity, we deduce that Cryepts are resilient.


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
ENVIRONMENTAL DEGRADATION caused by unsuitable land use is a worldwide problem that has revived the issue of sustainability (Pierce and Larson, 1993; Zinck and Farshad, 1995; Hurni, 1997; Hebel, 1998). In this context, during the last decade, a soil quality concept has emerged that considers the soil's capacity to carry out its functions of biological production, environmental protection, and human health sustenance (Doran and Parkin, 1994; Bezdicek et al., 1996; Karlen et al., 1997; Herrick, 2000). Although there has been some debate regarding the concept's definition and quantification (Sojka and Upchurch, 1999; Singer and Ewing, 2000; Singer and Sojka, 2001), most soil scientists regard soil quality as something critical to human and environmental sustainability (Wander and Bollero, 1999; Webb et al., 2000; Southorn and Cattle, 2000). Land use profoundly influences soil quality at multiple levels of the agroecosystem. The latter can be quantified as relative changes of biological, chemical, and physical soil properties (Wang and Gong, 1998; Brejda et al., 2000; Schipper and Sparling, 2000; Chan et al., 2001). Soil properties that can be changed in a short time by land use are dynamic soil quality indicators (Carter et al., 1997).

Previous studies in mountain environments have shown that there is no universal standard of soil quality and that establishing a specific set of soil quality indicators (soil properties or indices) with universal meaning seems futile (Powers et al., 1998; Burger and Kelting, 1998; Schoenholtz et al., 2000; Pagedumroese et al., 2000). The direction and degree of soil quality changes in managed mountain ecosystems depend on climate, soil conditions, and land use. The majority of studies consider individual aspects of soil quality (biological productivity–environmental quality) or discuss them separately (Hajabbasi et al., 1997; Pennock and van Kessel, 1997; Wang and Gong, 1998; Perie and Munson, 2000; Islam and Weil, 2000). Emphasis on focused, component-type soil quality studies is understandable given that (i) there is no single pure state of soil and (ii) the status of soil properties and soil functions occasionally are contradictory (Sojka and Upchurch, 1999). As a result, soil quality indices often have limited value with respect to evaluating agroecosystem performance. Consequently, to avoid misuse of soil quality paradigms, an assessment of dynamic soil quality indicators influenced by land use requires (i) separation of soil environments with narrow climatic and soil ranges, (ii) identification of native and current soil properties within each environment, (iii) description of the relative quantified changes of individual soil properties, and (iv) inference of the state of individual soil functions.

Commission VIII "Soil and the Environment" of the International Union of Soil Scientists (IUSS) emphasized that one of the principal environmental problems facing the regions bordering the Mediterranean Sea is the threat of progressive soil desertification (De Kimpe et al., 1999; Eswaran et al., 1999). The principal causes of degradation of Mediterranean ecosystems are soil erosion and deficit of water in the soil (Torrent, 1995; Hill et al., 1996; Rubio and Calvo, 1996; De la Rosa et al., 1999). Mediterranean mountain ecosystems are of great value in slowing desertification because of the presence of the few woodlands in the region. The quality of the soil in these sites is an important determinant of the quality of the resulting environment. Human pressure has modified the land use of mountain environments with production systems, resulting in unknown ecological effects. Ecological sustainability of land use requires that the functions of soil—biogeochemical cycling, partitioning of water, storage and release, buffering and energy partitioning—be maintained (De Kimpe and Warkentin, 1998; Shaxson, 1998).

The Sierra Nevada Mountains National Park, in the most southwestern part of the Mediterranean Basin (Fig. 1) , has various natural environments containing native and managed soils. The Sierra Nevada is a protected natural area that provides Europe's first barrier against the advance of desertification from North Africa. The objective of this study is to examine from an environmental perspective and on a regional scale soil quality under the various land uses in Sierra Nevada, so as to obtain an idea of the ecological sustainability of each land use. It is our hope that this approach will provide information concerning the effects of land use on soil properties and processes whereby land use distribution can be optimized for improving ecological soil functions.



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  		Fig. 1. Location of studied soils in Sierra Nevada (southern Spain, Europe) showing their distribution in the natural soil environments of Xerolls, Xerepts, Cryepts, and Orthents.

 

   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS AND DISCUSSION
 CONCLUSIONS
 REFERENCES
 
The research sites were in a climatic and elevational gradient of the Sierra Nevada Mountains (Fig. 1). Soil parent materials are carbonated and silicic metamorphic rocks. Elevations range from 700 to 3482 m, increasing from the edges to the inner zone. The climate is temperate-cool Mediterranean with generally hot, dry summers and cold wet winters, but with substantial variations in temperature and moisture because of elevation. Average annual precipitation ranges from 500 to 1500 mm. Average annual temperature ranges from 17 to 0°C, ascending from 700 to 3482 m.

A field judgement sampling was performed. Forty-seven sample points in benchmark soils were selected from a previous soil survey to represent different land uses with similar parent material, relief, and climate. The soils were Haploxerolls, Calcixerolls, Haploxerepts, Dystroxerepts, Dystrocryepts, Calcixerepts, Xerorthents, and Cryorthents. At each sample point, a soil pit was dug to determine the depth of the soil horizons and to carry out discrete depth sampling by natural horizons. Intact cores in density cylinders and bulked samples were collected. Soil horizon samples were air dried, and, in the case of bulked samples, crumbled and sieved through a 2-mm screen. All samples were stored in suitable polythene receptacles.

The analytical characteristics of the soil horizon samples were determined in the following manner. We used the sieving and the pipette methods to determine the percentage by weight of gravel, sand, silt, and clay. Soil bulk density was measured by use of a cylindrical core of known volume, and particle density with a pycnometer. Soil water release was determined at -33 and -1500 kPa in a Richards membrane. We measured the pH (1:1) in water by potentiometry. We determined organic C content by the Tyurin method and CaCO3 content with a Bernard calcimeter. Total N was measured with the Kjeldhal method, and available P by ammonium acetate extraction followed by colorimetry. To determine exchangeable ions and base status, we used ammonium and sodium displacement solutions (Page et al., 1982; Klute, 1986).

Because of our special interest, the organic material and structural state of some A soil horizons were studied by more specific techniques. The composition of the soil organic matter was studied by the fractionation procedure of Duchaufour and Jacquin (1975); humic acid samples were characterized by elemental composition with a Perkin-Elmer 240C microanalyzer (Perkin-Elmer, Wellesley, MA), distribution of molecular size by gel permeation with Sephadex G-50 (Pharmacia Biotech Inc., Piscataway, NJ), and visible and infrared spectra registered with Shimadzu UV-240 derivatographic spectrophotometer (Shimadzu Corporation, Japan) and Perkin-Elmer 683 spectrophotometer (Oyonarte et al., 1994). The arrangement of particles and pores on the surface and within microaggregates metalized with gold was observed with a Hitachi S-510 scanning electron microscope (SEM; Hitachi Scientific Instruments, San Jose, CA) with an acceleration voltage of 25 kV.

Twelve common quantitative soil characteristics directly related to the ecological functions of environmental protection of the soil were used as indicators to infer soil quality (Doran and Parkin, 1996; Powers et al., 1998; Brejda et al., 2000; Singer and Ewing, 2000). Rooting depth was inferred from soil depth and available water was studied for the whole solum. For the top 20 cm of soil, total porosity was estimated from the particle and bulk density, and macroporosity from total porosity less microporosity, the latter was measured as water content at field capacity. Erodibility or USLE K factor (Wischmeier and Smith, 1978); pH; cation exchange capacity; base saturation; and content of organic C, total N, extractable P, and exchangeable K were also averaged for the upper 20 cm of soil. For selected A horizon samples, the quality of the soil organic matter (humus characteristics) and the structure (SEM fabrics) were also considered.

The investigation was designed to compare soil characteristics of mature sites with various land uses with those of comparable native sites (reference conditions; Pennock and van Kessel, 1997; Burger and Kelting, 1998); a t-test was used to verify whether there were statistically significant differences. Changes in these indicators can be used to determine whether soil quality, from an environmental viewpoint, is improving, stable, or declining with changes in land use (Brejda et al., 2000).


   RESULTS AND DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
CONCLUSIONS
 REFERENCES
 
Native Soils
The distribution of native soils in the landscape, generally influenced by the geological substrate and altitude, permitted the identification of four natural soil environments (Fig. 1). The first environment was defined by Typic Calcixerolls and Calcic Haploxerolls over limestones and dolomites and under evergreen–oak (Quercus rotundifolia Lam.) woodland. The remaining three, within a larger zone, corresponded to soils derived from periglacial colluviae of mica-schists and quartzites with a concentric elevational distribution: (i) Humic Haploxerepts and Humic Dystroxerepts under oak (Quercus pyrenaica Willd.) and evergreen–oak woodland from 700 to 2000 m, (ii) Xeric Dystrocryepts under juniper shrubs (Juniperus sabina L., Juniperus communis L, and Genista versicolor Boiss.) from 2000 to 2600 m, and (iii) Typic Cryorthents under pasture Festuca clementei Boiss. at the highest altitudes. Thus, the four environments would be represented by Xerolls, Xerepts, Cryepts and Orthents, respectively.

In Table 1 some representative soil profiles show the most relevant properties of the natural soil environments established. All the soils showed a horizon sequence A-AC/Bw-C or A-C. The B horizon is absent from soils of the greatest slope, lowest vegetation cover or those in the upper limit of the corresponding elevation interval. The Xerolls had a loamy or silty loam texture, a slightly or moderately alkaline pH and a base saturation of 100%. On the other hand, Xerepts, Cryepts, and Orthents were of sandy loam, acidic, and desaturated, at least in some part of the solum. The elevational gradient of climate and vegetation led to changes in the soil properties, as occurs in other similar areas (Dahlgren et al., 1997). With increasing elevation, that is from Xerepts to Orthents passing through Cryepts, soil depth, pH, base saturation, and clay content all decreased. The organic C content of A horizon was similar in Xerepts and Cryepts, both with mollic or umbric epipedon, decreasing drastically in Orthents (ochric epipedon). Total N, cation exchange capacity, water retention, and degree of structure also decreased because of elevation. These trends reveal appreciable differences between the native conditions of soil quality in these environments in terms of the intensity of the erosion, weathering, contribution of organic material, humification, and leaching processes. The maximum potential of the soil for performing ecological functions in each environment would be represented by the properties of its native soils.


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  		Table 1. Selected properties of some profiles of native soils representative of the four natural environments (Xerolls, Xerepts, Cryepts, Orthents) of the Sierra Nevada Mountains National Park in southern Spain.

 
Assessment of Dynamic Soil Quality Indicators under Different Land Uses
In the Orthents environment, soil and climatic limitations rule out its use for farming and forestry. Within each of the three natural environments below 2600 m, traditional land uses have modified the soil properties of the natural systems. Table 2 shows the mean values of the physical and chemical indicators of soil quality under the main types of land use in each environment. About 17% of the land area is covered by irrigated and dryland crops. The irrigated terraced croplands are used for the cultivation of fruit and vegetables. The rainfed old croplands of potatoes (Solanum tuberosum L.) and cereals have largely been abandoned. The rangelands grazed by migrating livestock (33% of the land) support a degraded vegetation of Festuca scariosa (Lag.) Ascherson & Graebner, Festuca elegans Boiss., and Festuca pseudoeskia Boiss. in grasslands and of Thymus mastichina L., Ulex parviflorus Pourret subsp. parviflorus, Halimium viscosum (Willk.) P. Silva, and Cistus laurifolius L. in thyme scrub lands. The pinewoods (17% of the land) are plantations of P. nigra, P. sylvestris, and P. uncinata more than 40 yr old. The sites that the best preserve native soils (evergreen–oak woodlands in Xerolls environment, oak and evergreen–oak woodlands in Xerepts environment, and mature juniper shrubs in Cryepts environment) cover 18% of the land. These showed the most favorable levels of soil quality indicators for the total environmental functioning of the soils in these mountain areas, and were used to establish the reference conditions of each environment. The remainder is miscellaneous area (urban, water, rock outcrop, and rubble land).


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  		Table 2. Indicator data (mean and standard deviation) of soil quality under various land uses within the natural environments of Xerolls (Environ1), Xerepts (Environ2), and Cryepts (Environ3) in the Sierra Nevada Mountains National Park in southern Spain.

 
The soil characteristics most sensitive to land use, showing significant differences at 0.05 probability level (t-test) with respect to the reference conditions, were organic C, total porosity, macroporosity, erodibility, rooting depth, available soil water, and cation exchange capacity. These dynamic soil attributes could be used as indicators of the ability of soils to carry out biocycling, partitioning, storage and release of water, and buffering of soil solution. The values of total N, extractable P, and exchangeable K varied greatly within each land use group so that, in many cases, mean values significantly different from those of the reference sites could not be found. Base saturation and pH tended to increase slightly in the sites with a greater intensity of use.

Irrigated Terraced Cropland
The soil properties of the irrigated croplands on stonewall terraces within the Xerepts environment (Table 2) were relatively unaffected because of suitable conservation management, including stonewall repair and a continuous supply of water, solid soil materials, and organic and inorganic fertilizers. The only characteristics showing significant differences with respect to the reference sites were total porosity, macroporosity, and cation exchange capacity. Their average values decreased by 18, 49, and 48% respectively. Structural decline due to compaction, typical of agricultural systems (Bezdicek et al., 1996; Ball et al., 1997), specifically affected the transmission and drainage pores (macroporosity was reduced by 0.18 cm3 cm-3) while microporosity increased (0.07 cm3 cm-3). Structural details from scanning electron microscopy (SEM fabric observations) revealed that primary compaction had occurred at the surface of the microaggregates but not at their core, where a reticulated fabric of clusters with ample pore space is maintained (Fig. 2a, 2b, 2c) . In addition, the reduction in water infiltration into soil that might be expected with this decline in macroporosity was largely counteracted by the level terraced surface, which favors infiltration over surface runoff. Consequently, the irrigated terraces do not seem to prevent the soil from carrying out its ecological functions and can be catalogued as sustainable land use (Shaxson, 1998). Only the decrease in buffering potential as a result of the cation exchange capacity being affected is cause for concern.




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  		Fig. 2. SEM fabrics in structural units of A soil horizon samples. From irrigated terraced cropland within the Xerepts environment: a) compacted subangular blocky aggregate with fissures at the interfaces of smaller units; b) surface fabric of previous aggregate, packing of plate shaped particles, well cemented; c) internal fabric of previous aggregate, reticulum of clusters, each formed from laminar particles and concentrically cemented to a central pore of 10 to 20 µm. From rainfed old cropland within the Xerepts environment: d) compacted granular aggregate; e) surface of previous aggregate, sand and silt particles with face-face joins; f) interior of previous aggregate, porosity of 10 µm generated by face-face and face-edge joins of silt-size phyllosilicates without cements. From thyme scrub land within the Xerepts environment: g) granular microaggregate composed of skeleton grains (silt size <50 µm) without apparent bonds. From oak and evergreen–oak woodland within the Xerepts environment: h) set of granular microaggregates constructed with skeleton grains (silt size) joined by organic cements and clustered around roots. From planted pine forest within the Cryepts environment: i) fine crumb aggregate ordered in smaller structural units pressed by abundant fungal hyphae; j) structural detail of previous aggregate, mycelium enveloping a group of silt-sized particles with clayey matrix and cements. From mature juniper shrub within Cryepts environment: k) aggregate composed of well cemented microaggregates, between whose weakness planes pores for the transmission of water (>30 µm) can be seen; l) detail of the internal fabric of the previous aggregate, silt particles joined by clay and edaphic cements forming layers and partitions.

 
Rainfed Old Cropland and Grassland
The changes in soil characteristics in the rainfed old croplands and in the grasslands within the Xerepts environment were relatively unfavorable for the carrying out the environmental protection functions of soils. Relative to reference conditions (Table 2), the capacity for cropland and grassland soils to distribute water at the surface has changed: macroporosity has been reduced from 0.11 to 0.16 cm cm-3 (without corrections of slope), resistance to erosion has decreased by a factor of two (USLE K factor increased 0.15), and cation exchange capacity has been halved [-11 cmol(+) kg-1], reducing the soils' ability to act as an environmental filters. The SEM images (Fig. 2d, 2e, 2f) confirmed the low porosity, which would inhibit water conductance, and, especially, the erosive vulnerability of microaggregates. The surface and internal fabrics of microaggregates are composed of laminar skeletal grains (Fig. 2e, 2f) with far fewer structural binding agents than are found in the microaggregates of irrigated terraces (Fig. 2b, 2c). Furthermore, in the grassland environment, there was a 37% decrease in organic C and a 52% decrease in available soil water, which suggests biological activity and storage and release of water have been greatly altered.

In Cryepts environment, in contrast to the Xerepts environment, soil quality indicators in rainfed old cropland and grassland environments were generally within the original reference levels (Table 2). In croplands, rooting depth even increased because of previous cultivation management. Thus, it is unlikely that the soils are functioning much below their potentials, which suggests a high level of soil resilience and a greater ability of the soils in the Cryepts environment to return to their original dynamic equilibrium after disturbance (Szabolcs, 1994; Seybold et al., 1999). The original degree of soil quality and the soils' capacity to sequester organic matter could explain this.

On the one hand, the original level of the soil quality indicators in Cryepts (represented by soil properties in mature juniper shrub) was lower than those in Xerepts (represented by soil properties in oak and evergreen–oak woodland); those characteristics could therefore have been reestablished more easily. Only the mean value of organic C expressed on a volume basis was slightly higher in Cryepts (42.6 Mg ha-1) than in Xerepts (39.7 Mg ha-1). This was due to the differences in soil bulk density (mean in Cryepts 1.14 Mg m-3, mean in Xerepts 0.92 Mg m-3) and not to the mean percentage by weight of organic C in fine earth (3.4% in Cryepts and 3.6% in Xerepts).

On the other hand, the recovery of the soil organic matter in rainfed croplands and grasslands after the cessation of human activity seems to be more favored in the Cryepts environment than the Xerepts environment. Average contents of organic C were 38.4 and 43.0 Mg ha-1 in croplands and grasslands of Cryepts and 30.2 and 25.0 Mg ha-1, respectively, within Xerepts. This is probably because of a more rapid recovery of the natural vegetation, less erosion, and a slower oxidation of the new organic material added to the soil resulting from the colder and wetter climatic conditions (Sojka and Upchurch, 1999; Seybold et al., 1999). Soil organic matter, because of its impact on other soil physical and chemical properties (Reeves, 1997; Wick et al., 1998; Bolinder et al., 1999; Liebig and Doran, 1999; Brejda et al., 2000), plays a crucial part in the resilience of these mountain soils with their homogeneous texture and mineral composition.

Thyme Scrub Land
Of the three environments, the soils of lowest quality were found in the grazed thyme scrub lands (Table 2). Total porosity declined 21 to 30%, macroporosity declined 39 to 49%, and cation exchange capacity declined 62 to 70% within Xerolls and Xerepts environments. In addition, erodibility in Xerolls, Xerepts, and Cryepts environments increased 59 to 115%, while all the following decreased: organic C content by 52 to 81%, total N content by 65 to 69%, available soil water by 65 to 79%, and rooting depth by 68 to 78%. The substantial decrease in rooting depth indicates that the process of accelerated erosion is the principle cause of surface soil property changes in these areas, basically affecting soil functions that support plant growth and water supply and retention. The inefficiency of the soil at carrying out these functions minimizes its resilience because of the low contributions of organic material, which, in turn, leads to increased structural vulnerability and consequent erosion. Figures 2g and 2h show SEM images of microaggregates of A horizons (10 cm from the surface) of thyme scrub land and oak and evergreen–oak woodland (reference conditions) in Xerepts environment. The type and class of structure are similar, as is the internal porosity because of packing of silt-size primary particles. However, the greater stability of the microaggregates of woodland soils resulting from the action of the organic matter and roots which act as binding agents between particles is clear, as is the low resistance (easy crumbling) of the microaggregates of scrub land whose particles are almost unattached.

Planted Pine Forest
The planted pine forest soils maintained the levels of all the soil quality indicators with respect to the reference sites. Soil properties in these forests did not show significant differences from soil properties in evergreen–oak woodland of Xerolls or from soil properties in mature juniper shrub of Cryepts (Table 2); soil depth (no erosive processes) and the quantity of organic material and pores (structural status) in the upper solum was preserved. The maintenance of soil depth, organic matter, and porosity are the most effective ways of reducing the negative effects of changes on soil quality by land use (Rhoton and Lindbo, 1997; Gilley et al., 1997). Our results support the idea that conifers are good pioneer tree species for improving low quality soils (Wang and Gong, 1998). However, reforestation with pine is a controversial issue in the Mediterranean region because pines are not native to many of these zones. Consequently, it was deemed necessary to look beyond the total values of organic C and porosity, analyzing the quality of the soil organic matter and the soil fabric in the A horizon (10 cm from surface) of the profiles under planted pine and native juniper shrub within the same soil environment.

Table 3 shows the characteristics of the soil organic matter grouped by mean values for planted pine forest and mature juniper shrub. The high content of free organic matter implies insufficient decomposition of plant material in both pine and juniper soils. The greater content of nonextractable humin under pine reveals greater stability of organomineral complexes in these soils. For the rest of the soil humus fractions and elementary composition of humic acids, no significant differences (t-test) were found between the two vegetation biotypes, whose values reveal a low degree of transformation and maturation of the soil organic matter. The most noteworthy effects of pine reforestation seem to be a certain, although not significant, trend towards an increase in the molecular size of the humic acids (% fraction Kav = 0) and aromaticity (E4). The spectrophotometric characteristics of the soil humic acids showed a similar composition for all of them (Fig. 3) . The second derivative spectra suggest the presence of perylenequinonic pigments of fungal origin (valleys near 570 and 615 nm), which are common in the soils of Mediterranean forest ecosystems (Oyonarte et al., 1994). In the infrared spectra, the bands corresponding to amides (1440 and 1660 cm-1) and to the constituent groups of lignin (bands between 1460 and 1030 cm-1) stood out. These results indicate that the humus characteristics of soil organic matter in planted pine forest are similar to those of the reference native soils, possibly because of the influence of analogous climatic and edaphic factors. Thus, organic material contributes in a similar way to soil quality in native and reforested soils.


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  		Table 3. Characteristics (mean ± standard deviation) of the soil organic matter in A horizon of the soils under planted pine forest (n = 4) and mature juniper shrub (n = 5) within the Cryepts environment in the Sierra Nevada Mountains National Park in southern Spain.

 


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  		Fig. 3. Selected bands of IR (a) and 2nd derivative visible (b) spectra of humic acids from planted pine forest (1) and mature juniper shrub (2) revealing a similar composition: amides (1440 and 1660 cm-1), lignins (bands between 1460 and 1030 cm-1), and perylenequinonic pigments (valleys 570 and 615 nm).

 
In the same way, the soil fabric confirmed that the structural quality of soil under pine and juniper is comparable. The SEM images of the aggregates (Fig. 2i, 2k) and their structural details (Fig. 2j, 2l) show structural units of similar type and degree. These are especially porous and stable in appearance, formed from a high content of skeleton grains with some clays and cements. In addition to the beneficial effect on the structure of the visible cemented zones (possibly organic cements), both in the soil aggregates of juniper and pine land, the latter also show fungal hyphae which envelop groups of particles, helping to hold them together and thus reinforcing the aggregates. The increase in fungal hyphae length has been noted previously in soil conservation areas (Karlen et al., 1999). Pine soils in the field had a layer of plant remains (O horizon with L-F subhorizons) which seems to have favored fungal activity.


   CONCLUSIONS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
REFERENCES
 
Four soil environments in Sierra Nevada Mountains National Park were identified, each with a different natural potential of soil quality, represented by Xerolls, Xerepts, Cryepts, and Orthents. The first three have been used by farming, forestry, and grazing. Total porosity, macroporosity, cation exchange capacity, erodibility, organic C, rooting depth, and available soil water were the physical and chemical characteristics that varied significantly with change from natural land use. These are the most sensitive dynamic indicators to assess effects of land use on soil quality from an environmental viewpoint and on a regional scale in these mountain environments.

We conclude that ecologically relevant destruction took place in the grazed thyme scrub lands of the three environments and in the rainfed old croplands and grasslands within the Xerepts environment. With respect to the corresponding native soil, erodibility increased by as much as 59% (+0.16 USLE K factor), total porosity declined by more than 18% (-0.11 cm3 cm-3), macroporosity declined by more than 30% (-0.11 cm3 cm-3), and cation exchange capacity declined by more than 50% (-11 cmol+ kg-1). Substantial losses of soil organic C (37%, -14.7 Mg ha-1), available water (52%, -36.2 mm), total N (65%, -1.7 Mg ha-1), and rooting depth (68%, -39 cm) were also observed. These changes suggest that serious alterations in the soil functions as a protection against desertification must have taken place, particularly in the partition of water at the surface and storage and release of soil water, all of which influence drought and erosion. Similar changes did not occur in rainfed croplands and grasslands within Cryepts or in any planted pine forests because of soil renewal in organic matter and porosity. Soil recuperation was natural in croplands and grasslands and induced by reforestation in pine forests. Irrigated terraced soils with conservation management only decreased in macroporosity and cation exchange capacity.

Soil aggregation and organic matter exhibited dynamic behavior with land use in these mountain environments. We gained the information we needed to assess dynamic soil quality indicators through scanning electron microscopy, which was used to study the soil fabric as a way to evaluate its structural quality and humus characterization as a means of estimating the quality of the soil organic matter. The decrease in pore space and pore sizes, the modification of the arrangement of the particles, and the loss of binding agents between particles were noteworthy morphological characteristics in the soils which declined in the soil quality attributes mentioned previously. The predominance of nonextractable humin and free organic matter, the presence of amides, lignins, and perylenequinonic pigments in the humic acids, and the conformation of porous structural units with silt-size grains, some clays, cements and fungal hyphae, both in the A horizons of native soils and in the soils of planted pine forest, confirmed that the latter's soil quality had recovered to its maximum natural potential.

Our approach has provided information regarding the ability of the soils to return naturally to their original status after disturbance, the ecological sustainability of the land uses, and the identification of reversible soil properties affecting the ecological functioning of the soil controlled by land use. This will be useful in a further studies on optimizing land use from an environmental perspective.


   ACKNOWLEDGMENTS
 
The authors thank Dr. G. Almendros, Centro Superior de Investigaciones Científicas, for his assistance with the characterization of humus samples; Dr. M. J. Singer, University of California, Davis, for documents regarding soil quality paradigm; and three anonymous reviewers for critical reviews and/or for helpful ideas that sparked additional thinking. Our appreciation to Mr. R. Abrahams, for translating the original manuscript into English, and to Mr. W.R. Luellen, Fine-Tuning Your Writing, LLC, for checking the SSSAJ format and style requirements. This study was supported by Ministerio de Ciencia y Tecnología, Spain, through Projects PB98-1361.0 and BTE2000-1152.

Received for publication January 26, 2001.


   REFERENCES
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS AND DISCUSSION
 CONCLUSIONS
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