O R I G I N A L P A P E R
Influence of plant species on physical, chemical and biological soil
properties in a Mediterranean forest soil
A. Pe´rez-BejaranoÆJ. Mataix-SoleraÆ R. ZornozaÆC. GuerreroÆV. ArceneguiÆ J. Mataix-BeneytoÆ S. Cano-Amat
Received: 30 December 2007 / Revised: 6 July 2008 / Accepted: 11 September 2008 / Published online: 20 December 2008 ÓSpringer-Verlag 2008
Abstract In semiarid ecosystems plant cover plays an important role in the improvement of physical, chemical and biochemical soil properties. With the aim of studying the influence of different plant species on soil properties, and establishing the relationships between them, 160 soil sam-ples from under four different plant species (Pinus halepensis, Quercus coccifera, Juniperus oxycedrus and
Rosmarinus officinalis) were taken in a forest area of the province of Alicante (SE Spain). The following soil pro-perties were analyzed in all soil samples: organic carbon content, microbial biomass, soluble organic carbon, aggre-gate stability, basal respiration, and some eco-physiological ratios. In addition, the near infrared spectra (NIR) of all soil samples were obtained to verify the similarities or differ-ences between soil samples under the four species. Some differences in parameters such as organic carbon content or basal respiration were found mainly between the group of
P. halepensisandQ. cocciferawith respect toJ. oxycedrus
and R. officinalis. Despite this, the high organic carbon content found under the four plant species showed an influence on the rest of soil properties. Moreover, using a discriminant analysis with factorial scores from NIR absorbance data did not result in a good classification of
samples in terms of the species, reflecting some similarities between them. Our results show that the high contents observed in some parameters under the four species, and the lack of significant differences in most of them, prove the important role of shrubland in semiarid conditions, it being capable of promoting good soil conditions.
Keywords Soil organic carbonMicrobial biomass Soluble organic carbonBasal respiration
Aggregate stabilityEco-physiological indicators
Pinus halepensis Quercus coccifera
Rosmarinus officinalisJuniperus oxycedrus
Introduction
It is well known that vegetation is a key factor in soil genesis. Furthermore, it provides soil protection and contributes to enhance soil properties (Garcia et al. 1994), which are influenced by the type of vegetation. In arid and semiarid ecosystems, where variation in the spatial and temporal availability of water and nutrients is extreme, dominant plants cause changes in soil properties that lead to complex local interactions between vegetation and soil (Wilson and Agnew 1992). Vegetal debris contributes to soil organic carbon which plays an important role in soil functions: it has nutrient and pollutant retention capacity, it improves soil structure and stability and it is source of nutrients and sub-strate for soil microbial community (Nambiar1997; Vallejo et al. 2005) and has influence over their distribution and activity. Furthermore, biologically active fractions of soil organic matter are important in understanding decomposi-tion potential of organic materials, nutrient cycling dynamics, and biophysical manipulation of soil structure (Franzluebbers et al. 2001). In this way, soluble organic Communicated by A. Merino and A. Rubio.
This article belongs to the special issue ‘‘Plant–soil relationships in Southern European forests’’.
A. Pe´rez-BejaranoJ. Mataix-Solera (&)R. Zornoza
C. GuerreroV. ArceneguiJ. Mataix-BeneytoS. Cano-Amat GEA—Grupo de Edafologı´a Ambiental—Environmental Soil Science Group, Department of Agrochemistry and Environment, University Miguel Herna´ndez, Avenida de la Universidad s/n, 03202 Elche, Alicante, Spain
carbon is a useful tool as it represents the fraction easily decomposable by microorganisms and is closely related with microbial growth and activity (Jandl and Sollins1997). It can be used as a measure of the labile fraction of total soil organic mater, which is actually available for soil microor-ganisms (West et al. 1992; Tanaka et al. 1998). Soil microbial properties have proved to be powerful indicators of soil quality. This has led to a great expansion of research into the possibilities of using such parameters to assess degradation processes and the feasibility of restoration strategies (Goberna et al.2006). A better understanding of the ability of plants to promote soil microbial processes in these conditions is necessary for successful soil reclamation (Garcı´a et al.2005). In this sense, soil microbial respiration is a useful index for measuring soil microbial activity (Nannipieri et al. 1990), being specifically the activity related to the decomposition of the organic matter. Fur-thermore, the metabolic quotient (qCO2: ratio between
microbial respiration and microbial biomass), is a valid indicator of the microbial efficiency in the use of energy and the degree of substrate limitation for soil microbes (Dilly and Munch1998; Moscatelli et al.2005).
In other way soil aggregate stability (AS) is one of the most important properties controlling plant growth in semi-arid Mediterranean environments (Hillel 1982; Letey
1985). Aggregates provide good conditions for plant growth in soil, related to porosity, water movement, air circulation and soil erosion resistance (Hillel1982; Singer et al.1992). Plants also affect the composition of the soil microbial community (Rolda´n et al. 1994), which can influence soil AS (Lynch1981; Caravaca et al.2002a). The agents responsible for AS are mainly organic, and hence biological in origin (Rolda´n et al. 2006). A few studies have demonstrated the important relationship between soil organic carbon and soil microbial biomass, and the influ-ence of microbial biomass over soil structure (Harris et al.
1964; Allison 1968; Insam and Domsch1988). Moreover, many authors have studied the positive relationship between AS and organic matter (Pagliai et al.1981; Clapp et al.1986; Oades1993; Cerda´1998). There is a need for studies that look for changes in soil AS in relation to vegetation species.
An alternative method to measure soil characteristics is using near infrared reflectance (NIR) spectroscopy. In the near-infrared region, the radiation is absorbed by the dif-ferent chemical bonds, such as C–H, N–H, S–H, C=O and O–H of the compounds present in the sample. Moreover, the radiation is absorbed in accordance with the concen-tration of these compounds. As a consequence, NIR spectra contain information about the composition of a soil sample. In this sense, many authors have observed that NIR spectra contain information about physical (Moron and Cozzolino
2003), chemical (Reeves et al. 1999; Chang and Laird
2002; Confalonieri et al. 2001; Zornoza et al. 2008) and biological (Reeves et al. 2000; Zornoza et al.2008) prop-erties of soils. Such variables are well known to indirectly influence soil microbial activity and plant growth (Odlare et al. 2005). According to this, the NIR spectrum can be considered as an integrative measure of many soil char-acteristics. With the aim of establish the influence of vegetation on soil properties, samples under four different plant species (Pinus halepensis, Quercus coccifera, Juni-perus oxycedrusandRosmarinus officinalis) from a forest region in SE Spain, were taken. These selected species are the most dominant in the study area and in Mediterranean semiarid forests. We could expect some differences because different inputs of organic debris would be expected as we are comparing different species.
The main objectives of this study were: (1) to determi-nate the influence of different plant species on some soil properties in a Mediterranean semiarid area, and (2) to investigate the relationships among them.
Materials and methods
Study area
The study area is located in the ‘‘Sierra de la Taja’’ (38°230N; 0°590W) near Pinoso, in the province of Alicante
(southeast Spain). The region has a semiarid Mediterranean climate with a mean annual precipitation of 260 mm and a mean annual temperature of 15.8°C ranging from 7.8°C in
January to 24.1°C in August (average 1961–1990). The
whole area of the ‘‘Sierra de la Taja’’ is approximately 500 ha. The samples were taken within 3 ha, representative of the whole area, with homogeneous conditions with respect to soil type, geology, plant distribution and slope. The soil is a Lithic Xerorthent (Soil Survey Staff 2006), developed over Jurassic limestone, and has a loamy texture consisting of 13% clay, 42% silt, and 45% sand. The mean content of carbonates is 37% and the pH 8.2.
The tree stratus of the area is formed by P. halepensis
Miller of approximately 40 years old, and the shrub vege-tation comprises mainly Q. coccifera L., R. officinalis
L. and J. oxycedrus L., which are the species where soil samples were taken for this study. The range of height for these species in the study area is: 6–12 m forP. halepensis, 1–2.5 m for Q. coccifera, 0.5–2 m for J. oxycedrus, and less than 1 m for R. officinalis. Brachypodium retusum
Soil sampling
In October 2004, 40 soil samples were collected from the first 5 cm of the mineral A horizon at micro-sites under each of the four species (P. halepensis, Q. coccifera, J. oxycedrusandR. officinalis). The sampling was done by selecting stems randomly, and taking one sample per stem (pooling soil material from ten different points around the stem). The distance between the stems sampled was around 10 m. Each sample contained around 1.2–1.5 kg soil. All soil samples were air- dried at room temperature (20–25°C)
to a constant weight, carefully sieved through a 2-mm mesh, and the coarser material discarded and the remaining fine-earth fraction gently mixed until it appeared to be homogeneous. For AS measurements, aliquots of the samples were sieved between 0.25 and 4 mm.
Soil parameters analysed
Soil organic carbon (Corg) was determined by wet
oxida-tion with 1 N potassium dichromate in acidic medium and back titration with 0.5 N ferrous ammonium sulphate, as described by Walkley and Black (1934). Microbial biomass carbon was determined using the fumigation-extraction (Cmic–FE) procedure (Vance et al.1987), and the
substrate-induced respiration (SIR) method (Cmic–SIR) (Anderson
and Domsch1978). For the SIR method, soil samples were amended with glucose (optimum rate 4.5 g glucose g-1 soil) and were incubated at 25°C and 50% of water holding
capacity (WHC) during 4 h. Hourly rates of evolved CO2
(ll CO2g-1h-1) and consumed O2(ll O2g-1h-1) were
measured using a respirometer (Micro-Oxymax, Colum-bus, USA). Two equations were used to calculate the Cmic–
SIR (mg C kg-1): CmicSIR mg C kg1
¼40:04 ðll CO2 g 1
h1Þ
þ0:37 ð1Þ
CmicSIR mg C kg1
¼19:6 ðll O2 g
1h1Þ ð2Þ
Results of Cmic–SIR using Eqs.1 (Anderson and
Domsch 1978) and 2 (Sparling and West 1990) were statistically similar (paired-samples t test P[0.05), and closely related (r= 0.989). According to this, only the Cmic-SIR obtained with the Eq.2was used. Soluble organic
carbon (Csol, extracted with K2SO40.5 M) was measured by
oxidation with K2Cr2O7and measurement of absorbance at
590 nm (Sims and Haby 1971). Basal soil respiration (CO2–C) was monitored over 4 days at 55% WHC and
25°C with a multiple sensor respirometer (Micro-Oxymax,
Columbus, OH, USA). AS was measured with the method of Rolda´n et al. (1994). This method examines the proportion of aggregates that remain stable after a soil sample (sieved between 0.25 and 4 mm) is subjected to an
artificial rainfall of known energy (270 J m-2). Five eco-physiological ratios were also calculated: the metabolic quotient (qCO2: CO2–C/Cmic–FE), Cmic–FE/Corg, Csol/Corg,
CO2–C/Corgand Cmic–SIR/Cmic–FE.
NIR spectra of soil samples were obtained with the following procedure: aliquots of around 50 g of soil sam-ples were placed in glass Petri-dishes, and scanned on reflectance mode from 12,000 to 3,800 cm-1. For these measurements we used a Fourier-Transform near infrared (FT-NIR) spectrophotometer (MPA, Bruker Optik GmbH, Germany), equipped with quartz beamsplitter and PbS detector. It is also equipped with an integrating macros-ample sphere and rotating smacros-ample cup, allowing the scanning of large areas of the samples. In each of the reflectance measurements, 64 scans were averaged. Sam-ples were measured in duplicate, increasing the surface of soil sample scanned. After this, they were averaged again. The time employed for the spectral measurement was approximately 1 min per sample. The resolution used for spectral analysis was 8 cm-1. Background corrections were made before each sample scan. The xscale of each NIR spectrum was transformed from wavenumber to wavelength, obtaining a spectrum with 1,000 data of absorbance comprising from 830 to 2,630 nm.
Statistical analyses
Statistical analyses were performed with the software SPSS for Windows, Version 14.0.
Results
Differences between species
Significant differences (P\0.01) between some of the species were obtained in Corg, CO2–C, Cmic–FE/Corg, qCO2, Cmic–SIR/Cmic–FE, whereas no differences were
found for the rest of soil parameters (Fig.1, Table1). Mean values of Corg, Csol, AS and CO2–C, even though
being high in all samples, are relatively higher in samples underP. halepensisandQ. coccifera. Samples under these two species also showed higher mean values of Cmic–SIR/
Cmic–FE ratio but a clear statistical difference was only
observed betweenP. halepensisandR. officinalis. Soil samples under Q. coccifera showed the highest values in qCO2 (P\0.01). The ratio Cmic–FE/Corg was
significantly higher in samples under R. officinalis than under Q. coccifera and P. halepensis. This species also showed the highest mean values of the CO2–C/Corgratio,
although with no statistically significant differences with respect to the others.
The NIR spectra of soil samples collected under four plant species did not show great differences. The DA carried out with the factorial scores of NIR absorbance data, that explained 91.1% of variance (60.3% for the first discrimi-nant function [DF1] and 30.8% for DF2) did not result in a good classification of samples in terms of the species (Fig.2, Table2). A 75% of soil samples taken underR. officinalis
andJ. oxycedruswere correctly classified. However, clas-sification was poorer forP. halepensisandQ. cocciferawith 32.5 and 50% of soil samples, respectively, correctly clas-sified, reflecting that soil samples beneath these two species show similarities with the other species studied.
Relationships between soil parameters
The analysis of correlations (Table3) showed that Corgand
Cmic–FE were closely related. Corg also showed positive
correlations with Csoland CO2–C, although the latter seems
to be indirectly influenced by the relationship between Corg
and Cmic–FE (as observed by analysis of partial
correla-tions controlling the Cmic–FE effect). Furthermore Csolwas
also positively correlated with Cmic–FE and AS. In
addi-tion, a strong correlation between soil microbial biomass analysed by SIR and by fumigation-extraction method was found (r= 0.735;P\0.001).
Fig. 1 Mean values (±standard
deviation) of soil parameters compared between species.
Correlations among the studied parameters for each species separately were also developed. Positive significant correlations between Corg, Cmic–FE and Csolwere observed
in all species. Samples underJ. oxycedrusandR. officinalis
also showed positive correlations between Corg and AS
(r= 0.505; P\0.01 for J. oxycedrus, and r= 0.669;
P\0.001 forR. officinalis), while CO2–C was positively correlated with Cmic-FE in samples under J. oxycedrus
(r=0.648; P\0.001) and P. halepensis (r=0.682;
P\0.001). Samples under P. halepensis showed the strongest correlations between all parameters. In these samples, all parameters had a positive correlation with Corg
except for eco-physiological ratios and AS. Moreover, Csol
was correlated with Cmic–FE (r=0.682; P\0.001) and
CO2–C (r=0.523;P\0.01).
Discussion
We have not found great differences in soil parameters between species. P. halepensis, the only tree species present in the study area, is not the only species which guarantees the best conditions for the improvement of soil properties. The lack of clear differences in most parameters and the high microbial biomass and activity under shrub species, suggest the importance of shrubs for soil func-tioning under semiarid conditions.
The property most clearly influenced by the different species has been the Corg. We observed higher Corg
con-tents underP. halepensisandQ. coccifera.This probably is related to higher aboveground and belowground biomass with increased C inputs in soil as litter and root exudates. In this sense, a positive relation between vegetation size and Corgcontent was observed. The influence of the
vege-tation type on soil properties due to different organic matter dynamics has been shown by many authors (e.g.: Fyles and Cote´1994; Dell’Abate et al.1999). As reported very often, plant cover has a considerable influence on the quantity of exudates and plant debris the soil receives (Caravaca et al.
Table 1 Mean values (±standard deviation) of eco-physiological ratios compared between species
Soil parameter J. oxycedrus R. officinalis Q. coccifera P. halepensis Significance
Cmic–FE/Corg(%) 1.6±0.5 bc 1.8±0.3 c 1.3±0.3 a 1.5±0.4 ab ***
Csol/Corg(%) 0.58±17.5 0.64±20.3 0.64±16.9 0.58±17.7 ns
qCO2(mgC gCmic-1h-1) 2.9±0.7 a 3.3±1.6 a 4.5±3.8 b 3.3±1.0 a **
Cmic–SIR/Cmic–FE 0.83±0.26 ab 0.80±0.14 a 0.91±0.34 ab 0.97±0.26 b *
CO2–C/Corg(lgC gCorg-1h-1) 45±14 61±33 56±44 47±16 ns
Different letters indicate significant differences (P\0.05) among values after one-way ANOVA
Corgorganic carbon,Cmic–FEmicrobial biomass carbon by fumigation-extraction method,Csolsoluble carbon,CO2–Cbasal respiration rate, qCO2CO2–C/Cmic–FE,Cmic–SIRmicrobial biomass carbon by SIR method,nsnon significant
Significant differences at *P\0.05; **P\0.01; ***P\0.001
Fig. 2 Discriminant analysis with factorial scores from NIR
absor-bance data of samples taken under four different species (n=160).
Black circles Pinus halepensis, white circles Juniperus oxycedrus,
open triangles Rosmarinus officinalis,grey squares Quercus coccif-era.Black starsdenote the centroids for each group
Table 2 Percentage of soil
samples classified at each plant species group using
discriminant analysis. Values in bold indicate the percentage of correctly classified. Leave-one out validation method was applied
Original Predicted group
R. officinalis P. halepensis Q. coccifera J. oxycedrus
R. officinalis 75 7.5 12.5 5
P. halepensis 17.5 32.5 40 10
Q. coccifera 15 30 50 5
2002b; Garcı´a et al. 2005). Vegetation exerts significant influence on the accumulation and turnover of soil organic matter (Quideau et al. 2001). Nonetheless, the organic carbon content in soil under all species is high in com-parison with other researches based on natural forests or afforestations with similar environmental conditions (Maestre et al. 2003; Maestre and Cortina 2004; Bastida et al.2008).
Associated to the high content of Corg, a high AS is
present in the soil under all species ([89%). Other studies on forest soils of the region have also shown high values in AS (Guerrero et al.2001; Mataix-Solera and Doerr2004). We observed a positive correlation between Corgand AS under J. oxycedrus and R. officinalis. This correlation was not found forP. halepensisandQ. coccifera, nor by pooling all soil samples from all species. These results suggest that an increment of organic carbon in the samples with lowest values cause an increment of AS. In any case the organic content in soil is high enough to guarantee a correct aggre-gation, independently of the concrete species. Furthermore, a positive correlation was found between AS and Csol, the
labile organic fraction, pointing to their action as being one of the key cementing agents (Elliot and Lynch1984). Garcı´a et al. (2005) also found a positive correlation between labile organic carbon fractions and the percentage of AS. This high stability of aggregates indicates a good soil structure under all species, and so increases resistance to erosive processes (Rolda´n et al.1994). It is also probable that the contribution of other factors in the aggregation of particles in this area is as important as organic matter content. The carbonates content, clays, iron compounds, microbial activity and the presence of divalent cations must play an important role in the AS of these soils (Hillel1982; Singer et al.1992; Porta et al.2003; Lynch and Brugg1985).
The positive correlation found between Corgand Csolhas
also been observed in other studies (Garcı´a et al. 2005;
Bastida et al. 2007; Zornoza et al. 2007a, b). Thus, although differences in Csolare not significant, plant
spe-cies with higher biomass show high values in this parameter. Csolis the labile fraction of organic matter, and
root exudates and the amount of belowground biomass fallen as litter can contribute to the increases in this organic fraction.
The high content of organic matter under all species contributes to high levels of microbial biomass. Soil microbial biomass and activity are influenced by the amounts of organic compounds (Goberna et al. 2006). A positive correlation between Cmic–FE and Corg was
observed, indicating that organic matter content controls microbial biomass (Zornoza et al. 2007b). Usually, this correlation is found in soils at equilibrium status (Anderson and Domsch1993). Despite the fact that the soil under all species showed similar levels in Cmic–FE, we observed that
Cmic–FE/Corg ratio was higher in samples under R. offici-nalisandJ. oxycedrus. This ratio suggests that under these species higher populations of microorganisms are main-tained per unit of organic carbon. Thus, this brings to light the great importance of shrubs under semiarid Mediterra-nean conditions, as a high proportion of microbial biomass pool is established under these species. This ratio is influ-enced by climatic conditions (Insam 1990). In soils from dry and arid zones, microorganisms can only metabolize organic compounds during the brief periods when soil moisture is increased after rainfall episodes. In these cases, microbial populations can survive in starving situations until climatic conditions are favourable, and as a result, the ratio Cmic–FE/Corg is higher. In contrast, in humid zones,
microorganisms are more active most of the time and die when easily mineralized compounds are exhausted. Thus, this ratio could be indicating a more active soil microbial community under Q. coccifera and P. halepensis. This hypothesis was supported by the data of soil microbial
Table 3 Correlation coefficients (rvalues) for relationships between the studied parameters for all soil samples (n=160)
Soil parameter Corg log Cmic–FE log Csol CO2–C AS Cmic–FE/Corg Csol/Corg logqCO2 Cmic–SIR/Cmic–FE
log Cmic–FE 0.66* –
log Csol 0.76* 0.60* –
CO2–C 0.41* 0.34 ns 0.37 ns –
AS 0.34 ns 0.32 ns 0.46* 0.19 ns –
Cmic–FE/Corg -0.51* 0.26 ns -0.29 ns -0.07 ns -0.19 ns –
Csol/Corg 0.07 ns 0.21 ns 0.69* 0.15 ns 0.26 ns 0.16 ns –
logqCO2 0.05 ns -0.26 ns 0.06 ns 0.76* 0.05 ns -0.29 ns 0.05 ns –
Cmic–SIR/Cmic–FE 0.50* 0.09 ns 0.26 ns 0.28 ns 0.04 ns -0.43* -0.12 ns 0.21 ns – CO2–C/Corg -0.28 ns -0.09 ns -0.14 ns 0.75* -0.06 ns 0.31 ns 0.12 ns 0.82* -0.04 ns
Corgorganic carbon,Cmic–FEmicrobial biomass carbon by fumigation-extraction method,Csolsoluble carbon,CO2–Cbasal respiration rate, qCO2CO2–C/Cmic–FE,Cmic–SIRmicrobial biomass carbon by SIR method,ASaggregate stability,nsnon significant
biomass measured by the SIR method (Cmic–SIR). With
this method, microbial biomass is estimated from active heterotrophic microorganisms, while the fumigation-extraction method (Cmic–FE) is based on the estimation of
microbial biomass from the extracted carbon constituent of most microorganisms. Thus, the ratio Cmic–SIR/Cmic–FE
can inform about the proportion of active versus latent microbial biomass. The Cmic–SIR must be lower or equal
to Cmic–FE. We observed that Q. cocciferaandP. halep-ensis showed the highest values of the ratio Cmic–SIR/
Cmic–FE. These results may support the hypothesis that
microbial community under these two species is more active. Furthermore, differences found in this ratio also suggest differences in the microbial community structure under each species. It has often been reported that plant species condition microbial communities by the release of radical exudates (Grayston et al.1998; Yang and Crowley,
2000; Hackl et al.2005).
Basal respiration was higher under P. halepensis and
Q. coccifera. High positive correlations between this parameter and Corg have been found, as has also been
observed in other researches (Saviozii et al.2001; Garcı´a et al.2005; Zornoza et al.2007a). This suggests that total organic carbon controls microbial activity. A high content in organic carbon in soil provides more nutrients for microbial communities development, and so, activity is increased (Incla´n et al., this issue). This assertion is sup-ported by the ratio CO2–C/Corg, which showed no
significant differences between species, reflecting an equilibrium between microbial activity and organic matter. Studied species have similar levels inqCO2 except for Q. coccifera. It has been proved that soil microorganisms divert more energy from growth into maintenance as stress increases, and thus qCO2 can be a sensitive indicator of
stress (Killham and Firestone,1984). However, in our case, we do not think the increments ofqCO2underQ. coccifera
imply a stress for the soils. Metabolic quotient (qCO2) has
also been used as an indicator of efficiency in the use of carbon (Insam and Haselwandter 1989). Soil under this species has the highest content in Csol. It is well known that
Csol is the most easily mineralizable fraction of organic
matter, source of energy and nutrients, directly used by microbial populations. Thus, the high content in Csol may
have led to increases in theqCO2.
All results shown here demonstrate the importance of shrubs in semiarid environments, and question the tradi-tionally believed statement that trees are more effective for the improvement of soil conditions in comparison to shrub species. Under a semiarid climate, the establishment of trees is difficult owing to the high water deficit most of the year, and the natural autochthonous vegetation normally corresponds to shrub communities. Restoration policies carried out during the 20th century in the semiarid areas of
the Mediterranean Basin were based on the reintroduction of conifers, primarily P. halepensis (Maestre and Cortina
2004). Concretely, 87% of forests in the semiarid sector of the province of Alicante are dominated by P. halepensis
(Bautista1999). This species was chosen preferentially for reforestation because of low-technical requirements for nursery production, high-resistance to adverse climatic and soil conditions, and because it was also considered a pioneer species, favouring the establishment of late-suc-cessional species (Ruiz de la Torre 1973). Nonetheless, these plantations have resulted in a low average survival as well as low cover and productivity. Moreover, apart from the fact that soil properties do not show great differences between pines and shrub,P. halepensisplantations tend to decrease species richness and overall plant cover in com-parison to shrublands (Maestre and Cortina2004).
The results of the DA carried out with NIR spectra of soils also supported the idea that there are no great dif-ferences between the four species. The NIR spectra offer an integrated vision of soil conditions, as well as synthesizing information regarding mineralogy, soil chemistry, organic matter and physical attributes (Cohen et al. 2005). Dis-criminant analyses have been successfully applied in NIR analysis for the classification of soil texture (Mouazen et al.
2005), to differentiate soil spectra into different water content groups (Mouazen et al.2006), to differentiate soils burned at different temperatures (Arcenegui et al. 2008) and to discriminate soils under different land use (Zornoza
2007). The fact that soil samples taken under the four studied species are not correctly classified by the DA indicates that soil spectra are quite similar between species, and so, organic compounds and mineral composition of soil under all these species are also similar.
Although pines can facilitate the reactivation of nutrients cycles and biological activities in degraded soils, native shrubs have also proved to be effective, with overall posi-tive effects not only in soil function, but also in the whole ecosystem functionality. P. halepensis plantations often homogenize the landscape and reduce habitat diversity, key factors that negatively affect plants and animals diversity (Lindernmayer and Hobbs 2004). Some studies have also found lower soil organic matter content, AS or cation exchange capacity in P. halepensis plantations than in adjacent shrublands dominated byQ. coccifera, J. oxyce-drus, Rhamnus lycioidesorPistacia lentiscus(Cortina et al.
restoration practices in semiarid landscapes, where trees development is limited due to climate constraints, is prone to stimulate successional processes, increase ecosystem resilience against disturbances (Maestre and Cortina2004) and improve soil conditions.
Conclusions
Despite the fact that there are some differences in soil properties between P. halepensis and Q. coccifera with respect toJ. oxycedrusandR. officinalis, the results of this study demonstrated that the soil beneath the four species has very good qualities. The high organic carbon contents found under all species control most soil properties, pro-viding an adequate microbial biomass and activity, as well as a high stability of aggregates. These results prove the important role of shrubland in semiarid conditions, being capable of maintaining similar soil conditions to those under a tree species such asP. halepensis.
Acknowledgments This research was supported by the CICYT
co-financed FEDER program (Reference CGL2004-01335/BOS). A. Pe´rez-Bejarano and V. Arcenegui acknowledge the grants from ‘‘Caja de Ahorros del Mediterra´neo’’. The authors also acknowledge the ‘‘Aula de la Naturaleza’’ of Pinoso and the cerdocarpa team for their collaboration and Frances Young for improving the English.
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