Influence of orientation, vegetation and season on soil microbial and biochemical characteristics under semiarid conditions

applied soil ecology 38 (2008) 62–70

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Influence of orientation, vegetation and season on soil microbial and biochemical characteristics under semiarid conditions F. Bastida *, G.G. Barbera´, C. Garcı´a, T. Herna´ndez Centro de Edafologı´a y Biologı´a Aplicada del Segura (CEBAS-CSIC), Department of Soil and Water Conservation and Waste Management, Campus Universitario de Espinardo, Spain

article info

abstract

Article history:

The semiarid climatic conditions of southeast Spain prevent the growth of stable vegetation

Received 29 May 2007

that would help preserve soil fertility. The aim of this work was to evaluate the effect of

Received in revised form

orientation, vegetation and season on soil microbial and biochemical properties in a

20 August 2007

semiarid climate. For this purpose four experimental sites were chosen: two southerly

Accepted 9 September 2007

orientated (SO) and two northerly orientated (NO). One of the SO sites had a vegetal cover of shrubs (SOS) and the other of Pinus halepensis (SOP). Similarly, one of the north-facing sites had a cover of shrubs (NOS) and the other of P. halepensis (NOP). The vegetal cover of north-

Keywords:

facing sites was about 75% while on the south-facing sites it was less than 25%. Orientation

Enzyme activities

significantly influenced the C fractions, which were higher on north-facing sites than on

Microbiological activity

south-facing sites. Microbial biomass C (MBC) reached 400 mg C kg

Semiarid

sites. Microbial activity was also greater in NO than in SO sites, the NOP site showing basal

Vegetation

respiration of 15 mg CO2-C kg 1

soil

1

day

1

1

soil

1

1

soil in north-facing

day 1, while basal respiration did not reach 2 mg CO2-

in either SO sites. Urease, b-glucosidase and N-a-benzoyl-L-argininamide

Orientation

C kg

Seasonal changes

(BAA) protease showed higher values in NO sites than in SO sites, being lower in autumn than in summer and spring in NO sites. As regards factor analysis, the biochemical and microbiological parameters measured were able to separate the soils. The effects of vegetation type were orientation-dependent. With a southerly orientation, vegetation type influenced microbial activity sufficiently to generate different grouping in the factor analysis. However, NO soils were more influenced by climatic factors than by vegetation type. The results allow to conclude that orientation plays an important role in the biochemical and microbiological properties of soil, outweighing, even, the effects of vegetation type and season. # 2007 Elsevier B.V. All rights reserved.

1.

Introduction

The predominant semiarid climatic conditions of SE Spain, with an average temperature of about 16–18 8C in the lowlands and a mean annual rainfall of 250–300 mm, prevent the growth of a stable vegetation. Such vegetation would not only

generate organic matter, improving soil quality and fertility, but would also slow down erosive processes and avoid the advance of desertification phenomena (Albaladejo and Dı´az, 1990; Bastida et al., 2006). In such conditions, several reforestation programmes have been carried out, usually using Pinus halepensis Millar because of its supposed good

* Corresponding author. E-mail address: [email protected] (F. Bastida). 0929-1393/$ – see front matter # 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.apsoil.2007.09.002

applied soil ecology 38 (2008) 62–70

adaptation to adverse conditions (Elena-Rosello et al., 1990). Vegetal development has a positive effect on the underlying microbial community and its activity, elevating soil organic matter content. The slope orientation affects the soil climate and the C mineralization rates (Raubuch and Joergensen, 2002). Under semiarid conditions, this factor may influence on vegetal and microbial development. The length and intensity of solar radiation, which is greater in south-facing zones, particularly in summer, can lead to moisture loss, and heavily influence vegetation development and soil microbial activity (Li and Sarah, 2003). However, Goberna et al. (2007) observed that the importance of orientation on soil microbial properties is lower than other factors, such as vegetation. The microbiological activity and development of a soil directly influences ecosystem stability and fertility (Smith and Papendick, 1993). Microbial biomass carbon (MBC) has been considered in the evaluation of natural and degraded systems (Ross et al., 1982). Activity of soil microorganisms can be evaluated by parameters such as ATP and respiration. Enzymes are biological catalysts of essential processes for the life of microorganisms and the simultaneous measurement of several enzyme activities may be useful for assessing soil microbial activity (Nannipieri et al., 1990). Among these activities are those related with the N (BAA-protease), P (phosphatase) and C (b-glucosidase) cycles (Harris and Birch, 1989; Garcı´a et al., 2002). The aim of this work was to evaluate, in semiarid conditions, the influence of orientation, season, and vegetation type and cover on soil microbial and biochemical characteristics, as well as to establish the suitability of such microbiological and biochemical parameters for discriminating between soils of different quality.

2.

Materials and methods

2.1.

Study area

The study area was the Ca´rcavo catchment, located in a central position of Murcia province (southeast Spain). This area is in the rain shadow of the Betic mountains and is one of the driest areas of Europe. Climate is predominantly semiarid Mediterranean. Average rainfall in Ca´rcavo catchment is 300.5 mm and the average temperature is 16.7 8C. In July and most of August, there is virtually no rain. Potential evapotranspiration, measured by the Thornthwaite method, is close to 900 mm. The soil type was a calcisol (Soil Survey Staff, 1998). Vegetation is mainly dominated by grasslands of Stipa tenacissima and shrubs with Rosmarinus officinalis, Cistus clusii, Thymus membranaceus. These shrublands are usually mixed with woodlands and forests of P. halepensis, which has been widely used in reforestation programmes (about 30–35 years old). Vegetal cover in northerly orientated (NO) sites was about 75%, while in southerly orientated (SO) sites it was less than 25%. In the SO sites with P. halepensis (SOP) survivorship was poor and only some trees remain, most of them not growing above 2 m. The understorey in this site was poor and mostly

63

dominated by S. tenacissima. In the NO site with P. halepensis (NOP) trees measured 4–6 m and the understorey was dominated by the perennial grass Brachypodium retusum. On southerly orientated sites without P. halepensis (SOS) the vegetation cover was very low and mostly dominated by Salsola genistoides a shrub with a root system well adapted to surviving on banks with high erosion rates. On the northfacing soil without P. halepensis (NOS), Salsola genistoides was also present but B. retusum covered a large part of the soil together with the shrubs R. officinalis and Cistus albidus. Soil samples were collected from these sites (in the top 15 cm of soil) in three different seasons spring (April), summer (July) and fall (October) in order to study the influence of climatic factors on the measured properties.

2.2. Physical, chemical, biochemical and microbiological parameters Electrical conductivity and pH were measured in a 1/5 (w/v) aqueous solution in a Crison conductivimeter and Crison 2001pHmeter, respectively. Total carbonates were measured in a Bernard calcimeter according to the method of Guitian and Carballas (1976). Texture analysis was performed using the method of Guitian and Carballas (1976). N was determined by the Kjeldahl method modified by Bremmer and Mulvaney (1978), bioavailable phosphorus following the method described by Olsen and Sommers (1982), and bioavailable potassium by displacement of the exchange cations by ammonium. The soil texture, physical–chemical and chemical parameters are shown in Table 1. Water holding capacity (WHC) was calculated from the amount of water retained by the saturated soil with no drainage at 1/3 atmospheric pressure. Total organic carbon (TOC) was determined by oxidation with K2CrO7 in an acid medium and evaluating the excess of dichromate with (NH4)2Fe(SO4)2 (Yeomans and Bremmer, 1989). Humic substances C was determined in a filtered and centrifuged 1:20 (solid:liquid) sodium pyrophosphate extract (pH 9.8) with a Shimadzu TOC5050A Total Organic Carbon Analyzer. Watersoluble carbon (WSC) was measured in a 1:5 (solid:liquid) distilled water extract in a Shimadzu TOC5050A Total Organic Carbon Analyzer after shaking for 2 h and filtering through ashless filter paper (Albet 145 110). Microbial Biomass Carbon (MBC) was determined by the fumigation-extraction method (Vance et al., 1987) measuring the C extracted by K2SO4 in a Shimadzu TOC5050A Total Organic Carbon Analyzer. Microbial biomass C was calculated by multiplying the extracted carbon by 2.66. Soil respiration was measured in 15 g of soil moistened to 65% of field capacity in hermetically sealed flasks incubated at 28 8C for 30 days. The CO2 produced during incubation was measured daily during the first week and then weekly using an infrared gas analyser (Toray PG-100, Toray Engineering Co. Ltd., Japan) (Garcı´a et al., 2003). The ATP was extracted following the method described by Webster et al. (1984) and measured by the luciferine-luciferase assay in a luminometer (Optocom 1, MM Instruments Inc.). The method described by Garcı´a et al. (1997) was used to measure dehydrogenase activity, reducing INT (2-p-iodo-3-nitrophenyl-5-phenyl tetrazolium chloride) to INTF (iodonitrophenyl formazan), which was measured in a spectrophotometer at

64

applied soil ecology 38 (2008) 62–70

Table 1 – Soil physical and chemical characteristics South orientation a

North orientation a

a

NOPa

SOS

SOP

NOS

Texture % Coarse sand (g 100 g 1) % Fine sand (g 100 g 1) % Silt (g 100 g 1) % Clay (g 100 g 1)

Sandy loam 0.56 68.05 12.24 19.15

Sandy clay loam 3.03 54.78 15.12 27.07

Sandy loam 3.41 65.92 12.24 18.43

Sandy clay loam 0.81 59.16 18.72 21.31

pH Electrical conductivity (dS m 1) (25 8C) Total carbonate (g kg 1) Total nitrogen (g kg 1) Available K (mequiv. 100 g 1) Available P (mg kg 1) Water-holding capacity (g 100 g 1)

7.55a 0.24b 565.00a 1.00a 2.72ab 5.0a 40.64a

7.72a 0.22b 542.00a 1.10a 2.19a 19.4ab 40.85a

7.99ab 0.19ab 578.00a 1.60b 3.58b 17.5ab 43.70b

8.02b 0.15a 554.00a 1.10a 3.79b 35.8b 46.48b

a

Treatment: SOS, south-facing soil without Pinus halepensis; SOP, south-facing soil with P. halepensis; NOS, north-facing soil without P. halepensis; NOP, north-facing soil with P. halepensis. For each parameter, data followed by the same small letter are not significantly different according to the LSD test (P  0.05).

490 nm. Protease activity (BAA-protease) was determined using N-a-benzoyl-L-argininamide (0.03 M) as substrate and measuring the NH4+ released in the hydrolysis reaction (Nannipieri et al., 1980). Alkaline phosphatase and b-glucosidase activities were determined following the methods reported by Tabatabai and Bremmer (1969) and Eivazi and Tabatabai (1987), respectively.

and SOP showed the lowest WSC values, which were lower in summer than in spring and autumn (Table 2). It is important to note that the seasonal trends in the reforested zones (SOP and NOP) were similar, TOC increasing in autumn and WSC decreasing in summer. However, the natural shrub sites (SOS and NOS) did not show similar seasonal trends.

3.2. 2.3.

Microbiological and biochemical parameters

Statistical analysis

Data were submitted to three-way ANOVA. In order to determine pair-wise differences by post-hoc tests, the data were submitted to one-way ANOVA at each sampling time. Then, the four levels corresponding to 2 orientation  2 vegetation types were treated as a single treatment with four levels. The post-hoc test applied was Fisher’s least significant difference (LSD) method. A factor analysis was carried out for all the microbial, biochemical and C fraction variables measured to explore whether the measured variables showed covariant trends attending to vegetation, season and orientation. An ANOVA was also carried out to test differences between treatments in the factor analysis (using to factor 1, which explains the highest variability). Results for the biochemical, microbiological and C fractions were submitted to correlation analysis. The software used for the statistical analysis was Statgraphics Plus 2.1.

3.

Results

3.1.

Carbon fractions and WHC

WHC in SOS and SOP were significantly lower than those in NOS and NOP (Table 1). TOC, humic substance C and WSC were significantly affected by the orientation (P < 0.01) (Table 2). Significantly higher TOC values were observed in NOS and NOP than in SOS and SOP (Table 3). As regards humic substance C, SOS showed the lowest values, followed by SOP, while NOS and NOP showed the highest values (Table 3). WSC was significantly influenced by season (P < 0.01) (Table 3). SOS

MBC, basal respiration and ATP were significantly influenced by orientation and vegetation type (Table 2). Microbial biomass C and ATP were significantly influenced by season (P < 0.01) (Table 2). NO soils (NOS and NOP) showed significantly higher MBC values than south-facing soils, the highest MBC values being recorded in NOP (Fig. 1). NOS and NOP showed significantly higher ATP values than SOS and SOP (Fig. 1). Except NOS, all the zones showed lower ATP values in summer. As regards basal respiration, there were significant differences between the north and south-facing sites, the values being much higher in the former (Fig. 1). No seasonal trends were seen for basal respiration. There was a significant interaction between vegetation type and season for MBC, basal respiration and ATP content (Table 2). b-Glucosidase, BAA-protease, phosphatase and dehydrogenase activities were significantly influenced by orientation, vegetation type and season (Table 2). There was a significant interaction between orientation, vegetation type and season for these enzyme activities (Table 2). SOS showed significantly lower dehydrogenase activity than the other sites (Fig. 1). The activity of this enzyme increased in spring in both SOS and SOP. b-Glucosidase, BAA-protease and phosphatase behaved similarly, all showing higher activity in NOP and NOS and decreasing in autumn, especially on the north-facing sites (Fig. 1). In spring and summer, NOS showed the highest values of b-glucosidase and BAA-protease activities (Fig. 1).

3.3.

Correlation analysis

When considering data from the three seasons, positive correlations (P < 0.01) were observed between TOC and humic

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applied soil ecology 38 (2008) 62–70

Table 2 – Results of three-factors ANOVA (orientation, vegetation type and season) for carbon fractions and microbiological parameters Factorsa

Or V S Or  V Or  S VS Or  V  S

TOCb

WSCb

MBCb

BRESPb

Fc

Pd

F

P

F

P

F

P

F

94.46 7.16 2.63 2.83 1.45 10.29 5.41

<0.01 0.013 0.09 0.10 0.25 <0.01 0.01

1961.68 139.43 98.68 155.05 52.69 189.60 106.32

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

74.05 1.23 15.37 2.22 0.68 3.18 57.9

<0.01 0.28 <0.01 0.15 0.51 0.06 <0.01

407.75 36.79 25.49 10.70 18.31 28.55 26.16

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

1644.40 11.07 1.86 6.00 0.38 5.30 4.78

Factorsa

Or V S Or  V Or  S VS Or  V  S

CHUMb

DHb

ATPb

bGLUCb

BAAb

P <0.01 <0.01 0.17 0.02 0.68 0.01 0.02

PHOSPb

F

P

F

P

F

P

F

P

F

70.65 29.57 8.98 22.00 5.32 6.90 21.99

<0.01 <0.01 <0.01 <0.01 0.01 <0.01 <0.01

594.12 14.22 121.48 0.06 88.84 53.43 35.09

<0.01 <0.01 <0.01 0.80 <0.01 <0.01 <0.01

1240.09 67.41 53.78 30.06 36.34 64.74 45.30

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

832.03 73.41 18.04 28.56 23.37 13.55 30.46

<0.01 <0.01 <0.01 <0.01 <0.01 <0.01 <0.01

573.45 7.26 84.93 7.47 43.46 6.53 0.82

P <0.01 0.01 <0.01 0.01 <0.01 <0.01 0.45

a

Or, orientation; V, vegetation type; S, season Parameter: TOC, total organic C; CHUM, humic substances C; WSC, water-soluble C; MBC, microbial biomass C; BRESP, basal respiration; DH, dehydrogenase activity; BAA, BAA-protease; bGLUC, b-glucosidase activity; PHOSP, phosphatase activity. c F-ratio. d P values. b

substance C, MBC, basal respiration and ATP (Table 4). WSC was positively correlated with MBC and basal respiration. MBC showed a high correlation coefficient (greater than 0.70 at P < 0.001) with indicators of microbial activities (basal respiration, ATP and hydrolases). The enzymatic activities showed significant positive correlations with TOC, WSC, MBC, basal respiration and ATP (Table 4). However, in summer, there was no significant correlation between hydrolases (BAA-protease, b-glucosidase and phosphatase)

and humic substance C. WHC showed significant positive correlations with all biochemical and microbiological parameters (Table 4).

3.4.

Factor analysis

The factor analysis placed SOS and SOP on the negative side of factor 1 (which explained 71.45% of the variability observed), significantly (using to factor 1) separated from

Table 3 – Carbon fractions in the soils under study at different seasons South orientation a

SOPa

SOS

1

Total organic C (g 100 g ) Humic substances C (mg C kg 1 soil) Water-soluble C (mg C kg 1 soil)

Spring

Summer

Fall

Spring

Summer

0.34a 19.22a 28.98a

0.63a 20.80a 13.36a

0.40a 17.04a 36.38a

0.66ab 222.74b 37.57ab

0.59a 369.33b 24.45b

Fall 0.75b 341.24b 40.08a

North orientation a

NOPa

NOS

Total organic C (g 100 g 1) Humic substances C (mg C kg 1 soil) Water-soluble C (mg C kg 1 soil)

Spring

Summer

Fall

Spring

Summer

1.10c 868.67c 60.10bc

1.05b 819.15c 56.57d

0.90b 749.8c 52.85b

0.83bc 907.7c 67.45c

0.93b 905.33c 30.78c

Fall 1.43c 872.20c 67.85c

For each parameter, treatment and season, data followed by the same small letter are not significantly different according to the LSD test (P  0.05). a Treatment: SOS, south-facing soil without P. halepensis; SOP, south-facing soil with P. halepensis; NOS, north-facing soil without P. halepensis; NOP, north-facing soil with P. halepensis.

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applied soil ecology 38 (2008) 62–70

Fig. 1 – Microbial biomass carbon (MBC), basal respiration, ATP and enzyme activities in the soils studied at different seasons. For each parameter and season, data followed by the same letter are not significantly different according to the LSD test (P = 0.05).

the other sites (NOS and NOP) (Fig. 2). Factor 2 explained 9.16% of the variability observed. Hydrolases and basal respiration had the greatest weight in factor 1 (Fig. 2). Factor 1 was significantly (P < 0.01) influenced by orientation and season, but not by vegetation type. Significant differences were observed between the grouping of SOS and SOP, but not between the different seasons for these treatments (Fig. 2). Differences between seasons for NOS and NOP treatments were significant, except NOP spring and fall (Fig. 2).

4.

Discussion

The greater vegetation cover and biomass on the NO sites was responsible for the TOC content being greater in the SO sites (SOS and SOP). The most active part of the TOC from a biochemical point of view is the WSC, which contains labile substrates which act as energy source for microbial metabolism (Cook and Allan, 1992). These water-soluble derivates may arise from degradation of the more stable compounds of the TOC in the soil (Campbell and Zentner, 1993) or from the

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applied soil ecology 38 (2008) 62–70

Table 4 – Correlation matrix between soil parameters at three seasons (A) and in summer (B) TOC

CHUM

WSC

MBC

BRESP

ATP

DH

BAA

bGLUC

0.78*** 0.61*** 0.75*** 0.74*** 0.74*** 0.66*** 0.65*** 0.67*** 0.65*** 0.69***

0.45** 0.58*** 0.78*** 0.49** 0.60*** 0.69*** 0.74*** 0.59*** 0.69***

0.81*** 0.71*** 0.72*** 0.57*** 0.73*** 0.66*** 0.57*** 0.62***

0.77*** 0.75*** 0.65*** 0.76*** 0.75*** 0.78*** 0.79***

0.68*** 0.58*** 0.90*** 0.89*** 0.80*** 0.84***

0.58*** 0.58*** 0.51*** 0.37* 0.63***

0.55*** 0.61*** 0.55*** 0.49**

0.94*** 0.78*** 0.65***

0.85*** 0.67***

PHOSP

A CHUMa WSC MBC BRESP ATP DH BAA bGLUC PHOSP WHC

TOC

CHUM

WSC

MBC

BRESP

ATP

DH

BAA

0.78** 0.77** 0.86** 0.94*** 0.77** NS 0.79** 0.69* 0.67*

NS 0.65* 0.75** NS NS NS NS NS

0.92*** 0.84*** 0.90*** 0.65* 0.88*** 0.89*** 0.90***

0.82** 0.77** NS 0.82** 0.88** 0.86**

0.87*** NS 0.85*** 0.70* 0.70*

NS 0.95*** 0.85*** 0.85***

0.60* NS NS

0.86*** 0.87***

0.78***

bGLUC

B CHUM WSC MBC BRESP ATP DH BAA bGLUC PHOSP

0.95***

*, ** and *** significant at P  0.05, P  0.01 and P  0.001, respectively. NS, not significant. CHUM, humic substances carbon; TOC, total organic carbon; WSC, water-soluble carbon; MBC, microbial biomass carbon; BRESP, basal respiration; DH, dehydrogenase activity; BAA, BAA-protease activity; bGLUC, b-glucosidase activity; PHOSP, phosphatase activity; WHC, water holding capacity. a

root exudates released in the plant root system (Cook and Allan, 1992). Whatever the case, its content was greater in NOS and NOP and reflects the fact that vegetal development in these zones continuously contributes energetic substrates for use by microorganisms. Given the ‘‘metabolic function’’ that substrates in the WSC have for microorganisms, it is to be expected that in north-facing areas, with greater waterholding capacity, the microbial biomass C will be significantly higher than in south-facing sites, pointing to the greater size of the microbial community. The positive correlation coefficients between WSC and MBC (Table 4) support the idea that the WSC contains substrates capable of sustaining microbial development (Garcı´a et al., 2002). South orientation means that frequently microbial activity comes to a halt due to desiccation, thus the turnover processes during the moist times must be faster. This fact could be related to the vegetal production and C accumulation in humid seasons. It may be typical of soils from arid climates that decomposition is accelerated as soon as moisture conditions are adequate (Insam, 1990). In this work, we have observed (both for north and south-facing soils) an increase in water-soluble C in spring and autumn, comparing to summer. Indeed, WSC was significantly influenced by season. These results could indicate that seasonal changes are related to accumulation of this C fraction in soil. WHC in north orientated sites was significantly higher than in south sites. All the parameters that act as indicators of microbial activity (basal respiration, ATP and dehydrogenase activity) showed lower values in SOS and SOP than in NOS and NOP. These findings reflect that moisture content and WHC of

a soil influence the size of microbial populations and their activity (Bastida et al., 2007). In this sense, positive correlation coefficients between WHC and microbiological parameters support this theory. Some authors observed significant decreases in parameters such as MBC, respiration, ATP, phosphatase activity and dehydrogenase activity in air-dried soil samples (Raubuch et al., 2002; Devi and Yadava, 2006). No significant differences were found between the respiration of NOS and NOP. This fact could indicate that the organic matter provided by the vegetation in these two zones is similar in quantity and/or quality. Indeed, vegetation type did not significantly influence on WSC and TOC. This is not surprising if we bear in mind that the vegetal remains of P. halepensis have a high lignin content (31.63%) that is difficult to degrade (Rovira and Vallejo, 2002). In this sense, the organic matter preferentially used as substrate for respiration does not come from this vegetal species but from the shrubs of NOP, which is similar to that found in NOS. Season significantly influenced the values of some parameters. Of interest is the fact that MBC and ATP generally showed lower levels in summer than in spring and autumn. Other authors have observed that microbial biomass decreases in the dry season (Devi and Yadava, 2006) when rainfall is absent and soil water content minimal. Associated with microbial proliferation that occurred in the north-facing zones, the hydrolases analysed (BAA-protease, bglucosidase and alkaline phosphatase) also showed higher values in north than in the south-facing sites, indicating that orientation significantly influences the specific microbial activity of soil. There is no doubt that microbial development

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applied soil ecology 38 (2008) 62–70

Fig. 2 – Factor analysis from biochemical and microbiological properties in four soils analysed. TOC, total organic carbon; CHUM, humic substances carbon; WSC, water-soluble carbon; MBC, microbial biomass carbon; BRESP, basal respiration; DH, dehydrogenase activity; BAA, BAA-protease activity; bGLUC, bglucosidase activity; PHOSP, phosphatase activity. Symbols: (&) SOS; (~) SOP; (*) NOS and (^) NOP. Colour symbol indicates season: white (spring), grey (summer) and black (fall). Data followed by the same small letter are not significantly different according to the LSD test (P = 0.05).

would not be possible if there were no enzymatic machinery associated with the C, N and P cycles (Nannipieri et al., 1990) that sustain the viability of the microorganisms and plants. As regards the seasonal variability in the hydrolases, it was observed that all three decreased in autumn (especially in the NO soils). This might have been due to: (i) a decrease in microbial biomass, (ii) a decrease in the concentration of substrate available to this enzyme, and (iii) a change in temperature in autumn. Nevertheless, results indicate that MBC did not change in autumn and WSC not always decreased in such season. Thus, a temperature change could be probably the responsible for enzyme activity decreasing in autumn. The existence of a general reduction in microbiological activity in NOP during summer could be due to the stressed conditions in this season which is responsible to leave lost that can modify the soil chemico-physical conditions. Free enzymes in the soil can be rapidly denaturalised, degraded or irreversibly inhibited. However, a certain proportion of these free enzymes may be stabilised by adsorption to clays or humic substances in the soil (Burns, 1982), allowing

the microbial activity to persist in the soil (Marx et al., 2005). The possible interaction between humic substances and different enzymes is supported by the high correlation coefficients found between humic substance C and such hydrolases (Garcı´a et al., 1994), as was observed in this study (Table 4). However, in summer, no significant correlation coefficients were found between humic substance C and the hydrolases studied (BAA-protease, b-glucosidase and alkaline phosphatase). In the dry season, when climatic conditions are adverse and moisture low, the cell walls and membranes of microrganisms may undergo lysis (Kra¨mer and Green, 2000). Probably, cell lysis releases free extracellular enzymes not still associated to mineral and organic components, and, for this reason the correlation does not exist. A factor analysis carried out with all the samples and the biochemical and microbiological parameters analysed provided an overall view of the importance of orientation, vegetation and season on the biochemical and microbiological status of the soils in question. Factor 1 explained 71.45% of the variability observed and was related with the soil microbial activity, due to the high weight of hydrolase activity and basal respiration on this factor. The north-facing zones (NOP and NOS) were clearly differentiated from their south-facing equivalents (SOP and SOS) (Fig. 2), emphasising the importance of orientation on soil microbial activity. The distance observed between the soil with P. halepensis and soil without this tree species was lower than the distance between south and north orientation samples (Fig. 2). In addition the distance between samples with different orientation was higher than the distance between samples taken in different seasons. These findings indicate that orientation plays an important role in biochemical and microbiological properties, outweighing, even, the effects of vegetation structure and season. Indeed, three-way ANOVA showed that vegetation type did not significantly affect factor 1, which is related to microbiological activity. However, interactions of vegetation type with other factors (orientation or season) did so significantly. Within the south-facing zones (negative side of factor 1), there were significant differences between SOS and SOP (Fig. 2), although it should be emphasised that no seasonal differences were observed in the same treatment. This means, that given a southerly orientation, vegetation type influenced the properties analysed sufficiently for the factor analysis to determine a significantly distinct grouping between soils with different vegetation structure, while the season had a minor influence. Possibly south orientation was so constricting and it did not allow a seasonal variability. However, in north sites differences were observed between seasons and between soils with P. halepensis (NOP) and without this vegetal species (NOS). A seasonal pattern for north orientated soils was observed: NOP and NOS samples in autumn were separated from other seasons in factor analysis. These suggest that the north-facing soils are more influenced by vegetation structure and climatic factors (season) than their south-facing counterparts. The differences between south and north orientation are in the organic matter quantity and in the microbial biomass content and activity. These two aspects make the north orientation soil more dynamic and then more sensitive to management and environmental factors (season). Changes in soil microbial activity due to

applied soil ecology 38 (2008) 62–70

seasonal factors (temperature and moisture) have been pointed widely in bibliography (Schimel et al., 2004; Chen et al., 2004). However, only heavy and significant actions could have reasonably effects on south orientated soils. In this sense, Bardgett et al. (1999) observed that temperature change had a little effect on microbial communities and activity of poor soils.

5.

Conclusions

The biochemical and microbiological parameters measured were able to separate soils according to their quality. Factor analysis was seen to be a valid tool for establishing groups according to the biochemical and microbiological properties. Basal respiration and hydrolases activities were identified as the most important parameters in such analysis. The type of different plant communities with P. halepensis or xerophytic shrub influenced biochemical and microbiological properties of south-facing soils. Vegetal structure may have a negative or positive effect on north-facing slopes, depending on the parameter considered. In semiarid conditions, the effect of orientation on the biochemical and microbiological properties of soil outweigh the effect of vegetation type. North orientation favours a higher microbial biomass and activity than a south orientation. In addition, north orientated soils are more sensitive to seasonal changes than southern ones.

Acknowledgements This research was funded by the INDEX Project (Indicators and Thresholds for Desertification, Soil Quality, and Remediation) funded by the European Commission. Felipe Bastida wishes to thank the Spanish Ministry of Education and Science for his FPU fellowship. This research is part of the RECONDES (Conditions for Restoration and Mitigation of Desertified Areas Using Vegetation) project funded by the European Commission. G.G. Barbera´ was supported by I3P Programme of CSIC.

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