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    Phosphorus content in five representative landscape units of the Lomas de

    Arequipa (Atacama Desert-Peru)

    Andre Fabre a,*, Thierry Gauquelin a, Francisco Vilasante b, Aldo Ortega b, Henri Puig a

    a Laboratoire Dynamique de la Biodiversite, 29 Rue Jeanne Marvig, 31055 Toulouse Cedex, Franceb Universidad Nacional San Augustin, Instituto Regional de Ciencias Ambientales, Casilla 985, Arequipa, Peru

    Received 27 September 2004; received in revised form 21 September 2005; accepted 12 October 2005

    Abstract

    Phosphorus forms and content were studied in soils of the Lomas de Arequipa (Atacama desert, Peru) using a fractionation method. These

    Lomas are small hills periodically submitted to the El Nino-Southern Oscillation (ENSO) which causes heavy rainfall. Sample soils were

    randomly selected in five landscape types characterized by vegetation: cactaceae (Cac), cactaceae and herbaceous (CacHerb), shrubs (Shr),

    trees with cover 60%) (ShrTree). All the soils were strongly acidic and classified as loamy sand,

    sandy loam or silt loam. Organic carbon content was under 1% in Cac or CacHerb, then increased strongly in ShrTree (6.50%). Considering

    phosphorus, all the forms (labile as well resistant forms) increased markedly from Cac soils to ShrTree soils. In all the soils, the labile forms

    (Resin-P: range 45105 Ag g1; NaHCO3-Pi: 23123 Ag g1; or NaHCO3-Po: 10122 Ag g1) were very high. These high phosphorus

    contents were attributed to the specific climatic conditions of the Lomas that feature a long period of vegetation dormancy (very dry period)

    and a short period of growth, following ENSO-associated precipitation. We suggested that during the dry period, plant decay and microbial

    cells death lead to release and accumulation of labile P in the soil, the rainfall wetting the soil, permitting vegetation growth. In this respect,

    the Lomas climatic conditions contribute to soil fertility, especially as labile forms of phosphorus are chiefly concerned.

    D 2005 Elsevier B.V. All rights reserved.

    Keywords: Soils; Phosphorus fractionation; Lomas; Peru; ENSO; Atacama desert

    1. Introduction

    Coastal deserts such as Atacama (Coastal Peruvian desert

    continued by the Northern Chilean desert) present specific

    characteristics: a) they are the driest among all deserts; b) the

    general climate is mild and uniform; c) the temperature is

    fairly evenly distributed throughout the year; d) they are

    subject to winter fogs. These climatic conditions impart tocoastal arid regions unique characteristics compared to arid

    regions characterised by high mean and large amplitude

    temperature. The aridity results from several combined

    factors, especially the permanent high pressure area over

    the Pacific Ocean and atmospheric stability induced by the

    cold northward flowing Humboldt Current. This cold current

    makes the air become cool or cold but dry and very stable

    overall, unable to produce precipitation. At the same time,

    there is very little evaporation and humidity is confined to a

    low level, giving persistent haze. Whereas mist may occur

    any time throughout the year, there are some particularly

    foggy periods, generally at the end of the austral winter and in

    early spring (Zavala Yupanqui, 1993). Along the Chilean and

    Peruvian coasts, elevations between 600 and 1000 m are the

    most favourable for fog formation (Osses McIntyre, 1996).The Atacama desert is strongly affected by El Nino

    (disruption of the ocean atmosphere system in the Tropical

    Pacific with consequences for weather around the globe)

    which generates abundant rainfall. El Nino-Southern Oscil-

    lation (ENSO) is a coupled ocean-atmosphere phenomena

    that has a worldwide impact on climate.

    ENSO, which seems to occur with a cyclic rhythm in

    coastal Peru (every 10 years on average) induces excep-

    tional rainfall in these regions. However, since the nineties,

    ENSO has occurred every 2 to 7 years. The last very rainy

    0341-8162/$ - see front matterD 2005 Elsevier B.V. All rights reserved.

    doi:10.1016/j.catena.2005.10.004

    * Corresponding author.

    E-mail address: [email protected] (A. Fabre).

    Catena 65 (2006) 80 86

    www.elsevier.com/locate/catena

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    periods occurred in 1982, 1992 and 19971998. In several

    parts of the Atacama desert as in the Arequipa region (South

    Peru), the coast is dominated by low hills (elevation varying

    from some hundred to about 1200 m) termed Lomas in

    Spanish (geomorphological sense). The same term refers to

    the fog caught on these hills (climatic sense) and to the

    vegetation arising during the foggy season (phytological

    sense). In the following text, the term Lomas is used in the

    global sense, comprising all three of these notions.

    The vegetation is composed of numerous ephemeral but

    also of perennial species, ligneous plants and cactaceae.

    Some studies have been published on the Peruvian Lomas

    (Pefaur, 1982; Ferreyra, 1993).

    The Lomas are utilized for forage and to gather woody

    species for fuelwood (Ferreyra, 1977). They are periodically

    used for grazing livestock (cattle, sheep and goats), especially

    during ENSO events, and possibly as grazing land during

    seasonal livestock migration during the Spanish period.

    Considering the soils of deserts, studies are scarce and

    mainly concern hot deserts or arid ecosystems (Lajtha,

    1988; Lajtha and Schlesinger, 1988; Cross and Schlesinger,

    2001). At the moment, no information exists on the soil

    characteristics of the Atacama desert or of the Lomas. In this

    paper we consider some general soil characteristics and we

    emphasize the different forms of phosphorus in soils of fiverepresentative vegetation types (Lomas types) of Lomas de

    Arequipa (South Peru, Fig. 1). Hypothesis of a close

    relationship between labile phosphorus content in the soils

    and ENSO events inducing exceptional rainfall is discussed.

    2. Materials and methods

    2.1. Study site

    The study site was situated near the town of Mollendo, in

    the Arequipa region, on the south Peruvian coast (72-10

    71-40V W; 16-90V 17-40 S). In this region, average annual

    precipitation is only < 50 mm below 500 m alt. and several

    years may pass without rainfall. The driest period occurs

    from January February to April. From May to October,

    heavy fog (relative air humidity near 75%) permits

    vegetation growth. The average annual temperature is

    around 18 -C and the annual variation in temperature issmall with a minimum of 912 -C in July and a maximum

    of 25 -C in JanuaryFebruary (Zavala Yupanqui, 1993).

    When the coastal topography is flat, the seasonal fog

    dissipates inland but where isolated hills (150 to 1000 m)

    intercept the fog, a fog zone appears allowing the develop-

    ment of rich vegetation termed Lomas formations sepa-

    rated by areas without vegetation. In Peru, around 40 Lomas

    formations exist, among them the Lomas de Mollendo.

    The bedrock is acid igneous (granodiorite) with local

    clastic sediments (sand, clay, sandstone or conglomerates).

    The non-consolidated parent material (particles

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    phosphorus was extracted using an anion exchange resin

    (Resin-P) (Amer et al., 1955). Sodium bicarbonate 0.5 M

    (pH 8.5) removed labile Pi (NaHCO3-Pi) and Po (NaHCO3-

    Po) sorbed to the soil surfaces (Bowman and Cole, 1978a,b).NaHCO3-Po is easily mineralizable and can contribute to

    plant available P. Sodium hydroxide 0.1 M extracted Pi

    (NaOH-Pi) associated with amorphous and some crystalline

    Al and Fe oxides (Syers et al., 1969) and Po associated with

    humic compounds (NaOH-Po) (Fares et al., 1974). NaOH-Pi

    is relatively labile Pi (Bowman and Cole, 1978a,b) while

    NaOH-Po is considered to be involved in long term

    transformation of soil under temperate climates (Tiessen et

    al., 1983). Resin-P, NaHCO3-Pi, NaOH-Pi, NaHCO3-Po and

    NaOH-Po are considered as non-occluded forms (Walker

    and Syers, 1976). Phosphorus extracted with 1M hydro-

    chloric acid (HCl-P) is mainly apatitic phosphorus. It isunavailable in the short term. The residue containing the

    most chemically stable Po and Pi forms was digested using

    concentrated H2SO4+ H2O2 (Resid-P) (Thomas et al., 1967).

    Extracts containing organic phosphorus were digested for

    total P determination using a persulfate digestion method

    (Standard Methods, 1971). Phosphorus in the extracts or

    digests was determined after pH adjustment if necessary,

    using the ascorbic acid molybdenum blue method. A

    literature review of the Hedley P fractionation method was

    performed by Cross and Schlesinger (1995). All the chemical

    results were expressed on air dried basis.

    2.4. Statistical methods

    All the statistical analyses were performed using Systat

    8.0 software. Analysis of variance was used to compare P

    contents between landscape types. When global ANOVA p

    value was 0.05). Resin-P varied from

    44.7 Ag g1 in the Cac stands to 104.5 Ag g1 in the

    ShrTree stands. These values are significantly different from

    Table 2

    General characteristics of the soil for each Lomas type (4 replicates in each Lomas type)

    Lomas type pH % Organic C % Clay % Silt % Sand Soil textural classes

    Cacti 4.9T0.21 0.34T0.28 2.1T0.71 11.0T3.42 87.0T4.10 Loamy sand

    Cacti and herbaceous 4.5T0.19 0.68T0.16 6.1T0.37 39.0T3.64 54.9T3.97 Loamy sand sandy loam

    Shrubs 5.0T0.19 1.45T0.38 7.5T0.33 59.2T1.07 33.4T1.38 Silt loam

    Trees (cover < 60%) 4.6T0.15 2.40T0.47 11.2T0.85 59.3T 29.8T2.49 Silt loam

    Shrubs and trees (cover >60%) 4.7T0.27 6.50T0.42 12.7T0.83 60.8T2.08 26.5T2.87 Silt loam

    Table 1

    Floristic composition of each Lomas type

    Lomas type Altitude

    (range)

    % Plant

    cover

    Some dominant species

    Cacti 160 680 1 3 Neoraimundia arequipensis, Borzicactus sp., Islaya mollendoensis, Trichocereus sp.,

    Neoporteria islayensis , Tephrocactus sp., Pilocereus sp.

    Cacti and herbaceous 620790 2 10 Neoraimundia arequipensis, Borzicactus sp., Islaya mollendoensis, Tichocereus sp.,Neoporteria islayensis , Tephrocactus sp.

    Eragrostis peruviana , Cotula australis, Tillandsia sp., Poa sp., Urocarpidium sp., Atriplex sp.

    Shrubs 620 850 20 35 Phylla nodiflora, Citharexylum flexuosum, Grindelia glutinosa, Croton ruizianus,

    Heliotropium lanceolatum , Vigueria weberbaueri, Lycopersicum peruvianum,

    Urocarpidium peruvianum, Cotula australis

    Trees (cover 60%)

    690980 75100 Caesalpinia spinosa, Duranta armata, Heliotropium arborescens, Phylla nodiflora,

    Citharexylum flexuosum, Grindelia glutinosa, Croton ruizianus, Heliotropium lanceolatum,

    Vigueria weberbaueri, Lycopersicum peruvianum

    A. Fabre et al. / Catena 65 (2006) 80 8682

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    Resin-P contents in the other stands which did not present

    significant differences between each other. NaHCO3-Pi

    contents were lowest in Cac or CacHerb (23.4 and 33.4 Ag

    g1, respectively) and differed significantly with ShrTree,

    Tree and Shr (92.3, 109.9 and 122.9 Ag g1, respectively),

    themselves being not significantly different. NaHCO3-Po

    varied from 10.4 (Cac) to 122.4 Ag g1 (ShrTree). These

    contents were significantly different from the three other

    stands which were not significantly different between each

    other (range 62.484.9 Ag g1). NaOH-Pi contents were

    significantly different between Cac (33.4 Ag g1) and

    ShrTree (125.1 Ag g1). The three other stands presented

    intermediate values (range 70.994.8 Ag g1). NaOH-Po

    presented the lowest values in Cac, CacHerb and Shr (not

    significantly different; range 9.827.4 Ag g1) contrasting to

    the content in ShrTree (139.3 Ag g1). The content in Tree

    (49.9 Ag g1) was significantly different from the other

    stands. HCl-P opposed a low content in ShrTree (136.4 Agg1) to the other stands (range 256.4 386.3 Ag g1). Resid-

    P markedly opposed ShrTree stand (242.8 Ag g1) to the

    other stands (range 47.079.8 Ag g1).

    3.3. Relations between soil parameters

    Organic carbon was positively correlated (r=0.90) with

    %cover (Table 4). Except on HCl-P and Resid-P, all the other

    forms of phosphorus are significantly positively correlated

    with %clay or %silt or both, and negatively correlated with

    %sand. Likewise, except on NaHCO3-Pi or HCl-P, all the P

    forms were positively correlated with %cover.

    4. Discussion

    4.1. Carbon content

    The high correlation between organic carbon and

    vegetation cover has already been shown in several studies

    in arid areas (Le Houerou, 1986; Gauquelin et al., 1998).

    Nevertheless we can notice the high organic carbon content

    (around 6.50%) of the soils of the ShrTree stands, generally

    situated in the upper part of the Lomas, where the

    percentage of plant coverage is high (> 75%).

    4.2. Phosphorus content

    In all the soils, we found high labile P contents (Resin-P,

    NaHCO3-Pi and Po), in comparison with data from other

    arid or desert soils. Nevertheless, comparisons with litera-

    ture data are difficult because most of these data concern hot

    arid areas or deserts not periodically exposed to intense

    rainy periods (ENSO events). The high concentrations of the

    different forms of P in the Lomas can be ascribed to the

    combination of different and independent effects (Fig. 2).

    Table 4

    Correlation matrix between phosphorus forms and related parameters (in bold character: statistical significance at P

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    4.2.1. Land use and grazing effect

    Since the Spanish colonization, the Lomas has been

    grazed by sheep, goats and cattle. Nowadays, the Lomas are

    still grazed, especially during ENSO events and are used as

    a fuelwood source. Livestock foraging is important in

    pasture nutrient cycling because they convert nutrients from

    unavailable forms (natural fodder) to available forms

    (excreta) (Buschbacher, 1987). Moreover, the constant

    movement of the animals leads to a relatively regular

    distribution of faeces through the patchy landscape (Turner,

    1998).

    4.2.2. ENSO events

    During ENSO events, seeds lying within the soil,germinate and emerge into a continuous blanket. Then, this

    vegetation dies and decays quickly. In temperate or tropical

    ecosystems, many studies have shown that the different

    forms of P, and especially the more labile, present seasonal

    fluctuations. Generally, the more labile forms of P increase

    during winter and decrease during the growing season

    (Timmons et al., 1970; Saunders and Metson, 1971;

    Dormaar, 1972; Vaughan et al., 1986; Sarathchandra et al.,

    1989; Perrott et al., 1990; Magid and Nielsen, 1992).

    Likewise, in a mature tropical moist forest, inorganic P

    peaks during the dry season (Yavitt and Wright, 1996).

    These findings suggest that during the dormant vegetation

    season (winter or dry season) there is an increase and

    accumulation of the more labile P forms.

    Considering the mechanism, the literature yields

    conflicting reports. Some authors consider that accumula-

    tion of labile P results on the microbial mineralization of

    plant debris or to the release of Pi from the organic matter

    (Saunders and Metson, 1971). Others attribute the labile P

    increase to the microbial biomass killed by air-drying

    (Srivastava, 1997). Using New Zealand acid soils, Haynes

    and Swift (1985) showed that drying soils increased

    phosphate extractable with EDTA, resin or NaHCO3 and

    considered that drying soil conducive to the release of P

    associated with organic matterFe and Al complexes, and

    possibly from killed microbial cells. Similarly, Sparling etal. (1985), studying 18 pasture soil samples from New

    Zealand, showed that, in most of the soils investigated,

    drying led to an increase of NaHCO3-Pi. Williams (1996)

    showed that a greater concentration of P leached by CaCl2,

    extracted from spruce or pines humus, coincided with

    drying of the soil during summer. He considered that the

    enhanced Pi contents in the dried soils can be mainly

    accounted for by the release of Pi from the killed cells or to

    death of fine roots and microorganisms and concluded that a

    rainy period following a dry period, could contribute to

    plant growth following rewetting. In this respect a period of

    soil drying could benefit overall fertility levels.

    Fig. 2. Mechanism of distribution of soil phosphorus in the Lomas de Arequipa: flowchart.

    A. Fabre et al. / Catena 65 (2006) 80 8684

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    Foliar or plant residue or litter leaching is generally

    considered as a source of labile-P (Timmons et al., 1970;

    Bromfield and Jones, 1972; Duffy et al., 1985; Johnson and

    Todd, 1987; Weiss et al., 1991; Polglase et al., 1992). In the

    Lomas, at the end ENSO-related rainfall, and during

    the beginning of the dry period, death and decay of the

    vegetation (especially the ephemerals), possibly causes therelease and accumulation of labile P permitting the P pool to

    be rebuilt. During the dry period, this pool is not used by the

    seeds and the vegetation is dormant.

    4.2.3. Particle size distribution

    The distribution of the vegetation from Cac to ShrTree

    from near 160 to 980 m of altitude can be considered as a

    toposequence. Generally, in a toposequence, erosional

    processes bring about enrichment in fine particles from the

    top to the bottom of the relief. In the Lomas, we found the

    reserve with a higher fine particles content in soil from the

    upper part of the landscape (Table 2). This can be ascribed tothe increasing percentage plant cover from Cac (lower part)

    to TreeShr stands (upper part) where canopy and litter reduce

    erosion processes. The result is an increasing content of P

    labile forms from the lower to the upper part of the landscape

    corresponding to the general association between labile P

    and the finest soil particles. A positive relation between the

    finest soil particles and labile P was shown in cultivated and

    uncultivated soils (Tiessen et al., 1983) or wit h algal

    available P or P sorption in eutrophication studies (Syers et

    al., 1969; Dorich et al., 1984; Keulder, 1982). Nevertheless,

    in a toposequence from semiarid northeastern Brazil,

    Agbenin and Tiessen (1995) found a downslope decreasing

    total P concentration as in the Lomas. They concluded from

    the studied toposequence that in arid environments, the

    distribution of P results from complex interactions of

    lithology, weathering, colluvial actions and climatic con-

    ditions (moisture deficit followed by intense rainfalls).

    In the Lomas, the plant cover increases from the bottom

    to the top of the relief where heavy fogs (May to October)

    enable vegetation growth, limiting erosion processes at the

    top with subsequent accumulation of the different forms of

    phosphorus generally associated to finest soil particles.

    5. Conclusion

    The Lomas constitute an original landscape chiefly

    characterized by: a) the localization near the Pacific Ocean

    and the presence of the cold Humboldt Current; b) the

    topography (regular increase of the altitude from around 100

    to 1000 m) which acts as a barrier to Ocean influences,

    causing fogs, especially between 600 to 1000 m (May to

    October) or receiving heavy rainfalls during ENSO events.

    With regard to the vegetation, the specific climatic

    conditions leads to a strong contrast between long periods

    of seed dormancy then short periods of growth, the trigger

    mechanism being rainfalls associated to ENSO events.

    Considering phosphorus, two periods are particularly

    important: a) the beginning of the drought with release of

    labile P (plants decaying and microbial cells killed) and its

    accumulation in the soil; b) rainfall with wetting of the soil

    permitting the growth of vegetation, especially of the

    ephemeral burst in a continuous blanket. In this respect

    the Lomas characteristics, that is the rainy period followingthe long dry period contribute to the overall fertility of the

    soil, especially as the labile forms of phosphorus are

    concerned the most.

    Acknowledgments

    The authors thank M.F. Bellan and D. Lacaze for the

    field assistance and F. Barthelat, K. Saint-Hilaire and M.

    Saurat for help with many chemical analyses. The study

    received financial support from European Communities:

    Contrat U.E. n- TS3 CT 94 0324 (19951998): Fog as anew water resource for the sustainable development of the

    ecosystem of the Peruvian and Chilean coastal desert.

    Project Coordinator: Dr Roberto Semenzato (19951997)

    and Dr Mario Falciai (19971998).

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