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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
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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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