Saturday, 15 July 2006

Soil Properties along a Toposequence in Mountainous Cloud Forests in Sierra Juárez, Southern Mexico.

Pavel Krasilnikov1, Norma Eugenia García Calderón2, Noé Velázquez Rosas3, and Elizabeth Fuentes Romero2. (1) (1) Institute of Biology, KarRC, RAS (2) Facultad de Ciencias, UNAM, (1) Pushkinskaya str., 11 (2) Av. Universidad, 3000, (1) Petrozavodsk, Russia (2) México, Mexico, (2) Facultad de Ciencias, UNAM, Av. Universidad, 3000, México, Mexico, (3) Instituto de Ecología, UNAM, Av. Univrsidad, 3000, México, Mexico

The soils of mountainous cloud forests (MCF) are ones of the least studied ones in the tropics. Few papers available indicate high acidity, low nutrient content, and well-developed histic epipedon of these soils. Until now there are no studies of the effect of an altitudional gradient within MCF on soil profile morphology, chemical properties, and mineralogical composition of clays. We studied a toposequence of soils formed on the Atlantic slope of Sierra Juárez mountainous system in Southern Mexico. The zone is believed to be the most humid place in Mexico, receiving up to 5000 mm of annual precipitation. Seven sites were established on the altitudes from 1400 to 2800 m above sea level. The upper soil was formed under highland shrubs, the lower one – under disturbed tropical rain forest, and the least 5 – under MCF natural well-conserved vegetation. Al the soils form on the same, but heterogeneous parent material – mica-chlorite shales. We studied soil morphology, determined basic chemical properties of the soils, and made an X-ray study of the mineralogical composition of clay fractions. The upper soil profile under shrubs has a thin litter layer, shallow albic horizon (3 cm), and a spodic horizon. Since the spodic horizon is too close to the surface, the soil cannot be classified as a Podzol in the WRB system, and it was named Dystric Cambisol; however, Soil Taxonomy 1999 has no lower limit for spodic horizon depth, and the soil might be classified as Lithic Haplorthod. Within the MCF altitudional belt there are two distinct ecological zones: upland MCF (approximately at the altitudes 2000–2500 masl) and lowland MCF (1500–2000 masl). The profiles under the upland MCF vegetation had thick organic layer, deep albic horizon, and well-developed spodic horizon with gleyic colour pattern in its upper part. The soils found there were classified as Histic Stagnic Podzols (Histic Epiaquods) except of one profile, formed in a local depression and, thus, saturated with water. The latter was classifiesd as Histic Stagnic Cambisol (Histic Humaquept). The profiles under lowland MCF vegetation had thin litter layer, thin or absent albic horizon, and distinct evidence of clay illuviation (clay skins), and no gleyic colour pattern, The soils were classified as Dystric Alisols (Typic Haplohumult). The soil profile formed under disturbed tropical rain forest is a sequence of buried profiles formed due to periodical disturbance and activization of erosional processes. All the horizons in the sequence had eveidences of clay illuviation. We found the following changes in soil properties along the altitudional gradient. Soil acidity is equally high, and base saturation is equally low in all the studied profiles. Total reserve of organic C is the highest in the highest profiles, but most of C is found in histic horizons. The reserve of organic C is the lowest at the altitudes 1900-2000 masl, and lower on it increases again, due to accumulation of humus in umbric horizons. Nitrates content decreases, while ammonium content increases with altitude. The content of clay in surface mineral horizons decreases with altitude, and the difference between clay contents in the eluvial and illuvial horizons is the highest in the upland profiles: we hypothesized that clay is rather destroyed in the surface horizon, than illuviated. At the altitudes 1900-2000 masl there is no difference in the distribution of clay in the profile. In the lowland profiles a difference in clay content also can be observed; we ascribed it to clay illuviation because of the presence of clay skins. Clay fraction of the upland soils contains chlorite, mica and interstratified products of their degradation; kaolinite is also present in significant amounts. In the lowland profiles the dominant mineral in clay fractions is gibbsite, with minor admixture of chlorite and mica degradation products. Thus, we found a distinct difference in the properties of soils formed under lowland and highland mountainous cloud forests.

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