Vegetation influence on soil quality in a highly degraded tropical ...

14
SOTOMAYOR-RAMÍREZ
ET AL./
TROPICAL SOILS
minimum temperatures ranged from 16.7 to 25.5 °C. Mean precipitation
ranged from 14.0 mm (July 2000) to 399 mm (December 1999).
Two leguminous trees, Moca ( Andira inermes C.W. Wright DC.), and
Acacia ( Albizia procera Roxb. Benth.); two leguminous shrubs, forage
peanut ( Arachis glabrata Benth., also known as rhizoma perennial
peanut) and Centrosema ( Centrosema acutifolium Benth.); and two forage
grasses, Brachiaria ( Brachiaria humidicola Schweickt.) and
Limpograss ( Hemarthria altissima Poir.) were selected among fortyseven
species that were established in April 1996. Plant material was
planted from plugs previously propagated in the greenhouse.
The experimental arrangement was a randomized complete block
with seven treatments (six plant species and an unplanted bare soil as
control), with three replications and two sampling depths. The area for
each treatment was a plot (1.82 × 1.52 m) that was surrounded by 15cm-high
polyethylene edging installed to a depth of 6 cm to exclude
run-on and divert runoff to a collector point for measurement (Ramos-
Santana et al., 2000).
Beginning in August 1999, soil samples were collected at monthly intervals
at two depths (0- to 5-, and 5- to 15-cm) from within each plot.
Composite soil samples were obtained from ten and five subsamples of
the 0- to 5- and 5- to 15-cm depth intervals, respectively. Upon transport
to the laboratory, soils were passed through a 6-mm sieve and stored at
5 °C until analysis. Soil moisture was quantified gravimetrically for each
depth interval and sampling depth; soil bulk density was quantified from
known soil volume and oven-dried (105 °C) soil mass measurements.
Soil samples were split in two portions, one of which was either airdried
or humidified to attain 40% w/w moisture for microbial biomass
and soil respiration measurements, and the other was air-dried for
chemical measurements. This moisture level is the expected field capacity
for this soil (Snyder et al., 1993), and ensured that differences
among microbial measurements were not due to changes in the instantaneous
field soil moisture level. Thus, all microbial measurements
represent values under optimal soil water content.
Soil pH was determined by using a glass electrode immersed in the
supernatant of a 1:2 soil:water mixture after a 2-h shaking period. Soil
organic matter (SOM) was quantified by using the dichromate oxidation
technique (Nelson and Sommers, 1982) with a conversion factor from organic
C to SOM of 2.24. Extractable P was determined by the Bray 1
method (Bray and Kurtz, 1945). Extractable bases were exchanged with
a 1M
NH4OAc
(pH 7) solution followed by quantification by atomic absorption
spectrometry. Soil acidity was quantified by 1M KCl extraction
of exchangeable H+
and Al+3
followed by titration with standardized
0.1M
HCl solution (Thomas, 1982). Soil effective cation exchange capac-