Figure 1. General map of the study site, the experimental
plots, and sampling points. In the upper left and lower right corners of
the map, there are zoom layers displaying the four 100 x 100 m plots for
each condition (OG and CG fenced area).
In each zone (CG and OG), four plots of 100 x 100 m were randomly
established using the Geographic Data Analysis R package (Hijmans & Van
Etten, 2016). Then, at each plot, eight random sampling points were
selected using the same R package. Once the sampling points were
established in the field, in each one of the random plots, we stratified
four of these points in a bare soil condition and four under Araucaria
canopy to consider the heterogeneity derived from the high clustering of
tree coverage in the study area (Figure 1).
In each point, and to consider the spatial variation of the soils
(Kirwan et al., 2005), four auger samples per sampling point were
composited at two established depths (0 – 15 cm and 15 – 30 cm)
(n=128). The samples were collected in sterile and hermetic bags and
separated into two aliquots, one for keeping it at four °C for
NO3- and
NH4+. These were measured
colorimetrically within 48h after sampling (Forster, 1995; Miranda et
al., 2001). The second larger aliquot was air-dried (25°C) and sieved
(<2mm) for later analysis. Total C and N were quantified by
Dumbas combustion using a SERCON© EA total CHNS analyzer. Besides, we
determined N following the Kjeldahl distillation method (Sadzawka et
al., 2006). Total, organic and inorganic phosphorus were determined
following the protocol described by Bowman & Moir (1993), using EDTA as
the extraction solution. Available phosphorus was measured by the Olsen
extraction method adapted by Sadzawka et al. (2006). Permanganate
oxidizable carbon (POXC) was determined following the procedure
described in Weil et al. (2003) as an index for easily decomposable
carbon (Culman et al., 2012). Soil pH was measured with combined
electrodes in both 1:1 soil/water and 1:50 NaF suspension. Cation
exchange capacity (CEC) was determined following the protocol described
in Sadzawka et al. (2006) buffered (pH=7) ammonium acetate as the
extractant solution. All these analyses were performed at the Soil,
Water, and Forest Research Laboratory of the University of Concepción
(http://www.lisab.udec.cl).
For physical parameters, in each sampling point, infiltration tests were
conducted using a Mini-Disk infiltrometer (Decagon ©). Also and
following the protocol described in Mohanty et al. (1994), a 750cc
undisturbed soil sample was taken from the soil surface up to 10 cm
depth and carefully transported into the laboratory for soil saturated
conductivity measurements (Ksat) using the
”Constant-Head” method (Black et al.,1965). Following the
Ksat measurements, the samples were analyzed for soil
bulk density, particle density (Blake & Hartge, 1986), texture (Day,
1965), and water holding capacity as described in Burt (2004). Also,
undisturbed soil samples were taken at each sampling point from the top
5 soil cm and used to estimate aggregates stability using a wet sieving
apparatus (Eijkelkamp ®) following the methodology described by Kemper
& Rosenau (1986) and the adaptation described in Rivera & Bonilla
(2020) for the Eijkelkamp wet sieving apparatus.