4.1 Influence of seasonality on the soil hydrophysical
properties of páramos
The hydrophysical properties of soils under natural vegetation showed
gradients according to depth (except for Ds in the wet season). In soils
under agricultural use, none of the properties showed an evident pattern
or gradient related to depth. This shows that human practices, such as
ploughing and fertilization, change soil structure and properties, which
affects the natural hydrophysical occurrence (reduction of SOM and EC
with depth, and increase in Bd and pH with depth). This trend was
different only in the area of Ds, in which the pH increased with depth,
probably for being an area where fresh organic matter accumulates and
promotes an increase in weak organic acids in the surface.
In general, the soil hydrophysical properties under natural vegetation
did not exhibit statistically significant differences in relation to the
rainfall regime. The exception was Bd, in the area of páramo grasslands
with low slope (Ls) (average reduction in the three depths of 14.4%
from the wet season to the dry season). This difference of Bd in Ls is
counterintuitive with results from SOM in this area but can be
associated to the fact that this plot is located on top of a páramo
valley (3620–3660 m a.s.l.) with higher exposition to solar radiation,
winds and predominantly covered with low-size vegetation. The stability
of the soil hydrophysical properties despite the rainfall regime in the
other natural areas could be associated to the vegetation type. Plant
characteristics allow them capture water using different mechanisms and
limit evapotranspiration (Salamanca, 1986), while accumulating organic
matter and improving soil structure (Josse et al., 2009). This result in
high organic matter content, contributing to the preservation of soil
properties independently of seasonality (i.e., sustaining hydrological
regulation).
In contrast with natural vegetation, some hydrophysical properties in
soils under agricultural use showed changes in relation to the rainfall
regime. Comparing the wet and the dry season, F reduced SOM in about
38.7% (0–5 cm), possibly because of its land use history exhausting
soil conditions (Sandoval, 2004). In soils under agriculture subject to
ploughing, the micro-aggregates of the soil, where the organic matter is
mainly present (Six, Frey, Thiet, & Batten, 2006), are broken and they
are also exposed to the climate conditions of the páramo, which favors
the mineralization or loss of organic matter (Balesdent, Chenu, &
Balabane, 2000). Furthermore, the increase in runoff and erosion during
the wet season can favor the lixiviation of soil organic carbon as
observed in Oc (Otero, Figueroa, Muñoz, & Peña, 2011).
In cropped plots, Bd showed changes according to the season, in all the
assessed depths, 19.8% in in Oc, and 25.4% in Pc. These changes in Bd
are associated to soil structure alteration that can generate a loss in
the streamflow buffering capacity of páramo catchments
(Avellaneda-Torres et al., 2018; Dorel, Roger-Estrade, Manichon, &
Delvaux, 2000).
The results obtained here indicate that agricultural management
increases the sensitivity of the soil hydrophysical properties to
seasonal fluctuations (rainfall regime) in cropped areas (Hofstede,
1995; Ordoñez et al., 2015). The high stability of the soil properties
under natural vegetation, especially SOM, could explain the low
inter-seasonal variability observed.