4.2 Influence of agriculture on the soil hydrophysical
properties of páramos
The soil hydrophysical properties in the natural vegetation SUs did not
show significant changes according to depth. On the other hand,
comparing the effect of land use transformation (natural to
agricultural) on the soil hydrophysical properties, the following
conditions emerged:
The changes in SOM, comparing agricultural areas with natural vegetation
areas, were higher at the surface level. In this case, F had a higher
reduction in the median values (40.0% compared to Ls and Ss), followed
by Pc (31.1% compared to Ls and Ss) and Oc (27.3% compared to Ls and
Ss). The increase in the decomposition rate (Henry, Mabit, Jaramillo,
Cartagena, & Lynch, 2013), the rapid oxidation in unprotected areas
(Poulenard et al., 2003), and the mineralization of organic waste
(Sainju, Whitehead, & Singh, 2003) can reduce SOM, as observed in the
cultivated areas in this study. In addition, the agricultural practices,
in which organic amendments and biomass inputs are not considered,
prevent the preservation or restoration of the natural soil conditions,
leading to a slow passive recovery (Harden, 2006). The transformation of
páramos to agricultural lands changes the biological soil dynamics
(Avellaneda-Torres et al., 2018), resulting in a high enzymatic activity
(Li et al., 2018), favorable for crops but at the cost of losing soil
stability and the native microbiological diversity. These properties
directly influence soil carbon storage (Coonan et al., 2020) and the
biotransformation process of organic matter and nutrients. The use of
compost (Melero, Madejón, Ruiz, & Herencia, 2007) and zero tillage
(López-Bellido, Fontán, López-Bellido, & López-Bellido, 2010; Sainju et
al., 2003) increase enzymatic activity and surface organic carbon,
without an important alteration of soil structure, restoring soil
aggregation and preventing erosion. This yields better results than
traditional agricultural practices (e.g., the addition of unstable
organic matter to the soil) (Melero et al., 2011).
The greatest average changes in Bd, comparing agricultural land uses and
natural vegetation were observed at higher depths. The most significant
change occurred in Pc, where Bd was approximately homogenous at all
depths and decreased compared to natural vegetation (Ls and Ss) on
25.9%. In Oc, a reduction of 19.6% of Bd (compared to Ls and Ss)
occurred at the same depth. F had the higher increase in Bd at the
surface (0–5 cm; 19.8% compared to natural vegetation). In the study
area, the soils under Oc and Pc were recently transformed, for which,
the observed changes in Bd were relatively small. In contrast, F, an
abandoned area after an intense period of agricultural and livestock
use, had the worst hydrological conditions compared to all the studied
land uses.
pH was acidic in areas under natural vegetation as has been found in
other páramo studies (Daza-Torres et al., 2014; Estupiñán et al., 2009).
In contrast, in the agricultural areas, pH was more neutral, probably as
result of the application of amendments (e.g., agricultural lime) and
fertilizers. In this case, the greater changes in the pH of agricultural
areas were observed close to the surface, being higher in Oc (an
increase in average values of 31.1% compared to Ls and Ss) because of
an intensive use and diversity of agricultural inputs. In contrast to
Oc, the change was 11.1% in Pc at the surface level compared to Ls and
Ss.
In relation to EC, Oc and Pc had the greatest changes in the deep layer
(average increase in 323.9% and 201.4%, respectively compared to Ls
and Ss). This can be associated to an inefficacy of managerial
practices, for example, an unequal application of amendments and
fertilizers and excessive use of agrochemicals (Wei et al., 2009). In F,
the change occurred at the surface level, but it was subtler compared to
other anthropic uses (13% increase compared to Ls and Ss), which could
suggest signs of recovery in this degraded soil.
Ds generally behaved similar to other natural vegetation covers (Ls and
Ss) in the hydrophysical properties of SOM and Bd. However, pH and EC
had a dissimilar behavior, showing a lower pH (0.5 units in average
compared to Ls and Ss), and higher EC (in average 71.3% over Ls and
Ss). This suggests that Ds had higher ability to store water and
nutrients (Martinez, Vanderlinden, Ordóñez, & Muriel, 2009), together
with soil aggregation with continuous macropores which favor the
conduction of electric current (Lal, 1997).