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).