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.