3.3.1 Soil Organic Matter (SOM)
Figure 3 shows SOM results in the six SUs for the three studied depths.
A decrease in SOM with depth is observed for the natural vegetation
soils, while SOM was relatively constant with depth in soils under
anthropic land use. This is corroborated with the average difference in
SOM between 0–5 cm and 0–25 cm, with values of 12.4% for natural
vegetation and smaller values for F (3.9%), Oc (0.7%) and Pc (0.8%).
The SOM gradient in natural vegetation could be related with a higher
fraction on biomass in the superficial soil layers, and the limitation
of microbial activity with depth caused by scarce oxygen, temperature,
and nutrients (Dungait, Hopkins, Gregory, & Whitmore, 2012). In
contrast, in anthropic uses, the incorporation of fresh organic matter
from external sources together with ploughing can homogenize organic
matter in the ploughed soil profile.
[Insert Figure 3]
Natural vegetation SUs had average SOM values of 25.0% at the surface
and up to 52.3% in riparian areas (Ds). These values agree with reports
from natural vegetation in other dry páramos of 23.0–39.0%
(Peña-Quemba, Rubiano-Sanabria, & Riveros-Iregui, 2016). In contrast,
in humid páramo, SOM values up to 42.6% have been reported
(Agudelo-Cárdenas, 2016). The generally high SOM values observed in
páramos are possibly a consequence of a positive feedback between SOM
and WC (Buytaert, Célleri, De Bièvre, & Iñiguez, 2007). Podwojewski et
al. (2002) ascribe the high SOM in natural vegetation to three factors:
i) fresh organic matter originating from plant decomposition, ii) cold
conditions and microbial inhibition, and iii) protection from
mineralization by organometallic complexes.
With regards to agricultural land uses, F had the lowest SOM values
among the SUs (~15.8%). According to Hofstede (1995),
appreciable changes in the soils because of livestock grazing are
evident at cattle densities over 10 units per hectare. However, in our
study area, the livestock density was less than 2 units per hectare. The
observed low SOM levels could be more related to soil depletion during
its productive stage followed by a low recovery, which is typical of
páramo soils (Avellaneda-Torres et al., 2018).