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.
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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).