Inaccessible Andean sites reveal human-induced weathering in grazed soils

Abstract

Human activity affects properties and development of ecosystems across the globe, to such a degree that it is now challenging to get baseline values for undisturbed ecosystems. This is especially true for soil development, which is potentially affected by land-use history and holds a legacy of past human interventions. Therefore, it is still largely unknown for most ecozones how soil would have developed ‘naturally’. Here, we show undisturbed soil development, i.e. the processes of weathering and accumulation of soil organic matter (SOM), by comparing pristine with grazed sites in the high Andes (4500 m) of southern Peru. We located study plots on a large ledge (0.2 km2) that is only accessible with mountaineering equipment. Plots with pristine vegetation were compared to rangeland plots that were presumably under relatively constant grazing management for at least four millennia. Vegetation change, induced by grazing management, led to lower vegetation cover of the soil, thereby increasing soil surface temperatures and soil acidification. Both factors increased weathering in rangeland soils. Formation of pedogenic oxides with high surface area explained preservation of SOM, with positive feedback to acidification. Higher contents of pyrophosphate extractable Fe and Al oxides indicated the importance of organo-mineral associations for SOM stabilization on rangeland sites, which are likely responsible for a higher degree of humification. This higher degree of humification induced melanization (darker colour) of the rangeland soils which, together with sparse vegetation cover, also feeds back to soil temperature. With this work, we present a conceptual framework of positive feedback links between human-induced vegetation change, soil development and accumulation of SOM, which is only possible due to the unique baseline values of a pristine ecosystem. Using ‘inaccessibility’ as a tool to quantify human impact in future interdisciplinary studies may push research forward on evaluating anthropogenic impact on Earth’s ecosystems.

Relict high-Andean ecosystems challenge our concepts of naturalness and human impact

“…5a) usually reduce the effect of Al through complexation: While organic matter in soils can bind to many different substances, it is especially reactive towards the complexation of cations in general and, in particular, to multi-charged cations like Al 3+ and Fe 3+  43 . As all the soils studied had a high organic matter content, and thus a strong binding to Al remaining in the organic matter matrix of the soil, Al 3+ is therefore relatively accumulated in grassland because the less charged cations are more easily leached 26 . This organic matter matrix also buffers the pH lowering effect of Al and explains why there were no significant differences in pH between the different soil types.…”

An updated checklist and key to the open-panicled species of Poa L. (Poaceae) in Peru including three new species, Poa ramoniana, Poa tayacajaensis, and Poa urubambensis

“…This species was found during a large scale ecological study attempting to reconstruct the potential natural vegetation (PNV) and soils of the high-elevation Puna grasslands (see Heitkamp et al 2014 and Sylvester et al 2014 for pilot studies). In this research, pristine zonal vegetation, only accessible with mountaineering equipment, was compared with surrounding slopes which have been grazed and burnt consistently over millennia (Thompson et al 1988; Chepstow-Lusty et al 1996, 2009; Kuentz et al 2011).…”

Indicators for assessing tropical alpine rehabilitation practices

“…Given that the puna ecosystem's vegetation dynamics have been modified for thousands of years, existence of an undisturbed reference of this vegetation is controversial or according to some studies only found in inaccessible places (Villota and Behling , White , Heitkamp et al. , Sylvester et al. ).…”

“…Furthermore, because grazing intensities are variable even within the same area, the resulting vegetation patterns are heterogeneous and respond to site-specific environmental factors (Stohlgren et al 1999). Given that the puna ecosystem's vegetation dynamics have been modified for thousands of years, existence of an undisturbed reference of this vegetation is controversial or according to some studies only found in inaccessible places (Villota and Behling 2013, White 2013, Heitkamp et al 2014.…”