Towards understanding of carbon stocks and stabilization in volcanic ash soils in natural Andean ecosystems of northern Ecuador

Abstract

Volcanic ash soils contain very large stocks of soil organic matter (SOM) per unit area. Consequently, they constitute potential sources or sinks for the greenhouse gas carbon dioxide. Whether soils become a net carbon source or sink with climate and/or land-use change depends on the stability of SOM against decomposition, which is influenced by stabilization mechanisms in the soil. To quantify organic carbon stocks and to clarify the importance of chemical and physical soil characteristics for carbon stabilization in volcanic ash soils, we applied selective extraction techniques, performed X-ray diffraction analysis of the clay fraction and estimated pore-size distribution of soils under natural upper montane forest and grassland (páramo) in the Ecuadorian Andes. Our results show that organic carbon stocks under both vegetation types are roughly twice as large as previously reported global averages for volcanic ash soils. SOM stabilization is suggested to be dominantly influenced by the following chemical and physical soil characteristics: (i) direct stabilization of SOM in organo-metallic (Al-humus) complexes, explaining at most 40% of carbon accumulation, (ii) indirect protection of SOM (notably aliphatic compounds) through low soil pH and toxic levels of Al, and probably also (iii) physical protection of SOM caused by a very large micro-porosity. Moreover, in the case of the forest soils, inherent recalcitrance of OM itself was responsible for substantial accumulation in ectorganic horizons. Both vegetation types contributed to soil acidification, thus increasing SOM accumulation.

Opportunities and limitations related to the application of plant-derived lipid molecular proxies in soil science

“…The n-alcohol predominance also varied but to a much smaller extent, not affecting the predominance of a specific n-alcohol (Tulloch, 1973). Esters gradually showed an increase in esters of trans 2,3-unsaturated C 23 and C 24 acids with plant age (Tulloch, 1973). The variation was related to the development of the plant, in particular that of flag leaves and sheets between 55 and 66 days (Tulloch, 1973).…”

“…Tulloch (1973) also observed variation in the leaf waxes of several Triticum species with age. In particular, the whole plant n-alkane predominance shifted from C 31 at 24 days after germination to C 29 at 100 days after germination (Tulloch, 1973). Furthermore, Wiesenberg et al (2004 and Wiesenberg and Schwark (2006) observed changes in the nalkane and n-fatty acid compositions of a variety of temperate crop species with plant age.…”

“…Variations in the n-alkane composition could be observed during the growing season among all investigated plants, but general trends of increasing or decreasing chain length and n-alkane contents have not consistently been determined. The n-alcohol predominance also varied but to a much smaller extent, not affecting the predominance of a specific n-alcohol (Tulloch, 1973). Esters gradually showed an increase in esters of trans 2,3-unsaturated C 23 and C 24 acids with plant age (Tulloch, 1973).…”

“…For instance, much of the available information about variation in leaf wax lipid composition is presented in the plant physiological literature in studies that were not conducted with the application of such lipids as a molecular proxy for past vegetation composition from soil archives in mind (e.g., Tulloch, 1973;Avato et al, 1984;Kim et al, 2007). The fragmentation of the information makes it difficult for researchers to assess the potential influence of constraining factors on the application of molecular proxies.…”

“…For instance, in the clinical sciences molecular proxies among other applications are used as indicators of the presence of a disease or response to treatment (Brennan et al, 2013;Van Bon et al, 2014). In toxicology they are used to assess the effect of toxicant exposure on biota (Clemente et al, 2014), and in the forensic sciences they can link suspects to a crime scene (Concheri et al, 2011).…”