Aluminium stabilization controls organic carbon levels in Chilean volcanic soils

Abstract

Chilean volcanic soils are known to contain large amounts of organic matter (OM) and amorphous (e.g., allophanic) clay. Here, we test the hypothesis that Al, rather than clay content and climatic conditions, is the most important factor for OM levels in volcanic soils. This was achieved by compiling a total of 225 pedons from two national-wide datasets of south-Central Chilean soils sampled in the first 20 cm depth. All selected soils are of volcanic origin, previously under native Nothofagusforest and now used as pasture or arable land. The relationships between soil organic C (from now soil C) and environmental and soil characteristics (clay-allophane, Al content) were analysed by simple and multiple linear regression analyses. Soils classify as Alfisols, Andisols, Inceptisols and Ultisols. Al was extracted by KCl (Al k ), acid ammonium acetate (Al a ) and acid ammonium oxalate (Al o ). The simple regression models showed that Al a correlated best (R 2 = 0.52) with soil C variations for the four soil orders studied, while Al k (R 2 = 0.08) and Al o (R 2 = 0.17) were poorly correlated in all soils types. Especially in Inceptisols, Al a was highly correlated with soil C (R 2 = 0.88). Clay content, and hence allophanic content, was poorly correlated with soil C (R 2 = 0.14), as was the mean annual precipitation (R 2 = 0.21) and temperature (R 2 = 0.15). These patterns were supported by the multiple regression analyses. Results therefore support the idea that Al is the principal factor for immobilization of OM in acid volcanic soils. As ammonium acetate extractable Al at pH 4.8 correlated well with OM, and as this is a rarely used method to interpret pedogenic processes, this extraction method warrants further attention. D

Soil microorganisms and enzyme activity at different levels of organic matter stability

“…However, in kaolinitic soils, more crystallinity clay exerted a small stabilizing effect. In previous studies, Al-SOM complexes was the primary factor explaining soil C variation in similar soils rather than climatic variables and clay content (Percival et al, 2000;Matus et al, 2006). Neculman et al (2012) found an inverse relationship between soil pH and C pyrophosphate, supporting the hypothesis that the Al-SOM complexes and allophane formation are inverse, although complementary processes are mainly regulated by soil pH (Garrido and Matus, 2012;Panichini et al, 2012).…”

“…Regression analysis to test the effect of soil properties on C mineralization were conducted following the approach used by Matus et al (2006).…”

“…The volcanic soil is located in the Puyehue National Park (PNP) (40°26 '-40° 58'S and 72°22'-71°50 'W) at 800 m.a.s.l. (Godoy et al, 2001;Oyarzún et al, 2004;Matus et al, 2006) in a virgin forest (Nothofagus betuloides (Mirb.) with a mean annual precipitation >4,000 mm and mean annual temperature (MAT) 9.2 °C (Oyarzún et al, 2004).These soils display unique morphological, physical and chemical properties attributed to the composition of their mineral phase consisting of short range ordered (SRO) materials like allophane, imogolite, ferrihydrite and an important amount of Al-and Fe-SOM complexes (Matus et al, 2008).…”

Calcium-mediated stabilisation of soil organic carbon

“…It can occur whenever organic compounds become adsorbed to mineral surfaces (Kaiser and Guggenberger 2000;Kalbitz et al 2005), but also when the concentration of cations becomes sufficient for soluble organic polymers to flocculate and precipitate (Baldock and Skjemstad 2000). Much of the research on flocculation thresholds has focused on acid soils dominated by Al chemistry (Boudot 1992;Matus et al 2006;Rasmussen et al 2006). In these soils, it has been shown that extensive flocculation and precipitation can be expected at a C:Al ratio in the order of 10-30 or lower (Jansen et al 2003;Scheel et al 2007;Skjemstad et al 1992).…”

Arbuscular Mycorrhizal Fungi and Soil Aggregation

“…Aggregates containing clays and Al and Fe oxides or hydroxides promote soil organic C incorporation conferring aggregate stability, especially in volcanic soils. Recently, Matus et al (2006) have demonstrated that soil organic matter accumulation in Chilean volcanic soils is produced by Al stabilization rather than climatic conditions and clay content of soils. Bivalent cations such as Ca and Mg also improve soil aggregation in the same way as trivalent cations above mentioned.…”