Relationship between extractable Al and organic C in volcanic soils of Chile

In previous studies, Al extracted by acid ammonium acetate (Al_a) or Na-pyrophosphate (Al_p), rather than silt or clay content and climate conditions, was the most important factor that controls organic matter (OM) levels in volcanic soils. Here, the hypothesis was tested that Al_a is a comparable method (as much as CuCl₂) to quantify the proportion of Al bound to OM in allophanic soils. As far as we know, there are no previous antecedents in which selective dissolution method has been compared with this extractant. Secondly, we examine the effects of (a) Al, (b) silt plus clay content (particles size 0-53 µm) and (c) clay mineralogy on the control of organic carbon (OC) level in Chilean volcanic soils. This was achieved by sampling 16 soils series (11 Andisols, one Alfisol and four Ultisols, USDA classification) including 48 soil pedons up to 0.4 m depth. Soils were analyzed for Al_a, Al_p, oxalate (Al_o, Si_o and Fe_o), cold NaOH (Al_n) and un-buffered salts, CuCl₂ (Al_Cu), LaCl₃ (Al_La) and KCl (Al_k). We also measured the Al-humus as soluble C fraction after pyrophosphate extraction and the C associated to the silt plus clay fraction after sonication and gravity decantation. The statistical package (S)MATR was used to examine bivariate linear regressions among soil properties by computing the standardized major axis (SMA). Our results indicate that Al_a had a good correspondence with Al_p (R² = 0.76) in the top soil with Al_a/Al_p ratio of 0.19 and both extractans presented significant and positively relationship with soil OC (R² > 0.62). Acid ammonium acetate was as effective as Al_Cu to determine the Al-OM in allophanic soils. It is cheaper than Al_Cu and Al_p and 0.5 h shaking is required compared to 2 h of Al_Cu and 16 h of Al_p. The efficiency of the extraction was: Al_n ≥ Al_o > Al_p > Al_Cu ≥ Al_a > Al_La > Al_k. We also found that allophane content (estimated by Al/Si ratio) was strongly correlated (R² = 0.82) with the OC in the fine silt plus clay and that Al-humus together with C in the finest particles explained (R² > 0.60) the largest proportion of variation of soil OC across studied soils.