Anthropogenic alteration of available, amorphous, and total iron in an Andisol from dairy slurry applications over a 12-year period

Abstract

Iron is one of the most abundant elements in agricultural soils, but it is mostly present in non-assimilable forms. The dynamics of Fe is determined by several factors, such as organic matter (OM). Dairy slurry is used to increase total OM content in soils. The objective of this study was to determine the effect of applying dairy slurry over a 12-yr period on the levels of available, amorphous, and total Fe in an Andisol soil as indicators of pedogenic alteration. The contents of available (Fed), amorphous (Feox), and total (Fet) Fe were evaluated by selective extractions. A completely randomized experimental design with repeated measures was used, which consisted of six treatments (2, 4, 6, 8, 10, 12 yr) of slurry application and four replicates. A control treatment (no slurry application) was also included with permanent Lolium perenne L. and Trifolium repens L. grasslands. Dairy slurries were applied at a maximum rate of 150 m3 ha-1. Slurry application in the soil significantly increased Fed and OM contents up to 8 yr in the A (8.2 g kg-1 and 15.7%) and B (7.49 g kg-1 and 10.3%) genetic horizons under study; there was a positive correlation between Fed and OM. This would indicate that increased OM would accelerate the pedogenesis of this soil. In general, Fed-ox values were low and there was a significant decrease (p ≤ 0.05) in the 2-, 4-, and 6-yr treatments with values ranging between 1.0 and 0.7 for the Feox:d ratio, indicating increased pedogenesis.