Agricultural loess soils along a climosequence evidenced different susceptibility to acidification by simulated N-fertilization

Abstract

Agricultural loess soils of the central region of Argentina show acidification evidences linked to both climatic conditions and N-fertilization. Because of that, simulations to estimate the future acidification trends under continuing N-fertilization, considering the different neutralization capacities of the soils in this region, were performed. An equivalent number of protons to that produced by a constant application of 180 kg urea ha−1 year−1 (84 kg N ha−1 year−1) during 1, 10, 30 and 50 years of fertilization, was added to unfertilized topsoil samples of agricultural Haplustolls, Hapludolls and Argiudolls. Mostly proton additions did not modify neither CEC nor the contents of both amorphous and crystalline Al, Mn and Fe oxides. However, the Hapludolls, located in the transition zone of the climosequence, showed decreases in their phyllosilicates crystallinity with the most acidifying treatments equivalent to 30 and 50 years of N-fertilization. This effect was less pronounced in those soils placed in both the driest (Haplustolls) and the moist (Argiudolls) environments, due to the amount and composition of the substances and/or systems with acid neutralizing capacity that prevailed. Thus, the Haplustolls were the less affected soils by acidification due to their high amount of free lime- and soil organic matter (SOM) as well as the smectitic mineralogy of their fine mineral fractions, clay and silt. The Argiudolls were the soils with stronger neutralizing mechanisms given by both their high SOM and fine mineral fractions contents, though illitic. Therefore, the Hapludolls were the most susceptible soils to being acidified if N-fertilization continues, according to the low quantity of SOM and fine mineral fractions of illitic mineralogy. From these results, the development and validation of mathematical models were assessed in order to predict the soil buffer capacity and the future pH of the soils. The soil buffer capacity was explained 78% by both the cation exchange and dissolution reactions of minerals accumulated in clays and silts, while pH values were explained 75% by the cation exchange capacity as well as by the SOM, free-lime and clay and silt contents. The pH that the soils would have in the future was predicted with an accuracy of 75% by the outcomes of the simulations, and in a 57% by the pH values of no-tilled and urea fertilized soils.